EPA-600/R-02/021
January 2002
Costs of Urban Stormwater Control
By
James P. Heaney
David Sample and Leonard Wright
University of Colorado
Boulder, CO 80309
Contract No. 68-C7-0011
Project Officer
Chi-Yuan Fan
Water Supply and Water Resources Division
National Risk Management Research Laboratory
Edison, NJ 08837
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
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Notice
The information in this document has been funded by the U.S. Environmental Protection
Agency (EPA) under Contract No. 68-C7-0011 to Science Applications International
Corporation (SAIC) and its consultant. It has been subjected to the Agency's peer and
administrative review and has been approved for publication as an EPA document.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with
protecting the Nation's land, air, and water resources. Under a mandate of national
environmental laws, the Agency strives to formulate and implement actions leading to a
compatible balance between human activities and the ability of natural systems to support
and nurture life. To meet this mandate, EPA's research program is providing data and
technical support for solving environmental problems today and building a science
knowledge base necessary to manage our ecological resources wisely, understand how
pollutants affect our health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's
center for investigation of technological and management approaches for preventing and
reducing risks from pollution that threatens human health and the environment. The
focus of the Laboratory's research program is on methods and their cost-effectiveness for
prevention and control of pollution to air, land, water, and subsurface resources;
protection of water quality in public water systems; remediation of contaminated sites,
sediments and ground water; prevention and control of indoor air pollution; and
restoration of ecosystems. NRMRL collaborates with both public and private sector
partners to foster technologies that reduce the cost of compliance and to anticipate
emerging problems. NRMRL research provides solutions to environmental problems by:
developing and promoting technologies that protect and improve the environment;
advancing scientific and engineering information to support regulatory and policy
decisions; and providing the technical support and information transfer to ensure
implementation of environmental regulations and strategies at the national, state, and
community levels.
This publication has been produced as part of the Laboratory's strategic long-term
research plan. It is published and made available by EPA's Office of Research and
Development to assist the user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research
Laboratory
in
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Abstract
This report presents information on the cost of stormwater pollution control facilities in
urban areas, including collection, control, and treatment systems. Information on prior
cost studies of control technologies and cost estimating models used in these studies was
collected, reviewed, and evaluated. The collection phase involved identifying, screening,
and consolidating publications associated with capital costs of stormwater conveyance
systems and control technologies. The resulting data were evaluated to develop a critical
review of costs for urban stormwater control technologies, including identification of cost
information gaps and research needs.
IV
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Contents
Foreword iii
Abstract iv
Figures vii
Tables viii
Acknowledgment x
Chapter 1 Introduction 1
Chapter 2 Cost Estimation Methods-Literature Review 2
2.1 Forms of the Cost Equations 2
2.1.1 Single explanatory variable 2
2.1.2 Multiple explanatory variables 2
2.2 Pipe Costs 3
2.3 Manholes 6
2.4 Other Sewer Pipe Related Costs 7
2.5 Storage Costs 7
2.6 Multipurpose Facilities 7
2.7 Integrated Approaches 8
2.8 Process-Oriented Approaches 9
2.9 Stormwater Cost Optimization 9
2.10 Summary and Conclusions 11
Chapter 3 Cost Estimates for Stormwater Systems 12
3.1 Stormwater Pipelines 12
3.1.1 Pipeline installation 12
3.1.2 Trench excavation costs 14
3.1.3 Bedding costs 15
3.2 Manholes 17
3.3 Open Channels 19
3.4 Pump Stations 19
3.5 Pavement and Creation of Impervious Surfaces 19
3.6 Conclusions 21
Chapter 4 Cost Effectiveness of Stormwater Quality Controls 22
4.1 Objectives of Control 22
4.2 Control Descriptions and Construction Costs 22
4.2.1 Offline storage-release systems 22
4.2.2 Swirl concentrators 23
4.2.3 Screens 24
4.2.4 Sedimentation basins 25
4.2.5 Disinfection 25
4.2.6 Best management practices 25
4.3 Operation and Maintenance Costs for Controls 33
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Chapters Process-Level Cost Estimation 35
5.1 Case Study 35
5.1.1 Calculate the design flows into the drainage system 35
5.1.2 Sizing the sewer pipes and their slopes 45
5.1.3 Sewer system cost evaluation 48
5.2 Scenario Analysis 50
5.2.1 Management of the demand for imperviousness 52
5.2.2 Management of land use 52
5.2.3 Effect of recurrence interval 53
5.2.4 Effect of climate 53
5.2.5 Effect of assumed minimum inlet flow time 53
5.2.6 Required minimum depth of cover 53
5.2.7 Effect of pipe material 54
5.2.8 Possible number of scenarios 54
5.3 Results for the Selected Scenarios 56
5.4 Effect of Uncertainty in the Estimates 57
5.5 Summary and Conclusions on Scenarios 59
Chapter 6 Cost-Effectiveness of Alternative Micro-storm Management Options 63
6.1 Introduction 63
6.2 Literature Review 64
6.2.1 Land use/control options 64
6.2.2 Hydrology in SLAMM 67
6.2.3 NRCS method and initial abstraction 67
6.2.4 Costs of controls in Pitt's work 67
6.2.5 Control devices in SLAMM 69
6.2.6 Limitations of SLAMM 69
6.2.7 Low impact development 69
6.3 Proposed Approach 69
6.3.1 Introduction 69
6.3.2 Hydrologically functional landscaping 70
6.3.3 Cost of CN modifications 70
6.3.4 Land valuation 70
6.4 Hypothetical Study Area 77
6.4.1 Study area attributes 78
6.4.2 Unit costs 80
6.4.3 Summary of costs for each parcel 90
6.4.4 Summary of costs for each right-of-way 98
6.5 Estimated cost of BMP controls 98
6.5.1 Determination of runoff volumes using SCS method 99
6.5.2 Breakdown of calculated volumes per function 99
6.5.3 Estimated unit costs of various functional land use options 102
6.6 Results of BMP Optimization for Happy Acres 105
Chapter 7 Summary and Conclusions 106
References 108
VI
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Figures
2-1. Profile view of the vertical alignment of a stormwater system 10
2-2. Layout of the 20 pipe stormwater problem 11
3-1. Cost of storm drainage pipe 14
3-2. Trench excavation costs 15
3-3. Manhole costs, as a function of excavation depth 18
4-1. Construction costs, off-line storage 23
4-2. Construction costs for swirl concentrators, screens, sedimentation basins,
and disinfection 24
4-3. Construction cost, detention and retention basins, and off-line surface units 27
4-4. Construction cost, infiltration trenches and basins 29
4-5. Operation and maintenance costs for CSO controls 34
5-1. Study area topography 36
5-2. Study area sewer network 37
5-3. Study area land use 39
5-4. Intensity-duration-frequency curves for Boulder, CO 42
5-5. Intensity-duration-frequency curves for Houston, TX 42
5-6. Intensities vs. recurrence interval for Boulder, CO and Houston, TX
for a 20-min duration 43
5-7. Results of the five design scenarios 58
5-8. Cumulative total cost distribution 60
5-9. Tornado plot of uncertainty in scenario 61
6-1. Illustrative rainfall-runoff relationship 68
6-2. Conventional storm drainage 71
6-3. Illustration of hydrologically functional landscape 72
6-4. Study area GIS 78
6-5. Study area soils 79
6-6. Allocation of available storage for initial abstraction and land use 102
Vll
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Tables
2-1. Average Non-Pipe Costs as Percent of Total In-Place Pipe Costs
for Sanitary Sewers 7
2-2. Estimated Capital Cost of Storage as a Function of Volume 8
3-1. Lookup Table for Corrugated Metal Pipe 13
3-2. Lookup Table for Reinforced Concrete Pipe 13
3-3. Trench Excavation Costs, (Includes Backfill and Blasting) 14
3-4. Bedding Costs 16
3-5. Precast Concrete Manholes Costs 18
3-6. Capital Costs of Sewage Pump Stations 19
3-7. Paving Costs 20
4-1. BMP Pollutant Removal Ranges 32
4-2. An Assessment of Design Robustness Technology for Several BMPs 33
5-1. Sewer Network Design Hydrology 38
5-2. Mix of Land Uses in Happy Acres 40
5-3. Imperviousness for Various Land Uses 40
5-4. Runoff Coefficients for Various Areas 41
5-5. Comparison of Design Rainfall Intensities for 20-Minute
Duration Storms in Boulder, CO 43
5-6. IDF Curve Parameters for Boulder, CO 44
5-7. Sewer Network Design Hydraulics 46
5-8. Sewer Network Design Cost 47
5-9. Lookup Table for Corrugated Metal Pipe 48
5-10. Lookup Table for Reinforced Concrete Pipe 49
5-11. Excavation Costs 49
5-12. Bedding Costs 51
5-13. Summary of Cost Scenarios 56
6-1. Source Areas in SLAMM 65
6-2. Other Information Needed in a Source Area 65
6-3. Sample SLAMM Output for Toronto, ON, Canada 66
6-4. Initial Abstraction as a Function of Curve Numbers (CN) 70
6-5. Breakdown of the Cost of a Typical House 75
6-6. Breakdown of the Cost of Housing in 1984 and 1988 75
6-7. Estimated Housing Costs 76
6-8. Right-of-Way Characteristics 79
6-9. Lot Characteristics for Residential Parcels 79
6-10. Aggregate Characteristics for Commercial, Apartments, and Schools 80
6-11. Land Valuation for Medium Density Lot 80
6-12. Cost Analysis of Landscaping for Medium Density Lot 82
6-13. Land Valuation for Low Density Lot 83
6-14. Cost Analysis of Landscaping for Low Density Lot 83
6-15. Land Valuation for Commercial Areas 84
6-16. Cost Analysis of Landscaping for Commercial Areas 84
6-17. Land Valuation for Apartments 85
Vlll
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6-18. Cost Analysis of Landscaping for Apartments 85
6-19. Land Valuation for Schools 86
6-20. Cost Analysis of Landscaping for Schools 86
6-21. Costs of Pavement, Curb and Gutter, and Sidewalks 87
6-22. Cost Analysis for 50 ft Right-Of-Way 88
6-23. Cost Analysis for 60 ft Right-Of-Way 88
6-24. Cost Analysis for 70 ft Right-Of-Way 89
6-25. Cost Analysis for Parking 89
6-26. Cost Analysis for Sidewalks and Patios 90
6-27. Cost Analysis for Driveways 90
6-28. Parcel Development Costs 91
6-29. Right-of-Way Costs 98
6-30. SCS Hydrologic Classifications, and Calculation of Unit Storage Values 100
6-31. Calculation of Developed and Predevelopment Stormwater Volumes 101
6-32. Calculation of Unit Costs for Controls, Including Land Opportunity Costs 103
6-3 3. Calculation of Unit Costs for Controls, Excluding Land Opportunity Costs 104
6-34. Range of Costs for Storage 105
IX
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Acknowledgments
This report was prepared for the U.S. Environmental Protection Agency (EPA) Office of
Research and Development by Science Applications International Corporation and its
consultant under EPA Contract No. 68-C7-0011. The project was sponsored by the
National Risk Management Research Laboratory Environmental Engineering Economics
Program.
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Chapter 1
Introduction
The purpose of this report is to provide information on the cost of stormwater quantity
and quality control facilities. Information on prior cost studies of control technologies
and the cost estimating models used in these studies was collected, reviewed, and
evaluated as part of this effort. The collection phase involved identifying, screening, and
consolidating published literature, papers, reports, etc. associated with capital costs,
operation and maintenance costs, performance, and effectiveness of stormwater control
technologies. The resulting data were evaluated to develop a preliminary critical review
of stormwater control technologies. This review discusses cost-effectiveness, delineates
technology gaps, and develops a list of research needs in these areas. The prototype cost
model is presented as a spreadsheet model.
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Chapter 2
Cost Estimation Methods
2.1 Forms of the Cost Equations
2.1.1 Single explanatory variable
The traditional way to present summary results of cost estimation data is to approximate
the cost with a single variable power function shown in equation 2.1. This power
function is linear in the log transform. Thus, the data should plot as a straight line on log-
log paper. The two parameters («0 and al) can be estimated from the log-log graph or
found using linear regression on the log-transformed data. Contemporary spreadsheets
such as Excel fit the function automatically.
C = a0xai (2-1)
where
C = cost, $
a0 = site specific coefficient, e.g., location and land use
x = independent variable, i.e., some measure of component size
The exponent, <2j, represents the economies of scale factor. If a^ is less than 1.0, then
unit costs decrease as size increases. A generic economies of scale factor that has been
used for years is av = 0.6 (Peters and Timmerhaus, 1980). When av = 1, the power
function simplifies to a linear relationship and no economies of scale are present. If
av >1, then diseconomies of scale are evident.
A key reason for the popularity of the power function approximation was that it was an
efficient way to replace a database with a single equation. This feature was very
important before the widespread use of computers. The negative side of this simple
approximation is that the fit may not be that accurate. Cost is seldom a function of only
one explanatory variable.
2.1.2 Multiple explanatory variables
The cost estimation problem can be expressed in a general form as:
C = f(xl,x2,...xi,...xn) (2-2)
where
C = cost, $
xi = independent variable that is a measure of component size
If a database of historical cost estimates as a function of n explanatory variables is
available, then an approximating equation can be developed using a variety of multiple
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regression approaches. The most popular form of the estimating equation is simply to
use multiple linear regression. However, the relationship of cost to several explanatory
variables is seldom this simple.
Below is a review of historical cost relationships for materials of interest to this analysis.
These sections illustrate the development of functional forms to match the cost data at the
time, and the general development of cost estimation techniques. However, no attempt
has been made to update these equations to the present because the results are 20-30 yr.
old, and many of the key assumptions and limitations are not presented. All of the
regression models presented here assume that the independent variable is exact, i.e., that
all the error is in the independent variable, and that the error follows a normal
distribution.
2.2 Pipe Costs
Dajani et al. (1972) estimated wastewater collection network costs by fitting regression
models to data from actual construction bids. The following functional form was
assumed:
C = a + bD2+cX2 (2-3)
Where
C = construction cost, $
D = pipe diameter, ft
X= average depth of excavation, ft
Merritt and Bogan (1973) used a graphical relationship to estimate pipe construction cost
as a function of diameter and invert depth. No database accompanied this graph. Grigg
and O'Hearn (1976) present storm drainage pipe costs as a function of pipe diameter
based on data for Englewood, CO. Tyteca (1976) presents cost functions for wastewater
conveyance systems. For pipe systems, he uses functions of the following form:
(2-4)
-t
Where
C = total capital cost, $
L = length of pipe, m
K = fixed cost, $
D = diameter, m
a, b = parameters
According to Tyteca, values of b range from 1 .2 to 1.5. For the Belgium case studied by
Tyteca (1976), he developed three cost functions depending on whether the terrain is
"meadows," "river banks," or a "river in urban area." A positive fixed cost was included
in. each of these three equations and b ranged from 1.0 to 1.68. These regression
equations have little transferability in space or time.
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Han, Rao, and Houck (1980) estimated storm drainage costs as part of an optimization
model they developed. They used the following equations for estimating storm sewer
pipe costs:
For#<20,£><36 C = 1.93D + 1.6SSH-12.6 (2-5)
For H>20,D<36 C = .692D + 2.14H+ .559DH-13.56 (2-6)
For£»36 C = 3.63SD + 5.11H-111.12 (2-7)
Where
C = installation cost of the pipe, 1980 $/ft
D = diameter, in.
H = invert depth, ft
The U.S. Army Corps of Engineers (1979) MAPS software was the first to use a process
engineering oriented approach for estimating the cost of water resources infrastructure.
For gravity pipes, MAPS estimated the cost as follows:
The required input is as follows:
• Flow (maximum and minimum), MGD
• Length, ft
• Initial elevation, ft
• Final elevation, ft
• Terrain multipliers
• Design life (default = 50 yr)
• Manning's n (default = 0.015)
• Number and depth of drop manholes
• Rock excavation, % of total excavation
• Depth of cover, ft (default = 5 ft)
• Dry or wet soil conditions
• Cost overrides
The average annual cost is calculated as:
AAC = AMR + TOTOM (2-8)
Where
AAC = average annual cost, $/yr
AMR = amortized capital cost, $/yr
TOTOM= annual O&M cost, $/yr
The amortized capital cost is:
AMR = CRF*PW
(2-9)
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Where
CRF = capital recovery factor
PW= capital cost, $
The capital costs are estimated as
PW = CC + OVH + PLAND
(2-10)
Where
CC = construction costs, $
OVH= overhead costs, $
PLAND = land costs, $
Overhead costs are estimated as
OVH = 0.25* CC
(2-11)
(1+Rock* 2}
CC =AVC* WETFAC* DEPFAC* XLEN* SECI* CITY* CULT*
255.6
(2-12)
Where
AVC = unit cost of pipe for average conditions, $/ft
WETFAC = wetness factor
= 1.2 for wet soil
= 1.0 for average soil
= 0.8 for dry soil
DEPFAC = depth of cover factor
= 0.725 + 0.048 * DEPTH
(2-13)
DEPTH = depth of cover, ft
XLEN= length of pipe, ft
SECI = EPA sewer index (1957-59 = 100)
CITY = city multiplier
CULT= terrain multiplier
Rock= rock excavation percent of total excavation, in decimal form
The EPA sewer index is no longer available. The Engineering News-Record (ENR)
Construction Cost Index has been used in this report. The terrain multiplier is calculated
as:
(C7*0.8131+C2* 0.6033+C3* 0.6985+CW*0.7169+C5* 0.7911+ C6* 1.3127)
CULT= (2-14)
100
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Where
Cl = % open country
C2 = % new residential
C3 = % sparse residential
C4 = % dense residential
C5 = % commercial
C6 = % central city
The MAPS formulation is an interesting blend of regression equations and cost factors.
Unfortunately, the database for the regression equations such as for estimating terrain
effects was never presented. Thus, the user must take these equations at face value.
Moss and Jankiewicz (1982) promote the use of life cycle costing to determine the best
type of storm sewer pipe to buy. For their case study of Winchester, Virginia, three types
of sewers were being considered: reinforced concrete (service life = 75 year), aluminum
coated steel (service life = 25 year), and asphalt-coated galvanized steel (service life = 20
year). As the authors point out, service life is difficult to estimate. It depends on material
durability, in-place structural durability, abrasive characteristics of the drainage, and
corrosive characteristics of both ground water and drainage. In the case of different
service lives, the comparison should be done using a least common multiple of years, 300
yr in this case. Thus, the present worth is calculated by comparing the cost of the original
installation and three replacements for the steel pipe, 11 replacements for the aluminum
steel pipe, and 14 replacements for the galvanized steel pipe. The salvage value for each
replacement should be included. Alternatively, the equivalent uniform annual cost of
each option could be determined with the lowest annual cost used as the decision
criterion.
2.3 Manholes
For individual manholes, Han, Rao, and Houck (1980) used the following equation:
Cm= 259.4 + 56 Ah
(2-15)
Where
Cm = manhole cost, 1980 $
h = depth of manhole, ft
Dames and Moore (1978) estimate manhole costs indirectly as 36 to 38% of the total in-
place pipe cost.
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2.4 Other Sewer Pipe Related Costs
Dames and Moore (1978) present estimates of added costs associated with sanitary sewer
pipes. Their results are shown in Table 2-1. The above results indicate the vital
importance of site-specific cost data since the total additional cost is over 100%.
Table 2-1. Average Non-Pipe Costs as Percent of Total In-Place Pipe Costs for Sanitary Sewers
(Dames and Moore, 1978)
Category
Sanitary sewer miscellaneous appurtenances
Manholes
Drop manholes
Thoroughfare crossings
Stream crossings
Rock excavation
Pavement removal and replacement
Special bedding
Miscellaneous costs not categorized
Utility reconnection and removal
Total
Pipe Cost (%)
7
32
2
13
1
2
13
1
28
1
100
2.5 Storage Costs
Storage is used to detain or retain peak stormwater flows for later release at a slower rate.
Storage can improve or degrade downstream water quality, depending upon how it is
operated. Stahre and Urbonas (1993) present a detailed evaluation of urban stormwater
storage systems. Nix and Heaney (1988) show how to find the optimal mix of storage
and release or treatment rate.
Storage costs depend heavily upon land costs. Land costs range from zero, if the land is
assumed part of an easement or "donated" by the developer, to "full costs," based on the
highest alternative use of the land. A summary of selected storage cost estimation
equations is presented in Table 2-2.
Inspection of the storage estimating equations reveals that the economies of scale factor
ranges from a low of 0.40 for large reservoirs to a high of 0.83 for a combined sewer
overflow (CSO) storage basin. In addition, earthen basins cost less than 10% of the cost
of the same size concrete basin.
2.6 Multipurpose Facilities
The cost of storm drainage systems is affected by other purposes that the system serves.
For example, a combined sewer system provides the dual purposes of transporting both
wastewater and stormwater. Storm drainage systems provide local flood control but may
exacerbate water quality problems and degrade downstream receiving waters.
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Stormwater detention systems may serve as both quantity and quality controls. Streets
serve as traffic conduits and transport stormwater. An acceptable way to apportion the
costs of a multipurpose facility to individual purposes is to design systems for each
purpose independently, and then design the multipurpose system. The go-it-alone costs
and the costs for the multipurpose facility are prorated to determine the apportioned costs
(Heaney, 1997).
rable 2-2. Estimated Capital Cost of Storage as a Function of Volume
Type
Reservoir
Covered concrete tank
Concrete tank
Earthen basin
Clear well, below ground
Clear well, ground level
CSO storage basin
CSO deep tunnel
Equation
C = 160V04
C = 614V081
C = 532V61
C = 42V061
C = 495V61
C = 275V061
C = 3637V083
C = 4982V080
C ($ Units)
1,000
1,000
1,000
1,000
1,000
1,000
1,000
1,000
V (Range)
104-106
1 -10
1-10
1-10
1-10
0.01-10
0.15-30
1.8-2,000
V(Units)
Acre-ft
Mgal
Mgal
Mgal
Mgal
Mgal
Mgal
Mgal
Year
1980
1976
1976
1976
1980
1980
1993
1993
Reference
1
2
2
2
2
2
2
3
C = capital cost; V= volume
References: 'U.S. Army Corps of Engineers (1981); 2Gummerman et al. (1979); 3U.S. EPA (1993b)
2.7 Integrated Approaches
Rawls and Knapp (1972) gathered data from 70 stormwater systems in the United States
and used linear and nonlinear regression analysis to estimate total system costs as a
function of the explanatory variables shown below:
• Recurrence interval, yr
• Average ground slope, ft/100 ft
• Runoff coefficient, C
• Number of manholes and inlets
• Smallest pipe size, in.
• Largest pipe size, in.
• Total capacity, ft3/s
• Total length of lines, ft
• Total drainage area, acre
• Total developed area, acre
This approach is useful for aggregate comparative analysis among cities but the results
are quite dated.
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Earle and Farrell (1997) recently presented a mathematical model for estimating sanitary
sewer costs. They used construction cost data from R.S. Means "Site Work and
Landscape Cost Data." The output of their model is an estimate of the average cost per
house for the collection system under study. The following factors are used to estimate
the final cost per house:
City Cost Index
Bidding Conditions Factor
Hazen Williams "C" Factor
Restoration Complexity
Location (in or out of right-of-way)
Soil Conditions (influence of rock)
Ground Water
K1
K2
K3
K4
K5
K6
K7
.85-1.12
.95-1.05
1.0-1.04
.85-1.25
1.0-1.05
1.0-1.75
1.0-1.26
By selecting values of each of the above seven factors (K), the final cost per house is
estimated as:
c final = C,(K1*K2*K3*K4*K5*K6*K7)
'final
(2-16)
This approach is a big improvement over the regression approach. The R.S. Means
database is a reliable source of current information on sewer costs. The use of factors is a
way to incorporate site attributes. The major limitation of this approach is that factor
selection remains subjective. For example, the Soil Conditions Factor varies from 1.0 to
1.75. Which value should we choose? The effect of rock depends not only on its
presence but also on its location in the pipe network.
2.8 Process-Oriented Approaches
In a process-oriented approach, the cost estimation model is linked directly to a process
simulator. In the case of urban stormwater, the cost model can be linked directly to the
hydrologic and hydraulic simulators. The only current model we found that incorporates
this feature is the HYDRA computer program available as part of the Federal Highway
Administration's HYDRAIN program (FHWA 1991). This model only does simple links
between pipe costs and an assumed design. Storm sewer optimization is not included.
