Estimating Change in Impervious Area (IA) and Directly Connected
Impervious Areas (DCIA) for New Hampshire Small MS4 Permit
Small MS4 Permit Technical Support Document, April 2011
Draft NPDES Permit Focuses on DCIA
The 2010 NPDES Small MS4 permits for New
Hampshire require regulated communities to estimate the
number of acres of impervious area (IA) and directly
connected impervious area (DCIA) that have been
added or removed each year due to development,
redevelopment, and or retrofitting activities (Draft Permit
Section 2.3.6.8 (c)). Beginning with the second year
annual report, IA and DCIA estimates must be provided
for each subbasin within your regulated MS4 area. This
technical support tool outlines accepted methods for
estimating and reporting IA and DCIA in three steps:
Accepted Methods for Estimating IA & DCIA
Stepl
Establish
Baseline
Use EPA
estimates
or refine
with local
data
Step 2.
Calculate
Annual
Change
Add/remove
lA/DCIAfor
new projects
completed in
reportingyear
Summarize
in annual
NPDES
report by
Subbasin
Step 1.
Establish
Baseline
Use the estimates of existing IA and DCIA
provided by EPA to establish the baseline
acreage from which future additions or
reductions of impervious cover can be
tracked and measured.
For each regulated municipality in New Hampshire, EPA
will provide graphical and tabular estimates of IA/DCIA
ordered by land use type and subbasin. Permittees may
simply use these baseline estimates as is, or develop
more accurate estimates when justified. This may
include using local data to refine EPA's estimates or the
direct measure of IA (Figure 1). If the EPA estimates are
not used for the baseline, permittees must provide in the
annual report a description of the alternative methodology
used.
What does DCIA really mean?
Impervious surfaces such as roadways, parking lots,
rooftops, sidewalks, driveways, and other pavements
impede stormwater infiltration and generate surface
runoff. Research has shown that total watershed IA is
correlated with a number of negative impacts on our
water resources such as increased flood peaks and
frequency, increased sediment, nutrient, and other
pollutant levels, channel erosion, impairments to aquatic
biota, and reduced recharge to groundwater (Center for
Watershed Protection, 2003). Typically watersheds with
4-6% IA start to show these impacts, though recent work
has found lower % IA threshold values for sensitive
species (Wenger et al., 2008). Watersheds exceeding
12% IA often fail to meet aquatic life criteria and
narrative standards (Stanfield and Kilgore, 2006).
For the purposes of the MS4 permit, DCIA is considered
the portion of IA with a direct hydraulic connection to the
permittee's MS4 or a waterbody via continuous paved
surfaces, gutters, drain pipes, or other conventional
conveyance and detention structures that do not reduce
runoff volume. DCIA does not include:
• IA draining to stormwater practices designed to meet
recharge and other volume reduction criteria.
• Isolated IA with an indirect hydraulic connection to
the MS4, or that otherwise drain to a pervious area.
• Swimming pools or man-made impoundments, unless
drained to anMS4.
• The surface area of natural waterbodies (e.g.,
wetlands, ponds, lakes, streams, rivers).
Figure 1. EPA will use statewide land use data (GRANIT),
subbasin boundaries, and land use impervious coefficients to
estimate baseline IA for each MS4 jurisdiction (upper).
Communities may choose to refine these estimates with direct
measure of IA where local GIS capacity is available, as shown
here from Somersworth, NH (lower).
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Why Quantify Your IA & DCIA?
New construction, redevelopment, and restoration
activities can change existing IA and DCIA - potentially
exacerbating or reducing existing watershed impairments.
Understanding watershed imperviousness is important for
communities because it:
• Informs management of impaired waterbodies and
prioritization of watershed restoration efforts;
• Facilitates investigation of existing chronic flooding
and stormwater drainage problems, and avoidance of
new problems;
• Indicates potential threats to drinking water
reservoirs/aquifers; commercial fisheries, and
recreational waters;
• Demonstrates progress toward achieving future Total
Maximum Daily Load (TMDL) allocations based
on impervious cover thresholds;
• Serves as an educational tool for encouraging
environmentally sensitive land use planning and Low
Impact Development (LID);
• Facilitates equitable derivation of possible
stormwater utility fees based on parcel-specific
impervious cover; and
• Provides guidance for stormwater retrofit efforts.
Table 2. Sutherland Equations to Determine DCIA [%]
Based on the established IA, DCIA can be estimated
using empirical formulas developed by Sutherland as a
function of watershed type (CWP, 2000). Table 1
provides approved IA coefficients to be used for this
approach. These coefficients were derived from previous
studies and used by EPA to establish baseline conditions
for regulated New Hampshire communities using
Equations 1 and 2.
