aFPA United States
Environmental Protection Agency
EPA/600/R-19/076
August 2019
NATIONAL STORMWATER CALCULATOR WEB
APP USER'S GUIDE - VERSION 3.2.0
By
Lewis A. Rossman (retired)
Water Systems Division
National Risk Management Research Laboratory
Cincinnati, OH 45268
Jason T. Bernagros
Water System Division
National Risk Management Research Laboratory
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OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
DISCLAIMER
The information in this document has been funded wholly by the U.S. Environmental Protection Agency
(EPA). It has been subjected to the Agency's peer and administrative review, and has been approved for
publication as an EPA document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
Although a reasonable effort has been made to assure that the results obtained are correct, the
computer programs described in this manual are experimental. Therefore the author and the U.S.
Environmental Protection Agency are not responsible and assume no liability whatsoever for any results
or any use made of the results obtained from these programs, nor for any damages or litigation that
result from the use of these programs for any purpose.
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ACKNOWLEDGEMENTS
Paul Duda, Paul Hummel, Jack Kittle, and John Imhoff of Aqua Terra Consultants developed the data
acquisition portions of the National Stormwater Calculator desktop application under Work Assignments
4-38 and 5-38 of EPA Contract #EP-C-06-029. They, along with Alex Foraste (EPA/OW), provided many
useful ideas and feedback throughout the development of the calculator. Scott Struck, Dan Pankani, and
Kristen Ekeren of Geosyntec, and Marion Deerhake of RTI International developed the cost estimation
components of the Stormwater Calculator under Task Orders 0019 (PR-ORD-14-00308) and 026 (PR-
ORD-15-00668). Jason Bernagros served as the Task Order Contracting Officer's Representative (TOCOR)
for the cost estimation task order and the TOCOR for the mobile web application (a.k.a., app). Jason
Bernagros served as the Work Assignment Manager for the maintenance of the mobile web app.
Michael Tryby and Michelle Simon, in EPA's Office of Research and Development (ORD), provided
project guidance and valuable feedback throughout the development of the cost estimation procedures
and the mobile web app. Dawn Bontempo, Fahim Chowdhury, Marie Calvo, Martina Donati, Jason
Lowengrub, Anthony Passamonti, Seth Pennington, Catherine Sweeney, and Natasha Virdy of Attain,
LLC., Uyen Tran of Tetra Tech, and developed the mobile web app version of the Stormwater Calculator
under Task Order HHSN316201200117W (EP-G15H-01113). Colleen Barr, was supported in part by an
appointment to the Postdoctoral Research Program at the U.S. Environmental Protection Agency, Office
of Research and Development, National Risk Management Research Laboratory, administered by the
Oak Ridge Institute for Science and Education through Interagency Agreement No. (DW-8992433001)
between the U.S. Department of Energy and the U.S. Environmental Protection Agency, and provided
programming support for the maintenance of the mobile web application. Brad Cooper, Mike Liadov,
and Naveen Tharalla of Eastern Research Group (ERG) (under Work Assignment 0-52 (EP-C-17-041) also
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ACRONYMS AND ABBREVIATIONS
ASCE
= American Society of Civil Engineers
BLS
= United States Bureau of Labor Statistics
CMIP3
= Coupled Model Intercomparison Project Phase 3
CREAT
= Climate Resilience Evaluation and Awareness Tool
EPA
= United States Environmental Protection Agency
GCM
= General Circulation Model
GEV
= Generalized Extreme Value
Gl
= Green Infrastructure
HSG
= Hydrologic Soil Group
IMD
= initial moisture deficit
IPCC
= Intergovernmental Panel on Climate Change
Ksat
= saturated hydraulic conductivity
LID
= low impact development
NCDC
= National Climatic Data Center
NRCS
= Natural Resources Conservation Service
NWS
= National Weather Service
OW
= Office of Water
SCS
= Soil Conservation Service
SSURGO
= Soil Survey Geographic Database
SWAT
= Soil and Water Assessment Tool
SWC
= Stormwater Calculator
SWMM
= Storm Water Management Model
UDFCD
= Urban Drainage and Flood Control District
US
= United States
USDA
= United States Department of Agriculture
WCRP
= World Climate Research Programme
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TABLE OF CONTENTS
DISCLAIMER ii
ACKNOWLEDGEMENTS iii
ACRONYMS AND ABBREVIATIONS iv
TABLE OF CONTENTS v
LIST OF FIGURES vi
List of Tables viii
1. Introduction 9
2. How to Run the Calculator 11
Location 12
Soil Type 16
Soil Drainage 18
Topography 19
Precipitation/Evaporation 20
Climate Change 23
Land Cover 25
LID Controls (including cost estimation options) 27
Results 32
3. Interpreting the Calculator's Results 35
Summary Results 35
Rainfall / Runoff Events 37
Rainfall / Runoff Frequency 38
Rainfall Retention Frequency 39
Runoff by Rainfall Percentile 41
Extreme Event Rainfall/Runoff 42
Cost Summary 44
Printing Output Results 48
4. Applying LID Controls 49
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5. Example Application 53
Pre-Development Conditions 53
Post-Development Conditions 57
Post-Development with LID Practices 59
Cost Summary 65
Climate Change Impacts 69
6. Computational Methods 75
SWMM's Runoff Model 75
SWMM's LID Model 76
Site Model without LID Controls 78
Site Model with LID Controls 81
Precipitation Data 82
Evaporation Data 85
Climate Change Effects 86
Cost Estimation 89
Post-Processing 102
7. References 104
LIST OF FIGURES
Figure 1. The calculator's (a) opening page and (b) Location icon page 11
Figure 2. The calculator's Location icon page 13
Figure 3. Bird's eye map view with a bounding circle 14
Figure 4. Bird's eye map view with a bounding polygon 15
Figure 5. The calculator's Soil Type page 17
Figure 6. The calculator's Soil Drainage icon page 19
Figure 7. The calculator's Topography icon page 20
Figure 8. The calculator's Precipitation/Evaporation icon page 21
Figure 9. The calculator's Precipitation/Evaporation icon page (weather station) 22
Figure 10. The calculator's Climate Change icon page 24
Figure 11. The calculator's Land Cover page 26
Figure 12. The calculator's LID Controls icon page 28
Figure 13. The Calculator's Project Cost icon page showing the Re-Development pop-out window (shown
by clicking Re-Development) 29
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Figure 14. The Calculator's Project Cost icon page showing the Site Suitability - Poor pop-out window
(shown by clicking Poor) 30
Figure 15. Map of BLS Regional Cost Centers (pop-out window) used for computing regional multipliers.
A multiplier for each Regional Center applied within a 100-mile radius (inside blue circles). A National
value of 1 used otherwise (in green areas) 31
Figure 16. Drop down list of closest BLS Regional Cost Centers and national value 32
Figure 17. The calculator's Results page icon 33
Figure 18. An example of the calculator's Summary Results report 36
Figure 19. The calculator's Rainfall / Runoff Event report 38
Figure 20. The calculator's Rainfall / Runoff Frequency report 39
Figure 21. The calculator's Rainfall Retention Frequency report 40
Figure 22. The calculator's Runoff by Rainfall Percentile report 41
Figure 23. The calculator's Extreme Event Rainfall / Runoff report 43
Figure 24. Graphical output option of the calculator's estimate of average capital costs 46
Figure 25. Graphical output option of the calculator's estimate of average annual maintenance costs... 48
Figure 26. Example of an LID Design dialog for a street planter 51
Figure 27. Pre-development conditions land cover 54
Figure 28. Runoff from different size storms for pre-development conditions on the example site 56
Figure 29. Rainfall retention frequency under pre-development conditions for the example site 57
Figure 30. Rainfall retention frequency for pre-development (Baseline) and post-development (Current)
conditions 59
Figure 31. Low Impact Development controls applied to the example site 60
Figure 32. Design parameters for Rain Harvesting and Rain Garden controls 61
Figure 33. Design parameters for the Infiltration Basin and Permeable Pavement controls 62
Figure 34. Daily runoff frequency curves for pre-development (Baseline) and post-development with LID
controls (Current) conditions 64
Figure 35. Contribution to total runoff by different magnitude storms for pre-development (Baseline)
and post-development with LID controls (Current) conditions 64
Figure 36. Retention frequency plots under pre-development (Baseline) and post-development with LID
controls (Current) conditions 65
Figure 37. Graphical output option of the calculator's estimate of capital costs 67
Figure 38. Graphical output option of the calculator's estimate of maintenance costs 69
Figure 39. Climate change scenarios for the example site 70
Figure 40. Daily rainfall and runoff frequencies for the historical (Baseline) and Warm/Wet climate
scenarios 72
Figure 41. Target event retention for the historical (Baseline) and Warm/Wet climate scenarios 73
Figure 42. Extreme event rainfall and runoff for the Warm/Wet climate change scenario and the
historical record (Baseline) 74
Figure 43. Conceptual representation of a bio-retention cell 76
Figure 44. NWS rain gage locations included in the calculator 83
Figure 45. NRCS (SCS) 24-hour rainfall distributions (USDA, 1986) 84
Figure 46. Geographic boundaries for the different NRCS (SCS) rainfall distributions (USDA, 1986) 84
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Figure 47. Locations with computed evaporation rates (Alaska and Hawaii not shown) 85
Figure 48. CMIP3 2060 projected changes in temperature and precipitation for Omaha, NE (EPA, 2012).
87
Figure 49. Conceptual overview of cost estimate ranges derived from cost curves 98
Figure 50. Sample regression cost curve for Rain Gardens 99
List of Tables
Table 1. Definitions of Hydrologic Soil Groups (USDA, 2010) 18
Table 2. Tabular representation of the calculator's estimate of capital costs 45
Table 3. Tabular output option of the calculator's estimate of annual maintenance costs 47
Table 4. Descriptions of LID practices included in the calculator 50
Table 5. Editable LID parameters 52
Table 6. Void space values of LID media 52
Table 7. Summary results for pre-development conditions on the example site 55
Table 8. Land cover for the example site in developed state 57
Table 9. Comparison of runoff statistics for post-development (Current) and pre-development (Baseline)
conditions 58
Table 10. Runoff statistics for pre-development (Baseline) and post-development with LID controls
(Current) scenarios 63
Table 11. Tabular output option of the calculator's estimate of capital costs 66
Table 12. Tabular output option of the calculator's estimate of maintenance costs 68
Table 13. Summary results under a Warm/Wet (Current) climate change scenario compared to the
historical (Baseline) condition 71
Table 14. Depression storage depths (inches) for different land covers 79
Table 15. Roughness coefficients for different land covers 80
Table 16. Infiltration parameters for different soil types 81
Table 17. Cost Variables Selected for Cost Estimation Procedure 90
Table 18. LID Control Cost Curve Regression Equations 92
Table 19. Project Complexity Computation Based on User Input 93
Table 20. Regionalized Cost Model Coefficients for BLS Center 96
Table 21. BLS Regional Centers 100
Table 22. National BLS Variables and Model Coefficients 101
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t. Introduction
The National Stormwater Calculator (https://www.epa.gov/water-research/national-stormwater-
calculator) is a simple to use tool for computing small site hydrology for any location within the US. It
estimates the amount of stormwater runoff generated from a site under different development and
control scenarios over a long-term period of historical rainfall. The analysis takes into account local soil
conditions, slope, land cover, and meteorology. Different types of low impact development (LID)
practices (also known as green infrastructure) can be employed to help capture and retain rainfall on-
site. Future climate change scenarios taken from internationally recognized climate change projections
can also be considered. The calculator provides planning level estimates of capital and maintenance
costs which will allow planners and managers to evaluate and compare effectiveness and costs of LID
controls.
The calculator's primary focus is informing site developers and property owners on how well they can
meet a desired stormwater retention target. It can be used to answer such questions as the following:
• What is the largest daily rainfall amount that can be captured by a site in either its pre-
development, current, or post-development condition?
• To what degree will storms of different magnitudes be captured on site?
• What mix of LID controls can be deployed to meet a given stormwater retention target?
• How well will LID controls perform under future meteorological projections made by global
climate change models?
• What are the relative cost (capital and maintenance) differences for various mixes of LID
controls?