2.9 Stormwater Cost Optimization
While accurate cost data are essential for cost estimation, the total project cost depends
heavily upon the quality of the selected solution. Various optimization techniques for
finding the optimal design for a stormwater drainage system have been proposed, but
because of the inherent complexity of the problem these classical optimization
approaches have had very limited success.
Literature on this subject has been reviewed by Miles and Heaney (1988) who present a
spreadsheet-based trial and error approach for solving the problem. A profile view of the
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vertical alignment of a stormwater drainage system is shown in Figure 2-1 (Miles and
Heaney, 1988). The basic tradeoff is that between pipe and excavation costs. The larger
the pipe diameter, the shallower the slope that can be used, reducing excavation costs,
albeit at the expense of additional pipe costs.
Miles and Heaney (1988) reanalyzed the twenty-pipe problem shown in Figure 2-2. They
were able to demonstrate that the spreadsheet method provided a superior solution
because it depicted the pipe hydraulics more accurately and used a relatively efficient
trial and error procedure. For each trial, the spreadsheet calculates the total cost of the
design and checks to see whether the design constraints have been satisfied.
The problem is actually relatively complex. Typically, the drainage network must
discharge at a specified elevation at the outfall. For each section, the designer must select
from 8 to 10 pipe diameters among a large range of pipe slopes. If 10 pipe diameters and
10 slopes are available at each section, then 100 possible combinations need to be
checked. If one starts at the headwaters, then the calculations can proceed relatively
easily until this branch intersects another branch. For example, we can design branches
12-32 and 32-42 in Figure 2-2. Similarly, we can design branches 11-22, 22-33, and 33-
42. However, the two independently designed branches may result in different invert
elevations at node 42. The invert elevation for node 42 affects the cost of the entire
downstream pipe network. Thus, we quickly end up with thousands of possible
combinations to evaluate. Conventional designers typically evaluate very few options
and then stop once they have found a feasible solution.
MH1
MH2
Constraints:
* Maximum velocity
* Minimum velocity
* Minimum cover
ground
MH3
pipe invert
Figure 2-1. Profile view of the vertical alignment of a stormwater system (Miles and Heaney, 1988)
(Reproduced with permission of the American Society of Civil Engineers).
10
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t
32
12
42
41
52
51
71
22
33
43
T
61
53
23 34
44
91
81
out
10
62
Figure 2-2. Layout of the twenty pipe stormwater problem (Miles and Heaney, 1988)
(Reproduced with permission of the American Society of Civil Engineers).
Because existing designs are not optimized, it is difficult to compare them. It is also
difficult to do sensitivity analysis because we don't know how good the solutions are.
The lack of a systematic way to optimize sewer design is a major impediment for
improved cost-effectiveness evaluations. We have developed a new way to do this
evaluation using intelligent search techniques (Heaney et al., 1998d).
2.10 Summary and Conclusions
Virtually all cost estimates in the literature are based on the conventional approach of
fitting regression equations to cross sectional data on "as-builts." Before the widespread
availability of microcomputers, these approaches were the only viable alternative.
Unfortunately, even since the advent the personal computer, little research funding has
been available to develop the databases necessary for detailed cost estimation procedures.
Curve fitting methods are inefficient given the available technology for computerized
design calculations. An improved method is to link the cost estimator directly to the
hydraulic simulator, and then develop cost estimates relative to the fundamental
processes of an urban drainage system.
11
-------
Chapter 3
Cost Estimates for Stormwater Systems
The goal of this section is to provide the tools and data necessary to accurately estimate
the costs of conventional Stormwater systems; pipeline installation; excavation; bedding,
and manhole installation. Section on open channels, storage, pumps, and paving costs are
included as well for future reference
3.1 Stormwater Pipelines
This section describes the cost components of pipeline installation, i.e.:
1. Pipeline Installation: The pipelines themselves, and the material, labor, and
equipment necessary for installation.
2. Trench Excavation Costs: The cost of excavating and constructing the trench into
which the pipeline is installed. Backfill and rock blasting are included within this
category.
3. Bedding Costs: These include the material, labor, and equipment necessary to install a
simple compacted bedding system prior to backfilling the trench.
3.1.1 Pipeline installation
The costs of two different types of pipe were tabulated based on the data from RS Means
(1996a). All values are updated to 1/99 $ using the ENR index of 6000 for January 1999,
and 5584 for July 1995. The costs include fixed operations cost and profit, and the pipe
materials, labor, and equipment. Because of the relative cost of the materials, pipes
typically chosen for Stormwater systems are corrugated metal (CMP), and reinforced
concrete (RCP). The RS Means data was chosen for this analysis because of the longevity
of this source of data (the user of this spreadsheet can easily swap databases, however).
A plot of the total installed costs (excluding excavation and backfill) vs pipe diameter for
the CMP and RCP pipes is shown in Figure 3-1. A nonlinear relationship is readily
apparent, and a power function was fit to the data. The resulting equation below is for
CMP pipe, using the updated RS Means data:
Cp=0.54D13024 (3-1)
Where
Cp = construction cost, 1/99 $/ft
D = pipe diameter, in.
Although Equation 3.1 has a relatively high correlation coefficient (R2) of .98, it is not a
close fit for larger pipe diameters. A better way to estimate pipe costs is to use a lookup
table, which is a standard feature in spreadsheets. Lookup tables are particularly useful
for discrete data such as pipe diameters, and avoid the problem of trying to find a single
equation that fits well over a wide range of pipe sizes.
12
-------
The lookup tables for the design model is shown as Tables 3-1 and 3-2 for CMP and RCP
pipe, respectively. A major disadvantage of using equations instead of direct cost data
can be seen in Figure 3-1. The power function, although providing a good overall fit, can
deviate from the actual cost/ft data point significantly, leading to an underestimation of
project costs. However, an important advantage is that the equations provide a shorthand
method of storing the relationship between costs and capacity. Equations facilitate the
economic analytical evaluation of the component under consideration. With the use of a
spreadsheet model, however, it becomes less necessary to make simplifying assumptions
necessary to make regression fits possible, because simple lookup functions can replace
these approximating equations.
Table 3-1. Lookup Table for Corrugated Metal Pipe (updated from RS Means, 1996a)
Diameter
(in.)
8
10
12
15
18
24
30
36
48
60
72
Cost
(1/99$/ft)
9.40
11.80
14.40
18.40
20.90
30.10
37.20
54.80
81.60
118.20
179.50
Table 3-2. Lookup Table for Reinforced Concrete Pipe (updated from RS Means, 1996a)
Diameter
(in.)
12
15
18
21
24
27
30
36
42
48
60
72
84
96
Cost
(1/99$/ft)
15.70
16.60
19.00
23.00
27.60
32.90
55.80
74.40
85.40
102.30
146.70
192.60
288.90
355.60
13
-------
g 200.00
o
O 150.00
D RS Means CMP Pipe
O RS Means RCP Pipe
RS Means RCP Pipe (Power fit)
RS Means CMP Pipe (Power fit)
,0'
0.19D1 E
C= 0.54D
20
30
40 50
Diameter, in.
70
80
90
100
Figure 3-1. Cost of storm drainage pipe.
3.1.2 Trench excavation costs
Various trench excavation cost data were updated from RS Means (1996a) and plotted in
Figure 3-2. Included are such fixed operations costs as labor, equipment, and materials
costs. Although the excavation costs generally vary with depth and backhoe bucket size
(not shown here), there was no statistical relationship that could explain this variation
easily. For the purposes of the model, an average of this data was taken, which results an
average excavation cost in $/yd3 for a "moist loam" type of soil. Then, using productivity
estimates from RS Means (1996a) for various soils, the excavation costs in Table 3-3
were obtained.
Table 3-3. Trench Excavation Costs, Includes Backfill and Blasting (updated from RS Means, 1996a)
Soil Type
Clay
Moist loam
Rock
Sand
Silt
Horizontal
1
2
0
2
1.5
Vertical
1
1
1
1
1
Excavation Cost
(1/99 $/yd3)
7.09
5.87
86.29
6.12
6.72
14
-------
8.00-
7.00-
6.00-
•|5.00-
W
|
tf 4.00 -
O
O
1
O 3.00 - -
2.00-
1.00-
0.00-
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00
Excavation Depth, ft
Figure 3-2. Trench excavation costs (Updated from RS Means, 1996a).
3.1.3 Bedding costs
Bedding provides sufficient compacted material necessary to protect the pipe from
external loading forces. Bedding costs in the RS Means (1996a) system vary with
diameter and side slope of the trench. The bedding material is compacted bank sand
filled to 12 in. above the pipe. These costs were updated to 1/99 $ and can be found in
Table 3-4. This table relates the horizontal and vertical side slope, the diameter, and the
width to bedding costs, which include fixed operations cost and profit. Although several
regression relationships were evaluated, it was decided that the most accurate model of
these costs would be a two-way lookup table, relating the horizontal:vertical ratio and the
pipe diameter to the projected cost.
15
-------
Table 3-4. Bedding Costs (updated from RS Means, 1996a)
Horizontal
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1.5
1.5
1.5
1.5
1.5
Vertical
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
HA/
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1.5
1.5
1.5
1.5
1.5
Diameter
(in.)
6
8
10
12
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
14
Trench
Width (ft)
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
3
Cost
(1/99$/ft)
0.92
2.00
2.07
2.12
3.47
3.51
3.57
3.62
5.25
5.29
5.44
5.55
9.72
9.98
13.01
16.23
23.39
31.80
1.90
3.16
3.43
3.67
5.25
5.39
5.55
5.88
1.11
7.95
8.52
9.56
14.06
15.08
20.58
26.81
37.47
49.71
2.90
4.36
4.77
5.25
7.06
7.30
7.56
8.14
10.28
10.59
11.61
13.50
18.46
20.17
28.17
37.40
51.76
67.70
3.91
5.69
6.15
6.81
8.83
16
-------
Horizontal
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Vertical
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
HA/
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Diameter
(in.)
15
16
18
20
21
24
30
32
36
48
60
72
60
72
84
6
8
10
12
14
15
16
18
20
21
24
31
32
36
48
60
72
84
Trench
Width (ft)
3
3
3
4
4
4
4
6
6
7
8
10
8
10
12
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
Cost
(1/99$/ft)
9.18
9.56
10.38
12.80
13.24
14.63
17.64
22.77
25.23
35.76
48.21
65.65
48.21
65.65
86.16
5.01
6.73
7.49
8.37
10.59
11.04
11.54
12.66
15.32
15.89
17.71
21.61
27.15
30.22
43.22
58.67
79.32
103.94
3.2 Manholes
Manhole cost data, updated from RS Means (1996a), are tabulated in Table 3-5. The
costs include fixed operations cost and profit, and labor, equipment, and materials costs
for installation of precast concrete manholes. A plot of this data can be found in Figure
3-3. A power relationship was plotted and the following equation obtained:
0.9317
(3-2)
Where
Cmh = cost of manhole, 1/99 $
H = height of manhole, ft (maximum difference between the ground
elevation and the invert elevations of sewers entering the manhole)
In general, the fit of the power equation was good, particularly at the lower heights.
Some inaccuracies are introduced due to the regression relationship, however this is
mitigated by the desire within the system model for a continuous function providing cost
as a function ofH. An alternative method is to use a lookup table and interpolate
between the values of Table 3-5.
17
-------
Table 3-5. Precast Concrete Manholes Costs (updated from RS Means, 1996a)
7,000
6,000
5,000
2 4,000
3,000
2,000
1,000
Riser Internal
Diameter (ft)
4
4
4
4
4
4
5
5
5
5
5
5
6
6
6
6
6
6
Depth
(ft)
4
6
8
10
12
14
4
6
8
10
12
14
4
6
8
10
12
14
Cost
(1/99 $/ft)
1,860
2,460
3,250
3,970
4,830
6,060
2,310
3,120
3,970
5,070
6,260
7,600
3,150
4,070
5,340
6,710
8,350
9,990
C = 482H0'9
10
12
16
Excavation Depth, ft
Figure 3-3. Manhole costs, as a function of excavation depth.
18
-------
3.3 Open Channels
The cost of open channels needs to be estimated on a case by case basis since cut and fill
calculations are required. Excavation costs are an important component of the
construction of an open channel. MAPS (US Army Corps of Engineers, 1979) provides a
general template for doing these calculations. The data presented in Table 3-3 on
excavation costs may assist in this effort.
3.4 Pump Stations
Two different sized sewage pump stations are available in the RS Means database, as
shown in Table 3-6. The costs include fixed operations cost and profit, and labor,
equipment, and materials costs. An alternative method for calculating a pump station
cost would be to develop a generic design of the structure that would be scaled based
upon capacity and head, and include the appropriate pump costs. This work is beyond
the scope of this effort.
Table 3-6. Capital Costs of Sewage Pump Stations (updated from RS Means, 1996a)
Description
Sewage Pump Station
Sewage Pump Station
Flow Rate
(gpm)
200
1000
Cost
(1/99 $)
59,000.00
112,000.00
3.5 Pavement and Creation of Impervious Surfaces
Fairly good data are available on the cost of various types of pavement, including porous
pavement. Table 3-7 lists the main activities associated with paving and creation of
impervious areas within developments. The costs include fixed operations cost and profit,
and labor, equipment, and materials costs. An example of the use of this data is the
following: Using a 32 ft wide subdivision street, with 6 in. crushed stone base material of
ll/2 in. in diameter, a primer, and a wearing course of ll/2 in. of asphaltic concrete
pavement, and curb and gutter (both sides) sums to a total of $58.80 per linear foot of
pavement. This is shown below:
Base course:
Prime:
Paving:
Curb:
5.85
$
yd2 9/r
•*32ft = 20.SOS///1
$ . yd
1.82 * * 32ft = 12.94 $/ft
gal 9ft
3.14-
X
9/r
•* 32 ft = 11.16$/'ft
6.95 $//?* 2 = 13.90 $/ft
Total per linear ft: $20.80+ $12.94+ $11.16+ $13.90 = $58.80
(3-3)
(3-4)
(3-5)
(3-6)
(3-7)
19
-------
Table 3-7. Paving Costs (updated from RS Means, 1996a)
Activity
Prepare and Roll Subbase >2500 yd'
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Base Course
Prime and seal
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Asphaltic Concrete Pavement
Curb and Gutter, machine formed
Material
Crushed Stone
Crushed Stone
Crushed Stone
Crushed Stone
Crushed Stone
Crushed Stone
Crushed Stone
Crushed Stone
Bank run gravel
Bank run gravel
Bank run gravel
Bituminous
concrete
Bituminous
concrete
Bituminous
concrete
Bituminous
concrete
Binder Course
Binder Course
Binder Course
Binder Course
Wearing Course
Wearing Course
Wearing Course
Wearing Course
Wearing Course
Concrete
Diameter
(in.)
0.75
1.5
24
Unit
yd'
yd^
yd'
yd^
yd'
yd^
yd'
yd'
yd'
yd'
yd'
yd'
yd^
yd^
yd'
yd'
gal
yd'
yd'
yd'
yd^
yd'
yd^
yd'
yd^
yd'
LF
Depth
(in.)
3
6
9
12
4
6
8
12
6
9
12
4
6
8
10
1.5
2
3
4
1
1.5
2
2.5
3
Cost
(1/99 $)
0.88
3.39
6.07
8.92
11.49
3.52
5.85
7.82
12.36
2.63
3.22
5.10
8.37
12.04
15.86
19.58
1.82
3.14
4.09
5.91
111
2.31
3.44
4.52
5.47
6.51
6.95
Note: gal = gallon; yd = square yards; LF = linear foot.
This unit cost ($/ft) is for a lightly traveled subdivision street. As the projected traffic
increases, the thickness used increases, thereby increasing the cost per linear foot.
This data is presented so that the cost of transportation related impervious surfaces is
included in the system model.
20
-------
3.6 Conclusions
In summary, detailed databases exist that can provide accurate cost information. The use
of lookup tables, database functions, and regression (limited use where appropriate), a
system model providing generic costing relationships can be built. Systematic evaluation
of different designs through simulation enables repeated testing of various designs,
leading to a method for optimization.
21
-------
Chapter 4
Cost Effectiveness of Stormwater Quality Controls
4.1 Objectives of Control
Stormwater quality control is used to reduce pollutant loadings from urban runoff events.
In most cases, the volume and peak flow of the event has a direct bearing on the
discharge quality. Some facilities, where the local regulatory focus was on peak flow
reduction are now being reevaluated for quality control as well.
4.2 Control Descriptions and Construction Costs
Predominant Stormwater quality controls are outlined in the following sections and
available cost information on them is provided. Detailed cost data were not available for
most of these systems, and so design guidance cost curves were updated from several
references. This approach would be more viable if the sample size was large. However,
the sample sizes are not available for the bulk of these data.
4.2.1 Offline storage-release systems
Storage-release systems are designed to intercept effluent and retain it for a
predetermined time-period prior to its discharge into receiving waters. Before the
effluent is released from the storage unit, it has undergone some physical settling, and,
perhaps some biological treatment. The two main types of storage systems evaluated
here are surface storage and deep tunnels.
4.2.1.1 Surface storage
Surface storage units are offline storage, at or near the surface, and are typically made of
concrete. Typically, large diameter culverts are used. The best source of empirical cost
data on surface storage can be found in US EPA (1993), which relates cost as a function
of size, or volume of the facility. This relationship has been updated to 1/99 $ and is
found in equation 4.1:
C = 4.546F°'826
(4-1)
Where
C = construction cost, millions 1/99 $
V= volume of storage system, Mgal (where 0.15< F<30 Mgal)
Equation 4.1 has been plotted in Figure 4-1 for the applicable range of volumes.
22
-------
1000
w
1 100
ist. Cost, Million
o
o
O
1
0 1
-•
^x
#
X
*
X
„
X
*
X
^ *
,'
.^
f *~
• V
X
x
-
x
','
, • '.s*
x^
tr
c =
* '
4.i
x
4t
x
V
*s
c = e.;
-i>
#g>x^
^
28V
#*X*
.79J
x1
.1*
J#
,x^
EPA CSO Deep Tunnels
EPA CSO Storage
0.1
10 100
Volume, Mgal
1000
10000
Figure 4-1. Construction costs of offline storage. (Updated to 1/99 $, ENR = 6000, Adapted from US
EPA, 1993)
4.2.1.2 Deep tunnels
Deep tunnels, bored into bedrock have been used increasingly in urban areas because
space is unavailable for surface storage units. Although they function similarly to surface
storage units it is difficult to add biological treatment enhancements or baffling to
tunnels. US EPA (1993) is currently the best source of data on the cost of deep tunnels.
This source relates cost as a function of size, or storage volume. This relationship has
been updated to 1/99 $ and is expressed in equation 4.2:
C = 6.228F
0.795
(4-2)
Where
C = construction cost, million 1/99 $
V = volume of storage system, Mgal (where 1.8 < V< 2,000 Mgal)
Equation 4.2 has been plotted in Figure 4-1 for the applicable range of volumes.
4.2.2 Swirl concentrators
Swirl concentrators use centrifugal force and gravitational settling to remove the heavier
sediment particles and floatables from urban runoff. They are typically used in CSO
23
-------
situations, but may also be used in general urban runoff events (US EPA 1993). These
devices alone do not provide any means to reduce peak discharge, they are commonly
used in conjunction with some form of storage, and their performance varies (Urbonas,
1999).
The best source of data on swirl concentrators is currently US EPA (1993), which relates
cost as a function of size, or, in this case, design flow. This relationship has been updated
to 1/99 $ and is expressed in equation 4.3:
C = 0.22Q
(4-3)
Where
C = construction cost, millions 1/99 $
Q = design flow rate, MOD (where 3 < Q < 300 MOD)
Equation 4.3 has been plotted in Figure 4-2 for the applicable range of flows.
•Sedimentation
-Swirl
-Screens
-Disinfection
0.1 1 10 100
Design Flow Rate, MGD
Figure 4-2. Construction costs for swirl concentrators, screens, sedimentation basins, and
disinfection. (Updated to 1/99 $, ENR = 6000, Adapted from US EPA, 1993).
4.2.3 Screens
Coarse screens are used to remove large solids and some floatables from CSO discharges.
US EPA (1993) is the best current source of available cost data. Cost is expressed as a
24
-------
function of size, or design flow. This relationship has been updated to 1/99 $ and is
shown in equation 4.4:
C = 0.09g°843 (4-4)
Where
C = construction cost, millions 1/99 $
Q = design flow rate, MOD (where 0.8 < Q < 200 MOD)
Equation 4.4 has been plotted in Figure 4-2 for the applicable range of flows.
4.2.4 Sedimentation basins
Sedimentation basins detain stormwater to allow physical settling prior to its discharge.
These basins are usually baffled to eliminate short circuiting of the flow. US EPA (1993)
is the best current source of cost data on sedimentation basins. This source relates cost as
a function of size, or design flow. The relationship has been updated to 1/99 $ and is
expressed in equation 4.5:
C = 0.2810°'668 (4-5)
Where
C = construction cost, millions 1/99 $
Q = design flow rate, MOD (where 1 < Q < 500 MOD)
Equation 4.5 has been plotted in Figure 4-2 for the applicable range of flows.
4.2.5 Disinfection
Disinfection is used to kill off pathogenic bacteria prior to a CSO discharge. The best
current source of data on disinfection (chlorination without dechlorination) is US EPA
(1993). This source relates cost as a function of size, or design flow. This relationship
has been updated to 1/99 $ and is expressed in equation 4.6:
C = 0.1610°'464 (4-6)
Where:
C = construction cost, millions 1/99 $
Q = design flow rate, MOD (where 1 < Q < 200 MOD)
Equation 4.6 has been plotted in Figure 4-2 for the applicable range of flows.
4.2.6 Best management practices
The term "Best Management Practices" (BMPs) is used for any practice meant to control
and manage the quality or quantity of urban runoff (Urbonas, 1999). This definition
delineates stormwater BMPs as structural and nonstructural. Structural BMPs include
25
-------
such devices as detention basins, retention basins, infiltration trenches or basins. They
are typically constructed as part of the urban development process to mitigate the
deleterious effects of urban runoff. A key BMP, minimizing the directly connected
impervious area, is not included in this analysis as very little data is available on its cost
(Urbonas, 1999). The more typical, nonstructural BMPs, include such activities as street
sweeping and public education on the disposal of pollutants, e.g., oils. These methods are
more difficult to assess.
4.2.6.1 Detention basins
Detention basins are storage basins designed to empty after each storm. These basins are
most common in rapidly developing urban areas. They use an undersized outlet which
causes water to back up and fill the basin (Ferguson, 1998). The rate of discharge
depends upon the outlet size and is usually set by local standard. Detention basins
attenuate the peak runoff from the developed area. These basins perform well in
controlling local water quantity impacts of urban runoff. If the outlet is designed
appropriately, water quality can also be controlled to some extent.
The best current source of cost information is Young et al. (1996), which gives cost as a
function of storage volume as shown in equation 4.7:
C = 55,OOOF069
(4-7)
Where:
C = construction cost, 1/99 $
V= volume of basin, Mgal
The construction costs have been updated to 1/99 $. Land costs were excluded. This
relationship is plotted in Figure 4-3. Off-line surface storage for CSO controls is plotted
alongside these for comparison purposes. The basis for this relationship is a study done
for the Metropolitan Washington Council of Governments (Wiegand et al., 1986).
26
-------
°
o
o
°
o
8
1
o
^^X
^
-
^
* \
o
93.001
* **
^
0
X
^
--
0.01
x
^
•1
X
r
P
X
«•
x
^
,^
^
A\
0.1
i'
C = 4.5^
j<
b(b
xf
x"
fO(
j(<
x^
3)\'
^'
•'
— •
1
— ~S*
,X^
*''
**^^*
^-
'_4*
if-
X
^
,'
^
J ~
Retention Pond
- - Detention Pond
- - EPA CSO Storage
^^^
:x*
x
x
069
10 100 10C
Volume, Mgal
Figure 4-3. Construction costs of detention, retention, and offline surface units (Adapted from
Young et al, 1996).
4.2.6.2 Retention basins
Retention basins are similar to detention basins, except that the permanent pool is
increased. By increasing the permanent pool, (i.e., the point at which discharge occurs),
in the storage volume (and typically increasing the storage size as well), increased
physical and biological treatment occurs due to the longer residence time in the basin.
These types of basins are called retention basins, or wet ponds. The amount of physical
storage available is determined by the difference between the height set as the permanent
pool volume and the height above the top of the weir or outlet structure available, or
freeboard. Because cost depends upon volume, retention basins are more costly in
controlling the same amount of peak discharge as a dry detention basin from a quantity
standpoint.