Eq. 1 IALu,= Total acresLu, * %IA
Eq. 2 Total Subbasin IA=E; = : IALu,
Table 1. Estimating DCIA as a function of Land Use1
Land Use
Commercial
Industrial
High density residential
Med. density residential
Low density residential
Institutional
Agricultural
Forest
Open Urban Land
%IA
76
56
51
38
19
342
2
1.9
11
1 IA coefficients taken from Rouge River Study/EPA
Institutional land use coefficient from Cappiella and Brown, 2001
Table 2 summarizes the appropriate Sutherland equations
to apply for estimating DCIA from IA for average, highly
connected, totally connected, somewhat connected, and
mostly disconnected watersheds. Permittees may opt to
refine DCIA estimates to better reflect actual basin
conditions where justified.
Watershed Selection
Criteria
Average: Mostly storm
sewered with curb &
gutter, no dry wells or
infiltration, residential
rooftops not directly
connected
Highly connected: Same
as above, but residential
rooftops are connected
Totally connected: 100%
storm sewered with all IA
connected
Somewhat connected:
50% not storm sewered,
but open section roads,
grassy swales, residential
rooftops not connected,
some infiltration
Mostly disconnected:
Small percentage of urban
area is storm sewered, or
70% or more
infiltrate/disconnected
Assumed
Land Use
Commercial,
Industrial,
Institutional/
Urban public,
Open land, and
Med. density
residential
High density
residential
--
Low density
residential
Agricultural;
Forested
Equation
(where IA(%) >1)
DCIA=0.1(IA)15
DCIA=0.4(IA)1 2
DCIA=IA
DCIA=0.04(IA)1 7
DCIA=0.01(IA)2
Step 2.
Calculate
Annual
Change
Once baseline IA/DCIA is established for
each subbasin, permittees must annually
track the change in IA and DCIA acreage
from development, redevelopment, and
retrofit projects completed that year.
To account for the estimated annual change in DCIA,
permittees will need to determine how much IA and
DCIA have been added or removed as a result of
individual development, redevelopment, or retrofit
projects completed during the reporting period.
The acres of DCIA for each project will be based on two
factors: (1) the amount of site IA, and (2) the
effectiveness of stormwater best management
practices (BMPs) employed to reduce associated runoff.
Practices that reduce runoff volume will lower DCIA.
Note that practices that remove stormwater pollutants but
do not provide runoff reduction benefits are not
considered effective at reducing DCIA.
This information must be obtained from site plans and
verified by as-built drawings or site inspection upon
project completion. For all completed projects:
(1) Determine the former and new IA for each site.
(2) Determine the number and type of existing and/or
new BMP(s) used, and calculate the amount of IA
removed, managed, and umnanaged draining to each
BMP.
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(3) For each BMP designed in accordance with
specifications provided in New Hampshire
Stormwater Manual Storm water Handbook (Vol. 2,
Ch. 4), select the appropriate "disconnection"
multiplier from Table 3. For infiltration trenches or
basins, determine appropriate runoff volume
reduction using Tables 4 and 5 depending on site-
specific soil infiltration rates and runoff depth
captured as derived from the EPA 2010 BMP
Performance Curves. Use Equation 3 to generate the
"disconnection" multiplier.
Eq. 3 Multiplier = 1 - % Runoff Reduction Volume/100
(4) Calculate DCIA for each BMP using Equation 4 if
adding newly created IA at new construction or
redevelopment site, OR by using Equation 5 if
reducing existing IA in a retrofit or redevelopment
scenario.
Eq. 4 Added DCIABMp,= IABMp, * BMP Multiplier
Eq. 5 Reduced DCIABMP,= IABMP, * (1 - BMP Multiplier)
(5) Calculate DCIA for entire project site draining to
BMPs by summing DCIA for individual BMPs using
Equation 6.
Eq. 6 SiteDCIAadded= D = : DCIABMP, + New Unmanaged IA
Table 3. Determining DCIA based on Interim Default BMP
Disconnection Multiplier or EPA's Infiltration Curves
BMP Description
Removal of pavement;
restore infiltration capacity
Redirection of rooftop
runoff to infiltration areas,
rain gardens or dry wells
Permeable pavement,
bioretention, dry/vegetated
water quality swales
Infiltration trenches
Infiltration basins
Non-runoff reduction
practices (i.e., detention
ponds, wetlands, sand
filters, hydrodynamic
separators, etc)
% Runoff
Volume
Reduction1
100%
85%
75%
15-100%
13-100%
0%
BMP
"Disconnection"
Multiplier2
0
0.15
0.25
0.85-0
0.87-0
1.0
1 Interim default values for % runoff reduction are based on
Schueler 2009 and are subject to change as more data
becomes available. Values for infiltration trenches and
basins are based on soil infiltration rates and depth of runoff
treated. See Tables 3 and 4 to determine the site specific
values to apply.