The calculator seamlessly accesses several national databases to provide local soil and meteorological
data for a site. The user supplies land cover information that reflects the state of development they wish
to analyze and selects a mix of LID controls to be applied. After this information is provided, the site's
hydrologic response to a long-term record of historical hourly precipitation, possibly modified by a
particular climate change scenario, is computed. This allows a full range of meteorological conditions to
be analyzed, rather than just a single design storm event. The resulting time series of rainfall and runoff
are aggregated into daily amounts that are then used to report various runoff and retention statistics. In
addition, the site's response to extreme rainfall events of different return periods is also analyzed.
The calculator uses the EPA Storm Water Management Model (SWMM) as its computational engine
(https://www.epa.gov/water-research/storm-water-management-model-swmm). SWMM is a well-
established, EPA developed model that has seen continuous use and periodic updates for 40 years. Its
hydrology component uses physically meaningful parameters making it especially well-suited for
application on a nation-wide scale. SWMM is set up and run in the background without requiring any
involvement of the user.
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The calculator is most appropriate for performing screening level analysis of small footprint sites up to
12 acres in size with uniform soil conditions. The hydrological processes simulated by the calculator
include evaporation of rainfall captured on vegetative surfaces or in surface depressions, infiltration
losses into the soil, and overland surface flow. No attempt is made to further account for the fate of
infiltrated water that might eventually transpire through vegetation or re-emerge as surface water in
drainage channels or streams.
The remaining sections of this guide discuss how to install the calculator, how to run it, and how to
interpret its output. An example application is presented showing how the calculator can be used to
analyze questions related to stormwater runoff, retention, and control. Finally, a technical description is
given of how the calculator performs its computations and where it obtains the parameters needed to
do so.
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2. How to Run the Calculator
The National Stormwater Calculator mobile web app is an HTML5, platform neutral and responsive
mobile version of the desktop version of the calculator. The mobile version supports the existing
functionality of the desktop version of the calculator. It may be used with publicly available internet
browsers on laptop and desktop computers, smartphones, and tablets—you must have an internet
connection to run the calculator. The mobile web app functions best on the following web browsers:
Google Chrome, Microsoft Edge, Apple Safari, and Mozilla Firefox. The mobile web app may be accessed
from the following web page: https://swcweb.epa.gov/stormwatercalculator The opening and main
windows of the calculator are displayed in Figure 1. The main window uses a series of tabbed pages to
collect information about the site being analyzed and to run and view hydrologic results. A Bing Maps
display allows you to view the site's location, its topography, selected soil properties and the locations
of nearby rain gages and weather stations.
National Stormwater Calculator
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Figure 1. The calculator's (a) opening page and (b) Location icon page.
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The various pages of the calculator are represented by icons and used as follows:
1. Location icon - establishes the site's location
2. Soil Type icon - identifies the site's soil type
3. Soil Drainage icon - specifies how quickly the site's soil drains
4. Topography icon - characterizes the site's surface topography
5. Precipitation/Evaporation icon - selects a nearby rain gage to supply hourly rainfall data and a
nearby weather station to supply evaporation rates
6. Climate Change icon - selects a climate change scenario to apply
7. Land Cover icon - specifies the site's land cover for the scenario being analyzed
8. LID Controls icon - selects a set of LID control options, along with their design features, to
deploy within the site and specifies site and project considerations for cost estimation purposes
9. Project Costs icon - specifies site and project considerations for cost estimation purposes
10. Results icon - runs a long term hydrologic analysis and displays the results including estimates of
capital and average annual maintenance costs.
Six command options shown along the top of the web app can also be selected at any time:
1. U.S. EPA logo: This command takes you back to the homepage of the web app.
2. New: This command will discard all previously entered data and take you to the Location page where
you can begin selecting a new site to analyze. You will first be prompted to save the data you entered
for the current site.
3. Save: This command is used to save the information you have entered for the current site to a disk
file. This file can then be re-opened in a future session of the calculator by selecting the Open command.
4. Open: This command allows you to open a previously saved site.
5. Resources: This command will show you helpful resources, such as the User's Manual, general LID and
green infrastructure information from the U.S. EPA, fact sheet, and climate change.
6. Contact: This command provides the SWC@epa.gov email address
You can move back and forth between the calculator's icon pages to modify your selections. Most of the
pages have a Help command that will display additional information about the page when selected. Text
displayed as blue on the interface can generally be clicked to display more information. After an analysis
has been completed on the Results icon page, you can choose to designate it as a "baseline" scenario,
which means that its results will be displayed side-by-side with those of any additional scenarios that
you choose to analyze. Each of the calculator's icon pages will now be described in more detail.
Location
The Location icon page of the calculator is shown in Figure 2. You are asked to identify where in the U.S.
the site is located. This information is used to access national soils and meteorological databases as well
as Bureau of Labor Statistics (BLS) data for cost estimation purposes. It has an address lookup feature
that allows you to easily navigate to the site's location. You can enter an address or zip code in the
Search box and either click on the Search icon, or press the Enter key to move the map view to that
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location. You can also use the map's pan and zoom controls to hone in on a particular area. Once the
site has been located somewhere within the map's viewport, move the mouse pointer over the site and
then left-click the mouse to mark its exact location with a drop-down point.
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Figure 2. The calculator's Location icon page.
The map display can be toggled between a standard road,, aerial, bird's eye, and streetside views. Figure
3 shows the site located in Figure 2 with a zoomed-in aerial view selected with the site bounded by an
orange circle. You can specify the area of the site, which will result in a bounding orange circle or a
polygon being drawn on the map. Figure 4 illustrates how a user may click on the polygon draw tool to
draw out polygon points that create a connected polygon boundary around the project site. The project
area cannot be larger than 12 acres. Entering the size of your site is optional because the calculator
makes all of its computations on a per unit area basis.
You can also click on Open o previously saved site to read in data for a site that was previously saved to a
file to continue working with those data (every time you begin analyzing a new site or exit the program
the calculator asks if you want to save the current site to a file). Once you open a previously saved site,
the calculator will be populated with its data.
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NEW SAVE OPEN RESOURCES CONTACT
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Figure 3. Bird's eye map view with a bounding circle.
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Figure 4. Bird's eye map view with a bounding polygon.
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Soli Type
Figure 5 shows the Soil Type icon page of the calculator, which is used to identify the type of soil
present on the site. Soil type is represented by its Hydrologic Soil Group (HSG). This is a
classification used by soil scientists to characterize the physical nature and runoff potential of a soil.
The calculator uses a site's soil group to infer its infiltration properties (Table 1).
You can select a soil type based on local knowledge or by retrieving a soil map overlay from the U.S.
Department of Agriculture's Natural Resources Conservation Service (NRCS) SSURGO database
("https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm"). Simply select the soil type icon on
the left side of the screen to retrieve SSURGO data. (There will be a slight delay the first time that
the soil data are retrieved and the color-coded overlay is drawn). There is an option to hide the soil
polygon data under the soil type menu box. Figure 5 displays the results from a SSURGO retrieval.
You can then select a soil type directly from the left panel or click on a color shaded region of the
map.
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Location
Soil Type
Soil Type
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Soil Drainage
Select s soil type and runoff potential from the
choices listed or by clicking a shaded region of
the map to select its value
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Figure 5, The calculator's Soil Type page.
The SSURGO database houses soil characterization data for most of the U.S. that have been collected
over the past forty years by federal, state, and local agencies participating in the National Cooperative
Soil Survey. These data are compiled by "map units" which are the boundaries that define a particular
recorded soil survey. These form the irregular shaped polygon areas that are displayed in the
calculator's map pane.
Soil survey data do not exist for all parts of the country, particularly in downtown core urban areas;
therefore, it is possible that no data will be available for your site. In this case you will have to rely on
local knowledge to designate a representative soil group.
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Figure 6. The calculator's Soil Drainage icon page.
There are several options available for assigning a hydraulic conductivity value (in inches per hour) to
the site;
a) The edit box can be left blank, in which case, a default value based on the site's soil type will be
used (the default value is shown next to the edit box).
b) As with soil group, conductivity values from the SSURGO database are displayed on the map
when the soil drainage icon is selected. Clicking the mouse on a colored region of the map will
make its conductivity value appear in the edit box.
c) If you have local knowledge of the site's soil conductivity you can simply enter it directly into the
edit box. This is preferred over the other two choices.
It should be noted that the hydraulic conductivity values from the SSURGO database are derived from
soil texture and depth to groundwater and are not field measurements. As with soil type, there may not
be any soil conductivity data available for your particular location.
Topography
Figure 7 displays the Topography icon page of the calculator. Site topography, as measured by surface
slope (feet of drop per 100 feet of length), affects how fast excess stormwater runs off a site. Flatter
slopes results in slower runoff rates and provide more time for rainfall to infiltrate into the soil. Runoff
rates are less sensitive to moderate variations in slope. Therefore, the calculator uses only four
categories of slope - flat (2%), moderately flat (5%), moderately steep (10%) and steep (above 15%). As
Soil Drainage
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Directions
Enter your own conductivity value directly
into the input field below or dick a shaded
region on the map to select its conductivity
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with soil type and soil drainage, any available SSURGO slope data will be displayed on the map when the
topography icon is selected. You can use the resulting display as a guide or use local knowledge to
describe the site's topography.
National Stormwater Calculator
OPEN RESOURCES CONTACT
Location
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Topography
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Figure 7. The calculator's Topography icon page.
Preci pitation/Evaporation
The Precipitation/Evaporation icon page of the calculator is shown in Figure 8. It is used to the select
rain gage location that will supply rainfall data for the site and a National Weather Service Station as a
source for evaporation rates. Rainfall is the principal driving force that produces runoff. The calculator
uses a long term continuous hourly rainfall record to make sure that it can replicate the full scope of
storm events that might occur. In addition, it identifies a set of 24-hour extreme event storms
associated with each rain gage location. These are a set of six intense storms whose sizes are exceeded
only once every 5, 10, 15, 30, 50 and 100 years, respectively.
Topography
Directions
Select a slope from the choices listed below i
dick a shaded region on the map to select its
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National Stormwater Calculator
NEW SAVE OPEN RESOURCES CONTACT
Soil Type
Directions
Soil Drainage
Select a rain gage location to use as a
source of hourly rainfall data and a
weather station to use as a source for
evaporation rates
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Topography
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Climate Change
ATHENS BEN EPPS AIRPORT
Land Cover
LID Controls
ATHENS BEN EPPS AIRPORT
Project Cost
Rainfall and Evaporation Information
Record Start Date: 1970/01/01
Record End Date: 2006/12/31
Annual Rainfall: 47.05
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Figure 8. The calculator's Precipitation/Evaporation icon page.
The calculator contains a catalog of over 8,000 rain gage locations from the National Weather Service's
(NWS) National Climatic Data Center (NCDC). Historical hourly rainfall data for each station have been
extracted from the NCDC's repository, screened for quality assurance, and stored on an EPA file server.
As shown in Figure 8, the calculator will automatically locate the five nearest gages to the site and list
their location, period of record and average annual rainfall amount. You can choose what you consider
to be the most appropriate source of rainfall data for the site by selecting one of the available rain gages
in the drop-down list or the map icons.
The Precipitation/Evaporation icon page of the calculator, also allows the user to select a weather
station that will supply evaporation rates for the site. Evaporation determines how quickly the moisture
retention capacity of surfaces and depression storage consumed during one storm event will be
restored before the next event.
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Figure 9. The calculator's Precipitation/Evaporation icon page (weather station).
Over 5,000 NWS weather station locations throughout the U.S. have had their daily temperature records
analyzed to produce estimates of monthly average evaporation rates (i.e., twelve values for each
station). These rates have been stored directly into the calculator. The calculator lists the five closest
locations that appear in the table along with their period of record and average daily evaporation rate
(the average of the twelve monthly rates). Note that these are "potential" evaporation rates, not
recorded values (there are only a few hundred stations across the U.S. with long term recorded
evaporation data). The rates have been estimated for bare soil using the Penman-Monteith equation;
and thus, transpiration or vegetative land cover is not explicitly represented. More details are provided
in the Computational Methods section of this document.