The best available cost data on retention basins is found in Young et al. (1996), which
gives cost as a function of the total volume of the pond (not the available storage). This
relationship is:
C = 61,OOOF075
(4-8)
27
-------
Where:
C = construction cost, 1/99 $
V= volume of pond, Mgal
The construction costs have been updated to 1/99 $. Land costs were excluded. This
relationship is plotted in Figure 4-3. The basis for this relationship is a study done for the
Metropolitan Washington Council of Governments (Wiegand et al., 1986). The data
behind this relationship was not reported.
4.2.6.3 Infiltration trenches
Infiltration is the process of runoff water soaking into the ground. Since infiltrated water
is removed from surface waters, it represents a complete control for that fraction of
stormwater that can be infiltrated (Ferguson, 1998). An infiltration trench is used in areas
where space is a problem. It usually consists of excavating a void volume, lining the
volume with filter fabric to keep out fine material, installation of conveyance piping, and
filling the void with gravel or crushed stone. The trench's performance depends greatly
upon the soil characteristics of the area, and operating and maintenance practices
(Urbonas, 1999).
The best available cost data on infiltration trenches is found in Young et al. (1996), which
gives cost as a function of the total volume of the trench. This relationship is:
C = 157F°'63
(4-9)
Where:
C = construction cost, 1/99 $
V= volume of trench, ft3
The source did not list the data for this relationship. The construction costs have been
updated to 1/99 $. Land costs were excluded. This relationship is plotted in Figure 4-4.
4.2.6.4 Infiltration basins
Infiltration basins are similar to retention ponds; however, they are typically used in
flatter terrain, and discharge only in low frequency events. Permeable soils underlying
the basin and high rates of evapotranspiration are the major prerequisite for using these
basins. The water typically can only leave via percolation into the groundwater, or
evapotranspiration. Performance in buffering runoff water quality is high; however, from
a quantity standpoint, a large land area must be used to control significant runoff events.
A major disadvantage is the high maintenance involved due to clogging of the basin.
28
-------
4
4
/P
0
x
('
/
~^
^~~
^ '
100
X
'
'
/
/
,''
/
\
^
1000
^^
,f
t*
,'
t
/
,
t
/
t
C= 1
J
,x
«
^
<
'
57(
^
/
Vl)°6
^
. /
^
1
/
^
^
X
C = 15. 3('
- - Inf
'
J
X
/,)'
^
69
,,
/'
t
j '
X*
s
>'
/
Itration Basin
Itration trench
X
r1
j
X
^L —
/
10000 100000 1000000 10000000 100000000
Volume, ft3
Note: Y!= trench volume; V2=basin volume
Figure 4-4. Construction cost, infiltration trenchs and basins (Adapted from Young et al., 1996).
The best available cost data on infiltration basins is found in Young et al. (1996), which
gives cost as a function of the total volume of the basin. This relationship is:
= 15.3F
0.69
(4-10)
Where:
C = construction cost, 1/99 $
V= volume of infiltration basin, ft3
The construction costs have been updated to 1/99 $. Land costs were excluded. Equation
4.10 is plotted in Figure 4-4. The basis for this relationship is a study done by Schueler
(1987). The data that this relationship was based upon were not reported.
4.2.6.5 Sand filters
Sand filters remove sediment and pollutants from runoff. Usually the filters have a
presettling chamber to induce settling of the larger solids that would typically clog the
29
-------
sand filter itself. The filtered outflow is collected, rather than infiltrated, and either
discharged, or treated further. Performance of these systems is typically good in space-
limited areas and in arid climates (Young et al., 1996).
The best available cost data on sand filters is found in Young et al. (1996), which gives
cost as a function of the total impervious surface area draining to the filter. This
relationship is found in equation 4.11:
C = KA
(4-11)
Where:
C = construction cost, 1/99 $
A = impervious surface, acres
K = constant, ranging from 11,200 to 22,400
The construction costs have been updated to 1/99 $. Land costs were excluded. The
basis for this relationship is a study done for the Metropolitan Washington Council of
Governments (Schueler 1994). The data behind this relationship was not reported.
4.2.6.6 Water quality inlet
Water quality inlets are inlets modified for the control of some solids, oil, and grease.
These are sometimes referred to as oil and grit separators. According to Urbonas (1999),
the performance of these devices has not been very good.
The best available cost data on water quality inlets is found in Young et al. (1996).
Updated to 1/99 $, the costs range from $7,200 to $21,500. The basis for this relationship
is a study done by Schueler (1987). The data behind this relationship was not reported.
4.2.6.7 Grassed swales
Grassed swales are vegetated channels used in lieu of the traditional concrete curb and
gutter typical of urban areas. Pollutants are removed through filtration by vegetation,
settling, and infiltration into the soil (Young et al., 1996). The performance of these
systems is highly variable. The use of swales is not recommended in dense urban areas
where space is at a premium, or in commercial/industrial areas where contamination of
groundwater can occur due to oils and grease in the effluent (Urbonas, 1999).
The best available cost data on grassed swales is found in Young et al. (1996), in which
cost is found to vary as follows:
C = KL
(4-12)
Where:
C = construction cost, 1/99 $
L = length of swale, ft
30
-------
K= constant, 5 to 14
The construction costs have been updated to 1/99 $. No land costs were included in this
analysis. These costs can be significant because an increased right-of-way is needed to
include the swale. The basis for this relationship is a study done by Schueler (1992).
The data behind this relationship was not reported.
4.2.6.8 Vegetated filter strip
Vegetated filter strips are located adjacent to an impervious surface and gradually sloped
to allow overland flow to run slowly across the vegetation. Pollutants are adsorbed and
filtered by the vegetated material. High volumes or velocities are not appropriate for
these types of areas (Young et al., 1996). Good removal of pollutants can be achieved,
assuming the width of the strip is sufficient (Urbonas 1999). No cost information is
available, because the designs are highly variable (Young et al., 1996).
4.2.6.9 Wetlands
Wetlands are a modification of the retention pond/infiltration pond to include a broad,
shallow, shelf that is inundated periodically under low frequency events. Under these
conditions a littoral wetland ecosystem is planted, or allowed to form. The design of
wetlands is similar to that of the retention pond, but because of relatively high adsorption
surfaces and high levels of biological productivity, wetland pollution removal rates tend
to be better (Young et al., 1996). Cost information is not given, as the designs are highly
variable (Young et al., 1996)
4.2.6.10 Porous pavements
Porous pavements are a modification to asphalt pavements to allow some infiltration to
occur. A berm is used to trap water and contain it on site. Typically, porous pavement
infiltration rates are much lower than rates in infiltration basins, although similar
treatment characteristics can occur. This method is reserved for low traffic areas because
high vehicular traffic can damage the pavement due to "pumping" of groundwater.
(Young et al., 1996). Porous pavements can also be negatively affected by freezing
temperatures due to frost heave. Cost information is not given, because the designs are
highly variable (Young et al., 1996)
4.2.6.11 Nonstructural BMPs
Nonstructural BMPs include such management practices as street sweeping, and
educational programs, e.g., on oil recycling. Although important benefits may result
from these activities, they are typically difficult to measure, and when measured, usually
the constituent measured may have not a causal relationship with a variable that directly
affects receiving water quality (Urbonas, 1999). Because of their indirect nature, detailed
cost information is not available (Heaney et al., 1998c).
31
-------
4.2.6.12 Assessment of BMP control performance
An overall assessment of structural BMP control performance can be found in Table 4-1
(Urbonas, 1999). The table lists expected removal ranges for total suspended solids, total
phosphorus, total nitrogen, zinc, lead, BOD, and bacteria, compiled from several different
sources. Urbonas (1999) however, cautions the use of the table alone, he argues that the
definition of "effectiveness is fundamentally flawed, as it is typically a snapshot in time,
and ignores the performance of the control over time, and the variability of maintenance
to the control." For example, porous pavement is excellent at removal of solids, but is
certainly not designed to do so and will clog very quickly if a high solids loading is
applied to it.
Table 4-1. BMP Pollutant Removal Ranges (Urbonas, 1999)
Structural BMP
Porous Pavement
Grass Buffer Strip
Grass Lined Swale
Infiltration Basin
Percolation Trench
Retention Pond
Extended Detention
Wetland Basin
Sand Filters (fraction
flowing through filter)
Removal Range (%)
TSS
80-95
10-20
20-40
0-98
98
91
50-70
40-94
14-96
Total P
65
0-10
0- 15
0-75
65-75
0-79
10-20
(-)4 - 90
5-92
TKN
75-85
0-10
0- 15
0-70
60-70
0-80
10-20
21
0)129 - 84
Zinc
98
0-10
0-20
0-99
95-98
0-71
30-60
0)29 - 82
10-98
Lead
80
N/A
N/A
0-99
N/A
9-95
75-90
27-94
60-80
BOD5
80
N/A
N/A
0-90
90
0-69
N/A
18
60-80
Bacteria
N/A
N/A
N/A
75-98
98
N/A
50-90
N/A
N/A
Note: The above-reported removal rates represent a variety of site conditions and influent-effluent concentration
ranges. It is not appropriate to use the averages of these rates for any of the reported constituents as design objectives
for expected BMP performance or for its permit effluent conditions. Keep in mind that influent concentrations, local
climate, geology, meteorology and site-specific design details and storm event-specific runoff conditions affect the
performance of all BMPs.
Urbonas (1999) advocates a more design-oriented approach in assessing control
performance. An example of this approach is found in Table 4-2. While subjective, this
approach does provide the designer with enough information to evaluate the control
under a wider range of conditions than the regulatory approach found in Table 4.1.
However, much more work needs to be done in this area to properly assess the expected
benefits of the BMP control in question.
32
-------
Table 4-2. An Assessment of Design Robustness Technology for Several BMPs (Urbonas, 1999.)
Structural BMP
Swale
Buffer (filter) stripb
Infiltration basin0
Percolation trench
Extended detention (dry)
Retention pond (wet)
Wetland
Media filter
Oil separator
Catch basin inserts
Monolithic porous pavementb
Modular porous pavementb
Hydraulic
Design8
High
Low - Moderate
Low - High
Low - Moderate
High
High
Moderate - High
Low - Moderate
Low - Moderate
Uncertain
Low - Moderate
Moderate - High
Removal of Constituents in
Stormwater
TSS
Low - Moderate
Low - Moderate
High
High
Moderate - High
High
Moderate - High
Moderate - High
Low
N/A
Moderate - High
Moderate - High
Dissolved
None - Low
None - Low
Moderate - High
Moderate - High
None - Low
Low - Moderate
Low - Moderate
None - Low
None - Low
N/A
Low - Highc
Low - High0
Overall Design
Robustness
Low
Low
Low - Moderate
Low - Moderate
Moderate - High
Moderate - High
Moderate
Low - Moderate
Low
N/A
Low
Low - Moderate
aWeakest design aspect, hydraulic or constituent removal, governs overall design robustness.
bRobustness is site-specific and very much maintenance-dependent.
°Low-to-Moderate whenever designed with an underdrain and not intended for infiltration;
4.3 Operation and Maintenance Costs for Controls
Operation and maintenance cost data for controls are only available for a limited number
of CSO-type controls; i.e., sedimentation, disinfection, and screens. CSO-type controls
are expected to be significantly more expensive in terms of operating and maintenance
costs than those controls that handle only stormwater, however, no data were available
(beyond anecdotal) for non CSO-type controls. These relationships can be found in
Figure 4-5 from US EPA (1993). For a complete cost/benefit analysis of each control,
one needs operating and maintenance costs to complete a life-cycle cost analysis (LCA).
LCA is done by bringing all controls to the same design life (by including replacements
as necessary), amortizing the control over the same period, and including in this annual
cost the annual operating and maintenance cost for each control. LCA is then compared
to the benefits of the control.
33
-------
o
£
n
^H
sr
^
«
iis
«
in
«
Sc
Scree
reens, ;
ns, 10 C
^^
Sa
I
»OF
Fevc
*»'"' *
ims
eve
sits/
i^
nta
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nts/
yr
lor
I
/r
^
w
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^
&
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3rrical treatm
^^ M
<*'
/
/
ant /--
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^
.«-
Sa
^
Jime
-^
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nta
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t
ior
Screens, 10CFevents/yr
Screens,
Osinfect
• • Sedimen
30 CF events/yr
on
tation
i Chemical Treatment
i_f
10
100
1000
Design Row, MGD
Note: OF = overflow
Figure 4-5. Operation and maintenance costs for CSO controls (Adapted from US EPA 1993).
34
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Chapter 5
Process-Level Cost Estimation
The utility of the cost estimation models presented herein can be illustrated by applying
them to a proposed design. By automating the design in a spreadsheet model, many
different designs can be evaluated.
5.1 Case Study
During the literature review, several urban stormwater design case studies were evaluated
to determine if any of them were suitable to demonstrate process-level cost estimation.
Tchobanoglous 1981 presents calculations for designing sanitary and storm sewers for
the same study area. The total area is approximately 106 acres. The topography of the
study area is shown in Figure 5-1. The highest part of the drainage area is on the north
side. All drainage ultimately goes to a local brook. The layout of the storm sewer system
is shown in Figure 5-2. The entire study area is divided into 54 sub-areas that range in
size from 0.8 to 3.4 acres in size. A spreadsheet was designed to incorporate all of the
necessary information for design by trial and error. The calculations are presented in
tables 5.1, 5.2 and 5.3, which are described below.
5.1.1 Calculate the design flows into the drainage system
Table 5-1 consists of 69 rows and 20 columns. Each row designates a link in the
drainage network. The land use for the total area is shown in Figure 5-3. Total land use
consists of the mix of uses shown in Table 5-2. The dwelling units/acre for each link is
listed in column 7 of Table 5-1 (except for commercial and schools, which are listed as -
1 and -2, respectively). The percent imperviousness is related to land use as shown in
Table 5-3. Two cases will be considered: existing zoning practices, and low impact
development (LID) land use practices. If LID is used, the imperviousness for all land
uses is assumed reduced by 30%. Column 8 of Table 5-1 is then computed, listing the
impervious percentage for each link. Column 9 of Table 5-1 is the multiplication of the
drainage area in acres from column 6 times the impervious percentage of column 8.
Column 10 of Table 5-1 is the impervious coefficient of the impervious area, nominally
1.0. Column 12 of Table 5-1 is the impervious area, or column 9, totaled within each
branch. Column 13 is the permeable area within the link, or the total area minus the
impervious area. Based upon the land use, through a lookup table, a runoff coefficient is
assigned in column 14. A cumulative runoff coefficient is calculated in column 15.
Column 16 is computed by assuming an initial flow time of 20 min, and summing the
previous link in the branch's time in column 17. Column 17 is calculated by dividing the
distance in column 5 by the Manning velocity for the design pipe diameter. Column 18 is
the sum of columns 17 and 16. Column 19 is the rainfall intensity for the given time in
column 18. Column 20 is computed by the Rational Method, to be explained later.
35
-------
Z7,
(elevations in meters; 1 m = 3.281 ft)
Figure 5-1. Study area topography (Adopted from Tchobanoglous, 1981).
(Reproduced with permission of The McGraw-Hill Companies).
36
-------
Figure 5-2. Study area sewer network.
37
-------
Table 5-1. Sewer Network Design Hydrology
•
5 ='»
s s ? B a
^ o y m » o
i
CM CM CM
3 CO P3
CJ CNJ OJ
?•* CN 00 CO 1^
. V -r co co M
CM CN CM CM CN CM
CM'CM'CNCMCM'CN'CMCNCM'CNC
si 8 as
s ;
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S3S8
CSS 83S S
sis s sss s
2 S SS
Si Si
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si si
E
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to T- o o o
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R S S 5 8 3 S
o d d i- d T-
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5 SI
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d ^ d d ^ ^ W
1- T- T- O
85
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88
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8 S 3 §8
88
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8 8
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iou
, IA
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SUES
5$
8 S 8 S
SS
S & 5 S S S
c ay co to g g oo
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SS
SS
SS
SS
in m in in in
10 m in m 10 in
in m in u>
CNCMCNCNCNCNC
8 8 S S
CM CO CM O
S 2 S85C 5
CM CM CM O T^ T-; T-:
•^ CM i^ d CM CM ^
as
s. sas
(M CM CM ^
ss
SS
in
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s'
z E
i- CN co •*• in
SSSSoSSSS
6
f
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ss
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1
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Si g 5 S
1 i
0 C
-g -s
< <
s s s
38
-------
Location of.
tmnk norm
tew
Figure 5-3. Study area land use (Adapted from Tchobanoglous, 1981).
(Reproduced with permission of The McGraw-Hill Companies).
39
-------
Table 5-2. Mix of Land Uses in Happy Acres
Land Use
Residential, low density
Residential, medium density
Apartments
School
Commercial
Total
Area
(acres)
20.8
51.7
10.0
5.7
18.4
106.6
Dwelling
Density
(units/acre)
2-3
5
10
N/A
N/A
Table 5-3. Imperviousness for Various Land Uses (Heaney et al., 1998d.)
Dwelling
(units/acre)
1
2
3
4
5
6
7
8
9
10
11
12
Commercial
School
Imperviousness
(%)
30
35
40
43
46
48
50
52
54
56
58
60
80
35
The runoff coefficients for impervious and permeable areas are shown in Table 5-4.
Runoff coefficients for permeable areas depend on the soil type.
The expected peak runoff is calculated using the Rational Method, or
= CiA
Where
Q = estimated peak flow, ftVs
C = runoff coefficient
/' = rainfall intensity, in./hr
A = contributing drainage area, acres
(5-1)
40
-------
Table 5-4. Runoff Coefficients for Various Areas
Description
Directly connected impervious area
Other impervious area
Pervious areas Soil Type:
Sand
Silt
Clay
Rock
Runoff
Coefficient
1
0.7
0.2
0.3
0.5
0.7
The runoff coefficient is calculated in Table 5-1 as the weighted average of the runoff
coefficients from the impervious and permeable areas. The total drainage area is
calculated by summing the contributing drainage areas. The design rainfall intensity is
established by calculating the time of concentration of the runoff. The time of
concentration is:
=t<
(5-2)
where
tc = time of concentration, min
ti = time to inlet, min
tp = time in pipe, min
The flow time in the pipe is simply
t =L/
p /i
(5-3)
where
L = length of pipe, ft
v = velocity, ft/s
However, it is less clear how to estimate the inlet time. For urban areas, inlet times from
5-20 min are used. Following the Tchobanoglous, 1981 protocol, 20 min is used here as
the inlet time.
Intensity-duration-frequency (IDF) curves for Boulder, CO and Houston, TX are shown
in Figures 5-4 and 5-5 (Bedient and Huber, 1989). A summary of the values of intensity
for 20 min in duration for Boulder, CO and Houston, TX is presented in Table 5-5.
41
-------
Figure 5-4. Intensity-duration-frequency curves for Boulder, CO (From US SCS, 1973).
Houilon, !«»<»«. 191O-1951
Figure 5-5. Intensity-duration-frequency curves for Houston, TX (Bedient and Huber 1989).
(Reprinted by permission of Pearson Education, Inc. Upper Saddle River, New Jersey).
42
-------
Table 5-5. Comparison of Design Rainfall Intensities for 20-min Duration Storms in Boulder, CO,
and Houston, TX
Recurrence Interval
(yrs)
2
5
10
25
50
100
Boulder, CO
(in./hr)
2
2.9
3.9
4.5
4.9
5.5
Houston, TX
(in./hr)
3.6
4.7
5.6
6.1
6.8
7.3
These two cities will be used for the cost analysis as representing a wet area with annual
precipitation greater than 40 in. and a semi-arid area with annual precipitation of less than
20 in.
A plot of the intensities vs. recurrence intervals is shown in Figure 5-6. Several
observations can be made. First, intensities are about 1.8x larger in Houston, TX than in
Boulder, CO. Second, the design intensities increase at a decreasing rate as the
recurrence intervals increase. Urban storm drainage designs are usually sized to handle a
5-yr or 10- yr storm. Flood control systems are typically designed to provide protection
for the 100-yr storm. For this example, a 5-yr recurrence interval will be used for the
initial calculations. The design recurrence interval can then be varied to see its effect on
total cost.
30 40 50 61
Recurrence interval, yrs
70 80
90 100
Figure 5-6. Intensities vs. recurrence interval for Boulder, CO and Houston, TX for a 20-min
duration.
43
-------
Intensity-Duration-Frequency (IDF) curves can be approximated by equations of the
form:
= ktb
(5-4)
where
/' = rainfall intensity, in./hr
t = time of concentration, min
k, b = parameters
The parameters of the IDF equation can be determined by forcing the curve through two
points. Using Boulder, CO as an example, intensities for durations of 10 min and 60 min
were estimated from the IDF graphs for various recurrence intervals. These estimates of
two data points yield the necessary two equations and two unknowns. The two
parameters can be calculated using:
In
b = -
In
k = \
Values ofk and b for Boulder, CO are shown in Table 5-6.
Table 5-6. IDF Curve Parameters for Boulder, CO
(5-5)
(5-6)
Recurrence
Interval (yrs)
2
5
10
25
50
100
k
12.169
17.234
25.072
29.655
33.127
38.796
b
-0.6228
-0.6131
-0.6433
-0.6526
-0.6569
-0.6697
Using approximating equations allows the design intensity to be easily recalculated as the
time of concentration changes.
The estimated peak flow rates using the Rational Method are shown in the last column of
Table 5-1. A much better way to estimate peak flows is to use real storm hydrographs
and route these hydrographs through the drainage system using a simulator such as the
Stormwater Management Model (SWMM) developed by the US EPA. However, this
example will use the "standard practice" of the simpler Rational Method approach.
44
-------
Table 5-7. Sewer Network Design Hydraulics
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5.1.2 Sizing the sewer pipes and their slopes
The calculations for selecting a feasible solution to the storm drainage design are shown
in Table 5-7. Using this template, the engineer selects from among 20 available sewer
sizes and 20 assumed slopes. It is convenient to number these options 1-20 and then use
a lookup table to input the associated pipe diameters and slopes. The design
requirements for this sewer network are as follow:
1. The minimum depth of cover is 4 ft.
2. Stormwater can flow in the street for the first section only.
3. The flow capacity of the pipe must exceed the estimated peak flow.
4. No downstream pipe can be smaller in diameter than its upstream pipe.
5. Where multiple pipes enter a single manhole, the depth of the manhole is the
maximum required depth.
6. The minimum velocity of flow in the sewer under design conditions is 3.0 ft/s.
The model computes the velocity for full or partially full pipes as appropriate.
Using a trial and error procedure, the pipe diameters and slopes are varied until all of the
above conditions are satisfied. This spreadsheet template is an advanced way to do storm
sewer design. In a typical design, only a few scenarios are evaluated before settling on a
final design. The feasible solution shown in Table 5-7 is based on several trials that
included evaluation of the system cost. Thus, diameters and slopes were varied in order
to reduce the total cost. The basic tradeoff in conventional storm sewer design is that a
larger pipe can be laid at a flatter slope. Thus, added pipe costs are offset by reduced
excavation costs.
Column 8 of Table 5-7 is the design diameter in inches, which is restricted to a given set
of diameters within a lookup table. Column 6 is the slope of the pipe, which is also
restricted to a group of slopes from a lookup table. Column 10 is the computed upstream
crown elevation. Column 11 is the computation of the downstream crown elevation.
Each of the calculations within each link (row) is matched to subsequent downstream
elevations, and a check is made for the minimum cover constraint in column 11.
Column 18 is the computation of the full flow within the pipe, and column 19 is the
computation of the pipe flowing under the design conditions. The choice of pipe
diameter is restricted such that the capacity of a pipe is not exceeded. Next the ratios of
2/2/are calculated in column 16, which then leads to the v/v/in column 17 using the
ratios for a partially full pipe. The velocity of the pipe flowing full is calculated by
dividing the full flow rate in column 12 by the cross sectional area of the pipe(function of
the pipe diameter) and is listed in column 17. Column 18 is velocity of the partially full
pipe, calculated from the ratio in column 16. Details of the pipe hydraulics are described
in Miles and Heaney (1988).