2 BMP multiplier = 1 - %Runoff Volume Reduction/100
Step 3.
Report Net
Change in IA
&DCIA
Starting in year 2, permittees must include
a summary of net changes in IA/DCIA by
subbasin and document methodology in
its annual report.
Permittees will be required to summarize IA and DCIA
estimates for all completed construction, redevelopment,
and retrofit projects within each subbasin. EPA will
provide a tracking spreadsheet to assist in the
calculation and tracking of this information. For
individual BMPs at each site, permitees will need to track
the type of practice, the IA captured, and the % runoff
reduction and "disconnection" multiplier assigned to that
practice. Consider incorporating these DCIA accounting
elements into your program's existing BMP tracking
database.
Table 4. Infiltration Trench: Percent Runoff Reduction
based on EPA's Infiltration Curves
Depth of
Runoff
Treated
(inches)
0.1
0.2
0.4
0.6
0.8
1.0
1.5
2.0
Soil Infiltration Rate (in/hr)
0.17
15%
28%
49%
64%
75%
82%
92%
95%
0.27
18%
32%
55%
70%
79%
85%
93%
96%
0.52
22%
38%
62%
76%
84%
89%
95%
97%
1.02
26%
45%
68%
81%
88%
92%
97%
98%
2.41
34%
55%
78%
88%
93%
96%
99%
100%
8.27
54%
76%
93%
97%
99%
100%
100%
100%
TableS. Infiltration
on EPA's Infiltration
Basin: Percent Runoff Reduction based
Curves
Depth of
Runoff
Treated
(inches)
0.1
0.2
0.4
0.6
0.8
1.0
1.5
2.0
Soil Infiltration Rate (in/hr)
0.17
13%
25%
44%
59%
71%
78%
89%
94%
0.27
16%
30%
51%
66%
76%
82%
91%
95%
0.52
20%
36%
58%
73%
81%
87%
94%
97%
1.02
24%
42%
66%
79%
87%
91%
96%
98%
2.41
33%
54%
78%
88%
93%
96%
99%
100%
8.27
55%
77%
93%
98%
99%
100%
100%
100%
Are We Required to Follow This Protocol?
Permittees are encouraged to refine IA and DCIA
baseline estimates where local data is more accurate;
however the general methodology for calculating annual
change in IA and DCIA should be applied. Deviations
from the methodology are subject to review by EPA and
must be described in the annual report.
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Example Subbasin DCIA Calculations
Baseline conditions for subbasin #54203 were
estimated to include 100 acres IA and 50 acres DCIA.
By the second year of NPDES reporting, two
construction projects were completed that resulted in an
overall change in the amount of subbasin IA and DCIA
as follows:
Project 1: New 5-acre residential townhome complex with 4
acres of new I A, of which, 0.9 acres drain to a bioretention
facility, 3 acres drain to an infiltration basin, and 0.1 acres
drain untreated to the main road. The infiltration basin is
designed based on a soil infiltration rate of 0.52 in/hr and 0.8
inches of runoff captured.
Step 1. Establish new IA to add to baseline = 4.0 ac
Steps 2 -4. Determine DCIA per BMP
Eq. 3 Multiplierlnf basm =1-81/1 00 = 0.19
Eq. 4 DCIAbloretentlon = 0.9 ac * 0.25 = 0.23 ac
DCIAmf basm = 3.0 ac * 0.19 = 0.57 ac
Step 5. Sum DCIA for entire site
Eq. 6 Total Project DCIA= 0.23 ac + 0.57 ac + 0. 1 ac^,,^
= 0.9 ac DCIA to add to baseline
Project 2: Redevelopment of an 8-acre retail outlet with 5.5
acres of existing IA. After redevelopment, there are now 6.0
acres total IA. 3.0 acres of IA continues to drain to an
existing detention pond, but 1.0 acre of overflow parking was
converted to pervious pavement. A new bioretention retrofit
now captures 0.7 acres of IA that used to drain to the pond, as
well as 0.5 acres of newly added I A. The remaining 0.8 acre
of site IA remains untreated.
Step 1. Establish new IA to add to baseline = 6.0 ac - 5.5 ac
= 0.5 ac
Steps 2 -4. Determine DCIA per BMP to be added or
subtracted from baseline.
= 0.5 ac * 0.25 = 0.13 ac
Eq. 4 Added DCIAbloretentlon.new IA
Eq. 5 Reduced DCIAporous pavement = 1 ac *(1 -0.25) = 0.75 ac
Reduced DCIA^^ = 3.0 ac *(!-! .0) = 0 ac
Reduced DCIAblo.exlstmgIA = 0.7 ac *(1-0.25) = 0.53 ac
Step 5. Sum DCIA for entire site.