If the Download rainfall/evaporation data ... command label is clicked, a Save As dialog window wili
appear allowing you to save the rainfall data to a text file in case you want to use those data in some
other application, such as SWMM. Each line of the file will contain the recording station identification
number, year, month, day, hour, and minute of the rainfall reading and the measured hourly rainfall
intensity in inches/hour. If this option is selected, data will be written to a plain text file of your choice
with the twelve monthly average rates appearing on a single line.
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Directions
Select a rain gage location to use as a
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Climate Change
The 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states
that changing of the climate is now unequivocal (IPCC, 2007). Some of the impacts that such changes
can have on the small-scale hydrology addressed by the calculator include changes in seasonal
precipitation levels, more frequent occurrence of high intensity storm events, and changes in
evaporation rates (Karl et al., 2009). A climate change component has been included in the calculator to
help you explore how these impacts may affect the amount of stormwater runoff produced by a site and
how it is managed.
Figure 10 displays the Climate Change icon page of the calculator. It is used to select a particular future
climate change scenario for the site. The scenarios were derived from a range of outcomes of the World
Climate Research Program's CMIP3 multi-model dataset (Meehl et al., 2007). This dataset contains
results of different global climate models run with future projections of population growth, economic
activity, and greenhouse gas emissions. The results have been downscaled to a regional grid that
encompasses each of the calculator's rain gage and weather station locations. Three different scenarios
are available that span the range of changes projected by the climate models: one is representative of
model outputs that produce hot/dry conditions, another represents changes that come close to the
median outcome from the different models, and a third represents model outcomes that produce
warm/wet conditions. Projections for each scenario are available for two different future time periods:
2035 and 2060.
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^ EPA National Stormwater Calculator
NEW SAVE OPEN RESOURCES CONTACT
Soil Type
.-O
Soil Drainage
Topography
O Precipitation/Evaporation
Climate Change
Land Cover
LID Controls
Project Cost
Land Cover
Directions
Describe the site s land cover tor the
development scenario being analyzed
Click on a category to see a more detailed
description
|> Bing O
Figure 11. The calculator's Land Cover page.
You are asked to supply the percentage of the site covered by each of four different types of pervious
surfaces:
1. Forest - stands of trees with adequate brush and forested litter cover
2. Meadow - non-forested natural areas, scrub and shrub rural vegetation
3. Lawn - sod lawn, grass, and landscaped vegetation
4. Desert - undeveloped land in arid regions with saltbush, mesquite, and cactus vegetation
-------
You should assign land cover categories to the site that reflects the specific condition you wish to
analyze: pre-development, current, or post-development. A pre-development land cover will most likely
contain some mix of forest, meadow, and perhaps desert. Local stormwater regulations might provide
guidance on how to select a pre-development land cover or you could use a nearby undeveloped area as
an example. Viewing the site map in bird's eye view, as shown in Figure 11, would help identify the land
cover for current conditions. Post-development land cover could be determined from a project's site
development plan map. Keep in mind that total runoff volume is highly dependent on the amount of
impervious area on the site while it is less sensitive to how the non-impervious area is divided between
the different land cover categories.
LID Controls (including cost estimation options)
The LID Controls icon page of the calculator is depicted in Figure 12. It is used to deploy low impact
development (LID) controls throughout the site. These are landscaping practices designed to capture
and retain stormwater generated from impervious surfaces that would otherwise run off the site. There
are seven different types of green infrastructure (Gl) LID controls available (Figure 12). You can elect to
apply any mix of these LID controls by simply telling the calculator what percentage of the impervious
area is treated by each type of control. Each control has been assigned a reasonable set of design
parameters, but these can be modified by clicking on the name of the control. You have the option to
specify a 24-hour design storm to assist you with sizing the selected LID controls. More details on each
type of control practice, its design parameters and sizing it to retain a given design storm are provided in
the LID Controls section of this user's guide. For the purposes of cost estimation, the calculator factors
in the cost implications of construction feasibility and site suitability, and adjusts the cost of the LID
Controls based on regional cost differences associated with a site's location. Refer to the Cost Estimation
section of this user guide (page 89) for a brief discussion of the cost curve approach used to generate
estimates of probable capital and maintenance costs in the calculator. By indicating whether the project
is new- or re-development and selecting from poor, moderate, or excellent for site suitability for placing
LID controls along with other user input information, the calculator computes and applies the
appropriate cost curve for the project.
For additional help with selecting the options that influence project site complexity, click the blue
underlined text labeled Re-Development, New Development, Poor, Moderate, and Excellent on the LID
controls tab, to show a help window explaining the conditions that warrant the selection of each of
those options. An example of the help window for Re-Development is shown in Figure 13 and the help
window for Poor (Site Suitability - Poor) is show in Figure 14.
The calculator uses Bureau of Labor Statistics (BLS) data to compute regional cost adjustment factors
and allows the user to choose from the various computed factors as follows:
• National - this is the default selected value if your site is more than 100 miles from any of the 17
BLS Regional Centers distributed across the country (including centers from the Northeast,
Midwest, South, and West)
• Nearest 3 BLS Regional Centers - arranged in ascending order of distance from your project site.
You have the option of selecting one of the nearest three BLS Regional Centers.
-------
• Other - select this if you are an advanced user and want to specify your own regional cost
adjustment factor
Click on Cost Region for a map of the BLS Regional Centers (Figure 15). Regional cost multipliers for each
Region are selected as the default multiplier for areas within a 100-mile radius of the regional center
(see light blue circles in Figure 15). Areas that are not within a 100-mile radius of any regional center are
assigned a default National value of 1 (see green areas in Figure 15). The user can override the default
selection by selecting one of the three closest regions to their location from the Cost Region drop down
menu (Figure 16). Note that regional cost multipliers that are greater than 1 increase costs, while
multipliers that are less than 1 decrease costs compared to the National average. Additional information
about the cost estimation procedure, including the BLS regional centers is provided on page 89.
National Stormwater Calculator new save open resources contact
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Location
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Soil Type
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Soil Drainage
A
Topography
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Climate Change
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Project Cost
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Figure 12. The calculator's LID Controls icon page.
Directions
Enter the percentage of your site's
impervious area would like to be treated
by the listed LID Controls.
Click a practice to learn more about it or to
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yy£^!^National Stormwater Calculator
Location
SB
Soil Type
NEW SAVE OPEN RESOURCES CONTACT
rO
Soil Drainage
Topography
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Precipitation/Evaporation
Climate Change
Project Cost
Project Cost
Directions
Atlanta{60 miles)
Re-Development
Re-Development is construction that is a change in existing development (land cover, land use, or similar development alteration) which
requires new or alteration of existing stormwater management facilities
Costs of removal, decommissioning, or alteration of existing structures or additional (new) infrastructure is typically required to connect
existing structures and results in costs that are greater than what would be anticipated with a new development site.
Re-development and extensive retrofit costs are typically higher than
new development costs because existing structures might have to be removed or new structures may be required but may not be located in
a preferred location.
Selecting "Re-development" on the "Project Cost" tab of the National Stormwater Calculator influences the site complexity, and shifts the
costs towards a higher complexity cost estimation.
Re-development combined with information on site suitability, topography, and soil drainage determines whether complex, typical, or simple
cost curves apply. See User Guide for more information.
Figure 13. The Calculator's Project Cost icon page showing the Re-Development pop-out window
(shown by clicking Re-Development)
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Site suitability is a measure of construction feasibility and includes factors such as topography, soil type, slope, and other physical features
that might result in higher implementation costs.
Poor site suitability refers to sites that have a number of the following characteristics
• Physical obstructions,
• Utility conflicts.
• Other features that are likely to make construction of stormwater management infrastructure challenging and more costly.
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Sites determined to have poor suitability for LID practices are typically higher in cost because of the potential need for additional excavation,
accommodation for physical obstructions, required retaining walls, challenging access, distant haul locations, required dewatering the
addition of engineered or custom media blends, and need to address geotechnical or groundwater concerns.
Selecting "Site Suitability - Poof on the "Project Cost' tab of the National Stormwater Calculator influences the site complexity, and shifts
the costs towards a higher complexity cost estimation.
Poor site suitability combined with information on development type, topography, and soil drainage determines whether complex, typical, or
simple cost curves apply. See User Guide for more information.
Figure 14. The Calculator's Project Cost icon page showing the Site Suitability - Poor pop-out window
(shown by clicking Poor).
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^>EPANat,onal Stormwatej^g}oi}atQ^^
Cost Regions
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Project Cost
Your "region" has been determined from the Location tab. Using data from the Bureau of Labor Statistics (BLS) a multiplier has been
computed representing the relative regional differences in costs for your nearest region (unless 'National" is shown) compared to National
costs. Three regions are reported from 20 of the major cities for which BLS data is available Users can select another region or select
"National* to apply a multiplier of 1. representing a national average. If you prefer to apply your own multiplier, select "Other and enter the
multiplier in the Regional Multiplier field (a multiplier >1 would adjust above the National average, while a multiplier < 1 would adjust below
the National average). The default multiplier for your region is shown in the Regional Multiplier box. The light blue circles in the figure below
represent areas within a 100-mile radius of each major city. See User Guide for more information
0.92
"New York
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Figure 15. Map of BLS Regional Cost Centers (pop-out window) used for computing regional
multipliers. A multiplier for each Regional Center applied within a 100-mile radius (inside blue circles).
A National value of 1 used otherwise (in green areas).
Green infrastructure (Gl), similar to LID controls, is a relatively new and flexible term, and it has
been used differently in different contexts. However, for the purposes of EPA's efforts to
implement the Gl Statement of Intent, EPA intends the term Gi to generally refer to systems
and practices that use or mimic natural water flow processes and retain stormwater or runoff
on the site where it is generated. Gl can be used at a wide range of landscape scales in place of,
or in addition to, more traditional stormwater control elements to support the principles of LID.
-------
National Stormwater Calculator
$
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Soil Type
Soil Drainage
Topography
o Precipitation/Eva poration
Climate Change
Land Cover
Jjjfcp LID Controls
Project Cost
NEW SAVE OPEN RESOURCES CONTACT
Figure 16. Drop down list of closest BLS Regional Cost Centers and national value.
Results
The final page of the calculator is where a hydrologic analysis of the site is run; its results are displayed
along with estimates of probable capital and maintenance costs. As shown in Figure 17, by selecting the
Site Description report option you can first review data that you entered for the site and go back to
make changes if needed.
Project Cost
Choose a Project Type
Directions
Verify cost estimation variables below. Click
eacfi option to learn more
Allanta(60 miles)
Detroit(581 miles)
Miami(595 miles)
NATIONAL(NA)
Other (NA)
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The Actions section of the page contains commands that perform the following actions:
Refresh Results - runs a long-term simulation of the site's hydrology and updates the output displays
with new results (it will be disabled if results are currently available and no changes have been made to
the site's data).
Use as Baseline Scenario - uses the current site data and its simulation results as a baseline against
which future runs will be compared in the calculator's output reports (this option is disabled if there are
no current simulation results available).
Remove Baseline Scenario - removes any previously designated baseline scenario from all output
reports.
Print Results to PDF File - writes the calculator's results for both the current and any baseline scenario
to a PDF file that can be viewed with a PDF reader at a future time.
The Reports section of the page allows you to choose how the rainfall / runoff results for the site should
be displayed. A complete description of each type of report available will be given in the next section of
this guide.
When the calculator first loads or begins to analyze a new site the following default values are used:
Soil Group: B
Conductivity: 0.4 inches/hour
Surface Slope: 5%
Rainfall Station: Nearest cataloged station
Evaporation Station: Nearest cataloged station
Climate Change Scenario: None
Land Cover: 40% Lawn, 60% impervious
LID Controls: None
Years to Analyze: 20
Event Threshold: 0.10 inches
Ignore Consecutive Days: No
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3, Interpreting the Calculator's Results
The Results page of the calculator (Figure 17) contains a list of reports that can be generated from its
computed results. Before discussing what these reports contain it will be useful to briefly describe how
the calculator derives its results. After you select the Refresh Results command, the calculator computes
an estimate of probable capital and maintenance costs and internally performs the following operations:
1. A SWMM input file is created for the site using the information you provided to the calculator.
2. The historical hourly rainfall record for the site is adjusted for any climate change scenario
selected.
3. SWMM is run to generate a continuous time series of rainfall and runoff from the site at 15-
minute intervals for the number of years specified.