46
-------
Table 5-8. Sewer Network Design Cost
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5.1.3 Sewer system cost evaluation
Finally, Table 5-8 does the cost estimation for the entire system. Column 10 is the pipe
slope from Table 5-7. Based upon the soil type of column 8, and a lookup table, a side
slope ratio of horizontal to vertical is chosen in column 11. Column 12 is the upstream
invert elevation, computed by multiplying the slope of column 10 by the pipe length of
column 5 and subtracting this from column 13. Column 13 is the downstream invert
elevation, computed by using the pipe diameter, the previous link invert elevation, and
the slope from column 10.
The ground elevations, soil types, cost of the pipes and manholes, and excavation costs
are calculated for a mix of selected pipe diameter and slope scenarios. Pipe costs are
estimated using lookup table values in Tables 5.9 or 5.10 for CMP or RCP pipe,
respectively. These costs are computed in column 14 of Table 5-8.
Table 5-9. Lookup Table for Corrugated Metal Pipe (Adapted from RS Means, 1996a)
Diameter
8
10
12
15
18
24
30
36
48
60
72
CMP Pipe Cost
(1/99$/ft)
9.40
11.80
14.40
18.40
20.90
30.10
37.20
54.80
81.60
118.20
179.50
Excavation costs depend on the volume of excavation and the unit excavation costs. The
lookup table values for these costs are listed in Table 5-11. The volume of excavation is
calculated as follows:
V =
LWH
27
(5-7)
Where
V= excavation volume, yd3
L = distance between manholes, ft
W= the average of the trench top and bottom widths (bottom is D+1.5), ft
H = average excavation depth, ft
48
-------
Table 5-10. Lookup table for reinforced concrete pipe (Adapted from RS Means, 1996a).
Diameter
(in.)
12
15
18
21
24
27
30
36
42
48
60
72
84
96
RCP Pipe Cost
(1/99 $/ft)
15.70
16.60
19.00
23.00
27.60
32.90
55.80
74.40
85.40
102.30
146.70
192.60
288.90
355.60
The average depth of the excavation is computed in column 15 of Table 5-8. The
average width of the excavation is calculated as an average of the top and bottom widths,
and is listed in column 16 of Table 5-8. The total volume is computed using equation 5.8
and results listed in column 17.
Excavation costs, (7 are calculated as:
= cV
(5-8)
The unit excavation cost, cex; is a function of the soil type, which were explained in Table
3-2, and are listed again in Table 5-9. These costs are computed in column 18 and 19 of
Table 5-8.
Table 5-11. Excavation Costs (Adapted from RS Means, 1996a)
Soil Type
Clay
Rock
Sand
Silt
Horizontal
1
0
2
1.5
Vertical
1
1
1
1
Excavation Cost
(1/99$/yr3)
7.09
86.29
6.12
6.72
49
-------
Bedding costs are evaluated based upon a two variable lookup function that uses side
slope and diameter to determine costs. The lookup values are presented in Table 5-12,
and the results are presented in column 20 of Table 5-8.
Manhole costs are estimated using the following equation:
Cmh=482H°'9311
(5-9)
Where
Cm = cost of manhole, 1/99 $
H = height of manhole, ft
(maximum difference between the ground elevation and the invert
elevations of sewers entering the manhole)
These costs are computed in column 22 of Table 5-8.
5.2 Scenario Analysis
The results shown in Tables 5-1, 5-7, and 5-8 reflect the costs for sets of such assumed
input conditions as topography, land use, design storm, performance criteria, and pipe
cost. The power of the spreadsheet is its ability to enable what-if design analysis. By
systematically changing one or more of the input variables the impact of these variables
on the total cost is more easily assessed. The classic approach is to change and assess
one variable at a time. However, sensitivity analysis may be performed for a finite
number of scenarios wherein many, if not all, of the input assumptions are allowed to
vary, and to find the cost for each scenario. The potential number of scenarios for this
problem is huge. For this initial effort, only a very small number of scenarios were
selected, in order to show the impact of various scenarios on the total cost. An important
caveat in this sensitivity analysis is that the base solution's effectiveness is unknown.
Thus, the sensitivity analysis is done for a solution of unknown quality. This limitation
can be removed in future work by using intelligent search techniques to find very good, if
not optimal, solutions for each scenario. The following sub-sections describe a limited
number of input variables that can be assessed to test the sensitivity of the final cost to
various assumptions.
50
-------
Table 5-12. Bedding Costs (Adapted form RS Means, 1996a)
Horizontal
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1.5
1.5
1.5
1.5
Vertical
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
HA/
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1.5
1.5
1.5
1.5
Diameter
(in.)
6
8
10
12
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
Trench
width
(ft)
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
Cost
(1/99 $/ft)
0.92
2.00
2.07
2.12
3.47
3.51
3.57
3.62
5.25
5.29
5.44
5.55
9.72
9.98
13.01
16.23
23.39
31.80
1.90
3.16
3.43
3.67
5.25
5.39
5.55
5.88
7.77
7.95
8.52
9.56
14.06
15.08
20.58
26.81
37.47
49.71
2.90
4.36
4.77
5.25
7.06
7.30
7.56
8.14
10.28
10.59
11.61
13.50
18.46
20.17
28.17
37.40
51.76
67.70
3.91
5.69
6.15
6.81
51
-------
Horizontal
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Vertical
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
HA/
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Diameter
(in.)
14
15
16
18
20
21
24
30
32
36
48
60
72
84
6
8
10
12
14
15
16
18
20
21
24
31
32
36
48
60
72
84
Trench
width
(ft)
3
3
3
3
4
4
4
4
6
6
7
8
10
12
1
2
2
2
3
3
3
3
4
4
4
4
6
6
7
8
10
12
Cost
(1/99 $/ft)
8.83
9.18
9.56
10.38
12.80
13.24
14.63
17.64
22.77
25.23
35.76
48.21
65.65
86.16
5.01
6.73
7.49
8.37
10.59
11.04
11.54
12.66
15.32
15.89
17.71
21.61
27.15
30.22
43.22
58.67
79.32
103.94
5.2.1 Management of the demand for imperviousness
Imperviousness can be reduced by designing narrower streets and driveways, reducing
parking requirements, etc. Two cases are:
Case
1
2
Imperviousness
Present values
0.7
* Present values
5.2.2 Management of land use
The assumed land use for this example is representative of a typical mix of residential,
commercial, and public land use. Two other scenarios are all low density and all high
density. Thus, the three land use scenarios are:
Case
1
2
o
J
Land Use
Mixed
All residential
All residential
at
at
2 dwelling units/acre-commercial and school are the same.
10 dwelling units/acre-commercial and school are the same.
52
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5.2.3 Effect of recurrence interval
The selected design storm recurrence intervals are assumed to range from 2 to 100 yr. A
2 yr level represents a minimum level of service, whereas a 100 yr level would represent
an upper limit on drainage systems. Three cases can be considered:
Case
1
2
3
Recurrence Interval
(yrs)
2
5
100
5.2.4 Effect of climate
The sophistication of the drainage system is expected to vary widely from the wetter
areas of the country with high intensity storms to very arid areas with low intensities.
For this analysis, IDF curves from Boulder, CO and Houston, TX are used. Peak
intensities in Houston are about 40% higher than in Boulder. Thus, two cases are:
Case
1
2
City
Boulder, CO
Houston, TX
5.2.5 Effect of assumed minimum inlet flow time
Our preliminary simulations indicate the importance of the assumed inlet time. Inlet time
should be calculated. In our case, the calculated inlet time was only 2-3 min. If this inlet
time is used, then very high intensities result. The usual assumption in stormwater
manuals is to use a 5 - 20 min inlet time. For the base case, a 20 min inlet time was used.
Two other cases can be considered:
Case
1
2
o
J
Inlet Time
(min)
Calculated
5
20
5.2.6 Required minimum depth of cover
The minimum depth of cover is a function of local climate, groundwater conditions, the
presence of basements, etc. For this example, two minimum depths can be used:
53
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Case
1
2
Minimum Depth of Cover
(ft)
4
6
5.2.7 Effect of pipe material
The unit cost of pipe depends on the type of material. The optimal type of pipe is a
complex issue and a life cycle cost analysis should be done to decide which pipe material
is better for a given location. Pipe cost information has been developed for corrugated
metal pipe and reinforced concrete pipe. These two options provide two sets of
scenarios.
Case
1
2
Pipe Material
Corrugated Metal
Reinforced Concrete
5.2.8 Possible number of scenarios
The number of cases enumerated above are just a small percentage of the possible cases
that could be considered. The combinations are listed below:
Input Variable
Imperviousness
Land Use
Recurrence Interval
Climate
Inlet Flow Time
Depth of Cover
Pipe Material
Cases
2
3
o
6
i
3
2
2
The number of possible combinations is the product of the above seven cases, or 432
possible scenarios, far more than we can deal with in this introductory evaluation. The
selected initial five scenarios are presented below:
Scenario 1. Boulder, CO typical
Input Variable
Imperviousness
Land Use
Recurrence Interval
Climate
Inlet Flow Time
Depth of Cover
Pipe material
Value
Present imperviousness
Existing mixed land use
5yrs
Boulder, CO
20min
4ft
RCP
54
-------
Scenario 2. Houston, TX typical
Input Variable
Imperviousness
Land Use
Recurrence Interval
Climate
Inlet Flow Time
Depth of Cover
Pipe material
Value
Present imperviousness
Existing mixed land use
5yrs
Houston,TX
20min
4ft
RCP
Scenario 3. Boulder, CO major flood
Input Variable
Imperviousness
Land Use
Recurrence Interval
Climate
Inlet Flow Time
Depth of Cover
Pipe material
Value
Present imperviousness
Existing mixed land use
100 yrs
Boulder, CO
20min
4ft
RCP
Scenario 4. Boulder, CO 5-yr storm with calculated inlet time
Input Variable
Imperviousness
Land Use
Recurrence Interval
Climate
Inlet Flow Time
Depth of Cover
Pipe material
Value
Present imperviousness
Existing mixed land use
100 yrs
Boulder, CO
Calculated
4ft
RCP
Scenario 5. Boulder, CO typical with different pipe material
Input Variable
Imperviousness
Land Use
Recurrence Interval
Climate
Inlet Flow Time
Depth of Cover
Pipe material
Value
Present imperviousness
Existing mixed land use
5 yrs
Boulder, CO
20min
4ft
CMP
55
-------
5.3 Results for the Selected Scenarios
Upon selection of the assumed scenario parameters (land use, design event, inlet time,
minimum depth of cover), appropriate design variables (pipe diameter, pipe slope) are
entered into the spreadsheet in a trial and error fashion. For the first scenario, a 5-yr.
storm was selected from the IDF relationship for the local Boulder, CO area. The inlet
flow time was assumed 20 min for all sub-basins. After these hydrologic and hydraulic
assumptions were made, a feasible design was found by entering slopes and pipe
diameters for each section. The feasibility of the design is established through design
constraints built into the spreadsheet template (i.e., minimum pipe velocity, minimum
cover depth).
The cost for this design is calculated based on the selected feasible design parameters of
slope and pipe diameter. It is likely that this problem will have many feasible solutions,
that can be improved upon only by further trial and error. The final cost for the first
scenario is $975,000, including a geographic location factor of 92% to account for local
deviation from the national average (the cost functions are based on national averages).
This total is broken into pipe costs, excavation costs, bedding costs, and manhole costs in
Table 5-13 and Figure 5-7.
Table 5-13. Summary of Cost Scenarios
Total
Pipe
Excavation
Bedding
Manhole
1. Boulder
5-Yr
(1/99 $)
975,000
439,000
287,000
161,000
88,000
2. Houston
5-Yr
(1/99 $)
1,174,000
556,000
338,000
187,000
93,000
3. Boulder
100-Yr
(1/99 $)
1,264,000
600,000
374,000
193,000
97,000
4. Boulder 5-Yr
Calculated Inlet
Time
(1/99 $)
1,444,000
749,000
386,000
216,000
93,000
5. Boulder
5-Yr With
Corrugated
Steel (1/99 $)
1,029,000
456,000
319,000
163,000
91,000
Min. Total
Max. Total.
Std. Dev.
6. Scenario 1
5-Yr With
Uncertain Costs
(1/99 $)
975,000
439,000
287,000
161,000
88,000
538,000
1,299,000
111,000
Scenario 1 was then altered to reflect an identical design done for the Houston, TX area.
The Houston, TX area receives approximately 80% more rainfall than Boulder, CO so the
design must reflect a higher 5-yr peak flow rate. The selected feasible design for
Houston, TX had steeper slopes and larger pipe diameters to convey the increased flow.
This resulted in an increase of 20% over the Boulder, CO design, including the reduced
location factor of 90.2% for Houston, TX. The final Houston design costs were
calculated to be $1,174,000.
The rainfall intensities for the Houston, TX Scenario increased by 66% over the Boulder,
CO 5-yr storm scenario. The product of length of sewer (ft) and the diameter of sewer
(in.) increased by 15%, and the average slope increased 10%, from .0082 to .0090. Pipe
costs increased 27%, from $439,000 to $556,000. Excavation costs increased 18%, from
$287,000 in scenario 1 to $338,000 in Houston, TX.
The third scenario reflected an increased level of service for the Boulder, CO 5-yr design
in Scenario 1. A 100-yr design storm was selected, resulting in a larger capacity design
56
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over the 5-yr design Boulder, CO design storm. The rainfall intensity increased 91%
from the 5-yr storm to the 100-yr design storm. Consequently, the final costs increased
by 30% to $1,264,000. The excavation costs increased 30% over the 5-yr design storm
scenario. For the selected design, pipe costs increased 37%. The average slope of the
system increased from 0.0082 to 0.098.
The fourth scenario demonstrates the dependence of the final design cost on initial
hydrologic assumptions. The first three scenarios assumed a 20 min rainfall inlet time.
In a basin with developed land use, the flow paths taken to the first drainage inlet may be
over permeable or impervious surfaces, with widely different slopes, roughness factors,
etc. For this scenario, the inlet design time was calculated based on the actual
dimensions of the sub-basin, and substituted for the assumed 20 min inlet time in
scenario 3. The average calculated inlet time was 10.5 min for this scenario. The altered
hydrologic assumptions increased the total cost 48%, from $975,000 to $1,444,000. The
majority of the cost increase was in increased pipe costs; however, a cheaper solution
may exist that uses smaller pipes and steeper slopes.
The selected design also failed to meet design constraints for one pipe section. Because
of a steep ground surface slope for one section, a severe slope was necessary to maintain
the minimum depth of cover of 4 ft over the crown of the pipe. This severe slope caused
maximum design velocities to exceed the maximum velocity constraint of 10 ft/s by 1
ft/s.
A fifth scenario was included to demonstrate the use of different pipe materials. A design
using corrugated steel pipe was done for the Boulder, CO 5-yr storm. This scenario also
included the increased head loss in the pipe caused by the higher roughness coefficient of
the steel pipe. The Manning coefficient was increased from 0.013 for reinforced concrete
pipe to 0.025 for corrugated steel pipe. This change in hydraulic performance resulted in
a need for larger pipes and steeper slopes. Also, for diameters greater than 18 in. steel
pipes are more expensive than concrete pipes, therefore, the final cost increased 5.5% to
$1,029,000. It is likely that this design can be improved upon by further trial and error.
The results of the five design scenarios are shown in Figure 5-7.
5.4 Effect of Uncertainty in the Estimates
Once the design is fixed, the uncertainty in the cost for that design can be estimated using
Monte Carlo simulation. In this initial evaluation, we consider only uncertainty in the
assumed input cost parameters since these values do not affect the final design when one
is doing what-if analysis. Changes in the assumed cost would affect the design in an
optimization or what's best analysis. When using risk analysis software such as @Risk
or Crystal Ball, it is straightforward to introduce uncertainty into the cost estimates and
then run, say 1,000 simulations to estimate the variability in the final cost estimate that is
attributable to the uncertainty in the input cost estimates. In order to do Monte Carlo
simulation, one needs to input a probability distribution for each input variable that is
assumed to have uncertainty. For this evaluation, Scenario 1 will be used, and the
57
-------
o
o
(0
c
a)
o
re
c
u
o
(0
•S
c«
at
u
<
OJ
I
-------
Pipe cost per inch diameter per foot: A normal distribution of the form, Normal (1, 0.25),
is used to define a coefficient with a mean of one and standard deviation of .25. This
coefficient is then multiplied by the mean pipe unit cost ($/ft). The following table shows
the unit excavation cost for different type of soils:
Soil
Clay
Sand
Silt
Rock
Unit Excavation Cost
(1/99 $/yd3)
Triangular* (5.67; 7.09; 8.50)
Triangular* (4.87; 6.12; 7.34)
Triangular* (5.38; 6.72; 8.06)
Uniform** (69; 104)
* Triangular (minimum; mean; maximum)
**Uniform (minimum; maximum)
Monte Carlo simulation is done by repeatedly sampling from the above distributions.
Each trial is a set of assumed values of the inputs. The output is the system cost for that
realization. The process is repeated 1,000 times resulting in 1,000 estimates of the
system cost. Finally, the cumulative distribution of these costs is determined and the
results reported. Monte Carlo simulation allows us to see how uncertainty in inputs
affects the final answer. It is assumed that there is no covariance between the variables.
The minimum cost recorded in the 1,000 Monte Carlo simulations was $538,000 and the
maximum was $1,299,000. The mean cost of $974,867 compared well with the cost of
scenario 1, $975,000. The standard deviation of the 1,000 simulation results was
$111,000. The cumulative distribution of total costs is shown in Figure 5-8. The source
of the variance is shown in a tornado plot depicted in Figure 5-9. The majority of the
uncertainty in the final cost is due to the uncertainty assumed for the pipe costs, despite
being a smaller fraction of the total costs.
5.5 Summary and Conclusions on Scenarios
The results of the five scenarios and the uncertainty analysis are shown in Table 5-13.
The effect of design assumptions and initial conditions on the final outcome of the design
is evident. However, hidden within these what-if analyses is the fact that the selected
designs are merely one feasible solution of the many possible designs that satisfy the
design constraints. When a design assumption was changed, say from the 5-yr event in
scenario 1 to the 100-yr event in scenario 3, the physical design was altered greatly to
convey the added flowrate. It is possible that a nearly optimal solution is compared
against a sub-optimal solution in Table 5-13. Therefore, direct comparisons of design
costs are impossible. While valuable, the what-if analysis does little to illuminate the
optimal solution.
59
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Results of Monte Carlo Analysis on Boulder 5 Year Design Event
100
600000 700000 800000 900000 1000000
Construction Costs ($)
Figure 5-8. Cumulative total cost distribution
1100000 1200000 130C
60
-------
Monte Carlo Tornado Plot
.Q
.5
1
Silt
Clay
Rock
Sand
Pipe Costs
Excavation Cost
Excavation Cost
Excavation Cost
Excavation Cost
H
n
-1.00
-0.75
-0.50
-0.25 0.00 0.25
Regression Sensitivity
0.50
0.7
Figure 5-9. Tornado plot of uncertainty in scenario.
61
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A more robust comparison of design costs would include optimization techniques to find
optimal designs for each scenario. Then the true costs of increasing the level of service
from a 5-yr storm to a 100-yr storm could be measured. To illustrate this point, assume
that the design selected for Scenario 1 is very nearly optimal and the design for Scenario
3 is grossly over designed. For illustrative purposes, assume that the optimal design for
the 100-yr storm in Scenario 3 is $1,000,000, and that the increased benefit from flood
damage is estimated to be $250,000. That is, an estimated $250,000 will be saved over
the life of the project if the drainage system is designed for a 100-yr event instead of only
a 5-yr event. Under the designs found in Scenarios 1 and 3, the increased level of service
($1,264,000 - $975,000 = $289,000) would not be warranted because the costs of the
increased project exceed the estimated benefits ($250,000). However, if optimal
solutions were to be found for each scenario, the increased level of service in Scenario 3
would be worthwhile, because the costs of increasing the level of service from Scenario 1
to Scenario 3 ($1,000,000 - $975,000 = $25,000) would be exceeded by the expected
increase in benefits ($250,000). While the example is simplified by the exclusion here of
such variables as possible increased maintenance costs and uncertainty in increased
benefits, it does illustrate the importance of obtaining optimal design solutions to enable
direct comparison among alternatives.
62
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Chapter 6
Cost-Effectiveness of Alternative Micro-storm Management Options
6.1 Introduction
In a recently completed project sponsored by the U.S. Environmental Protection Agency
titled Innovative Urban Wet-Weather Flow Management Systems (Heaney etal., 1999a),
many methods for improving urban stormwater quality were described. Approaches
range from traditional end-of-pipe treatment methods, to sophisticated source control
BMPs, including land use controls. A summary of many of these approaches is presented
by Heaney et al. (1999b). However, innovative wet weather flow (WWF) quality
management programs must be designed in concert with the need to provide adequate
flood protection and drainage. Interestingly, innovative methods for flood control and
drainage focus similarly on source controls and non-structural options (Heaney et al.,
1998c). The resulting new paradigm for WWF management is that the analyst will need
to evaluate a very large variety of management options that include land use
modifications. Fundamental questions arise regarding how to develop effective methods
for the evaluation of alternatives and how to prioritize among them. In one report,
Heaney et al. (1998c) describe the optimization methods used to prioritize among
options, While in another (Heaney et al., 1999b) they evaluates the role of geographical
information systems (GIS) in providing the essential spatial information for these new
approaches. The interested reader is referred to the other cited reports for a more
complete description of these other aspects.
In this section we describe some preliminary results in developing cost estimates for
land-intensive BMP urban stormwater control systems. Developing reliable cost
estimates is also complicated because many BMPs are designed to serve multiple
purposes. For example, if the yard of a house is retrofitted to replace one half of the lawn
with infiltration and wooded areas, does the homeowner perceive a loss of the use of this
yard, or welcome the fact that there will be less lawn to maintain? Controls for
stormwater quality management using micro-storm (i.e., storms of a low return period of
say, two months) design criteria also have value for larger design storms for drainage and
flood control. Thus, these costs need to be allocated among purposes. A robust solution
is one that works well over the entire spectrum of future scenarios. Traditional
stormwater management systems have been designed to function well under a single
design condition, e.g., the 100-yr flood (major storm) or the 10-yr storm (minor storm).
Unfortunately, designing a control systems around a single extreme event is myopic
because the design may not perform well under other scenarios. Major floodways
designed for the 100-yr event degrade the natural stream system, overdrain the system
during more frequent storms, and degrade downstream water quality by transporting
pollutants rapidly through urban areas. Concern for water quality and receiving stream
integrity in urban stormwater systems demonstrates the importance of a stormwater
system that performs well in managing the runoff from frequent, or "micro" storms that
occur on a regular basis, e.g., weekly or monthly. Lippai and Heaney (1998) present
63
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principles for doing cost allocations across purposes (micro, minor, and major storms)
and groups (residential, commercial, transportation, etc.) for water supply systems. The
same principles can be applied to stormwater systems. What is needed for WWF systems
is an efficient method for optimizing stormwater control systems for micro to major
storms. The companion report by Heaney et al. (1998c) shows how this can be done for
simpler cases. More research is needed to develop fully functional models for more
realistic scenarios.
Below we review previous efforts to evaluate micro-storm systems, present methods for
estimating the unit costs of these BMPs, and display the results of using these cost
estimates for finding the optimal mix of BMPs.
6.2 Literature Review
Pitt (1987) showed the importance of evaluating smaller storms with regard to urban
stormwater quality protection. He initiated the development of the Source Loading and
Management Model (SLAMM) used to estimate the efficacy of various urban nonpoint
source water quality management options (Pitt and Voorhees, 1995). SLAMM
emphasizes small storm hydrology and its associated particulate washoff. The predictive
equations in SLAMM are based on extensive field data. Below is a brief description of
the relevant components of SLAMM.
6.2.1 Land use/control options
SLAMM depicts urban land use as falling into the following major categories:
1. Residential Areas
2. Institutional Areas
3. Commercial Areas
4. Industrial Areas
5. Open Space Areas
6. Freeways
The first five of these areas contain up to the 14 source area types shown in Table 6-1.
The area in acres is needed for each of these source areas. Finally, the additional
information shown in Table 6-2 is needed for some of the source areas.
64
-------
Table 6-1. Source Areas in SLAMM (Pitt and Voorhees 1995)
Source Area
Roofs
Paved Parking/Storage
Unpaved Parking/Storage
Playgrounds
Driveways
Sidewalks
Street Areas/Alleys
Large Landscaped Areas
Undeveloped Areas
Small Landscaped Areas
Isolated Areas
Other Permeable Area
Other Directly Connected Impervious Area
Other Partially Connected Impervious Area
Paved Freeway and Shoulder Area (F)*
Large Turf Area (F)*
Number Available
in Each Land Use
5
3
2
2
3
2
3
2
1
3
1
1
1
1
5
1
' (F) indicates available in Freeway Land Use only
Table 6-2. Other Information Needed in a Source Area (Pitt and Voorhees 1995)
Type of roof-pitched or flat
Source area connectedness-unconnected or
draining to a permeable area.