Eq. 6 Total Project Added DCIA = 0.13 ac + 0 acnewunmamgedIA
= 0.13 ac DCIA to add to baseline
Eq. 6 Total Reduced DCIA = 0.75 ac + 0 ac+0.53 ac
= 1.28 ac DCIA to subtract from baseline
End of Year Report: Totals for Subbasin #54203:
IA =100 acbaselme + 4.0 acproject { + 0.5 acproject 2
= 104.5 ac (net gain of4.Sac)
= 50 acbaseime +0.9 acproject l + 0.
= 49.75 ac DCIA (net reduction of 0.25 ac)
DCIA = 50 acbaseime +0.9 acproject l + 0.13 ac project 2 - 1.28 acproject 2
Checklist of What to Expect EPA to Provide
EPA will provide all regulated MS4 communities in New
Hampshire with the following information:
• Delineation of subbasin boundaries.
• Baseline estimates of IA and DCIA for each subbasin in
your regulated area in tabular format.
• DCIA calculation and tracking spreadsheet.
How Does LID Influence IA and DCIA?
Incorporating LID techniques into site design can reduce
IA & DCIA, protect natural areas, and minimize
alterations to existing hydrology on site. The use of
BMPs that maximize runoff reduction benefits (e.g.,
practices with low BMP Multipliers in Table 2 and those
shown in Figure 2) can result in a higher "disconnection"
factor than if using traditional detention ponds. Your
community can help reduce total IA and DCIA by:
• Adopting LID design requirements for new development
projects.
• Requiring documentation of design methods used to
minimize site IA and to disconnect IA.
• Requiring site designers to calculate and submit %IA
and %DCIA for each site.
• Retrofitting existing, unmanaged impervious areas.
Figure 2. BMPs such as the bioretention, porous pavers, and
infiltration trenches seen here are designed to provide water
quality treatment and maximize runoff reduction through
improved infiltration, evapotranspiration, and plant uptake.
These are effective practices for reducing DCIA.
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What are the Costs of Annual DCIA Tracking?
The cost will vary depending on the size of the regulated
area, amount of existing IA, sophistication of existing
GIS, number of new projects requiring tracking, and the
level of effort required to obtain information for each site.
Refining the EPA-provided baseline estimates of IA and
DCIA may require collecting new data, purchasing new
software/GIS, and additional staff time. This effort may
not be worth the cost if the annual net change in IA and
DCIA is the true measure of interest. Factors that will
add to overall effort may include:
• Refining EPA's baseline estimates, particularly if local
IA mapping doesn't already exist.
• Over-complicating the analysis by refining given
equations.
• Not easily obtaining required IA and BMP information
from proposed site plans. Determine the most efficient
method to obtain this information as soon as possible -
changing applicant reporting requirements may be a
solution.
• Verifying as-built conditions with individual site visits.
Consider alternatives (e.g., occupancy certifications).
• Maintaining an updated impervious and stormwater
infrastructure layer in GIS, particularly if new projects
have to be hand-digitized. Possibly require applicants to
submit plans electronically.
• Not integrating effort with other existing programs (i.e.,
plan review, building inspection, or stormwater utility).
Stanfield and Kilgour, 2006. Effects of Percent Impervious
Cover on Fish and Benthos Assemblages and Instream
Habitats in Lake Ontario Tributaries. American Fisheries
Society Symposium 48: 577-599.
Sutherland. 2000. Methods for Estimating Effective
Impervious Cover. Article 32 in The Practice of Watershed
Protection, Center for Watershed Protection, Ellicott City,
MD.
Wenger, S. et al., 2008. Stream fish occurrence in response to
impervious cover, historic land use, and hydrogeomorphic
factors. CanJ. Fish Aquatic Sci. 65 1250-1264.
Where Can I go for More Information?
For more information regarding the new permit
requirements, go to the New Hampshire Small MS4
webpage at:
www.epa.gov/ne/npdes/stormwater/MS4 2008 NH.html
Here you will find links to relevant permit documents;
community-specific mapping and statistics for baseline
IA and DCIA estimates; detailed descriptions of methods
used to calculate IA and DCIA estimates; and the
calculation and tracking spreadsheet template.
References
Cappiella K. and K. Brown. 2001. Impervious Cover and Land
Use in the Chesapeake Bay Watershed.
Center for Watershed Protection. 2003. The Impacts of
Impervious Cover on Aquatic Systems. Watershed
Protection Research Monograph No. 1. Ellicott City, MD.
www.cwp.org/Resource Library/Center Docs/IC/Impacts
1C AcLSvstems.pdf
EPA, 2010. Stormwater BMP Performance Analysis.
www.epa.gov/regionl/npdes/stormwater/assets/pdfs/BMP-
Perfbrmance-Analvsis-Report.pdf
Schueler, T. 2009. Guidance for meeting NPDES Permit
Requirement in Montgomery County, MD
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