4. The 15-minute time series of rainfall and runoff are accumulated into daily values by calendar
day (midnight to midnight).
5. Various statistics of the resulting daily rainfall and runoff values are computed.
6. The SWMM input file is modified and run once more to compute the runoff resulting from a set
of 24-hour extreme rainfall events associated with different return periods. The rainfall
magnitudes are derived from your choice of climate change scenario or from the historical
record if climate change is not being considered.
Thus for the continuous multi-year run, the rainfall / runoff output post-processed by the calculator are
the 24-hour totals for each calendar day of the period simulated. A number of different statistical
measures are derived from these data, some of which will be more relevant than others depending on
the context in which the calculator is being used.
Summary Results
The calculator's Summary Results report, an example of which is shown in Figure 18, contains the
following items:
• A pie chart showing the quantity of total rainfall that infiltrates, evaporates, and becomes runoff.
Note that because the calculator does not explicitly account for the loss of soil moisture to
vegetative transpiration, the latter quantity shows up as infiltration in this chart.
• Average Annual Rainfall: Total rainfall (in inches) that falls on the site divided by the number of
years simulated. It includes all precipitation amounts recorded by the station assigned to the site,
even those that fall below the Event Threshold.
• Average Annual Runoff: Total runoff (in inches) produced by the site divided by the number of
years simulated. It includes all runoff amounts, even those that fall below the Event Threshold.
• Days per Year with Rainfall: The number of days with measurable rainfall divided by the number
of years simulated (i.e., the average number of days per year with rainfall above the Event
Threshold).
-------
As an example of how to interpret this plot, look at the bar in Figure 22 associated with the 90th to 95th
percentile storm interval (daily rainfalls between 1.37 and 1.75 inches). Storms of this magnitude make
up 15% of the total runoff (for this particular site and its land cover). Note that by definition the number
of events within this 5th percentile interval is 5 % of the total number of daily rainfall events.
Extreme Event Rainfall/Runoff
The Extreme Event Rainfall/Runoff report shows the rainfall and resulting runoff for a series of extreme
event (high intensity) storms that occur at different return periods. An example is shown in Figure 21.
Each stacked bar displays the annual max day rainfall that occurs with a given return period and the
runoff that results from it for the current set of site conditions. The max day rainfalls correspond to
those shown on the Climate Change page for the scenario you selected (or to the historical value if no
climate change option was chosen).
Note that the max day rainfalls at different return periods are a different statistic than the daily rainfall
percentiles that are shown in the Runoff by Rainfall Percentile report (Figure 20). The latter represents
the frequency with which any daily rainfall amount is exceeded while the former estimates how often
the largest daily rainfall in a year will be exceeded (hence its designation as an extreme storm event).
Most stormwater retention standards are stated with respect to rainfall percentiles while extreme event
rainfalls are commonly used to define design storms that are used to size stormwater control measures.
The extreme event rainfall amounts are generated using a statistical extrapolation technique (as
described in the Computational Methods section) that allows one to estimate the once in X year event
when fewer than X years of observed rainfall data are available.
-------
Extreme Event Rainfall I Runoff
Extreme Event Rainfall / Runoff Depth
Rainfall Runoff Rainfall Baseline Runoff Baseline
15 30
Return Period (years)
Extreme Event Peak Rainfall / Runoff
Rainfall
Runoff
Rainfall Baseline
Runoff Baseline
10
15 30
Return Period (years)
50
100
Figure 23. The calculator's Extreme Event Rainfall / Runoff report.
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Cost Summary
The final report produced by the calculator shows estimates of probable LID construction and annual
maintenance costs. Tables and charts in the results tab, show construction and annual maintenance
costs applied to the site. All the cost estimates produced after February of the current year are adjusted
to be current for the previous year. For instance, running the calculator after February 2018 produces
cost estimates in 2017 dollars. Site complexity and suitability variables that affect costs and the cost
regionalization option selected by the user are also shown below. Table 2 is an example of the tabular
output option of capital costs. All costs are presented as a range (low and high values). Note that if a
baseline scenario is provided, the calculator shows the differences in costs between the baseline
scenario and the current scenario. The tabular and graphical examples provided do not account for
baseline levels. Figure 24 shows a graphical output option of the average capital costs. Similarly, Table 3
shows a tabular output option of a range of annual maintenance costs, whereas Figure 25 shows a
graphical output option of the average annual maintenance costs. Note that the annual maintenance
costs are estimates of current average annual maintenance and are not based on an assumed life span
or lifecycle for the LID controls. In other words, the annual maintenance costs shown do not represent
annualized present value estimates of the cost of maintenance over the life of the LID control. Other
tools such as the Water Research Foundation (WRF) BMP and LID Whole List Cost Models may be useful
for estimating lifecycle costs. The numbers shown in the tables and charts represent the results using
the example described in Section 5.
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Table 2. Tabular representation of the calculator's estimate of capital costs.
Cost Summary
Estimate of Probable Capital Costs (estimates in 2017 US.$)
Maintenance Costs [ Graphical View
Drainage Area Has Pre- Baseline
% Treatment? Current Scenario (C) Scenario (B) Difference (C - B)
Cost By LID Control
Type
Current /
Baseline
Current /
Baseline
Low
High
Low
High
Low
High
Disconnection
0/0
No/Mo
SO. 00
SO. 00
SO.OO
SO.OO
SO.OO
SO.OO
Rainwater
Harvesting
10/0
No / Wo
S9,962.59
S11.676.35
SO.OO
SO.OO
59,962.69
511,676.35
Rain Gardens
15/0
Yes / NA
S13.223.93
S17.512.37
SO.OO
SO.OO
S13.223.93
S17,512.37
Green Roofs
0/0
No / No
SO. 00
SO. 00
SO.OO
SO.OO
SO.OO
SO.OO
Street Planters
20/0
No/No
S26.179.02
S36,360.01
SO.OO
SO.OO
S26.179.02
S36.360.01
Infiltration Basins
4/0
Yes / NA
S12,800.14
S17,333.11
SO.OO
SO.OO
S12.800.14
S17,333.11
Permeable
Pavement
0/0
Yes / NA
SO. 00
SO.OO
SO.OO
SO.OO
SO.OO
SO.OO
Total
49 ,'0
Varies
S62,165.79
582,881.84
SO.OO
SO.OO
S62.165.79
S82.881.84
Note: site complexity variables that affect cost shown below:
Current Scenario
Baseline
Scenario
Chart Key
D - Disconnection IB - Infiltration Basins
Dev. Type Re-Development NA
RH - Rain Harvesting PP - Permeable Pavement
Site Poor NA
Suitability RG - Rain Gardens
Topography Mod. Steep (10% NA GR - Green Roofs
Slope)
SP - Street Planters
Soil Type B NA
Cost Region Atlanta(60 miles) 0.92 NA
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35000
30000
25000
Cost Summary
Estimate of Probable Capital Costs (estimates in 2017 US.$)
Maintenance Costs I Tabular View
I Current Scenario Baseline Scenario
Q
CO
z>
20000
15000
10000
5000
I
RH
RG
Note: site complexity variables that affect cost shown below:
Baseline
Current Scenario Scenario
GR
LID Controls
SP
PP
Dev. Type
Re-Development
NA
Site
Poor
NA
Suitability
Topography
Mod. Steep (10%
NA
Slope)
Soil Type
B
NA
Chart Key
D - Disconnection IE - Infiltration Basins
RH - Rain Harvesting PP - Permeable Pavement
RG - Rain Gardens
GR - Green Roofs
SP - Street Planters
Cost Region Atlanta{60 miles) 0.92 NA
Figure 24. Graphical output option of the calculator's estimate of average capital costs.
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Table 3. Tabular output option of the calculator's estimate of annual maintenance costs.
Cost Summary
Estimate of Annual Probable Maintenance Costs
Capital Costs | Graphical View
Drainage Area Has Pre- Current Scenario Baseline Scenario
% Treatment? (C) (B) Difference (C - B)
Cost By LID Control
Type
Current 1
Baseline
Current /
Baseline
Low
High
Low
High
Low
High
Disconnection
0/0
No/No
SO.OO
SO.OO
SO.OO
SO.OO
SO
SO
Rainwater Harvesting
10/0
No/No
S424.46
S1,018.63
SO.OO
SO.OO
S424.46
S1,018.63
Rain Gardens
15/0
Yes 1 NA
553.56
51,294.97
SO.OO
SO.OO
S53.53
51,294.97
Green Roofs
0/0
No / No
SO.OO
SO.OO
SO.OO
SO.OO
SO
SO
StreeS Planters
20/0
No / No
S57.13
S1,353.07
SO.OO
SO.OO
S57.13
51,358.07
Infiltration Basins
4/0
Yes / NA
S10.30
S374.29
SO.OO
SO.OO
S10.3
S374.29
Permeable Pavement
0/0
Yes / NA
SO.OO
SO.OO
SO.OO
SO.OO
SO
SO
Total 49/0 Varies S545.46 S4,045.97 SO.00 SO.OO S545.46 S4,045.97
Note: site complexity variables that affect cost shown below:
Baseline Chart Key
Current Scenario Scenario
D - Disconnection IB - Infiltration Basins
Dev. Type Re-Development NA
RH - Rain Harvesting PP - Permeable Pavement
Site Poor NA
Suitability RG - Rain Gardens
Topography Mod. Steep (10% NA GR - Green Roofs
Slope)
SP - Street Planters
Soil Type B NA
Cost Region Atlanta(60 miles) 0.92 NA
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4. Applying LID Controls
LID controls are landscaping practices designed to capture and retain stormwater generated from
impervious surfaces that would otherwise run off the site. The Stormwater Calculator allows you to
apply a mix of seven different types of LID practices to a site. These are displayed in Table 4 along with
brief descriptions of each. This particular set of Gl practices was chosen because they can all be sized on
the basis of just area. Two other commonly used controls, vegetative swales and infiltration trenches,
are not included because their sizing depends on their actual location and length within the site,
information which is beyond the scope of the calculator.
Each LID practice is assigned a set of default design and sizing parameters, so to apply a particular
practice to a site, you only must specify what percentage of the site's impervious area will be treated by
the practice (Figure 12). You can, however, modify the default settings by clicking on the name of the
particular practice you wish to edit. For example, Figure 26 displays the resulting LID Design dialog
window that appears when the Street Planter LID is selected. All the LID controls have similar LID Design
dialogs that contain a sketch and brief description of the LID control along with a set of edit boxes for its
design parameters. The Learn More ... link will open your web browser to a page that provides more
detailed information about the LID practice.
Table 3 lists the various parameters that can be edited with the LID Design dialogs along with their
default factory setting. Arguably the most important of these is the Capture Ratio parameter. This
determines the size of the control relative to the impervious area it treats. Note that because the
calculator does not require that the actual area of the site be specified, all sub-areas are stated on a
percentage basis. So, total impervious area is some percentage of the total site area, the area treated by
a particular LID control is some percentage of the total impervious area, and the area of the LID control
is some percentage of the area it treats.
Pressing the Size for Design Storm button on an LID Design form will make the calculator automatically
size the LID control to capture the Design Storm Depth that was entered on the LID Control page (Figure
12). This computes a Capture Ratio (area of LID relative to area being treated) for Rain Gardens, Street
Planters, Infiltration Basins, and Permeable Pavement by taking the ratio of the design storm depth to
the depth of available storage in the LID unit. For Infiltration Basins it also determines the depth that will
completely drain the basin within 48 hours. For Rainwater Harvesting it calculates how many cisterns of
the user-supplied size will be needed to capture the design storm. Automatic sizing is not available for
Disconnection, because no storage volume is used with this practice, and for Green Roofs, because the
ratio is 100% by definition. The methods used to automatically size the LID controls are described in the
Computational Methods section of this user's guide. Note that even when sized in this fashion, an LID
control might not fully capture the design storm because it may not have drained completely prior to
the start of the storm or the rainfall intensity during some portion of the storm event may overwhelm its
infiltration capacity. The calculator is able to capture such behavior because it continuously simulates
the full range of past precipitation events.
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Table 4. Descriptions of LID practices included in the calculator.
LID Practice
Description
Disconnection
Disconnection refers to the practice of directing runoff from impervious
areas, such as roofs or parking lots, onto pervious areas, such as lawns
or vegetative strips, instead of directly into storm drains.