Soil type-sandy (A/B) or Clayey (C/D).
Building density - low or medium/high
Pavement of alleys - yes or no
Pavement texture - smooth to very rough
Total street length - curb miles
Street dirt accumulation equation coefficients
Initial street dirt loading
Average daily traffic - vehicles/day
While SLAMM uses far more detail and represents a significant improvement over other
stormwater models, it still uses a highly aggregate representation of soil and land use
conditions, e.g., only two soil classifications are used, building densities are either low or
medium/high. An example printout of the input file for an analysis in Toronto, shown in
Table 6-3, gives a general idea of the amount of spatial aggregation. Thirty source area
categories are shown, 12 of which have positive amounts of acreage. Small landscaped
areas account for 436 out of a total of 730 acres
65
-------
Table 6-3. Sample SLAMM Output for Toronto, ON, Canada (Pitt and Voorhees, 1995)
(Reproduced with permission of Dr. Robert Pitt)
Citi lilt run* i CXAWU.OAT
Htln III* MMt LUtO.DM
tunoff Co*fMcl«nt flit n**«; tWJOff .«¥
Jtui/ perled mrtlng dtt«: 04/10/tC
Ottei 0*-OMW
tilt tnfenwtleoi Wtt rOlffl VIM STAJBMO CONTROL*
for tich Seurci .5S
0.04
9.00
0.00
<.$*
e.w
6.09
O.T4
0.00
O.CO
0,60
21. IZ
i*,.n
0.00
8,C9
O.CO
IS.M
5S.I2
a.oo
U$J
3,09
0,60
«35.n
O.K
0.03
1.52
33,«
0.00
8.60
7JS.S6
iMtttu- e
tlentt
Artti
0,03
d.,oo
o-.oo
0.00
O.M
0.00
0,04
0.08
0.00
o.oo
0,00
0.00
0.09
O.M
0,09
0.09
0.00
O.M
0.00
0.00
O.M
0.00
0.08
0.00'
0.00
0.06
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66
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6.2.2 Hydrology in SLAMM
Using field measurements, a rainfall-runoff relationship is established for the study area.
Such a relationship for clean, rough streets is shown in Figure 6-1. The 45° line
represents a 1:1 rainfall-runoff relationship. Losses can be partitioned as follows:
1. Initial losses, also known as initial abstraction, and
2. Maximum variable losses.
Three stages of rainfall-runoff response can be identified:
1. The amount of rainfall before any runoff is produced.
2. The rainfall range between no runoff and all of the losses being
satisfied, the nonlinear portion of the runoff curve.
3. The rainfall beyond stages land 2, wherein the rainfall and runoff
rates are equal.
Our main concern for water quality is with stages 1 and 2. Urbanization reduces
the initial abstraction. It also tends to reduce the total infiltration since the
infiltration capacity has been reduced by development.
6.2.3 NRCS method and initial abstraction
Pitt (1987) provided an excellent review of the literature on the nature of the initial
abstraction. Initial abstraction includes the following:
• Detention storage, e.g., on flat roofs
• Infiltration into the soil
• Interception by vegetation, particularly trees
• Evaporation from impervious surfaces such as streets.
Recognizing the uncertainty of the estimates of the total initial abstraction, we will use
this concept to illustrate our methodology.
6.2.4 Costs of controls in Pitt's work
Pitt (1987) estimated the following costs (as 1986 Canadian $) and needed to be revised.
• Street cleaning: $50 per curb-km cleaned
• Catchbasin cleaning: $50 per catchbasin cleaned
• Redirecting roof drains to permeable areas: $125 per house
• Infiltration trenches: $40,000 per ha paved area or roof
• Detention ponds: $200,000 per ha pond surface
Annual maintenance costs are 4% of initial construction costs
67
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Figure 6-1. Illustrative rainfall-runoff relationship (Pitt, 1987).
(Reproduced with permission of Dr. Robert Pitt)
Maxinua varlabl^-O
All loaoon satisfied at 13.5 oa ninot
20
25
68
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6.2.5 Control devices in SLAMM
SLAMM evaluates the following control devices (Pitt and Voorhees, 1995):
• Wet detention ponds
• Porous pavement
• Infiltration devices
• Other devices for source areas
• Street cleaning
• Catchbasin cleaning
• Grass swales
• Other outfall devices
6.2.6 Limitations of SLAMM
SLAMM is an improvement over other approaches that neglect the dynamics of small
storms. It also uses more refined spatial information and breaks land uses down into
functional units, or source areas. However, its cost evaluation is limited and has not been
updated since 1987. More importantly, it only does what-if analysis and cannot be used
to find optimal solutions.
6.2.7Low impact development
Some design guidelines are available for micro-storms. Extensive work has been done by
Prince George's County (1999) Maryland to develop designs for Low Impact
Development. They use the Natural Resources Conservation Service (NRCS) Curve
Number (CN) approach to evaluate the percentage of the development that must be set
aside in order to provide storage. Other design guidelines suggest capturing the first part
of the runoff, typically the first 0.5 to 1 in. of runoff.
6.3 Proposed Approach
6.3.1 Introduction
Heaney et al. (1998c) describe a proposed method for using the NRCS CN method for
evaluating micro-storms. The fundamental principle for the proposed approach is that
development should not reduce the initial soil moisture storage that existed prior to
development. This initial soil moisture storage is equivalent to the initial abstraction as
calculated using the NRCS CN method. The initial abstraction is a good measure of the
ability of the soil system to filter the stormwater. The initial abstraction, as a function of
CN, is shown in Table 6-4. Inspection of Table 6-4 reveals the importance of CN. A
low CN of 30 corresponds to an initial abstraction of 4.67 in. Even at a CN of 80, the
initial abstraction is still 0.5 in. If the original CN is fairly low, then a significant amount
of soil moisture storage is lost if this area is rendered impervious by development.
69
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Table 6-4. Initial Abstraction as a Function of Curve Numbers (CN)
CN
20
30
40
50
60
la
(in.)
8
4.67
3
2
1.33
CN
70
80
90
100
la
(in.)
0.86
0.5
0.22
0.02
Note: la = initial abstraction
The method presented here uses the concept of modifying the CN for the developed
condition so that the modified CN is the same as the natural CN. The more cost-effective
controls tend to focus on using the permeable area for more intensive infiltration.
Alternatively, we seek to design hydrologically functional landscapes as described in the
next section.
6.3.2 Hydrologically functional landscaping
Traditional landscaping relies on covering most, if not all, of the permeable area with
grass. The lot is graded so that stormwater drains to the street and/or the rear of the lot as
shown in Figure 6-2 (Dewberry and Davis, 1996). An example of a hydrologically
functional landscape is shown in Figure 6-3 (Prince George's County, 1999). The
general idea is to maximize the infiltration of stormwater by providing depressions,
draining runoff from impervious areas to permeable areas, providing more circuitous
routes for the stormwater to increase the time of concentration.
6.3.3 Cost of CN modifications
If the cost of modifying the CN can be determined, then cost-effective strategies can be
developed for maintaining the undeveloped CN for each parcel or combination of parcels.
Most BMPs are land intensive. Thus, if a BMP is installed within a right-of-way, or in a
backyard, or in open space land, should the cost of the land be included in the
calculation? What is the value of this land? This important topic is discussed below.
6.3.4 Land valuation
Land valuation is of critical importance for many controls because it constitutes a
significant, if not major, component of total costs. Traditional urban storm drainage
designs relied on subsurface sewer systems to carry WWF from the service area. Thus,
land costs were not an important factor because no land was used in the process.
However, once requirements for detention and retention systems were included in the
WWF designs, then the cost of the land became an issue. Various perspectives on the
cost of land are summarized below:
70
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a) Lot Grading: Drainage Directed Toward Front of Dwelling
b) Lot Grading: Drainage Directed Toward Rear ol Dwelling
c) Lot Grading: Drainage Directed Toward Front and Rear of Dwelling
Figure 6-2. Conventional storm drainage (Dewberry and Davis, 1996).
(Reproduced with permission of The McGraw-Hill Companies)
71
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100-Foot Maximum Overland Flow at Minimum 1 %
-Street
PLAN VIEW
ELEVATION
Figure 6-3. Illustration of hydrologically functional landscape (Prince George's County, 1999)
(Reproduced with permission of the Prince George's County).
72
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• The land should be valued at zero because it is part of the required
right-of-way that the developer must provide along with the
traditional right-of-way for streets and sidewalks, schools, and
parks. If the land is on private property and is being used as
landscaping, then it is viewed as being free for this other purpose.
• The land should be valued at full market value since the developer
would otherwise be able to use this land for additional
development of houses, commercial development, and/or other
uses.
This issue is of paramount importance in estimating the "true cost" of land-intensive
urban WWF BMPs whether they are located onsite or offsite. While little literature is
available on this subject for urban stormwater systems, this topic has been discussed
extensively with regard to evaluating the cost of transportation systems. This related
literature is reviewed in the next section.
6.3.4.1 Value of land for transportation
A relatively large body of literature exists that is directed at estimating the true costs of
various forms of transportation, particularly automobile-related transportation. Litman
(1998) summarizes this literature and recommends methods for properly estimating the
cost of transportation. Heaney et al. (1999a) quantify the impact of the automobile on
urban land use in general and urban stormwater systems in particular. Accommodating
the automobile requires committing a major portion of contemporary urban systems for
such constructs as streets, driveways, parking lots, and garages. Some of the cost of
providing land for transportation is paid by external subsidies from the state and federal
governments. Much of the cost of local street and parking systems are paid by property
and sales taxes. Thus, virtually none of these costs are directly assessed on the user. This
approach is in stark contrast to a water utility wherein the total cost is assigned to the
users, much of it in the form of commodity charges, so that they are aware of the full cost
and have direct incentives to reduce their demand. For the purposes of this section,
assume that a transportation utility exists in the urban area. This utility is responsible for
all aspects of transportation and parking. It must pay full cost for its network, and it
levies this cost directly on the transportation users. Litman (1998) defines roadway land
value as follows:
Roadway land value costs include the value of land used for rights-of-way and other
public facilities dedicated for automobile use. This cost could also be defined as the rent
that users would pay for roadway land if it were managed as a utility, or at a minimum,
the taxes that would be paid if road rights-of-way were taxed.
6.3.4.2 Rate of return on land investments
Real estate appraisers estimate market value, which can be defined as (Boyce 1981):
73
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The highest price in terms of money which a property will bring in a competitive and
open market under all conditions requisite in a fair sale, to the buyer and seller each
acting prudently, knowledgeably, and assuming the price is not affected by undue
stimulus.
The present value of a series of future annual income is:
PV = A
(6-1)
Where
PV= present value, $
A = annual income, $/yr
n = number of yrs
/' = annual interest rate
As n tends to infinity, equation 6.1 becomes capitalized present value of an infinite
stream of future benefits (PVC):
PVC = —
i
(6-2)
The present value of an infinite future stream of earnings is called the capitalized value of
the future income stream. For example, a detailed investigation of the rate of return for
muck farms north of Lake Apopka in Florida revealed an expected annual return of about
$460/acre (Heaney et al. 1998d). Using a discount rate of 10%, the expected value of this
land would be $4,600/acre. Detailed studies of comparable muck farmland indicated an
average selling price of $4,500/acre, very close to the farm budget analysis.
For urban land use, there is no similar simple metric of land value in terms of crop
productivity. However, a reliable estimate of the value of urban land can be obtained by
viewing the urban development as an investment opportunity. The first step is to
calculate the investment in raw land and its improvements exclusive of the building.
Then, a reasonable return on investment, say 8%, is assumed. Thus, the annual benefit of
committing this parcel of land to this use is 8% of the investment. The land is assumed to
hold its value over time. Thus, the present value of the future sales price equals the
original purchase price. Then, the cost of committing land to this use is the opportunity
cost as estimated as the investment cost times the rate of return.
It is instructive to trace the development of raw land into housing or other uses, and then
estimate the investment in raw and improved land. Dion (1993) provides a breakdown on
the components of cost for a typical house built in 1992 as shown in Table 6-5. Finished
land and labor/materials constitute 73% of the total cost. If the overhead and financing
are prorated to the land and the house then the land cost constitutes about 27% of total
cost, or 38% of construction costs.
74
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Table 6-5. Breakdown of the Cost of a Typical House (Dion 1993)
Item
Overhead
Financing
Finished land
Labor/materials
Total
% of Total
20
5
20
53
98
Cost
($)
24,000
6,000
24,000
63,600
120,000
The Urban Land Institute (1989) presents another breakdown of land development costs
for 1984 and 1988 as shown in Table 6-6. For 1988, land costs are about 76% of
construction costs while they are 51% of construction costs in 1984. A rule of thumb in
the home construction industry is that the house costs should be about twice the land
costs. Thus, we will use land costs to be 50% of construction costs.
Table 6-6. Breakdown of the Cost of Housing in 1984 and 1988 (Urban Land Institute 1989)
Item
Raw land
Land improvements
Financing
Labor
Marketing
Materials
Overhead
Profit
Advertising
Other
Total
% of development
(1988$)
19.3
12.6
4.4
17.4
4.3
24.1
6.5
8.1
1.2
0.4
98.3
% of development
(1984$)
17
7
6
18
4
29
7
9
2
2
101
Note: The totals do not sum to 100 in the source
A breakdown of housing costs by function for a typical house is shown in Table 6-7. The
total construction cost for the house is about $118,200. The total land value is estimated
to be 50% of the cost of the house. Each component is then allocated its value based
upon the proportion of area that it occupies. Unimproved land is assumed to be 2/3 of the
total land value. The costs of improvements for water, wastewater, and storm water are
estimated for each functional unit. For example, all of the wastewater costs are assigned
to the house. The result is a total land value attributable to the yard of $29,702. The
capital and operation and maintenance costs for the yard are shown in Table 6-7. Capital
costs consist of the initial preparation of topsoil plus landscaping, typically sod. Also, a
sprinkler system is included. This option can be dropped as appropriate. Operation and
maintenance costs consist of irrigation water, maintenance of the yard and the sprinkler
system, and the opportunity cost of the land. The total present value of these costs is
$87,880 or $6.76 per ft2 of yard area.
75
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Table 6-7. Estimated Housing Costs
Component
Roof-house
Roof-garage
Driveway
Yard
Patio
Total
Area
(ft2)
1600
400
800
9800
400
13000
%of
total
12.3%
3.1%
6.2%
75.4%
3.1%
100.0%
Cost
($/ft2)
56.25
34.00
4.00
1.00
4.00
Construction Cost
(1/99$)
90,000
13,600
3,200
9,800
1,600
118,200
Total Land
Cost (1/99$)
7,274
1,818
3,637
44,552
1,818
59,100
Unimproved
Land Cost (1/99 $)
4,849
1,212
2,425
29,702
1,212
39,400
Item
A. Initial Capital Investment
1 . Soil preparation
Initial cost of sod
Initial cost of topsoil, 6 in.
Spreading topsoil, 6 in.
Soil conditioners
Sprinkler system
2. Opportunity Cost of Land
Land Investment Cost, $
Opportunity cost investment rate
Annual cost, $/yr
Interest rate peryr
Present worth over 25 yr, $
Cost in $/ft2
Total of initial capital investment
B. Operation & Maintenance Costs, $
Lawn watering
In./yr
% of permeable area that is irrigated
Cost of water, $/1 ,000 gal.
Present worth factor
Present worth, $/ft2
Lawn maintenance
Weeks/yr.
$/week
Maintenance area, ft2
Present worth, $/ft2
Sprinkler system maintenance
Total operation and maintenance costs, $
C. Total Cost, $/ft2
Portion attributable to stormwater
Assumed %
D. Cost for Stormwater
Input
Data
44,552
6%
2,673
0.06
34,172
20
80%
1.50
12.78
26
17
7840
10%
Good
(1/99 $/ft2)
0.43
0.50
0.64
0.03
0.62
2.22
3.49
5.71
0.24
0.72
0.25
1.21
6.92
0.69
Fair
(1/99 $/ft2)
0.34
0.40
0.51
0.02
0.44
1.71
3.49
5.20
0.15
0.50
0.15
0.80
6.00
0.60
Poor
(1/99 $/ft2)
0.26
0.30
0.38
0.01
0.00
0.95
3.49
4.44
0.09
0.35
0.00
0.44
4.88
0.49
76
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6.3.4.3 Value of land for WWF systems
We support the view that land value should be included in the cost of WWF systems.
The amount to be charged should be based on the opportunity cost of this land. This
charge is an essential part of the analysis because most of the onsite or neighborhood
BMPs are land intensive, e.g., detention systems, functional landscapes. The incidence of
these costs is also critical in order to reward customers for onsite controls and to properly
assess all users for their fair share of the total cost.
6.3.4.4 Customers in the WWF system
The customers of the urban WWF system can be viewed as the individual parcels served
by the system. However, this taxonomy ignores perhaps the largest generator of urban
WWFs, especially during micro-storms. This large customer is transportation that takes
place in the rights-of-way of cities. This right-of-way consists of about 25% of total land
use. However, it constitutes a disproportionately large amount of the directly connected
impervious area that is critical in reducing the natural initial abstraction. Transportation
systems also constitute a major portion of the WWF quality loads. Thus, they should be
included as separate customers in order to evaluate their share of the cost of the WWF
system.
6.4 Hypothetical Study Area
The study area shown in Figure 5-3 was digitized, and a parcel level GIS developed
based upon each graphic object. The available themes are the following:
1. Land Use
2. Parcels
3. Storm Sewer Lines
4. Manholes
5. Soils
6. Spot Elevations
7. Street Right-of-way
8. Rooflines
9. Driveways
The study area GIS is shown in Figure 6-4. The land use classifications of the study area
are shown in shaded colors. Rights of way are shown in shaded blue, rooflines are
outlined in purple, driveways are in solid magenta, the storm sewer system is outlined in
red, manholes for the storm sewers are in solid black, and the parcel boundaries are in
black outline.
A representation of the soils for the site is shown in Figure 6-5. The three soil
classifications are shown as green for rock, light brown for clay, and dark brown for silt.
The soil classification is based upon the values given in Table 5-8.
77
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Associated with each graphic object, grouped according to type, is a relational database.
Attributes associated with parcels are address and land area; and with streets are right-of-
way width, length, land area, and street name. Soils and land use exist in separate tables,
and this information is combined with the parcel and street databases by performing an
intersection query on the two themes. The results of the query can also be output to an
Excel spreadsheet by using Arc View's Avenue® script language and Microsoft's
Dynamic Data Exchange® (DDE). This procedure was used to extract the relevant
attribute information for parcels and streets.
6.4.1 Study area attributes
The rights-of-way identified in Figures 6.4 and 6.5 were assigned widths based upon the
following criteria. Most streets within the development have a 50 ft right-of-way, a
minor arterial is given a 60 ft right-of-way, and a major arterial a 70 ft right-of-way. The
profile for each right-of-way is given in Table 6-8.
78
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Table 6-8. Right-of-Way Characteristics*
R/W
(ft)
50
60
70
Length,
(ft)
28680
1124
2741
Curb
(ft)
4
4
4
Parking
(ft) _
8
16
16
Landscaping
strip (ft)
10
10
18
Sidewalk
(ft)
8
8
8
Traffic
Lanes (ft)
20
22
24
* Some of the parameters are summed from both sides of the street.
Lot characteristics for the two single lot residential land use classifications are tabulated
in Table 6-9. Lots were aggregated in this manner for the optimization, but not for the
detailed cost analysis.
Table 6-9. Lot Characteristics for Residential Parcels
Land Use
MD Residential (6-8 DU/AC)
LD Residential (2-5 DU/AC)
#of
Parcels
255
51
Roof
Area
(ft2)
1,600
2,000
Patio
(ft2)
200
400
Driveway
(ft2)
600
800
Landscaping
(ft2)
3,600
9,800
Total
Area
(ft2)
6,000
13,000
79
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For the apartments, commercial, and school land uses, an aggregated analysis was used.
This is because these land uses exhibited multi-parcel characteristics, such as for parking
uses. A summary of these characteristics is found in Table 6-10.
Table 6-10. Aggregate Characteristics for Commercial, Apartments, and Schools
Land Use
Apartments
Commercial
School
Number
of
parcels
2
6
3
Stories
2
1
1
Parcel
Area
(ft2)
162,680
481,070
149,407
Roof
Area
(ft2)
46,927
152,839
69,080
Parking
Area
(ft2)
75,083
304,678
51,807
Landscaping
(ft2)
40,670
23,553
28,521
6.4.2 Unit costs
Next, unit costs were developed for each development component. The results are
presented below.
6.4..2.1 Landscaping costs
Landscaping costs depend upon several factors, including opportunity costs, the cost of
soil preparation including topsoil, sod, and soil conditioners, and an irrigation system. In
order to determine the opportunity cost, a land valuation analysis must be done for each
land use. Land valuation analysis for a medium density residential lot is presented in
Table 6-11. The area of each component of the medium density lot is listed in column 2
of Table 6-11. The percentage of each component is calculated in column 3.
Table 6-11. Land Valuation for Medium Density Lot
Component
Roof-house
Roof-garage
Driveway
Yard
Patio
Total
Area
(ft2)
1,200
400
600
3,600
200
6,000
%of
total
20.0
6.7
10.0
60.0
3.3
100.0
Cost
(1/99 $/ft2)
56.25
34.00
4.00
1.00
4.00
Construction Cost
(1/99$)
67,500
13,600
2,400
3,600
800
87,900
Total Land
(1/99 $)
8,790
2,930
4,395
26,370
1,465
43,950
Unimproved
Land (1/99 $)
5,860
1,953
2,930
17,580
977
29,300
An estimate of the cost in $/ft is found in column 4. Next, the construction cost is
obtained by multiplying column 2 by column 4, and listing this in column 5. Next, the
percentage in column 3 is multiplied by the total of column 5 to obtain an estimate of the
land cost, in column 6. Column 7, the unimproved land cost, is obtained by multiplying
the values in column 6 by 2/3. The value of the 3,600 ft of land for the yard function is
$26,370.
Next, opportunity costs must be calculated. This procedure is illustrated in Table 6-12.
The value of $26,370 is annualized, using an interest rate of 6%, and an infinite term (as
in equation 6.2), to obtain $l,582/yr. Then, this value is spread over 25 yrs at 6%, to
80
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obtain $20,226. Dividing this value by 3,600 ft2 gives $5.62/ft2. This value is used for
all grass types because the underlying value of the land is assumed constant irrespective
of the type of grass.
Landscaping costs were developed from RS Means (1996b), and updated to 1/99 $, using
the procedure shown in chapter 4 and are presented in Table 6-12 (for a medium density
residential lot). The initial capital investment consists of the cost of soil preparation
including sod, topsoil, and soil conditioners; and an irrigation system. For a good lawn,
the present value of the initial landscaping investment is $2.22/ft2. Costs for lesser
quality lawns drop to $1.7I/ft2 and $.95/ft2 for fair and poor quality lawns. For the good
lawn system, operation and maintenance costs add an additional $2.45/ft2 bringing the
total to $10.29/ft2. An estimated 10% of this total cost is allocated to stormwater
management. Similar estimates were made for fair and poor lawns. The resulting total
costs per ft2 vary from $0.70 to $1.03/ft2. Better lawns have a lower CN and are thereby
preferable from the viewpoint of being able to store more water. However, they also cost
more. A linear programming model will be used to find the least costly mix.
Similar estimates were made for the land valuation of low density residential lots,
commercial, apartments, and schools. A similar procedure was followed for these uses,
except commercial, apartment, and school uses are aggregated as one lot. These
valuations can be found in Table 6-13 for low density, Table 6-15, for commercial, Table
6-17 for apartments, and 6-19 for schools. Landscaping costs were determined the same
way, and are found in Table 6-14 for low density residential, Table 6-16 for commercial,
Table 6-18 for apartments, and Table 6-20 for schools.
81
-------
Table 6-12. Cost Analysis of Landscaping for Medium Density Lot
Item
A. Initial Capital Investment
1 . Soil preparation
Initial cost of sod
Initial cost of topsoil, 6 in.
Spreading topsoil, 6 in.