Rain Harvesting
Rain harvesting systems collect runoff from rooftops and convey it to a
cistern tank where it can be used for non-potable water uses and on-
site infiltration.
Rain Gardens
Rain Gardens are shallow depressions filled with an engineered soil mix
that supports vegetative growth. They provide opportunity to store and
infiltrate captured runoff and retain water for plant uptake. They are
commonly used on individual home lots to capture roof runoff.
Green Roofs
Green roofs (also known as vegetated roofs) are bioretention systems
placed on roof surfaces that capture and temporarily store rainwater in
a soil medium. They consist of a layered system of roofing designed to
support plant growth and retain water for plant uptake while
preventing ponding on the roof surface.
Street Planters
Street Planters are typically placed along sidewalks or parking areas.
They consist of concrete boxes filled with an engineered soil that
supports vegetative growth. Beneath the soil is a gravel bed that
provides additional storage as the captured runoff infiltrates into the
existing soil below.
Infiltration Basins
Infiltration basins are shallow depressions filled with grass or other
natural vegetation that capture runoff from adjoining areas and allow it
to infiltrate into the soil.
Permeable Pavement
Permeable Pavement systems are excavated areas filled with gravel and
paved over with a porous concrete or asphalt mix or with modular
porous blocks. Normally all rainfall will immediately pass through the
pavement into the gravel storage layer below it where it can infiltrate at
natural rates into the site's native soil
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Street Planters
1
Street Planters consist of concrete boxes filled with an engineered soil that supports vegetative growth. Beneath, "he soil is a gravel bed that
provides additional storage.
The walls of a planter extend 3 to 12 inches above the soil bed to allow for ponding wishing the unit. The thickness of the soil growing
medium ranges from 8 to 24 inches while gravel beds are S to 18 inches in depth.
The planter's Capture Ratio is the ratio of its area to the impervious area whose runoff s captures
Leam More
Ponding Height
Soil Media
Thickness:
Soil Media
Conductivity:
Gravel Bed
Thickness:
% Capture Ratio:
6
in
18
in_ I
10
inJhr '
12
' hJhr
6
% '
Save and Return
Restore Defaults
Figure 26. Example of an LID Design dialog for a street planter.
There are some additional points to keep in mind when applying LID controls to a site:
1. The area devoted to Disconnection, Rain Gardens, and Infiltration Basins is assumed to come
from the site's collective amount of pervious land cover whereas the area occupied by Green
Roofs, Street Planters, and Permeable Pavement comes from the site's store of impervious area.
2. Underdrains (slotted pipes placed in the gravel beds of Street Planter and Permeable Pavement
areas to prevent the unit from flooding) are not provided for. However, because underdrains
are typically oversized and placed at the top of the unit's gravel bed, the effect on the amount of
excess runoff flow bypassed by the unit is the same whether it flows out of the underdrain or
simply runs off of a flooded surface.
3. The amount of void space in the soil, gravel, and pavement used in the LID controls are listed in
Table 6. They typically have a narrow range of acceptable values and results are not terribly
sensitive to variations within this range.
WfHM
tmTftri
-------
5. Example Application
An example will now be presented to show how the calculator can be used to analyze small site
hydrology. The site shown earlier in Figure 3 will be used as our study area, although, because the
calculator is national in scope, we could have chosen any other location, as weil. It is a 2.64 acre
environmental research facility. Baseline data for the site were obtained as described above (Figure 4—
Figure 8. These identified the site's hydrologic soil group as B, its hydraulic conductivity as 0.11
inches/hour, its topography as moderately steep, its closest rain gage as having an annual rainfall of
47.05 inches and its closest weather station averaging 0.2 inches per day of potential evaporation. We
will simulate three different development scenarios (pre-development, post-development, and post-
development with LID controls) to show how one can both derive and evaluate compliance with
different stormwater retention standards. After that we will see the effect a future climate change
scenario might have on the site's ability to comply with the standard, such as the 99th percentile rainfall
event. Note that this example uses a 20-year simulation.
Pre-Development Conditions
Pre-development hydrology is often cited as an ideal stormwater management goal to attain because it
maintains a sustainable and ecologically balanced condition within a watershed. It is also commonly
used to define specific stormwater retention standards, as will be discussed below. To simulate a pre-
development condition for our study area, we must identify the land cover that characterizes the site in
its natural pre-developed state. In this example, if you pan the site's map display to the left, you will
observe an adjacent natural area that suggests a pre-development land cover of 80 percent Forest and
20 percent Meadow. These values are entered on the Land Cover page of the calculator (Figure 27).
ER^V National Stormwater Calculator
NEW SAVE OPEN RESOURCES CONTACT
Soil Type
Soil Drainage
Topography
O Precipitation/Evaporation
(^)) Climate Change
Land Cover
epfe LID Controls
(jgj) Project Cost
inijl Resu|ts
Figure 27. Pre-development conditions land cover.
Directions
Describe the site's land cover for the
development scenario being analyzed.
Click on a category to
description.
detailed
-------
For the next page of the calculator no LID Controls are selected because we are analyzing a pre-
development scenario. On the final page of the calculator, we select to analyze the latest 20 years of
rainfall data and to not ignore back to back storm events.
Running the calculator for these conditions produces the Summary Results report listed in Table 7. It
shows that there is an average of 71 days per year with rainfall, but only 7 of these produce measurable
runoff. Of the 47 inches of rainfall per year, 91 percent is retained on site. The Runoff by Rainfall
Percentile plot for this run, shown in Figure 28, indicates that it is mainly storms above 1 inch that
produce almost all the runoff.
Retention standards are developed by state and municipal governments and tailored to meet
stormwater control objectives unique to their jurisdiction. They stipulate the amount of rainfall
that must be "retained" on site and are used to determine the proper size of stormwater
controls. Standards are usually formulated in one of several ways including restoration of pre-
development conditions, rainfall depth retained, or percentile rainfall depth retained. A
summary of state post construction stormwater standards is available at:
https://www.epa.gov/sites/production/files/2016-08/documents/swstdsummary 7-13-
16 508.pdf (EPA 2016). Contact your local government to learn more about the retention
standards that apply in your area.
4>EPANational stormwater Calculator
Location
%
a.fi\ Soil Type
NEW SAVE OPEN RESOURCES CONTACT
.-O
Soil Drainage
/^\ Topography
O Precipitation/Evaporation
(^) Climate Change
Land Cover
¦j#
(©) Project Cost
Figure 27, Pre-development conditions land cover.
Directions
Describe the site's land cover for the
development scenario being analyzed.
¦ detailed
-------
ER/V National Stormwater Calculator
OPEN RESOURCES CONTACT
<3>
J.
fi
A
O
a
4ft
LID Controls
Enter the percentage of your site's
impervious area would like to be treated by
the listed LID Controls.
College Static
North Oconee River
North Oconee
© 2018 DfliaKaofce. ©20t8 HERE.© 2018 Mrnxcfl Corporation T<
Figure 31. Low Impact Development controls applied to the example site.
-------
Rain Harvesting
1
Rain harvesting systems collect runoff from rooftops and convey it to a cistern tank where it can be used for non-potable water uses and on-
site infiltration.
The harvesting system is assumed to consist of a given number of fixed-sized cisterns per 1000 square feet of rooftop area captured
The water from each cistern is withdrawn at a constant rate and is assumed to be consumed or infiltrated entirely on-site
Learn More
Cistern Size:
k 100
Gai.
Emptying Rate:
50
Gal/Day
Number per 1,000
A 23
/1,000 sq ft
sq. ft:
Size for Design Storm
Restore Defaults 1
a
Rain Gardens are shallow depressions filled with an engineered soil mix that supports vegetative growth. They are usually used on
individual home lots to capture roof runoff.
Typical soil depths range from 6 to 18 inches
The Capture Ratio is the ratio of the rain garden's area to the impervious area that drains onto it.
Leam More
Ponding Height: 6
Soil Media
Thickness:
Soil Media
Conductivity:
% Capture Ratio:
* Pre-Treatment
10 in./hf
Size for Design Storm
Save and Return ¦ Restore Details
Figure 32. Design parameters for Rain Harvesting and Rain Garden controls.
-------
Infiltration Basins
Inflow via a pipe^
or controlled
uiiitce flow
Infiltration basins are shallow depressions filled with grass or other natural vegetation that capture runoff from adjoining areas and allow it to
infiltrate into the soil.
The calculator assumes that the infiltration rate from the basin is the same as for site's native soil.
The basin's Capture Ratio is the area of the basin relative to the impervious area whose runoff it captures.
Learn More
Basin Depth:
% Capture Radio:
•* Pre-Treatment
Size for Design Storm
6
in.
92
%
Save arid Return I Restore Defaults
Permeable Pavement
i i- POROUS ASPHALT PAVEWE
1 . UNIFORMLY GRADED
UNCOMPACrED STONE AGGRtQAtt
" cy WITH
. , -...VOIDSPACE
" V " FOR STORMWATER STORAGE
** V ~ " AND RECHARGE
1-1 :
B
Sir
Continuous Permeable Pavement systems are excavated areas filled with gravel and paved over with a porous concrete or asphalt mix.
Modular Block systems are similar except that permeable block pavers are used instead
Normally all rainfall will immediately pass through the pavement into the gravel storage layer below it where it can infiltrate at natural rates
into the site's native soil.
Pavement layers are usually 4 to 6 inches in height while the gravel storage layer is typically 6 to 18 inches high.
The Capture Ratio is the percent of the treated area (street or parking lot) that is replaced with permeable pavement.
Leam More
Pavement . . 6 m
Thickness:
Gravel Layer 18
Thickness:
% Capture Ratio: 64 %
i<: Pre-Treatment
Size for Design Storm
Save and Return I Restore Defaults
Figure 33. Design parameters for the Infiltration Basin and Permeable Pavement controls
-------
Rainfall / Runoff Exceedance Frequency
Rainfall
Runoff
Rainfall Baseline
Runoff Baseline
Figure 34. Daily runoff frequency curves for pre-development (Baseline) and post-development with
LID controls (Current) conditions.
3 4 5
Depth (inches)
Runoff Contribution by Rainfall Percentile
| Current Scenario Baseline Scenario
70
60 I
I
1 40 |l
"5 30
10% 20% 30% 40% 50% 60% 70% 75% 80% 85% 90% 95% 99%
0.13 0.19 0.26 0.34 0.43 0.54 0.69 0.30 0.98 1.15 1.37 1.75 3.51
Daily Rainfall Percentile I Daily Rainfall Depth (inches)
Figure 35. Contribution to total runoff by different magnitude storms for pre-development (Baseline)
and post-development with LID controls (Current) conditions.
-------
T3
Si
cr
OS
:=
CL
100
90
80
70
60
50
40
30
20
10
0-
Rainfall Retention Frequency
I Current Scenario Baseline Scenario
^— -*
<
0.5 1.0 1.5 2.0 2.5
Daily Rainfall (inches)
3.0
3.5
4.0
Figure 36. Retention frequency plots under pre-development (Baseline) and post-development with
LID controls (Current) conditions.
Cost Summary
In addition to runoff results, the calculator also computes capital and maintenance cost estimates. The
capital costs computed for the pre-development condition (as Baseline) and the post-development with
LID controls (Current) conditions are shown in Table 11 and Figure 37; maintenance results are shown in
Table 12 and Figure 38.
-------
Table 11. Tabular output option of the calculator's estimate of capital costs.