Soil conditioners
Sprinkler system
2. Opportunity Cost of Land
Land Investment Cost, $
Opportunity cost investment rate, %
Annual cost, $/yr
Interest rate/yr, %
Present worth over 25 yr, $
Cost in $/ft2
Total of initial capital investment
B. Operation & Maintenance Costs, $
Lawn watering
in./yr
% of permeable area that is irrigated
Cost of water, $/1 ,000 gal
Present worth factor
Present worth, $/ft2
Lawn maintenance
Weeks/yr
$/week
Maintenance area, ft2
Present worth, $/ft2
Sprinkler system maintenance
Total operation and maintenance costs, $
C. Total Cost, $/ft2
Portion attributable to stormwater
Assumed %
D. Cost for Stormwater
Input
Data
26,370
6
1,582
6
20,226
20
80
1.50
12.78
26
8.46
2880
10
Good
1/99 $/ft2
0.43
0.50
0.64
0.03
0.62
2.22
5.62
7.84
0.24
0.98
0.25
1.46
9.31
0.93
Fair
1/99 $/ft2
0.34
0.40
0.51
0.02
0.44
1.71
5.62
7.33
0.15
0.50
0.15
0.80
8.13
0.81
Poor
1/99 $/ft2
0.26
0.30
0.38
0.01
0.00
0.95
5.62
6.57
0.09
0.35
0.00
0.44
7.01
0.70
82
-------
Table 6-13. Land Valuation for Low Density Lot
Component
Roof-house
Roof-garage
Driveway
Yard
Patio
Total
Area
(ft2)
1,600
400
800
9,800
400
13,000
%of
total
12.3
3.1
6.2
75.4
3.1
100.0
Cost
(1/99 $/ft2)
56.25
34.00
4.00
1.00
4.00
Construction
Cost (1/99$)
90,000
13,600
3,200
9,800
1,600
118,200
Total Land
Cost (1/99$)
7,274
1,818
3,637
44,552
1,818
59,100
Unimproved
Land Cost (1/99 $)
4,849
1,212
2,425
29,702
1,212
39,400
Table 6-14. Cost Analysis of Landscaping for Low Density Lot
Item
A. Initial Capital Investment
1 . Soil preparation
Initial cost of sod
Initial cost of topsoil, 6 in.
Spreading topsoil, 6 in.
Soil conditioners
Sprinkler system
2. Opportunity Cost of Land
Land Investment Cost, $
Opportunity cost investment rate, %
Annual cost, $/yr
Interest rate/yr, %
Present worth over 25 yr, $
Cost in $/ft2
Total of initial capital investment
B. Operation & Maintenance Costs, $
Lawn watering
in./yr
% of permeable area that is irrigated
Cost of water, $/1 ,000 gal
Present worth factor
Present worth, $/ft2
Lawn maintenance
Weeks/yr
$/week
Maintenance area, ft2
Present worth, $/ft2
Sprinkler system maintenance
Total operation and maintenance costs, $
C. Total Cost, $/ft2
Portion attributable to stormwater
Assumed %
D. Cost for Stormwater
Input
Data
44,552
6
2,673
6
34,172
20
80
1.50
12.78
26
17.00
7840
10
Good
$/ft2
0.43
0.50
0.64
0.03
0.62
2.22
3.49
5.71
0.24
0.72
0.25
1.21
6.92
0.69
Fair
$/ft2
0.34
0.40
0.51
0.02
0.44
1.71
3.49
5.20
0.15
0.50
0.15
0.80
6.00
0.60
Poor
$/ft2
0.26
0.30
0.38
0.01
0.00
0.95
3.49
4.44
0.09
0.35
0.00
0.44
4.88
0.49
83
-------
Table 6-15. Land Valuation for Commercial Areas
Component
Roof
Parking
Driveway
Yard
Patio
Total
Area
(ft2)
152,839
304,678
0
23,553
0
481,070
%of
total
31.8
63.3
0.0
4.9
0.0
100.0
Cost
(1/99 $/ft2)
150.00
1.50
1.50
1.00
4.00
Construction
Cost (1/99 $)
22,925,901
457,017
0
23,553
0
23,406,471
Total Land
Cost (1/99 $)
3,718,198
7,412,052
0
572,985
0
11,703,236
Unimproved
Land Cost (1/99 $)
2,478,799
4,941,368
0
381,990
0
7,802,157
Table 6-16. Cost Analysis of Landscaping for Commercial Areas
Item
A. Initial Capital Investment
1 . Soil preparation
Initial cost of sod
Initial cost of topsoil, 6 in.
Spreading topsoil, 6 in.
Soil conditioners
Sprinkler system
2. Opportunity Cost of Land
Land Investment Cost, $
Opportunity cost investment rate
Annual cost, $/yr
Interest rate/yr, %
Present worth over 25 yr, $
Cost in $/ft2
Total of initial capital investment
B. Operation & Maintenance Costs, $
Lawn watering
In./yr
% of permeable area that is irrigated
Cost of water, $/1 ,000 gal
Present worth factor
Present worth, $/ft2
Lawn maintenance
Weeks/yr
$/week
Maintenance area, ft2
Present worth, $/ft2
Sprinkler system maintenance
Total operation and maintenance costs, $
C. Total Cost, $/ft2
Portion attributable to stormwater
Assumed %
D. Cost for Stormwater
Input
Data
572,985
6
34,379
6
439,481
20
100
1.50
12.78
26
33.26
23553
10
Good
1/99 $/ft2
0.43
0.50
0.64
0.03
0.62
2.22
18.66
20.88
0.24
0.47
0.25
0.96
21.84
2.18
Fair
1/99 $/ft2
0.34
0.40
0.51
0.02
0.44
1.71
18.66
20.37
0.15
0.50
0.15
0.80
21.17
2.12
Poor
1/99 $/ft2
0.26
0.30
0.38
0.01
0.00
0.95
18.66
19.61
0.09
0.35
0.00
0.44
20.05
2.01
84
-------
Table 6-17. Land Valuation for Apartments
Component
Roof
Parking
Driveway
Yard
Patio
Total
Area
(ft2)
46,927
75,083
0
40,670
0
162,680
%of
total
28.8
46.2
0.0
25.0
0.0
100.0
Cost
(1/99 $/ft2)
84.38
1.50
1.50
1.00
4.00
Construction
Cost (1/99 $)
3,959,466
112,625
0
40,670
0
4,112,760
Total Land
Cost (1/99 $)
593,187
949,100
0
514,093
0
2,056,380
Unimproved
Land Cost (1/99$)
395,458
632,733
0
342,729
0
1,370,920
rable 6-18. Cost Analysis of Landscaping for A|
Item
A. Initial Capital Investment
1 . Soil preparation
Initial cost of sod
Initial cost of topsoil, 6 in.
Spreading topsoil, 6 in.
Soil conditioners
Sprinkler system
2. Opportunity Cost of Land
Land Investment Cost
Opportunity cost investment rate
Annual cost, $/yr
Interest rate/yr, %
Present worth over 25 yr, $
Cost in $/ft2
Total of initial capital investment
B. Operation & Maintenance Costs, $
Lawn watering
In./yr
% of permeable area that is irrigated
Cost of water, $/1 ,000 gal
Present worth factor
Present worth, $/ft2
Lawn maintenance
Weeks/yr
$/week
Maintenance area, ft2
Present worth, $/ft2
Sprinkler system maintenance
Total operation and maintenance costs, $
C. Total Cost, $/ft2
Portion attributable to stormwater
Assumed %
D. Cost for Stormwater
jartments
Input
Data
514,093
6
30,846
6
394,310
20
80
1.50
12.78
26
44.04
32536
10
Good
1/99 $/ft2
0.43
0.50
0.64
0.03
0.62
2.22
9.70
11.92
0.24
0.45
0.25
0.94
12.86
1.29
Fair
1/99 $/ft2
0.34
0.40
0.51
0.02
0.44
1
9.70
11.41
0.15
0.50
0.15
0.80
12.21
1.22
Poor
1/99 $/ft2
0.26
0.30
0.38
0.01
0.00
0.95
9.70
10.65
0.09
0.35
0.00
0.44
11.09
1.11
85
-------
Table 6-19. Land Valuation for Schools
Component
Roof
Parking
Driveway
Yard
Patio
Total
Area
(ft2)
46,927
75,083
0
40,670
0
162,680
%of
Total
28.8
46.2
0.0
25.0
0.0
100.0
Cost
(1/99 $/ft2)
84.38
1.50
1.50
1.00
4.00
Construction
Cost (1/99 $)
8,635,000
77,709
0
28,521
0
8,741,230
Total Land
Cost (1/99$)
2,020,799
1,515,482
0
834,334
0
4,370,615
Unimproved
Land Cost (1/99 $)
1,347,199
1,010,322
0
556,222
0
2,913,743
Table 6-20. Cost Analysis of Landscaping for Schools
Item
A. Initial Capital Investment
1 . Soil preparation
Initial cost of sod
Initial cost of topsoil, 6 in.
Spreading topsoil, 6 in.
Soil conditioners
Sprinkler system
2. Opportunity Cost of Land
Land Investment Cost, $
Opportunity cost investment rate, %
Annual cost, $/yr
Interest rate/yr, %
Present worth over 25 yr, $
Cost in $/ft2
Total of initial capital investment
B. Operation & Maintenance Costs, $
Lawn watering
In. per year
% of permeable area that is irrigated
Cost of water, $/1 ,000 gal
Present worth factor
Present worth, $/ft2
Lawn maintenance
Weeks/yr
$/week
Maintenance area, ft2
Present worth, $/ft2
Sprinkler system maintenance
Total operation and maintenance costs, $
C. Total Cost, $/ft2
Portion attributable to stormwater
Assumed %
D. Cost for Stormwater
Input
Data
834,334
6
50,060
6
639,935
20
80
1.50
12.78
26
32.38
22817
10
Good
1/99 $/ft2
0.43
0.50
0.64
0.03
0.62
2.22
22.44
24.66
0.24
0.47
0.25
0.96
25.62
2.56
Fair
1/99 $/ft2
0.34
0.40
0.51
0.02
0.44
1.71
22.44
24.15
0.15
0.50
0.15
0.80
24.95
2.49
Poor
1/99 $/ft2
0.26
0.30
0.38
0.01
0.00
0.95
22.44
23.39
0.09
0.35
0.00
0.44
23.83
2.38
86
-------
6.4.2.2 Right-of-way costs
Based upon the paving costs shown in Table 3-7 (explained in equations 3.4 to 3.8 and
the profile selected from Table 6-8), costs were assigned to each right-of-way. These
costs are presented as $/linear foot, assuming the widths from Table 6-8, and are
presented in Table 6-21.
Table 6-21. Costs of Pavement, Curb and Gutter, and Sidewalks*
Right-of-
Way
50
60
70
Curb
(1/99 $)
13.89
13.89
13.89
Pavement
(1/99 $)
33.63
45.64
48.04
Sidewalks
(1/99 $)
2.40
2.40
2.40
* Curbs are assumed to be both sides of the street with 2 ft in width
Unit costs are $3.47/ft2 for curbs, $1.20/ft2 for pavement, and $.30/ft2 for sidewalks.
Since the area of each paved surface is known, these ft2 estimates can be multiplied by
this area to obtain the total cost. Alternatively, the length (within each right-of-way type)
may be multiplied by the unit factors found in Table 6-21.
The total right-of-way costs are not just a function of pavement costs. There is an
opportunity cost to devoting land for right-of-way instead of for development. Several
different methods can be used for determining the value of the right-of-way; the one
selected here is that of using the lowest valued use, which is the opportunity cost for
undeveloped land for low density residential use, or $3.49/ft2. This method is consistent
with marginal cost analysis. Several street profiles were analyzed, and are shown in
Table 6-22. Street 1 is a standard street with curb and gutter. Street 2 is a street with
porous pavement and curb and gutter. Street 3 is a standard pavement street with swales.
Street 4 is a street with porous pavement and swales. Because the right-of-way must
remain constant, the travel lane was reduced in the case of streets using swales. These
costs are added to the opportunity cost and apportioned to stormwater as shown in Table
6-22.
87
-------
Table 6-22. Cost Analysis for 50 ft Right-of-Way
Item
A. Initial Capital Investment
Opportunity Cost: Low Density
Residential
B. Pavement Costs
width of street, ft
width of swales, ft
width of pavement, ft
Swales, $/ft^
curb and gutter, $/ft
pavement, $/ft
total, $/ft
total of B, $/ft^
C. Total, $/ft'
Portion attributable to
stormwater
Assumed, %
D. Cost for Stormwater
Input Data
32
3.00
5
Street 1
(1/99 $/ft2)
3.49
28
13.89
33.63
47.52
1.49
4.97
0.25
Street 2
(1/99 $/ft2)
3.49
28
13.89
42.04
55.93
1.75
5.23
0.26
Street 3
(1/99 $/ft2)
3.49
12
20
36.00
24.02
60.02
1.88
5.36
0.27
Street 4
(1/99 $/ft2)
3.49
12
20
36.00
30.03
66.03
2.06
5.55
0.28
Similar analysis can be performed for 60 and 70 ft right-of-way streets. These results are
presented in Tables 6.23 and 6.24.
Table 6-23. Cost Analysis for 60 ft Right-of-Way
Item
A. Initial Capital Investment
Opportunity Cost: Low Density
Residential
B. Pavement Costs
width of street, ft
width of swales, ft
width of pavement, ft
swales, $/ft^
curb and gutter, $/ft
pavement, $/ft
total, $/ft
total, $/ft"
C. Total, $/ft'
Portion attributable to
stormwater
Assumed, %
D. Cost for Stormwater
Input
Data
42
3.00
5
Street 1
(1/99 $/ft2)
3.49
38
13.89
45.64
59.54
1.42
4.90
0.25
Street 2
(1/99 $/ft2)
3.49
38
13.89
57.05
70.95
1.69
5.18
0.26
Street 3
(1/99 $/ft2)
3.49
12
38
36
45.64
81.64
1.94
5.43
0.27
Street 4
(1/99 $/ft2)
3.49
12
38
36
57.05
93.05
2.22
5.70
0.29
Proceeding from left to right in Tables 6.22 through 6.24, the streets have increasingly
better infiltration characteristics. This is reflected in the curve numbers for the street,
88
-------
however, the street becomes more expensive. A linear program model can be used to
determine the least costly mix.
Table 6-24. Cost Analysis for 70 ft Right-of-Way
Item
A. Initial Capital Investment
Opportunity Cost: Low Density
Residential
B. Pavement Costs
width of street, ft
width of swales, ft
width of pavement, ft
swales, $/ft^
curb and gutter, $/ft
pavement, $/ft
Total, $/ft
Total, $/ft"
C. Total, $/ft'
Portion attributable to
stormwater
Assumed, %
D. Cost for Stormwater
Input Data
44
3.00
5
Street 1
(1/99 $/ft2)
3.49
40
13.89
48.04
61.94
1.41
4.89
0.24
Street 2
(1/99 $/ft2)
3.49
40
13.89
60.05
73.95
1.68
5.17
0.26
Street 3
(1/99 $/ft2)
3.49
12
40
36.00
48.04
84.04
1.91
5.40
0.27
Street 4
(1/99 $/ft2)
3.49
12
40
36.00
60.05
96.05
2.18
5.67
0.28
6.4.2.3 Costs for other land functions
The costs of parking, sidewalks and patios, and driveways were determined using a
similar procedure. Parking lots were evaluated in the following forms: standard
pavement, and three types of porous pavement of gradually increasing permeability. The
cost analysis for parking is shown in Table 6-25. As the permeability of the parking area
increases, it is given a lower curve number, but the cost rises as well. This can be
investigated using a linear program model. A ratio of 5% was used to apportion the costs
to stormwater.
Table 6-25. Cost Analysis for Parking
Item
A. Initial Capital Investment
Opportunity Cost: Low Density
Residential
B. Pavement Costs
paving costs, $/ft2
C. Total, $/ft2
Portion attributable to
stormwater
Assumed %
D. Cost for Stormwater
Input Data
1.20
5
Parking 1
(1/99 $/ft2)
3.49
1.20
4.69
0.23
Parking 2
(1/99 $/ft2)
3.49
1.50
4.99
0.25
Parking 3
(1/99 $/ft2)
3.49
1.80
5.29
0.26
Parking 4
(1/99 $/ft2)
3.49
2.10
5.59
0.28
89
-------
Two types of sidewalks were evaluated, standard and porous, and two types of patios,
standard and porous. This analysis is shown in Table 6-26. Again, with the second
sidewalk (or patio), the curve number decreases as the infiltration performance increases,
however the cost also increases, albeit very slightly. A ratio of 5% was apportioned to
storm water costs.
Table 6-26. Cost Analysis for Sidewalks and Patios
Item
A. Initial Capital Investment
Opportunity Cost: Low Density
Residential
B. Pavement Costs
Sidewalk costs, $/ft2
C. Total, $/ft2
Portion attributable to stormwater
Assumed, %
D. Cost for Stormwater
Input
Data
0.30
5
Sidewalk*!/
Patiol
(1/99 $/ft2)
3.49
0.30
3.79
0.19
Sidewalk2/
Patio2
(1/99 $/ft2)
3.49
0.38
3.86
0.19
Two types of driveways were evaluated, standard and porous, and this analysis is shown
in Table 6-27. Again, with the second driveway, as the permeability increases, the curve
number decreases, but the cost increases. A ratio of 5% was apportioned to stormwater
costs.
Table 6-27. Cost Analysis for Driveways
Item
A. Initial Capital Investment
Opportunity Cost: Low Density
Residential
B. Pavement Costs
Paving costs, $/ft2
C. Total, $/ft2
Portion attributable to stormwater
Assumed, %
D. Cost for Stormwater
Input
Data
1.20
5
Driveway 1
(1/99 $/ft2)
3.49
1.20
4.69
0.23
Driveway 2
(1/99 $/ft2)
3.49
1.50
4.99
0.25
6.4.3 Summary of costs for each parcel
Based upon the landscaping costs shown in Tables 6-12, 6-14, 6-16, 6-18, and 6-20, the
costs for parking in Table 6-25, the cost for sidewalks in Table 6-26, and the cost for
driveways in Table 6-27, costs were assigned to each parcel. These costs are presented in
Table 6-28. These costs are based upon the rooflines calculated directly from the figures
or listed in Tables 6-9 (for single family residential lots) and the total parcel area. The
total landscaping costs for the developed area is $14.5 million. The parking areas total $2
million, and the driveways total $969,000. These costs include opportunity costs.