Cost Summary
Estimate of Probable Capital Costs (estimates in 2017 US J)
Maintenance Costs | Graphical View
Drainage Has Pre- Baseline
Area % Treatment? Current Scenario {C) Scenario (B) Difference (C - B)
Cost By LID
Control Type
Current/
Baseline
Current/
Baseline
Low
High
Low
High
Low
High
Disconnection
0/0
No /No
$0.00
$0.00
$0.00
$0.00
$0.00
$0.00
Rainwater
Harvesting
10/0
No / No
$33,275.96
$46,412.13
$0.00
$0.00
$33,275.96
$46,412.13
Rain Gardens
20/0
Yes / Yes
$50,599.22
$67,724.24
$0.00
$0.00
$50,599.22
$67,724.24
Green Roofs
0/ 0
No / No
$0.00
$0.00
$0.00
$0.00
$0.00
$0.00
Street Planters
0/0
No/No
$0.00
$0.00
$0.00
$0.00
$0.00
$0.00
Infiltration Basins
25/0
Yes / Yes
$103,332.66
$142,910.17
$0.00
$0.00
$103,332.66
$142,910.17
Permeable
Pavement
40/0
Yes / Yes
$216,583.16
$259,710.17
$0.00
$0.00
$216,588.16
$259,710.17
Total
95/0
Varies
$408,796.00
$516,756.70
$0.00
$0.00
$408,796.00
$516,756.70
Note: site complexity variables that affect cost shown Below:
Current Scenario
Baseline Scenario
Chart Key
Dev. Type
Re-Development
Re-Development
D - Disconnection
IB -
Infiltration Basins
Site
Suitability
Poor
Poor
RH - Rain Harvesting
PP-
Permeable Pavement
Dfi . Dain RarHane
Topography
Mod. Steep (10%
Slope)
Mod. Steep (10%
Slope)
GR - Green Roofs
Soil Type B B SP - Slreet Planters
Cost Region Atlanta(60 miles) 0.92 Atlanta(60 miles) 0.92
-------
Cost Summary
Estimate of Probable Capital Costs (estimates in 2017 US.$)
Maintenance Costs | Tabular View
I Current Scenario Baseline Scenario
250000
200000
n 150000
o
Cl
CO
Z)
S 100000
50000
I
D
RH RG
GR SP
IB PP
Note: site complexity variables that affect cost shown below:
LID Controls
Current Scenario
Baseline Scenario
Chart Key
Dev. Type
Re-Development
Re-Development
D - Disconnection
IB - Infiltration Basins
Site
Suitability
Poor
Poor
RH - Rain Harvesting
PP - Permeable Pavement
RG - Rain Gardens
Topography
Mod. Steep (10%
Slope)
Mod. Steep (10%
Slope)
GR - Green Roofs
Soil Type
B
B
SP - Street Planters
Cost Region Atlanta(60 miles) 0.92 Atlanta(60 miles) 0.92
Figure 37. Graphical output option of the calculator's estimate of capital costs.
-------
Table 12. Tabular output option of the calculator's estimate of maintenance costs.
Cost Summary
Estimate of Annual Probable Maintenance Costs
Capital Costs ] Graphical View
Drainage Area Has Pre- Baseline
% Treatment? Current Scenario (C) Scenario (B) Difference (C - B)
Cost By LID Control
Type
Current /
Baseline
Current /
Baseline
Low
High
Low
High
Low
High
Disconnection
0/0
No /No
$0.00
$0.00
$0.00
$0.00
$0.00
$0.00
Rainwater Harvesting
10/0
No / No
$2,440.67
$5,857.11
$0.00
$0.00
$2,440.67
$5,857.11
Rain Gardens
20/0
Yes 1 Yes
$557.03
$13,467.73
$0.00
$0.00
$557.03
$13,467.73
Green Roofs
0/0
No / No
$0.00
$0.00
$0.00
$0.00
$0.00
$0.00
Street Planters
0/0
NA / NA
$0.00
$0.00
$0.00
$0.00
$0.00
$0.00
Infiltration Basins
25/0
Yes / Yes
$1,185.05
$43,043.81
$0.00
$0.00
$1,185.05
$43,043.81
Permeable Pavement
40/0
Yes / Yes
$1,524.85
$8,328.46
$0.00
$0.00
$1,524.85
$8,328.46
Total
95/0
Varies
$5,707.60
$70,697.12
$0.00
$0.00
$5,707.60
$70,697.12
Note: site complexity variables that affect cost shown below:
Current Scenario
Baseline Scenario
Chart Key
Dev. Type Re-Development
Re-Development
D-
Disconnection
IB - Infiltration Basins
Site Poor Poor RH - Rain Harvesting PP - Permeable Pavement
Suitability
RG - Rain Gardens
Topography Mod. Steep (10% Mod. Steep (10%
Slope) Slope) GR - Green Roofs
Soil Type B B SP - Street Planters
Cost Region Atlarrta(60 miles) 0.92 Atlanta(60 miles) 0.92
-------
Rainfall / Runoff Exceedance Frequency
80
70
60
"d
1 50
o
X
LU
g 40
QJ
» 30
Q
20
10
0
0123456789
Depth (inches)
Figure 40. Daily rainfall and runoff frequencies for the historical (Baseline) and Warm/Wet climate
scenarios.
Rainfall
Runoff
Rainfall Baseline
Runoff Baseline
-------
35
30
25
20
15
10
5
0
40
35
30
I 25
i:
|> 15
10
5
0
Rainfall
Rainfall
Extreme Event Rainfall I Runoff
Extreme Event Rainfall i Runoff Depth
Runoff
I Rainfall Baseline
Runoff Baseline
tO 15 30
Return Period (years)
Extreme Event Peak Rainfall i Runoff
Runoff
I Rainfall Baseline
50
Runoff Baseline
100
10
15 30
Return Period (years)
50
100
Figure 42. Extreme event rainfall and runoff for the Warm/Wet climate change scenario and the
historical record (Baseline).
-------
6, Computational Methods
The National Stormwater Calculator uses SWMM 5 (EPA, 2010) as its computational engine. SWMM is a
comprehensive model that addresses surface runoff, infiltration, groundwater, snow melt, stormwater
detention, and full dynamic wave flow routing within any configuration of open and closed channels.
Only its runoff, infiltration, and LID sub-models are used by the calculator. This section describes how
SWMM carries out its hydrology calculations, how the calculator sets up a SWMM model for the site
being analyzed, how it populates the parameter values needed to run the model, and how it post-
processes the results produced by SWMM.
SWMM's Runoff Model
SWMM allows a study area to be subdivided into any number of irregularly shaped sub-catchment areas
to best capture the effect that spatial variability in topography, drainage pathways, land cover, and soil
characteristics have on runoff generation. An idealized sub-catchment is conceptualized as a rectangular
surface that has a uniform slope and drains to a single outlet point or channel or to another sub-
catchment. Each sub-catchment can be further divided into three sub-areas: an impervious area with
depression (detention) storage, an impervious area without depression storage, and a pervious area
with depression storage. Only the latter area allows for rainfall losses due to infiltration into the soil.
SWMM uses a nonlinear reservoir model to estimate surface runoff produced by rainfall over each sub-
area of a sub-catchment (Chen and Shubinski 1971). From conservation of mass, the net change in depth
per unit of time of water stored on the land surface is simply the difference between inflow and outflow
rates over the sub-catchment:
where d = depth of water on the land surface, i= rate of rainfall + any runoff from upstream sub-
catchments, e= evaporation rate, f= soil infiltration rate, q= runoff rate and t = time. Note that the
fluxes ij e, f, and ^are expressed as flow rates per unit area. By assuming that the overland flow across
the sub-area's width is normal, the Manning equation can be used to express the runoff rate qas:
where W= width of the sub-catchment's outflow face, S= sub-catchment slope, n = roughness
coefficient, A = sub-catchment area and ds = depression storage depth. The latter represents initial
rainfall abstractions such as surface ponding, interception by vegetation, and surface wetting. Note that
no runoff occurs when d\s below ds- How the calculator sets values for the parameters in this equation
is discussed later on in this section.
Substituting equation (2) into (1) produces an ordinary non-linear differential equation that can be
solved numerically fore/over a sequence of discrete time steps given externally imposed rainfall and
dd . _
— = i — e — f — q
dt / f
(1)
1.49WS1/2
(2)
-------
evaporation rates and a computed infiltration rate f. By knowing d, (2) can be evaluated to determine
the runoff qat each time step.
SWMM 5 offers a choice of three different methods for computing soil infiltration rates - the Horton,
Green-Ampt and Curve Number models. The Green-Ampt method was chosen for use in the calculator
because it is based on physical parameters that can be related to the site's soil type. SWMM uses the
well-known Mein-Larson form of this model (Mein and Larson, 1973):
f = ks (l + (3)
where Ks= saturated hydraulic conductivity, (f>- soil porosity, 6o = initial soil moisture content, y/=
suction head at the wetting front, and F= cumulative infiltration volume. Equation (3) applies after a
sufficient time has elapsed to saturate the top layer of soil. During wet periods the moisture content of
the uppermost layer of soil increases at a rate of //Lu where Lu is the layer depth equal to 4/VKS (for
Lu in inches and Ks in in./hr.). During dry periods the moisture content decreases at a rate of kr6o where
the rate constant kr is estimated as JKs/7S . At the start of the next wet period 6q is set equal to the
current moisture content.
SWMM's LID Model
SWMM 5 has been extended to explicitly model several types of LID practices (Rossman, 2009). Consider
a typical bio-retention cell in the form of a street planter as shown in the left panel of Figure 41.
Conceptually it can be represented by a series of three horizontal layers as depicted in the figure's right
panel.
I '
_ OVERRO-*
ELEVATION 0 0
CSOWING MEDIUM
DRAn RGCK
Overflow
Rainfall ET
t t
Runon
_
7 T
Surface I lnfi!tratic*n
Layer/
i
Soil Layer
Storage Layer
Underdrain
Percolat
T
on
Infiltration
Figure 43. Conceptual representation of a bio-retention cell.
-------
The surface layer receives both direct rainfall and run-on from other areas. It loses water through
infiltration into the soil layer below it, by evaporation of any water stored in depression storage and
vegetative capture, and by any surface runoff that might occur. The soil layer contains an amended soil
mix that can support vegetative growth. It receives infiltration from the surface layer and loses water
through evaporation and by percolation into the storage layer below it. The storage layer consists of
coarse crushed stone or gravel. It receives percolation from the soil zone above it and loses water by
either infiltration into the underlying natural soil or by outflow through a perforated pipe under drain
system.
The hydrologic performance of this LID unit can be modeled by solving the mass balance equations that
express the change in water volume in each layer over time as the difference between the inflow water
flux rate and the outflow flux rate. The equations for the surface layer, soil layer, and storage layer can
be written as
respectively, where di = depth of ponded surface water, 62 = soil layer moisture content, d3= depth of
water in the storage layer, i= rainfall rate, qo= upstream run-on rate, qi = surface runoff flow rate, Q3 =
underdrain outflow rate, ei= surface evaporation rate, e2= soil zone evaporation rate, fi = surface
infiltration rate, £2= soil percolation rate, /?= native soil infiltration rate, Lz= depth of the soil layer, and
(f>3 = porosity of the storage layer.
The flux terms (q, e, and f) in these equations are functions of the current water content in the various
layers (di, 62, and fife) and specific site and soil characteristics. The surface and native infiltration rates
are determined using the Green-Ampt model. The soil percolation rate decreases exponentially from Ks
with decreasing soil moisture: /2 = Ksex\)(—p(02 — ^2)) where p is a percolation constant
typically in the range of 5 to 15. Under drain outflow rate is modeled as a power function of head of
water above the drain outlet: q3 = d{d3 — d^ where a and 6 are constants and ddis the offset
distance of the drain from the bottom of the unit.
This set of equations can be solved numerically at each runoff time step to determine how an inflow
hydrograph to the LID unit is converted into some combination of runoff hydrograph, sub-surface
storage, sub-surface drainage, and infiltration into the surrounding native soil. In addition to Street
Planters and Green Roofs, the bio-retention model just described can be used to represent Rain Gardens
by eliminating the storage layer and also Permeable Pavement systems by replacing the soil layer with a
pavement layer.
UUl . r
-^=i + q0-e1-f1-q1
J dO'l £ £
L2~ST-fl~e2~f2
03 = f2~f3~Q3
(4)
(5)
(6)
-------
Site Model without LID Controls
To analyze a site's hydrology without any LID controls, the calculator creates a single SWMM sub-
catchment object and populates it with the following parameter values:
Site Area:
A nominal area of 10 acres is used. As mentioned earlier, because all results are expressed per unit of
area, there is no need to use an actual site area.
Width:
This is the width of the outflow face of a conceptual rectangular plane over which runoff flows. In most
SWMM models, it is initially set to the site area divided by the length of the overland flow path that
runoff follows, and is then refined by calibration against measured runoff hydrographs.