90
-------
Table 6-28. Parcel Develoi
Add-
ress
100
101
200
200
201
100
200
201
105
110
120
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
123
125
127
129
100
101
102
106
108
110
120
121
Street
Alpine Street
Alpine Street
Cedar Street
Ash mount Street
Ash mount Street
Highland Street
Birch Avenue
Birch Avenue
Center Street
Center Street
Center Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Maple Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Soil
Silt
Silt
Clay
Rock
Rock
Rock
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
)ment Costs
Land Use
Apartments
Apartments
Commercial
Commercial
Commercial
Commercial
Commercial
Commercial
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
Area
(ft2)
50320
112360
25957
154915
72968
80450
100139
46642
14235
18488
6844
15082
9927
11751
9742
11025
8744
11441
7667
12942
11518
11728
7707
12053
14291
17653
8015
13857
13778
11207
18674
15565
13029
14017
16758
19500
22449
14049
10172
11049
11131
11239
11681
11993
12611
Roof
(ft2)
0
46927
0
57707
0
0
95132
0
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
Parking
(ft2)
37740
37343
24659
89462
69319
76427
0
44810
Drive-ways
(ft2)
0
0
0
0
0
0
0
0
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
Patios
(ft2)
0
0
0
0
0
0
0
0
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
Impervious
(ft2)
37740
84270
24659
147169
69319
76427
95132
44810
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
Pervious
(ft2)
12580
28090
1298
7746
3648
4022
5007
1832
11035
15288
3644
11882
6727
8551
6542
7825
5544
8241
4467
9742
8318
8528
4507
8853
11091
14453
4815
10657
10578
8007
15474
12365
9829
10817
13558
16300
19249
10849
6972
7849
7931
8039
8481
8793
9411
Landscaping
(1/99 $)
159,000
354,000
29,000
168,000
79,000
87,000
109,000
40,000
77,000
106,000
26,000
83,000
47,000
60,000
46,000
55,000
39,000
58,000
31,000
68,000
58,000
60,000
32,000
62,000
77,000
101,000
34,000
74,000
74,000
56,000
108,000
86,000
69,000
75,000
94,000
113,000
134,000
76,000
49,000
55,000
55,000
56,000
59,000
61,000
66,000
Parking
(1/99 $)
177,000
176,000
116,000
420,000
325,000
359,000
0
211,000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Driveway
(1/99 $)
0
0
0
0
0
0
0
0
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
Total
(1/99$)
336,000
530,000
145,000
588,000
404,000
446,000
109,000
251 ,000
81 ,000
110,000
30,000
87,000
51,000
64,000
50,000
59,000
43,000
62,000
35,000
72,000
62,000
64,000
36,000
66,000
81 ,000
105,000
38,000
78,000
78,000
60,000
112,000
90,000
73,000
79,000
98,000
117,000
138,000
80,000
53,000
59,000
59,000
60,000
63,000
65,000
70,000
91
-------
Add-
ress
130
131
140
141
150
151
160
161
170
171
180
181
190
191
151
160
161
165
170
171
176
179
180
181
182
100
101
110
111
120
121
131
135
139
141
150
151
160
161
170
171
180
181
190
191
100
Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Oak Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Acorn Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash Street
Ash-Acorn Connec
Soil
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Land Use
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
LD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
Area
(ft2)
12127
12680
12646
12749
13048
12818
12950
12886
13016
12955
13412
13618
14363
11552
6019
5286
3926
3853
5543
3926
5800
3926
4788
3926
4783
5750
6785
6600
6765
6620
6744
6724
6703
6683
6662
3919
6642
4481
6621
4763
6601
4878
6581
4326
6560
3127
Roof
(ft2)
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
Parking
(ft2)
Drive-ways
(ft2)
800
800
800
800
800
800
800
800
800
800
800
800
800
800
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
Patios
(ft2)
400
400
400
400
400
400
400
400
400
400
400
400
400
400
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
Impervious
(ft2)
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
Pervious
(ft2)
8927
9480
9446
9549
9848
9618
9750
9686
9816
9755
10212
10418
11163
8352
3619
2886
1526
1453
3143
1526
3400
1526
2388
1526
2383
3350
4385
4200
4365
4220
4344
4324
4303
4283
4262
1519
4242
2081
4221
2363
4201
2478
4181
1926
4160
727
Landscaping
(1/99 $)
62,000
66,000
66,000
67,000
69,000
67,000
68,000
68,000
68,000
68,000
71,000
73,000
78,000
58,000
38,000
30,000
16,000
15,000
33,000
16,000
35,000
16,000
25,000
16,000
25,000
35,000
46,000
44,000
45,000
44,000
45,000
45,000
45,000
45,000
44,000
16,000
44,000
22,000
44,000
25,000
44,000
26,000
44,000
20,000
43,000
8,000
Parking
(1/99 $)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Driveway
(1/99 $)
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
Total
(1/99$)
66,000
70,000
70,000
71 ,000
73,000
71 ,000
72,000
72,000
72,000
72,000
75,000
77,000
82,000
62,000
41 ,000
33,000
19,000
18,000
36,000
19,000
38,000
19,000
28,000
19,000
28,000
38,000
49,000
47,000
48,000
47,000
48,000
48,000
48,000
48,000
47,000
19,000
47,000
25,000
47,000
28,000
47,000
29,000
47,000
23,000
46,000
1 1 ,000
92
-------
Add-
ress
101
111
121
131
141
150
151
154
155
161
165
166
170
171
180
181
190
191
100
101
110
111
112
116
120
121
131
141
151
161
180
190
101
111
121
131
141
181
191
100
110
120
130
140
150
160
Street
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Ash-Acorn Connec
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Soil
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Land Use
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
Area
(ft2)
3180
3039
3157
2994
3086
4739
3157
5648
3109
3089
3149
5648
4630
3349
4818
2948
4551
2686
6469
6554
6477
6522
6484
6492
6499
6490
6457
6425
6360
6328
6560
6568
6572
6580
6588
6595
6603
6663
6671
6481
6448
6416
6384
6351
6319
6286
Roof
(ft2)
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
Parking
(ft2)
Drive-ways
(ft2)
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
Patios
(ft2)
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
Impervious
(ft2)
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
Pervious
(ft2)
780
639
757
594
686
2339
757
3248
709
689
749
3248
2230
949
2418
548
2151
286
4069
4154
4077
4122
4084
4092
4099
4090
4057
4025
3960
3928
4160
4168
4172
4180
4188
4195
4203
4263
4271
4081
4048
4016
3984
3951
3919
3886
Landscaping
(1/99 $)
9,000
7,000
8,000
7,000
8,000
25,000
8,000
34,000
8,000
8,000
8,000
34,000
23,000
10,000
25,000
6,000
23,000
3,000
42,000
43,000
42,000
43,000
43,000
43,000
43,000
43,000
42,000
42,000
41,000
41,000
43,000
43,000
43,000
44,000
44,000
44,000
44,000
44,000
44,000
43,000
42,000
42,000
42,000
41,000
41,000
41,000
Parking
(1/99 $)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Driveway
(1/99 $)
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
Total
(1/99$)
12,000
10,000
1 1 ,000
10,000
1 1 ,000
28,000
1 1 ,000
37,000
1 1 ,000
1 1 ,000
1 1 ,000
37,000
26,000
13,000
28,000
9,000
26,000
6,000
45,000
46,000
45,000
46,000
46,000
46,000
46,000
46,000
45,000
45,000
44,000
44,000
46,000
46,000
46,000
47,000
47,000
47,000
47,000
47,000
47,000
46,000
45,000
45,000
45,000
44,000
44,000
44,000
93
-------
Add-
ress
170
106
101
111
120
140
141
150
151
100
101
120
121
141
161
100
101
121
140
100
101
120
141
100
101
120
141
101
100
101
110
111
120
121
130
131
140
141
150
151
156
160
161
165
166
170
Street
Elm Street
Forest Avenue
Main Street
Main Street
Main Street
Main Street
Main Street
Main Street
Main Street
Street A
Street A
Street A
Street A
Street A
Street A
Street B
Street B
Street B
Street B
Street C
Street C
Street C
Street C
Street D
Street D
Street D
Street D
Street E
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Soil
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Clay
Land Use
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
Area
(ft2)
6254
6428
4993
5154
6770
6636
6323
4939
6323
5072
4644
5072
4789
4934
5079
4787
4953
4953
4787
5609
4737
5609
4888
5254
5461
5254
5461
5192
6480
6511
6460
6712
6439
6470
6419
6492
6399
6514
6378
6536
6358
6337
6558
6580
6317
6296
Roof
(ft2)
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
Parking
(ft2)
Drive-ways
(ft2)
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
Patios
(ft2)
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
Impervious
(ft2)
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
Pervious
(ft2)
3854
4028
2593
2754
4370
4236
3923
2539
3923
2672
2244
2672
2389
2534
2679
2387
2553
2553
2387
3209
2337
3209
2488
2854
3061
2854
3061
2792
4080
4111
4060
4312
4039
4070
4019
4092
3999
4114
3978
4136
3958
3937
4158
4180
3917
3896
Landscaping
(1/99 $)
40,000
42,000
27,000
29,000
45,000
44,000
41,000
27,000
41,000
28,000
24,000
28,000
25,000
27,000
28,000
25,000
27,000
27,000
25,000
34,000
25,000
34,000
26,000
30,000
32,000
30,000
32,000
29,000
42,000
43,000
42,000
45,000
42,000
42,000
42,000
43,000
42,000
43,000
41,000
43,000
41,000
41,000
43,000
44,000
41,000
41,000
Parking
(1/99 $)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Driveway
(1/99 $)
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
Total
(1/99$)
43,000
45,000
30,000
32,000
48,000
47,000
44,000
30,000
44,000
31 ,000
27,000
31 ,000
28,000
30,000
31 ,000
28,000
30,000
30,000
28,000
37,000
28,000
37,000
29,000
33,000
35,000
33,000
35,000
32,000
45,000
46,000
45,000
48,000
45,000
45,000
45,000
46,000
45,000
46,000
44,000
46,000
44,000
44,000
46,000
47,000
44,000
44,000
94
-------
Add-
ress
171
180
181
190
191
193
101
110
120
130
140
150
156
158
160
170
180
190
161
130
170
190
141
160
180
181
190
191
160
161
171
190
191
180
181
190
191
100
120
151
171
190
191
126
130
136
Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Sycamore Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Ashmount Street
Main Street
Street A
Street A
Street A
Street B
Street B
Street B
Street B
Street B
Street B
Street C
Street C
Street C
Street C
Street C
Street D
Street D
Street D
Street D
Street E
Street E
Street E
Street E
Street E
Street E
Birch Avenue
Birch Avenue
Birch Avenue
Soil
Clay
Clay
Clay
Clay
Clay
Clay
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Rock
Silt
Silt
Silt
Land Use
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
Area
(ft2)
5931
6276
5744
6255
6274
5919
6649
5611
5524
6461
6805
6624
6875
6554
6693
6533
6461
5691
6323
5072
5072
5072
4953
4787
4787
4953
4787
4953
5609
5039
5189
5609
5340
5254
5461
5254
5461
6520
6520
5363
5533
6520
5704
6507
6515
6522
Roof
(ft2)
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
Parking
(ft2)
Drive-ways
(ft2)
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
Patios
(ft2)
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
Impervious
(ft2)
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
Pervious
(ft2)
3531
3876
3344
3855
3874
3519
4249
3211
3124
4061
4405
4224
4475
4154
4293
4133
4061
3291
3923
2672
2672
2672
2553
2387
2387
2553
2387
2553
3209
2639
2789
3209
2940
2854
3061
2854
3061
4120
4120
2963
3133
4120
3304
4107
4115
4122
Landscaping
(1/99 $)
37,000
40,000
35,000
40,000
40,000
37,000
44,000
34,000
33,000
42,000
46,000
44,000
47,000
43,000
45,000
43,000
42,000
34,000
41,000
28,000
28,000
28,000
27,000
25,000
25,000
27,000
25,000
27,000
34,000
28,000
29,000
34,000
31,000
30,000
32,000
30,000
32,000
43,000
43,000
31,000
33,000
43,000
35,000
43,000
43,000
43,000
Parking
(1/99 $)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Driveway
(1/99 $)
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
Total
(1/99$)
40,000
43,000
38,000
43,000
43,000
40,000
47,000
37,000
36,000
45,000
49,000
47,000
50,000
46,000
48,000
46,000
45,000
37,000
44,000
31 ,000
31 ,000
31 ,000
30,000
28,000
28,000
30,000
28,000
30,000
37,000
31,000
32,000
37,000
34,000
33,000
35,000
33,000
35,000
46,000
46,000
34,000
36,000
46,000
38,000
46,000
46,000
46,000
95
-------
Add-
ress
140
150
160
170
171
181
191
193
151
155
161
165
171
175
179
101
111
121
131
141
151
176
180
181
190
191
193
195
201
221
231
241
244
250
251
254
260
261
270
274
280
281
290
291
100
101
Street
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Birch Avenue
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Cedar Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Elm Street
Forest Avenue
Forest Avenue
Soil
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Land Use
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
Area
(ft2)
6530
6537
6545
6552
6345
6939
7911
5095
6610
6618
6625
6633
6641
6648
6656
6663
6667
6671
6676
6680
6684
6070
6675
6688
6941
6693
4843
4131
6416
6106
6452
6627
6706
6894
6665
6256
6865
6682
6463
6886
6909
6699
6765
6716
6312
7572
Roof
(ft2)
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
Parking
(ft2)
Drive-ways
(ft2)
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
Patios
(ft2)
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
Impervious
(ft2)
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
Pervious
(ft2)
4130
4137
4145
4152
3945
4539
5511
2695
4210
4218
4225
4233
4241
4248
4256
4263
4267
4271
4276
4280
4284
3670
4275
4288
4541
4293
2443
1731
4016
3706
4052
4227
4306
4494
4265
3856
4465
4282
4063
4486
4509
4299
4365
4316
3912
5172
Landscaping
(1/99 $)
43,000
43,000
43,000
43,000
41,000
47,000
57,000
28,000
44,000
44,000
44,000
44,000
44,000
44,000
44,000
44,000
44,000
44,000
45,000
45,000
45,000
38,000
45,000
45,000
47,000
45,000
26,000
18,000
42,000
39,000
42,000
44,000
45,000
47,000
44,000
40,000
46,000
45,000
42,000
47,000
47,000
45,000
45,000
45,000
41,000
54,000
Parking
(1/99 $)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Driveway
(1/99 $)
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
Total
(1/99$)
46,000
46,000
46,000
46,000
44,000
50,000
60,000
31 ,000
47,000
47,000
47,000
47,000
47,000
47,000
47,000
47,000
47,000
47,000
48,000
48,000
48,000
41 ,000
48,000
48,000
50,000
48,000
29,000
21,000
45,000
42,000
45,000
47,000
48,000
50,000
47,000
43,000
49,000
48,000
45,000
50,000
50,000
48,000
48,000
48,000
44,000
57,000
96
-------
Add-
ress
110
111
120
130
140
141
150
151
160
161
170
171
180
181
186
190
191
200
201
205
210
211
220
221
230
231
240
241
250
251
261
270
271
280
281
290
291
293
121
125
100
101
Street
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Forest Avenue
Main Street
Center Street
Walnut Street
Walnut Street
Soil
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Silt
Land Use
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
MD Residential
School
School
School
Area
(ft2)
6424
6971
6294
6313
6353
6998
6333
6875
6372
6694
6392
6619
8120
6724
6312
6079
6599
6558
6500
6389
6562
6266
6566
6326
6570
6133
6575
6025
6579
6193
6379
6583
6169
6587
5411
3196
5894
3230
5200
8600
97601
43206
Roof
(ft2)
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
1600
0
69080
0
Parking
(ft2)
8600
0
43206
Drive-ways
(ft2)
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
0
0
0
Patios
(ft2)
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
0
0
0
Impervious
(ft2)
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
2400
8600
69080
43206
Pervious
(ft2)
4024
4571
3894
3913
3953
4598
3933
4475
3972
4294
3992
4219
5720
4324
3912
3679
4199
4158
4100
3989
4162
3866
4166
3926
4170
3733
4175
3625
4179
3793
3979
4183
3769
4187
3011
796
3494
830
2800
0
28521
0
Landscaping
(1/99 $)
42,000
48,000
41,000
41,000
41,000
48,000
41,000
47,000
41,000
45,000
42,000
44,000
59,000
45,000
41,000
38,000
44,000
43,000
43,000
42,000
43,000
40,000
43,000
41,000
43,000
39,000
43,000
38,000
44,000
40,000
41,000
44,000
39,000
44,000
31,000
9,000
36,000
9,000
29,000
0
725,000
0
14,498,000
Parking
(1/99 $)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
41,000
0
203,000
2,028,000
Driveway
(1/99 $)
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
3,000
0
0
0
969,000
Total
(1/99$)
45,000
51,000
44,000
44,000
44,000
51,000
44,000
50,000
44,000
48,000
45,000
47,000
62,000
48,000
44,000
41,000
47,000
46,000
46,000
45,000
46,000
43,000
46,000
44,000
46,000
42,000
46,000
41,000
47,000
43,000
44,000
47,000
42,000
47,000
34,000
12,000
39,000
12,000
32,000
41 ,000
25,000
203,000
17,495,000
97
-------
6.4.4 Summary of costs for each right-of-way
A preliminary estimate of the right-of-way costs of development is obtained by:
1) Extracting the length and area attributes for each object using the "streets" theme
from the Arc View database;
2) Multiplying the unit costs found in Table 6-22 for 50 ft rights-of-way; Table 6-23
for 60 ft rights-of-way, and Table 6-24 for 70 ft rights-of-way by the area of each
right-of-way parcel.
The right-of-way cost data are presented in Table 6-29. Total paving costs for the
development are $2.3 million. Total opportunity costs for the area within the right-of-
way are $5.9 million. The total landscaping costs for the rights of way are $884,000.
Table 6-29. Right-of-Way Costs
Street Name
Acorn Street
Alpine Street
Ash Street
Ash-Acorn Connector
Ashmount Street
Ashmount Street ext.
Aspen Street
Birch Avenue
Cedar Street
Center Street
Elm Street
Forest Avenue
Highland Street
Main Street
Maple Street
Oak Street
Street A
Street B
Street C
Street D
Street E
stub between Elm
and Forest
Sycamore Street
Walnut Street
Total
RW
width,
(ft)
50
50
50
50
50
50
50
50
50
60
50
50
50
70
50
50
50
50
50
50
50
50
50
50
RW
length,
(ft)
1640
1125
1205
844
870
1620
851
2574
2899
1124
2639
2622
831
2741
2153
1751
490
465
517
415
397
519
1086
1167
Area
(ft2)
81990
56272
60251
42214
43492
80981
42537
128701
144940
67445
131944
131119
41568
191895
107667
87540
24491
23267
25829
20756
19875
25951
54281
58349
1 693357
Paving
Cost
(1/99 $)
114,000
78,000
84,000
59,000
61 ,000
112,000
59,000
178,000
201 ,000
92,000
183,000
182,000
58,000
230,000
149,000
122,000
34,000
33,000
36,000
29,000
28,000
36,000
76,000
81 ,000
2,315,000
Opportunity
Cost
(1/99 $)
286,000
197,000
211,000
148,000
152,000
283,000
149,000
449,000
506,000
236,000
461 ,000
458,000
145,000
670,000
376,000
306,000
86,000
82,000
91,000
73,000
70,000
91,000
190,000
204,000
5,920,000
Landscaping
Cost
(1/99 $)
42,000.00
29,000.00
31,000.00
22,000.00
22,000.00
41,000.00
22,000.00
65,000.00
73,000.00
29,000.00
67,000.00
66,000.00
21,000.00
124,000.00
55,000.00
44,000.00
13,000.00
12,000.00
13,000.00
11,000.00
10,000.00
14,000.00
28,000.00
30,000.00
884,000
Total
Cost
(1/99 $)
442,000
304,000
326,000
229,000
235,000
436,000
230,000
692,000
780,000
357,000
711,000
706,000
224,000
1 ,024,000
580,000
472,000
133,000
127,000
140,000
113,000
108,000
1 41 ,000
294,000
315,000
9,119,000
6.5 Estimated cost of BMP controls
The following sections describe the methodology used to determine runoff volumes,
evaluate the calculated difference in volume between the predevelopment and post
development scenarios, and lay out the procedure for estimating unit costs/gal of selected
controls for the optimization process.
98
-------
6.5.1 Determination of runoff volumes using SCS method
Each developed land use is assigned a curve number (CN) based upon work done by the
Soil Conservation Service (1986). The initial abstraction, or available storage is
estimated by the following equation:
/ =200-2
a CN
(6-3)
The final list of 10 permeable and 16 impermeable candidate land uses, with their
expected effectiveness as measured by their curve number (CN), and the associated initial
abstraction in inches, calculated using equation 6.3 are shown in Table 6-30. The CNs
range from 25 - 98. The initial abstraction associated with a CN of 25 is 6.00 in of
precipitation. Making this land impervious increases the CN to 98 with an associated
initial abstraction of only 0.04 in, a major loss of infiltration capacity. Using unit costs in
$/ft2, and having determined the appropriate abstraction, it is possible to convert the
control option costs to $/gal., which is done in the last four columns of Table 6-30. These
values are unique to the soil type heading the column. Unit costs expressed as $/gal are
useful for comparative purposes, as will be seen later.
6.5.2 Breakdown of calculated volumes per function
A functional analysis within each land use and soil classification is performed by adding
the total amounts of area for the functions of roof, lawns, driveways, and parking (for
non-right-of-way uses), and streets, curbs, parking, sidewalks, and lawns for rights-of-
way areas. Volumes of developed runoff can then be calculated by multiplying the initial
abstraction by the appropriate area. Predevelopment runoff can be calculated using the
composite curve number 63.07 for the area prior to development, determining an initial
abstraction for each soil group, and multiplying this again by the area as done for the
developed volumes. The result of this analysis is found in Table 6-31.
The functions are then compared across land uses by computing the difference between
the sum of the function's pre-development and post-development storage volumes. This
is plotted as a bar chart in Figure 6-6. The greatest impact by far is from streets and roofs,
with roughly equal values of storage volume reduction. Patios are insignificant in this
analysis. Lawns actually add a great deal of storage, somewhat offsetting the drastic
reductions from roofs and streets. Driveways and parking lots result in smaller
reductions in volume; however, because it is concentrated over smaller areas, the local
impact may be great.
99
-------
Table 6-30. SCS Hydrologic Classifications, and Calculation of Unit Storage Values (SCS, 1986)
No.
1
2
1
2
3
4
5
6
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Type
Permeable
Permeable
Impervious
Impervious
Permeable
Permeable
Permeable
Impervious
Impervious
Impervious
Impervious
Impervious
Impervious
Impervious
Impervious
Impervious
Impervious
Permeable
Impervious
Impervious
Impervious
Impervious
Permeable
Permeable
Permeable
Permeable
Cover Description
Cover type and hydrologic
condition
Aspen-mountain brush mixture:
Fair:30%-70% ground cover
Aspen-mountain brush mixture: Good:
>70% ground cover
Driveway
Driveway-porous pavement
Lawns, pasture, grassland: Fair
condition (grass cover 50%-75%)
Lawns, pasture, grassland: Good
condition (grass cover >75%)
Lawns, pasture, grassland: Poor
condition (grass cover < 50%)
Parking
Porous parking 1
Porous parking 2
Porous parking 3
Patio
Porous patio
Roof
Roof with detention
Sidewalks
Sidewalks with porous materials
Storage-offsite in infiltration/detention
basins
Street with curb and gutter
Street with curb and gutter and porous
pavement
Street with swales
Street with swales and porous
pavement
Swales 1
Swales 2
Woods:Fair: Woods are grazed but
not burned, and some forest litter
Woods:Good: Woods without grazing,
and adequate litter and brush
ID
Aspen F
Aspen G
Driveway 1
Driveway 2
Grass F
Grass G
Grass P
Parking 1
Parking 2
Parking 3
Parking 4
Patio 1
Patio 2
Roof 1
Roof 2
Sidewalk 1
Sidewalk 2
Storage
Street 1
Street 2
Street 3
Street 4
Swales 1
Swales 2
Woods F
Woods G
Curve Number
A
28
25
98
70
49
39
68
98
61
46
36
95
76
95
85
98
70
15
98
70
76
61
46
29
36
25
B
48
30
98
80
69
61
79
98
75
65
55
95
85
95
85
98
80
20
98
80
85
75
65
50
60
55
C
57
41
98
85
79
74
86
98
83
77
67
95
89
95
85
98
85
35
98
85
89
83
77
62
73
70
D
63
48
98
87
84
80
89
98
87
82
72
95
91
95
85
98
87
40
98
87
91
87
82
67
79
77
Initial Abstraction
(in.)
A
5.14
6.00
0.04
0.86
2.08
3.13
0.94
0.04
1.28
2.35
3.56
0.11
0.63
0.11
0.35
0.04
0.86
11.33
0.04
0.86
0.63
1.28
2.35
4.90
3.56
6.00
B
2.17
4.67
0.04
0.50
0.90
1.28
0.53
0.04
0.67
1.08
1.64
0.11
0.35
0.11
0.35
0.04
0.50
8.00
0.04
0.50
0.35
0.67
1.08
2.00
1.33
1.64
C
1.51
2.88
0.04
0.35
0.53
0.70
0.33
0.04
0.41
0.60
0.99
0.11
0.25
0.11
0.35
0.04
0.35
3.71
0.04
0.35
0.25
0.41
0.60
1.23
0.74
0.86
D
1.17
2.17
0.04
0.30
0.38
0.50
0.25
0.04
0.30
0.44
0.78
0.11
0.20
0.11
0.35
0.04
0.30
3.00
0.04
0.30
0.20
0.30
0.44
0.99
0.53
0.60
Unit cost
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.81
1.03
0.70
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
Unit Cost
(1/99 $/gal)
A
0.62
0.80
9.21
0.47
0.63
0.53
1.19
9.21
0.31
0.18
0.13
2.89
0.49
0.00
6.82
7.44
0.36
0.71
9.77
0.49
0.68
0.35
2.05
1.97
0.36
0.37
B
1.48
1.03
9.21
0.80
1.45
1.29
2.12
9.21
0.60
0.39
0.27
2.89
0.88
0.00
6.82
7.44
0.62
1.00
9.77
0.84
1.22
0.67
4.47
4.81
0.96
1.37
C
2.13
1.67
9.21
1.13
2.45
2.35
3.45
9.21
0.98
0.71
0.46
2.89
1.25
0.00
6.82
7.44
0.88
2.16
9.77
1.19
1.74
1.09
8.06
7.85
1.73
2.62
D
2.73
2.22
9.21
1.34
3.42
3.30
4.55
9.21
1.34
0.97
0.58
2.89
1.57
0.00
6.82
7.44
1.04
2.67
9.77
1.41
2.17
1.49
10.96
9.77
2.41
3.76
100
-------
Table 6-31. Calculation of Developed and Predevelopment Stormwater Volumes
Land Use
Apartments
Commercial
MD Residential
LD Residential
School
Streets
50
60
70
Function
Roof
Parking
Driveway
Lawns
Roof
Parking
Driveway
Lawns
Roof
Parking
Driveway
Lawns
Patio
Roof
Parking
Driveway
Lawns
Patio
Roof
Parking
Driveway
Lawns
ROW
Street with
curb and gutter
Parking
Sidewalks
curb
Lawns
ROW
Street with
curb and gutter
Parking
Sidewalks
Curb
Lawns
ROW
Street with
curb and gutter
Parking
Sidewalks
Curb
Lawns
Total
B
(ft')
46,927
75,083
0
40,670
95,132
44,810
0
6,839
140,800
0
52,800
353,666
17,600
102,000
0
40,800
491,233
20,400
69,080
51,806
0
28,521
659,728
105,556
105,556
105,556
52,778
52,778
87,540
1 1 ,672
23,344
11,672
5,836
5,836
13,195
1,508
3,016
1,508
754
754
D, Total
(ft')
0
0
0
0
57,707
259,868
0
16,714
267,200
0
100,200
538,755
33,400
0
0
0
0
0
0
0
0
0
774,288
123,886
123,886
123,886
61 ,943
61 ,943
0
0
0
0
0
0
189,531
21,661
43,321
21,661
10,830
10,830
Area, Total
(ft')
46,927
75,083
0
40,670
152,839
304,678
0
23,553
408,000
0
153,000
892,420
51 ,000
102,000
0
40,800
491 ,233
20,400
69,080
51 ,806
0
28,521
1,434,016
229,443
229,443
229,443
114,721
114,721
87,540
1 1 ,672
23,344
1 1 ,672
5,836
5,836
202,726
23,169
46,337
23,169
1 1 ,584
1 1 ,584
1,724,282
Developed
Volume, B
(ff)
412
255
0
4,334
834
152
0
729
1,235
0
180
37,686
154
895
0
139
52,344
179
606
176
0
3,039
359
359
359
180
3,952
40
79
40
20
437
5
10
5
3
56
Volume, D
(ff)
0
0
0
0
506
884
0
696
2,344
0
341
22,448
293
0
0
0
0
0
0
0
0
0
421
421
421
211
1,966
0
0
0
0
0
74
147
74
37
344
Total Dev.
Volume
(ff)
412
255
0
4,334
1,341
1,036
0
1,425
3,579
0
520
60,134
447
895
0
139
52,344
179
606
176
0
3,039
780
780
780
390
5,918
40
79
40
20
437
79
158
79
39
400
140,882
Undev.
B
(ff)
4,580
7,327
0
3,969
9,284
4,373
0
667
13,741
0
5,153
34,514
9,954
0
3,982
47,939
6,742
5,056
0
2,783
10,301
10,301
10,301
5,151
5,151
1,139
2,278
1,139
570
570
147
294
147
74
74
D
(ftJ)
0
0
0
0
49
86
0
68
229
0
33
2,191
0
0
0
0
0
0
0
0
41
41
41
21
192
0
0
0
0
0
7
14
7
4
34
Total Undev.
Volume
(ftJ)
4,580
7,327
0
3,969
9,333
4,459
0
735
13,969
0
5,186
36,705
0
9,954
0
3,982
47,939
0
6,742
5,056
0
2,783
10,342
10,342
10,342
5,171
5,343
1,139
2,278
1,139
570
570
154
309
154
77
107
21,0758
101
-------
140000.00
120000.00
100000.00
80000.00
60000.00
> 40000.00
20000.00
0.00
-20000.00
-40000.00
D Volume, post development,
(CF)
• Volume, predevelopment
(CF)
D Difference
Function
Figure 6-6. Allocation of available storage for initial abstraction and land use.