When assigning an overland flow path length, particularly for sites with natural land cover, one must
recognize that there is a maximum distance over which true sheet flow prevails. Beyond this, runoff
consolidates into rivulet flow with much faster travel times and less opportunity for infiltration.
There is no general agreement on what distance should be used as a maximum overland flow path
length. The NRCS recommends a maximum length of 100 ft (USDA, 2010), whereas Denver's Urban
Drainage and Flood Control District uses a maximum of 500 ft. (UDFCD, 2007). For the calculator, a
conservative value of 150 ft. is used. The resulting width parameter for the SWMM input file is therefore
set to the nominal area (10 acres) divided by this length.
Slope:
A value of 2% is used for flat slopes, 5% for moderately flat slopes, 10% for moderately steep slopes, and
20% for steep slopes. These values are derived from the SSURGO dataset (USDA, NRCS, 2019)
Percent Impervious:
SWMM only considers two types of land surfaces - impervious and pervious - each with its own
depression storage depth and surface roughness parameters. It does not explicitly consider the different
types of land covers that comprise these two categories and how their characteristics affect depression
storage and roughness. Impervious surfaces, such as roads, roofs, sidewalks, and parking lots show
minor variation in these parameters; therefore they are treated as a single category.
To provide more refinement in characterizing pervious areas, the calculator allows the user to specify
the percentage of the site's area devoted to four different sub-categories of land surface cover: Forest,
Meadow, Lawn, and Desert. These sub-categories were chosen from a distillation of categories used in
the Western Washington Hydrology Model (Clear Creek Solutions, Inc., 2006) and the National Green
Values Calculator (Center for Neighborhood Technology, 2009). The remaining area is assigned as
Impervious Cover.
-------
node is set to a nominal height of 48 inches (EPA, 2016). Its surface area equals the area of its
contributing sub-catchment times the number of cisterns per unit area (as supplied by the user) times
the area per cistern. The latter is found by dividing the user-supplied volume per cistern by the nominal
depth. Note that any nominal depth can be used because the area per cistern will adjust itself
accordingly to maintain an equal amount of total cistern storage volume. The rate at which the cisterns
empty is converted into an equivalent "infiltration" rate for the storage node, equal to the user-supplied
emptying rate (in gallons/day) divided by the area per cistern. When the cisterns become full, any
overflow shows up as node flooding in SWMM, which gets added to the runoff from other portions of
the site.
Other LID Controls
Rain Gardens, Green Roofs, Street Planters, and Permeable Pavement do not require additional sub-
catchments - they are all placed within the original sub-catchment used to model the site. The original
pervious area of this sub-catchment is reduced by the amount of area devoted to Rain Gardens, while
the original impervious area is reduced by the area taken up by any Green Roofs, Street Planters and
Permeable Pavement.
LID Sizing
When the user supplies a design storm depth, the LID controls can be automatically sized to retain this
depth. For Rain Harvesting, the number of cisterns required per unit area is simply the design storm
depth divided by the volume of a cistern. For the other controls, the Capture Ratio (CR), which is the
ratio of the LID control area to the impervious area being treated, is computed as
„ „ D storm
CR = : (7)
Dlid-(Dstorm-0.5Ksat)
where Dstorm is the design storm depth (inches over 24 hours), Dlidis the storage depth (inches)
provided by the LID control, and Ksatis the saturated hydraulic conductivity of the native soil
underneath the LID control (inches/day). The 0.5 factor accounts for the average amount of infiltration
occurring over the duration of the design storm. The LID storage depth consists of any ponding
depth, plus the depths of any soil and gravel layers, times their respective void fractions.
Precipitation Data
The SWMM model built by the calculator includes a single Rain Gage object that provides it with hourly
precipitation data. These data come from a nearby National Weather Service rain gage as selected by
the user. The calculator can access historical hourly rainfall data for 8,159 stations that are part of the
data holdings for EPA's BASINS system (https://www.epa.gov/exposure-assessment-models/basins ).
Data for each gage are contained in their own file on an EPA server, which is downloaded and made
available to the calculator. The national coverage provided by these gages is shown in Figure 44.
-------
Precipitation Locations
¦
YrCount
~ 5-9
~ 10- 19
~ 20+
States
~
Figure 44. NWS rain gage locations included in the calculator.
In addition to simulating a long term record of hourly rainfall, the calculator also computes the runoff
produced from a series of 24-hour rainfall events that represent extreme, high intensity storms with
different annual return periods. How the depths of these storms are estimated for each rain gage is
discussed in the Climate Change sub-section. To simulate each storm, the calculator uses NRCS (Soil
Conservation Service (SCR) 24-hour distributions (USDA, 1986) to disaggregate the event's total rainfall
depth into a series of rainfall intensities (measured in inches per hour) at six minute intervals, shows the
different NRCS distributions and Figure shows which distribution applies to each region of the US.
Each precipitation station is pre-assigned a distribution type (I, IA, II, or III) based on the region it falls in.
After the long term simulation is completed, the SWMM input file is modified as follows:
1. A time series object is added to the model which is the result of applying the appropriate SCS
distribution at a six minute interval to the total 24-hour rainfall amount being simulated.
2. The source of rainfall data for the model is set to the newly added time series.
3. The duration of the simulation is changed to three days starting on June 1.
4. After running the model, the only output recorded is the total runoff from the event.
These steps are repeated for each return period extreme event analyzed.
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Time (hours)
Figure 45. NRCS (SCS) 24-hour rainfall distributions (USDA, 1986).
' Rainfall
Distribution
~~ Type 1
~ Type IA
I I Type II
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5% different than those from the daily values. It was therefore decided to use just the monthly average
evaporation values for the calculator. Each NWS station is identified by its latitude, longitude, and
twelve monthly average evaporation rates that are contained in a table built into the calculator. This
table is used to supply evaporation rates to the SWMM model constructed by the calculator.
Climate Change Effects
The calculator obtains its climate change scenarios and their effect on local precipitation and
temperature directly from another EPA's CREAT 2.0 (Climate Resilience Evaluation and Analysis Tool)
(EPA, 2012). CREAT is a decision support tool to assist drinking water and wastewater utility owners in
understanding, evaluating, and addressing climate change risks. It contains a database of climate change
effects across the US localized to a grid of 0.5 degrees in latitude and longitude (about 30 by 30 miles).
These effects include changes in monthly average precipitation, monthly average temperature, and
extreme event 24-hour rainfall amounts for each of three different climate change scenarios in two
different future time periods.
CREAT 2.0 uses statistically downscaled General Circulation Model (GCM) projections from the World
Climate Research Programme (WCRP) Coupled Model Intercomparison Project Phase 3 (CMIP3) archive
(Meehl et al., 2007) as the source of its climate change data. The CMIP3 archive was chosen by CREAT
because:
• it contains 112 runs from 16 internationally recognized models using several emission scenarios;
• it supported model-based analyses presented in the IPCC Fourth Assessment Report (IPCC,
2007);
• it facilitates the comparison and diagnosis of model outputs by standardizing many of the
assumptions and boundary conditions used;
• it is downscaled to appropriate spatial (regional, watershed) and temporal (monthly) scales
using a proven downscaling technique;
• it contains well-documented model output that is widely available to researchers;
• it has a high degree of scientific credibility and the archive encompasses a broad range of
assumptions concerning demography, economic integration, technological advance, energy use,
and greenhouse gas emissions.
CREAT 2.0 limited its use of CMIP3 results to the nine GCM models that were most representative of US
climate conditions and used the IPCC's "middle of the road" projection of future economic growth. The
latter is characterized by (1) rapid economic growth, (2) global population that peaks in mid-century, (3)
the quick spread of new and efficient technologies, (4) the global convergence of income and ways of
life, and (5) a balance of both fossil fuel and non-fossil energy sources (IPCC, 2007).
Each of the nine models produces a different set of results for each future year within each downscaled
Vz degree grid cell. To represent this type of uncertainty inherent in predicting future climate conditions,
CREAT 2.0 defined three scenarios that span the range of results produced by the models for any given
projection year. The Warm/Wet scenario used the model that came closest to the 5th percentile of
annual temperature change and 95th percentile of annual rainfall change. The Median scenario selected
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Once the model output to use for each scenario in each projection year in each grid cell was identified,
CREAT 2.0 extracted its CMIP3 results to produce a database of percent changes in monthly average
precipitation and absolute changes in monthly average temperature for each scenario in each of the two
projection years in each grid cell across the U.S. For precipitation impacts, the National Stormwater
Calculator used these data to construct a table for each combination of climate scenario and projection
year (six in total) containing the change in monthly (January - December) average precipitation for each
of its 8,159 rain gages. When the calculator runs SWMM to evaluate the long-term rainfall / runoff for a
site under a particular climate change scenario, it first creates a new hourly rainfall file from the original
one downloaded from the EPA server. In this new file each historical hourly rainfall is adjusted by the
percent change (up or down) for the gage and month of the year contained in the appropriate climate
change scenario table.
Regarding temperature changes, the monthly changes in CREAT's database were used to generate new
sets of monthly average evaporation rates for the calculator. The same procedure described earlier,
using the SWAT model's Penman-Monteith procedure, was used to compute bare soil evaporation rates
for each day of temperature recorded at 5,236 different NWS weather stations. However, the daily
temperatures were first modified by applying the monthly temperature changes belonging to the
climate change scenario for the grid cell in which the weather station was located. The multi-year daily
evaporation values were then averaged into a set of twelve daily rates, one for each month of the year.
This process was repeated for each climate change scenario and projection year at each weather station
location. The result was another set of six tables, each containing a set of modified monthly evaporation
rates for all weather stations for a particular scenario and projection year.
It turned out the climate change modified evaporation rates showed little variation between the
different scenarios for a given month at any particular location, with most differences being 0.02
inches/day or less. One possible reason for this is that climate change effects for the other variables that
influence the Penman-Monteith estimates, such as wind speed, relative humidity, and solar radiation,
were not considered. Even though the variations are slight, the tables were still constructed and utilized
for each of the climate scenarios as was done for monthly precipitation. The monthly evaporation rates
appearing in the table for the user's choice of climate change scenario are inserted into the SWMM
input file for a particular site instead of the rates based on historical temperatures.
The third climate-influenced outcome the calculator considers is the change in the size and frequency of
intense precipitation events. CREAT 2.0 considered this effect of climate change by fitting a Generalized
Extreme Value (GEV) probability distribution to the collection of annual maximum 24-hour (midnight to
midnight) rainfall amounts over a 30 year period simulated by the CMIP3 GCM used for each scenario.
Under the cumulative GEV distribution, the annual maximum daily rainfall amount x that is exceeded
only once every Yyears is:
where ^uis a location parameter, cis a scale parameter, and £ is a shape parameter. These GEV
parameters can be estimated from a series of annual data.
(8)
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CREAT 2.0 estimated GEV parameters for both the historical record and all six of the future climate
scenarios for each rain gage location in the calculator's database. From these parameters, values of the
annual maximum 24-hour rainfall depths for return periods of 5, 10, 15, 30, 50, and 100 years were
calculated using Eq. 8 and were placed in a set of seven tables, one for the historical record and six for
the future climate change scenarios (three different model outcomes in each of two future years). Each
set of extreme event storms corresponding to the six return periods for either the historical record or
for a future climate change scenario at a given rain gage location was simulated in SWMM using the
procedure described earlier in the Precipitation Data sub-section of this guide.
Cost Estimation
The cost estimation procedure was developed by evaluating the input parameters to the calculator to
determine the type of information and the limits of user inputs supported by the tool that affect costs
(capital and maintenance). Critical and influential unit cost items were evaluated for how they could be
incorporated into costs estimates that accurately reflect changes in design input variables available in
the calculator. For critical cost items in which there was no existing design variable within the calculator,
these items were added as selectable options in the calculator. The following design variables selected
by the user influence itemized costs of the LID controls: footprint ratio (% capture ratio), cistern size,
number of LID controls per 1,000 ft2, soil media thickness, gravel bed thickness, basin depth, and
pavement thickness. For each of the design variables that affect costs, one or more corresponding line
items were included to account for the effect of that design variable. Other line items were added to the
cost estimates that are not directly related to the size of the LID control but are necessary to account for
other activities, design features, and processes necessary for construction such as mobilization.