6.5.3 Estimated unit costs of various functional land use options
BMP control costs are estimated in $/ft2. These costs are assumed incremental costs over
and above the costs of conventional systems. These unit cost estimates are preliminary in
that the proper definition of cost depends upon alternatives that provide "equivalent"
levels of service. For example, consider the following three options for a 6,000 ft2 lawn:
• Conventional lawn with a sprinkling system
• 3,000 ft2 of conventional lawn and 3,000 ft2 of forest
• 2,000 ft2 of conventional lawn; 2,000 ft2 of forest; and 2,000 ft2 of
swales
While it is possible to estimate the cost of each of these three options, the customer must
view these options as providing the same level of service for them to be considered
102
-------
equivalent. If the customer strongly prefers the conventional lawn, then it is inaccurate to
select other options based on lower cost if they are not perceived to be equivalent.
Further work is needed to provide a more accurate assessment of equivalent landscapes.
For this example customers are assumed to simply select the least costly combination of
BMP controls.
Using the procedures developed in section 6.4, unit costs for controls determined by
Table 6-30 were used for eight different land-use model: low and medium density
residential; commercial; school; apartment; and 50, 60, and 70 ft rights-of-way. The unit
costs, which include opportunity costs, are listed in Table 6-32. An alternative analysis
was performed which excluded the effect of opportunity costs. These unit costs are
presented in Table 6-33.
Table 6-32. Calculation of Unit Costs for Controls, Including Land Opportunity Costs
ID
Aspen F
Aspen G
Driveway 1
Driveway 2
Grass F
Grass G
Grass P
Parking 1
Parking 2
Parking 3
Parking 4
Patio 1
Patio 2
Roofl
Roof 2
Sidewalk 1
Sidewalk 2
Storage
Street 1
Street 2
Street 3
Street 4
Swales 1
Swales 2
Woods F
Woods G
LD Res.
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.60
0.69
0.49
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
MD Res.
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.60
0.69
0.49
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.29
3.00
6.00
0.80
1.40
Commer.
(1/99 $/ft2)
2.00
3.00
0.23
0.25
2.12
2.18
2.01
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
School
(1/99 $/ft2)
2.00
3.00
0.23
0.25
2.49
2.56
2.38
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
Apartm't
(1/99 $/ft2)
2.00
3.00
0.23
0.25
1.22
1.29
1.11
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
RW50
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.60
0.69
0.49
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
RW60
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.60
0.69
0.49
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.29
3.00
6.00
0.80
1.40
RW70
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.60
0.69
0.49
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.24
0.26
0.27
0.28
3.00
6.00
0.80
1.40
103
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Table 6-33. Calculation of Unit Costs for Controls, Excluding Land Opportunity Costs
ID
Aspen F
Aspen G
Driveway 1
Driveway 2
Grass F
Grass G
Grass P
Parking 1
Parking 2
Parking 3
Parking 4
Patio 1
Patio 2
Roofl
Roof 2
Sidewalk 1
Sidewalk 2
Storage
Street 1
Street 2
Street 3
Street 4
Swales 1
Swales 2
Woods F
Woods G
LD Res.
(1/99 $/ft2)
2.00
3.00
0.06
0.08
0.25
0.34
0.14
0.06
0.08
0.09
0.11
0.02
0.02
0.00
1.50
0.02
0.02
5.00
0.07
0.09
0.09
0.10
3.00
6.00
0.80
1.40
MD Res.
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.25
0.37
0.14
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
Commer.
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.25
0.32
0.14
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
School
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.25
0.32
0.14
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
Apartm't
(1/99 $/ft2)
2.00
3.00
0.23
0.25
0.25
0.32
0.14
0.23
0.25
0.26
0.28
0.19
0.19
0.00
1.50
0.19
0.19
5.00
0.25
0.26
0.27
0.28
3.00
6.00
0.80
1.40
RW50
(1/99 $/ft2)
2.00
3.00
0.06
0.08
0.25
0.34
0.14
0.06
0.08
0.09
0.11
0.02
0.02
0.00
1.50
0.02
0.02
5.00
0.07
0.09
0.09
0.10
3.00
6.00
0.80
1.40
RW60
(1/99 $/ft2)
2.00
3.00
0.06
0.08
0.25
0.34
0.14
0.06
0.08
0.09
0.11
0.02
0.02
0.00
1.50
0.02
0.02
5.00
0.07
0.08
0.10
0.11
3.00
6.00
0.80
1.40
RW70
(1/99 $/ft2)
2.00
3.00
0.06
0.08
0.25
0.34
0.14
0.06
0.08
0.09
0.11
0.02
0.02
0.00
1.50
0.02
0.02
5.00
0.07
0.08
0.10
0.11
3.00
6.00
0.80
1.40
The reasonableness of these estimates can be judged by comparing them to the unit cost
of storage systems reported in the literature; first converting them to unit costs in terms of
$/gal for a given soil type, as done in Table 6-30. Storage costs described in chapter 5 of
this report range from about $0.03 to $15.127 gal (Table 6-34, is calculated using the
equations from Chapter 5, obtaining a cost, then dividing the cost by the volume in gal.).
Using $5.007 ft2 in Table 6-32 and 6-33, the results range from $0.71-$2.67/gal, well
within an acceptable range. Costs for swales were estimated based upon the range of
equation 4.13. Costs for aspen and woods are estimated based upon typical landscaping
costs, and comparing the computed $/gal unit costs with others for reasonableness. The
incremental cost for roofed area is based on the added cost of directing this runoff toward
an appropriate permeable area, and again was checked for reasonableness.
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Table 6-34. Range of Costs for Storage
Volume
(1,000 gal.)
1
10
100
1,000
10,000
EPA CSO storage
(1/99 $/gal)
15.12
10.13
6.79
4.55
3.05
Detention Basin
(1/99 $/gal)
0.47
0.23
0.11
0.06
0.03
Retention Basin
(1/99$/gal)
0.34
0.19
0.11
0.06
0.03
Infiltration Basin
(1/99 $/gal)
0.45
0.22
0.11
0.05
0.03
6.6 Results of BMP Optimization for Happy Acres
The detailed results of the optimization can be found in the companion report Heaney et
al. (1998c). The optimal total system cost, including land opportunity costs for Happy
Acres is $4.2 million (calibrated to the Denver/Boulder, CO area). The total system cost,
neglecting opportunity costs is $3.9 million. This represents approximately 15%-19 % of
the total $26.6 million investment overall (not including buildings).
Direct comparison to the values obtained here for the micro-storm analysis with those for
the major storm analysis cannot be done, as it is normally expected that the total costs for
micro-storm drainage control would be less than that for minor and major storms
($915,000 and $1.21 million, respectively). A key issue here is that the allocation of a
fixed percentage of costs to stormwater control needs to be evaluated further. This
percentage is essentially unknown at present.
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Chapter 7
Summary and Conclusions
Cost estimation procedures for urban stormwater systems were primarily developed prior
to 1980. Simple equations using one or two explanatory variables were, and still are
being used to estimate costs. Modern hardware and software enable a move to data driven
approaches and a focus on developing good databases for developing cost estimates.
Equations are a very restrictive way to present information and should only be used for
simple summaries. The availability of computerized cost databases from companies such
as R.S. Means provide a very good source of information about current unit costs. In
order to significantly advance the state of the art in cost effectiveness modeling, unit cost
data need to be directly linked to process-simulators as demonstrated in chapter 5. This
spreadsheet model can be adapted to a wide variety of physical settings and used to do a
comprehensive evaluation of the nature of system costs and their relative importance.
Five scenarios were evaluated to illustrate the use of this model. However, the variety of
what-if analyses is virtually limitless. There are well over 1,000 variables for this 106
acre storm drainage design, thus, the number of combinations is very large. While this
process-oriented approach is a major improvement over existing practices, it is still
severely limited in that it only does what-if analysis and cannot systematically do what's-
best optimization analysis. Currently, such an approach using intelligent search
techniques is in development and this method has successfully been applied to
optimization of water distribution systems (Lippai et al., 1999).
The application of GIS technology allows for a more thorough, parcel based approach to
the analysis of water quality impacts during micro-storms from land use changes and
development. Although the hydrologic model used here is limited (as with the rational
model used in the sewer design model for the major and minor storms), it is apparent that
many impacts could then be traced directly to their origin
This initial exploration into storm drainage design cost estimation suggests the following
gaps in knowledge to be addressed by additional research:
1. A process-oriented approach to cost-effectiveness evaluations is essential. Curve
fitting approaches to cost estimation based on as-built systems are too aggregate and
the databases too inconsistent to provide the reliable estimates needed to enhance our
understanding of the underlying cause-effect relationships.
2. The unit cost data provided by companies such as R.S. Means are a valuable source
of the necessary cost data and should be an integral part of the overall cost-
effectiveness evaluations.
3. The spreadsheet model presented in this report should be expanded to implement
intelligent search techniques to determine optimal design.
4. An accurate representation of the system hydraulics is essential to meaningful system
optimization. While the spreadsheet can be used for simple hydraulic analysis, it is
essential to link the spreadsheet with hydraulic analyzers such as SWMM so that the
106
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hydraulics can be done more accurately. We have already done such linkages in
looking at water distribution systems by linking the optimizer with EPANET (Lippai
etal., 1999).
5. The Rational Method to estimate peak inflows to pipes is archaic and should be
replaced by data centered approaches. For example, the cost of this sewer system
design depends heavily on the assumed travel time to the sewer inlet. Yet, this value
is difficult to estimate accurately.
6. Conventional storm sewer design should also check how the system performs during
small storms when lower velocities might prevail and cause sediment accumulations
in the sewers.
7. The analysis needs to be expanded to include the effect of storage on the system
design. However, before evaluating storage, it is imperative to use more realistic
storm hydrographs and not continue to compound our ignorance by using simple
extensions of the Rational Method.
8. The method needs to be expanded to include onsite controls such as infiltration. Such
an analysis is not simple since storage routing is required at the parcel level in
addition to evaluating larger storage systems.
9. A database of flow and quality monitoring for small (100 acres or less) catchments is
needed to evaluate actual system response for small drainage areas. These
catchments can be used for overall cost-effectiveness evaluations.
10. The benefits of urban stormwater systems need to be quantified. Flood damages are
relatively easy to estimate. However, stormwater quality control benefits are more
elusive.
11. The overall system evaluation should include structural and non-structural BMPs as
well as conventional storm drainage systems.
12. The incidence of benefits and costs of alternative drainage systems needs to be
quantified. Residents who control their problems on site should receive fair credit for
reducing system cost.
13. Downstream receiving water impacts should be included in the evaluations.
14. A combined sewer design should be evaluated and its cost apportioned among
wastewater and stormwater. The effect of providing additional storage in the
combined sewer should be evaluated.
15. The cost optimization should be refined to take into account both the broader land use
optimization, and to allocate the costs down to each land use, and to each parcel.
Combined with GIS, this analysis should be done for several different scenarios
(micro-storms, minor storms, and major storms).
16. The impact of streets and parking as integral parts of the urban stormwater system
needs to be evaluated. Streets and parking comprise the majority of the directly
connected impervious areas for stormwater systems. Hence, they are a major source
of the problem. However, they also comprise an essential element of the stormwater
management system, especially during periods of very high runoff when the sewers
are overloaded. A significant part of the cost of streets and parking is for drainage.
This cost needs to be included in the overall cost of stormwater management systems.
A preliminary attempt has been made here to quantify these impacts in micro-storms.
More work at identifying these impacts, and assessing an allocated, true cost of
alleviating these impacts to these sources is essential for containment.
107
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References
American Society of Civil Engineers. 1996. Hydrology Handbook, 2nd Edition. Reston,
VA: American Society of Civil Engineers.
Bauer, W.F. 1962. Economics of Urban Drainage Design. Journal of the Hydraulics
Division, American Society of Civil Engineers 88:93-114.
Bedient, P.B. and Huber, W.C. 1989. Hydrology and Floodplain Analysis. , Reading,
MA: Addison-Wesley Publishers.
Boyce, B.N. (Ed.) 1981. Real Estate Appraisal Terminology. New York, NY: Harper
Business.
CH2M Hill. 1993. Cost of Providing Government Services to Alternative Residential
Patterns. Report for the Chesapeake Bay Program's Subcommittee on Population
Growth and Development. United States Environmental Protection Agency
Contract No. 68WO-0043.
Clark, R. 1983. Economics of Regionalization: An Overview. Reston, VA: American
Society of Civil Engineers.
Curtis, F.A., and Schlosser, M.W. 1986. Computer-Assisted Procedures for Subdivision
Cost Analysis. Journal of Urban Planning and Development 112:15-25.
Dajani, J.P., and Gemmell, R.S. 1971. Economics of Wastewater Collection Networks.
Research Report No. 43. Urbana, IL: University of Illinois Water Resources
Center.
Dajani, J.S., Gemmell, R.S., and Morlok, E.K. 1972. Optimal Design of Urban
Wastewater Collection Networks. Journal of the Sanitary Engineering Division,
American Society of Civil Engineers 98:853.
Dames & Moore. 1978. Construction Costs for Municipal Wastewater Conveyance
Systems: 1973-1977, United States Environmental Protection Agency Technical
Report, Office of Water, United States Environmental Protection Agency 430/9-
77-014.
Debo, T.N., and Reese, A.J. 1995. Municipal Storm Water Management. Boca Raton,
LA: Lewis Publishers, 756 pp.
Dewberry, and Davis. 1996. Land Development Handbook. New York, NY: McGraw-
Hill.
Dion, T.R. 1993. Land Development for Civil Engineers. New York, NY: John Wiley &
Sons, Inc.
Dodge Guide. 1998. New York, NY: McGraw Hill.
108
-------
Downing, P.B. 1969. The Economics of Urban Sewage Disposal. New York, NY:
Prager Publishers.
Earle III, G.A., and Farrell Jr., R.P. 1997. A Mathematical Model for Estimating Sewer
Costs. Boston, MA: New England Water Environment Association Conference.
Federal Highway Administration. 1991. HYDRAIN Documentation. Washington, DC:
Federal Highway Administration.
Ferguson, B. K. 1998. Introduction to Stormwater: Concept, Purpose, Design. New
York, NY: John Wiley & Sons, Inc.
Field, R., and O'Connor, T.P. 1997. Control and Treatment of Combined Sewer
Overflows. In: Moffa, P. (Ed.) Control and Treatment of Combined Sewer
Overflows. New York, NY: Van Nostrand Reinhold.
Frank, J.E. 1989. The Costs of Alternative Development Patterns: A Review of the
Literature. Washington, DC: Urban Land Institute.
Grigg, N.S., and O'Hearn, J. 1976. Development of Storm Drainage Cost Functions.
Journal of the Hydraulics Division, American Society of Civil Engineers, 8:102.
Gummerman, R.C., Gulp, R.L., and Hansen, S.P. 1979. Estimating Water Treatment
Costs. 600/2-79-162a. Cincinnati, OH: United States Environmental Protection
Agency.
Han, J., Rao, A.M., and Houck, M.H. 1980. Least Cost Design of Urban Drainage
Systems. Report no. 138. West Lafayette, IN: Purdue University Water
Resources Research Center.
Hazen and Sawyer and HydroQual, Inc. 1992. Cost Estimates for Select Combined
Sewer Overflow Control Technologies. Washington, DC: United States
Environmental Protection Agency.
Heaney, J.P. 1994. Conceptually Sound and Operational Methods for Environmental
Valuation. Water Resources Update, 96:7-31.
Heaney, J.P. 1997. Cost Allocation, in Design and Operation of Civil and Environmental
Engineering Systems. ReVelle, C., and McGarity, A.E. (Ed.), New York, NY:
John Wiley & Sons, Inc.
Heaney, J.P., Wright, L., and Sample, D. 1998a. Research Needs in Wet Weather Flows,
Draft, Alexandria, VA: Water Environment Federation.
Heaney, J.P., Peterka, J., and Wright, L. 1998b. Research Needs for Sustainable Natural
Disaster Management. Journal of Infrastructure Systems (submitted).
Heaney, J.P., Wright, L., and Sample, D. 1998c. Innovative Methods for Optimization of
Urban Stormwater Systems. Washington, D.C.: Report to United States
Environmental Protection Agency (draft).
109
-------
Heaney, J.P., Pitt, R., and Field, R. (Eds.) 1999a. Innovative Urban Wet-Weather Flow
Management Systems (EPA-600-R-99-029). Cincinnati, OH: United States
Environmental Protection Agency.
http:/www.epa.gov/ednnrmrl/publish/book/epa-600-r-99-029/index.html
Heaney, J.P., Wright, L., and Sample, D. 1999b. Research Needs in Urban Wet Weather
Flows. Water Environment Research (in press).
Knapp, G. 1998. The Determinants of Residential Property Values: Implications for
Metropolitan Planning. Journal of Planning Literature 12:3.
Linaweaver, F.G. and Clark, C.S., 1964. Costs of Water Transmission. Journal of the
American Water Works Association 60, No. 12.
Lippai, I, and Heaney, J.P., 1998. Efficient and Equitable Methods for Assessing Impact
Fees for Urban Water Systems. Journal of Water Resources Planning and
Management (submitted).
Lippai, I, Heaney, J., and Laguna, M. 1999. Robust Pipe Network Design Optimization
Using Genetic Algorithms. Journal of Computing in Civil Engineering (in press).
Litman, T. 1998. Transportation Cost Analysis: Techniques, Estimates and Implications.
Victoria, British Columbia, Canada: Victoria Transport Policy Institute.
Merritt, L.B., and Bogan, R.H. 1973. Computer-Based Optimal Design of Sewer
Systems. Journal of the Environmental Engineering Division, American Society
of Civil Engineers 99:35.
Miles, S.W., and Heaney, J.P. 1988. Better than Optimal Method for Designing Drainage
Systems. Journal of Water Resources Planning and Management, American
Society of Civil Engineers 114:477-499.
Moss, T., and Jankiewicz, E.J. 1982. What Type Sewer Pipe is Best? Life Cycle Cost
Analysis Yields Answer. Civil Engineering (October):75-76.
Nelson, A.C. 1995. System Development Charges for Water, Wastewater, and
Stormwater Facilities. Boca Raton, LA: CRC Press.
Nix, S.J., and Heaney, J.P. 1988. Optimization of Storage-Release Strategies. Water
Resources Research, 24:1831-1838.
Peters, M.S. and Timmerhaus, K.D. 1980. Plant Design and Economics for Chemical
Engineers. New York, NY: McGraw Hill.
Pitt, R.E. 1987. Small Storm Urban Flow and Particulate Washoff Contributions to
Outfall Discharges. Madison: University of Wisconsin, Department of Civil and
Environmental Engineering. Ph.D. Dissertation.
Pitt, R.E., Nix, S.J., Durrans, S.R., and Burian, S.J. 1997. Guidance Manual for
Integrated Wet Weather Flow (WWF) Collection and Treatment Systems for
110
-------
Newly Urbanizing Areas (New WWF Systems). First Year Project Report to
United States Environmental Protection Agency. Washington, D.C.: United States
Environmental Protection Agency.
Pitt, R.E., and Voorhees, J. 1995. Source Loading and Management Model (SLAMM).
Birmingham, AL: University of Alabama at Birmingham.
Prince George's County. 1999. Low-Impact Development Design Strategies: An
Integrated Design Approach, Department of Environmental Resources, Programs
and Planning Division, Prince George's County, MD;
http://www.epa.gov/owow/nps/urban.html
R.S. Means. 1996a. Heavy Construction Cost Data, 10th Annual Edition. [Copyright R.S.
Means Co., Inc., Kingston, MA, (781) 585-7800, all rights reserved].
R.S. Means. 1996b. Site Work & Landscaping Unit Cost Data. [Copyright R.S. Means
Co., Inc., Kingston, MA, (781) 585-7800, all rights reserved].
Raftelis, G.A. 1989. Water and Wastewater Finance and Pricing. Chelsea, MI: Lewis
Publishers.
Rawls, W.J., and Knapp, J.W. 1972. Methods for Predicting Urban Drainage Costs.
Journal of the Hydraulics Division, American Society of Civil Engineers
98:1575-1585.
Real Estate Research Corporation. 1974. The Cost of Sprawl. U.S. Washington, D.C.:
Government Printing Office.
SCS. 1986. Urban Hydrology for Small Watersheds. Technical Releases 55,
Washington, D.C.: U.S. Department of Agriculture, Soil Conservation Service,
Engineering Division.
Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and
Designing Urban BMPs. Washington, D.C.: Department of Environmental
Programs, Metropolitan Washington Council of Governments.
Schueler, T.R. 1994. Developments in Sand Filter Technology to Improve Stormwater
Runoff Quality. Water Protection Techniques, 1 (2).
Schueler, T. R., Kumble, P., and Heraty, M. 1992. A Current Assessment of Urban Best
Management Practices: Techniques for Reducing Nonpoint Source Pollution in
the Coastal Zone. Washington, DC: Anacostia Research team, Metropolitan
Washington Council of Governments.
Scott, K., Wang, D.C., Eralp, A.E., and Bingham, D.R. 1994. Cost Estimation
Methodologies for the 1992 CSO Needs Survey. In WEF A Global Perspective
for Reducing CSOs: Balancing Technologies, Costs, and Water Quality, a
Specialty Conference of the Water Environment Federation. Alexandria: WEF.
Ill
-------
Stahre, P., and Urbonas, B. 1993. Stormwater Detention. Englewood Cliffs, NJ:
Prentice-Hall.
Tchobanoglous, G. 1981. Wastewater Engineering: Collection and Pumping of
Wastewater. New York, NY: McGraw-Hill.
Tyteca, D. 1976. Cost Functions for Wastewater Conveyance Systems, Journal of Water
Polllution Control Federation 48:2120-2130.
United States Army Corps of Engineers. 1979. MAPS Users Guide and Documentation.
Draft Engineering Manual. EM-1110-2-XXX, Washington, D.C.
United States Army Corps of Engineers. 1981. Unpublished data on 87 reservoirs built
between 1952 and 1981.
United States SCS. 1973. Precipitation-Frequency Atlas of the Western United States,
Washington, D.C.
United States Environmental Protection Agency. 1976. Areawide Assessment
Procedures Manual, Appendix H, EPA-600/9-76-014. Cincinnati, OH: United
States Environmental Protection Agency.
United States Environmental Protection Agency. 1979. 1978 Needs Survey Cost
Methodology for Control of Combined Sewer Overflow and Stormwater
Discharges, EPA-430/9-79-003. Washington, D.C.: United States Environmental
Protection Agency.
United States Environmental Protection Agency. 1992. Evaluation of Wet Weather
Design Standards for Controlling Pollution from Combined Sewer Overflow,
EPA 230-R-92-006. Washington, D.C.: Office of Policy, Planning, and
Evaluation, United States Environmental Protection Agency.
United States Environmental Protection Agency. 1993a. 1992 Needs Survey: Report to
Congress. United States Environmental Protection Agency Office of Water (WH-
547). Washington, D.C.: United States Environmental Protection Agency.
United States Environmental Protection Agency. 1993b. Manual: Combined Sewer
Overflow Control, EPA-625/R-93-0007. Washington, D.C.: United States
Environmental Protection Agency.
Urban Land Institute 1989. Project Infrastructure Development Handbook. Washington,
D.C.: Urban Land Institute,
Urbonas, B. 1999. An Assessment of Stormwater BMP Technology. Chapter 7 in
Innovative Urban Wet-Weather Flow Management Systems. Heaney, J.P., Pitt,
R., and Field, R. Eds. 1999. Cincinnati, OH: United States Environmental
Protection Agency, National Risk Management Research Laboratory.
http://www.epa.gov/ednnrmrl/publish/book/epa-600-r-99-029/achap07.pdf
112
-------
Wiegand, C. Schueler, T., Chittendren, W., and Jellick, W. 1986. Cost of Urban Runoff
Controls in Urban Runoff Quality: Impact and Quality Enhancement
Technology. In: Urbonas B., and Roesner, (Eds). Reston, VA: American Society
of Civil Engineers.
Young, O.K., Stein, S., Cole, P., Kammer, T., Graziano, F., Bank, F. 1996. Evaluation
and Management of Highway Runoff Water Quality, Technical Report.
Washington, D.C.: the Federal Highway Administration.
113
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