Based on user input for select variables (soils, slope, and added variables such as pretreatment and site
suitability), three design scenarios (simple, typical, and complex) are available for selection by the user
for the project scenario the user is evaluating. Using the influential design variables and known
properties of the LID controls, cost curves (simple, typical, and complex) were developed for each of the
7 LID controls included in the calculator, using nationally available unit cost estimates.
The cost estimation approach implemented in the calculator is based on the use of previously developed
cost curves for each of the LID controls supported. The process of creating the cost curves is described in
detail in a report titled "Low Impact Development Stormwater Control Cost Estimation Analysis" (RTI
International and Geosyntec Consultants, 2015). The RTI International and Geosyntec Consultants (2015)
report included a literature review to develop a cost estimation procedure based on the unit cost
information to create curves for varying complexities of LID control implementation. The resulting cost
estimates report a range in costs to demonstrate the potential variability with LID control
implementation to communicate uncertainties in cost estimates.
The literature review included collection of cost data through web-based searches to determine and
document sources, including peer-reviewed publications, literature that is widely cited by the
stormwater LID community, and online data sources. In addition, existing cost tools and current or
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• Simple: Design criteria are generally lower than current design practices and site conditions are
conducive for BMP installation; likely representative of privately constructed and maintained BMPs in
new development, on a suitable parcel of land, sited as part of an effective site design process.
• Typical: Design criteria are consistent with typical design practices (e.g., sizing for capture of 85%
storm event or similar) found in current design manuals, and site conditions represent "median"
conditions for new construction; likely representative of BMPs designed per public maintenance
standards (generally more stringent) and sited as part of an effective site design process in new
development or large redevelopment.
• Complex: Design criteria are stringent and site conditions are difficult or constrained; cost curves
represent higher end estimates for all line items to meet project difficulty, may overpredict costs for
many sites that do not face these difficulties or constraints. Small redevelopment projects and retrofit
projects may tend toward this end of the range.
One of the primary benefits of the cost curve approach to cost estimation is the relative ease of
programming when properly implemented. The approach selected for curve development simplifies cost
estimation conceptually by incorporating the complexities related to the analysis using unit costs and
other critical design variables into curves based simply on LID footprint. The curves themselves can be
reduced to regression equations by plotting trend lines and obtaining equations for the trend lines. Once
regression equations have been developed, it is relatively straightforward to program the equations.
Cost curves were developed for three design scenarios (simple, typical, and complex) for each LID
control by varying the quantities of unit costs and other cost items commensurate with the intricacy of
implementation, LID control design parameters, and site feasibility constraints. Table 18 shows the
regression equations that were developed for the cost estimation procedure using the cost curve
production framework.
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Figure 50 for an example of a cost curve for a rain garden. The cost curves are plotted with LID control
footprint surface areas in square feet (cistern as storage capacity in gallons) on the x-axis and total
capital cost on the y-axis.
A brief summary of the steps taken to program and implement the cost estimation steps into the
calculator is provided below:
1. Define calculator user input limits and allowable LID control size variable limits.
2. Define and select design variables for LID controls, including calculator defaults for each
variable, and eliminate variables that do not significantly affect cost estimates.
3. Define and select simple, typical, and complex values for remaining variables that are influential
for costs.
4. Line item costs developed for variables that significantly affect magnitude of costs.
5. Use of an automated Excel spreadsheet to repeatedly size and estimate costs for all LID controls
under all three design scenarios (simple, typical, and complex) to produce regression cost curves
for each LID control.
The cost estimation procedure programmed into the calculator is based on the use of the regression
cost curves approach described above to produce both capital and annual maintenance costs. To
account for inflation and regional variability in costs, data from BLS have been used to compute regional
cost multipliers for BLS regional centers around the country (U.S. Department of Labor, BLS, 2017).
Many cost estimation techniques employ nationwide, disaggregated data to provide more robust,
tailored regional estimates. Several data sources such as Engineering News Record (ENR) and RS Means
(The Gordian Group) provide the ability to develop regionalized costs (e.g., for select cities). The
selected approach provides reasonable approximations to express national cost values in regional terms
using readily available BLS data. The BLS data set can be obtained online at monthly and annual
intervals, with calculated indices providing annual cost adjustments as well. Due to online accessibility,
the calculator can dynamically obtain BLS data in real-time during calculator program executions, as is
currently done with soil, precipitation, and evapotranspiration data. The end-product of this effort is a
regional cost multiplier that is applied to the calculator cost estimate to provide more current, tailored,
regionally representative cost.
All available data have been analyzed from all of the BLS regional centers where BLS Consumer Price
Index (CPI) data are available. BLS regional centers or areas are broken into four major regions, including
the Northeast, Midwest, South, and West. BLS publishes CPI data for 23 local regions, of which 17 local
regions have been programmed into the calculator based on availability of long term data (> 20 years)
pertinent to typical consumer expenditures on LID controls. More information on CPI and the regional
centers for which CPI data are maintained is available here.
BLS Producer Price Index (PPI) data categories/variables were assessed for costs that are most likely to
be included in LID controls construction. PPI variables are the outputs of industries such as service,
construction, utilities, and other goods-producing entities, and are only available on a national scale.
Documentation of data collection and quality assurance and quality control procedures for these data
are available from the BLS website at http://www.bls.gov/bls/qualitv.htm. Relevant PPI data include
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items/categories such as concrete storm sewer pipe, asphalt paving mixture, engineering services, and
construction sand and gravel.
When a user specifies their location in the calculator, the calculator computes a regional cost
adjustment factor for the three closest BLS regions. If all three BLS regions are more than 100 miles from
the users' location a National multiplier of 1 is selected as the default. On the LID controls tab the user
has the option of overriding the default selections and either choosing one the three nearest BLS centers
or specifying their own multiplier by choosing Other. The regionalized cost model shown in equation 9,
documents how BLS data for each BLS center is used to calculate a cost index value. Table 20 shows the
regional and national coefficient values for the shovel loader and fuels and utilities BLS data series. A
regional multiplier for each BLS center is calculated by dividing the cost index value of each BLS center
by the national index. A regional multiplier greater than one indicates that regional cost index for that
city is higher than the national average. A regional multiplier less than one indicates that the cost index
in that location is lower than the national average. The BLS centers used in the calculator are shown in
Table 21. The calculator directly accesses the BLS data using the BLS API (application program interface).
Using the BLS Regional Center and the model year from, the calculator queries the BLS API and retrieves
the values for the variables in the regionalization model as shown Table 22. More information about the
BLS API is available at http://www.bls.gov/developers/api_signature_v2.htm.
The final regionalized cost model is shown in equation 9.
Cost Indexyear n = —19.4 + (0.113 * Ready mix concreteyearn) + (0.325 *
Tractor shovel loaderyear n) +
(0.097 * Energyyearn) + (0.398 * Fuels and utilitiesyearn )
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using CPI and PPI variables for the current year. The inflation factor is calculated by dividing the current
National Index by the 2014 National Index.
In order to validate the model, data for five regional case studies (Dillwyn, VA, Chesterland, OH, Mission,
KS, and two in Portland, OR) were used to compare actual costs with the predicted SWC costs adjusted
by applying the regional cost multiplier. Three of the five cost estimates were within the range
estimated by the calculator. Of the two that were not well-predicted, one was under-predicted by 38%
(Mission, KS), and one was over-predicted by 37% (Portland, OR). There are potentially many causes for
these differences. This analysis did not complete a detailed design assessment to determine what may
have caused these differences for these locations. Although there are many factors that influence the
cost of actual projects, such as those that were highlighted in RTI International and Geosyntec
Consultants (2015), it is expected that the calculator's cost model with regional BLS-based cost indices
will provide a reasonable range of cost estimates for stormwater construction and operation and
maintenance costs.
The intent of these cost data and estimation procedure programmed in the calculator, is to produce
general estimates for relative comparisons of LID control alternatives. It is expected that in most cases,
planning-level estimates are sufficient for users of the calculator to evaluate LID control alternatives
based on relative cost differences of various LID controls as estimated using this procedure.
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CO
o
O
"ro
o
Area
Ae+
\G° ^
v^°
4^cie
¦ t>s
Complex design range
Typical design range
Simple design range
Figure 49. Conceptual overview of cost estimate ranges derived from cost curves.
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10,000,000
Simple RG(SF)
Simple(upper range) RG (SF)
Typ RG (SF)
Typ(upper range) RG (SF)
Complex RG (SF)
CompIex(upper range) RG (SF)
- LIT Complex RG (SF)
LIT Simple RG (SF)
rsj
10,000
1,000
100
10 100 1,000 10,000 100,000
Surface Area (square feet)
Figure 50. Sample regression cost curve for Rain Gardens.
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Post-Processing
For the long-term continuous simulation of rainfall / runoff, the calculator runs its site model through
SWMM using a 5-minute computational time step over each year of the period of record selected by the
user, and requests that SWMM use a 15-minute reporting interval for its results. SWMM writes the
rainfall intensity and the runoff results it computes at this reporting interval to a binary output file. The
calculator then reads this output file and aggregates rainfall and runoff into daily totals, expressed as
inches, for each day of the simulation period. It also keeps track of how many previous days occur with
no measurable rainfall, for each day with measurable rainfall. Measurable rainfall and runoff is taken as
any daily amount above the user-supplied threshold (whose default is 0.1 inches). For days that have
runoff but no rainfall, the runoff is added to that of the previous day. After the aggregation process is
complete, the long-term simulation results have been distilled down into a set of records equal in
number to the number of days with measurable rainfall; where each record contains a daily rainfall,
daily runoff, and number of antecedent dry days.
For extreme 24-hour storm events, SWMM makes a separate run for each event over a three-day time
period to allow for LID storage to drain down. Each run has different values in its time series of rainfall
intensities reflecting the different total depth associated with each extreme event return period. For
these runs the only output recorded is the total runoff from the site.
The Summary Results report produced by the calculator (Figure 13) comes from a direct inspection of
the long-term daily rainfall/runoff record. The Maximum Retention Volume statistic is simply the largest
difference between daily rainfall and its corresponding runoff among all records.
The Rainfall / Runoff Event scatterplot (Figure 14) is generated by plotting daily each daily rainfall and
its associated runoff for those days where rainfall exceeds the user-supplied threshold limit. For wet
days where the runoff is below the threshold value, the runoff value is set to zero (i.e., there is no
measurable runoff for those days).
The Rainfall / Runoff Frequency report (Figure 15) is generated by first sorting daily rainfall values by
size, ignoring consecutive rainfall days if the user selected that option. The days per year for which each
rainfall value is exceeded, is computed as (N -j) / Y, where N is the total number of rainfall values, j is
the rank order of the rainfall in the sorted list, and Y is the total years simulated. Then each rainfall -
exceedance frequency pair is plotted. The same set of operations is used to generate the runoff
exceedance frequency curve, except now N is the total number of runoff values and j is the rank order of
a runoff value in the sorted list.
The Runoff by Rainfall Percentile report (Figure 17) is generated as follows:
1. The daily measurable rainfall values are sorted by size and a set of different percentile values
are identified (the 10, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, and 99-th percentiles).
2. The days with rainfall that fall within each percentile interval are identified, honoring the user's
choice to either include or exclude consecutive wet days.
3. The total runoff from events in each interval, as a percentage of the total runoff from all events,
is computed and plotted.
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The Rainfall Retention Frequency report (Figure 21) is generated by taking the same set of rainfall
percentiles used in the Runoff by Rainfall Percentile report, only referring to them as retention
volumes. For each retention volume, the percentage of daily rainfall events providing that amount of
retention is computed. This is done by examining each day with observable rainfall, ignoring back to
back wet days if that option was selected. If there was no measurable runoff for the day, then the count
of retained events for the retention volume being analyzed is incremented. Otherwise, if the rainfall was
at least as much as the target retention and the difference between rainfall and runoff was also at least
this much, then the count of retained events is also incremented. The retention provided for the given
retention target is simply the number of retained events divided by the total number of daily events.
This process is repeated for each of the 13 pre-selected retention volumes and the resulting pairs of
retention volume - retention frequency values are plotted.
The Extreme Event Rainfall / Runoff report (Figure 23) is generated by simply plotting the rainfall and
accompanying computed runoff in stacked fashion for each extreme event return period.
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