i, Estimating Change in Impervious Area (IA) and Directly Connected
Impervious Areas (DCIA) for Massachusetts Small MS4 Permit
Small MS4 Permit Technical Support Document, April 2011
Draft NPDES Permit Focuses on DCIA
The 2010 NPDES Small MS4 draft permits for
Massachusetts 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 North
Coastal Permit Section 2.4.6.9). 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
Establish
Baseline
IA/DCIA
Use EPA
estimates
orrefine
with local
data
Calculate
Annual
Change
Add/remoue
[A/DCIAfor
new projects
completed in
reportingyear
Summarize
in annual
NPDES
report by
Subbasin
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).
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 Massachusetts, 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.
Figure 1. EPA will use IA extrapolated from 2005, 1-meter
orthoimagery provided by MassGIS (upper). A comparison of
a MassGIS-derived IA estimate (shown in purple) vs. a refined
direct measurement (shown in green) by the Town of Reading,
MA illustrates differences in precision (lower).
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Once IA has been established, DCIA can be estimated
using empirical formulas developed by Sutherland as a
function of IA for various watershed types (CWP, 2000).
Table 1 summarizes appropriate equations to apply for
average, highly connected, totally connected, somewhat
connected, and mostly disconnected watersheds. EPA
will provide each municipality with DCIA estimates
based on land use and assumed watershed conditions;
however, permittees may opt to refine these estimates
to better reflect actual basin conditions where
justified.
Table 1. Sutherland Equations to Determine DCIA [%]
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.
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,
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
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 directing stormwater retrofit efforts.
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 unmanaged draining to each
BMP.
(3) For each BMP designed in accordance with
specifications provided in MassDEP's Stormwater
Handbook (v.2, chp.2), select the appropriate
"disconnection" multiplier from Table 2.
For infiltration trenches or basins, determine
appropriate runoff volume reduction using Table 3
depending on site-specific soil infiltration rates and
runoff depth captured as derived from the EPA 2010
BMP Performance Curves. Use Equation 1 to
generate the BMP "disconnection" multiplier.
Eq. 1 Multiplier = 1 - % Runoff Reduction Volume/100
(4) Calculate DCIA for each BMP using Equation 2 if
adding newly created IA at new construction or
redevelopment site, OR by using Equation 3 if
reducing existing IA in a retrofit or redevelopment
scenario.
Eq. 2 Added DCIABMp,= IABMp, * BMP Multiplier
Eq. 3 Reduced DCIABMp,= IABMp, * (1 - BMP Multiplier)
(5) Calculate DCIA for entire project site draining to
BMPs by summing DCIA for individual BMPs
using Equation 4.
Eq. 4 SiteDCIAadded= D = : DCIABMP, + New Unmanaged IA
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Table 2. Determining DCIA based on Interim Default BMP
Disconnection Multipliers or EPA's Infiltration Curves
BMP Description
Removal of pavement;
restoration of infiltration
capacity
Redirection of rooftop
runoff to infiltration areas,
rain gardens or dry wells
Permeable pavement,
bioretention practices,
dry/vegetated water quality
swales
Disconnection to qualified
pervious area3
Infiltration trenches
Infiltration basins
Non-runoff reduction
practices (i.e., detention
ponds, wetlands, sand
filters, hydrodynamic
separators, etc)
% Runoff
Volume
Reduction1
100%
85%
75%
50%
15-100%
13-100%
0%
BMP
Disconnection
Multiplier2
0
0.15
0.25
0.50
0.85-0
0.87-0
1.0
1 Interim default values for % runoff reduction 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
3 Areas given MassDEP LID Site Design Credits per the MA
Stormwater Standards (Vol. 3, Chapter 1) are assigned % reduction
values based on upper estimates of rooftop disconnection to pervious
area as reported by Chesapeake Stormwater Network (2009).
Table 3. Percent Runoff Reduction based on EPA's
Infiltration Curves
2010
Depth of
Runoff
Treated
(inches)
Soil Infiltration Rate (in/hr)
0.17
0.27
0.52
1.02
2.41
8.27
Infiltration Trench
0.1
0.2
0.4
0.6
0.8
1.0
1.5
2.0
15%
28%
49%
64%
75%
82%
92%
95%
18%
32%
55%
70%
79%
85%
93%
96%
22%
38%
62%
76%
84%
89%
95%
97%
26%
45%
68%
81%
88%
92%
970/Ea
98%^
34%
55%
78%
88%
93%
96%
99%
' 100%
54%
76%
93%
97%
99%
100%
100%
100%
Infiltration Basin
0.1
0.2
0.4
0.6
0.8
1.0
1.5
2.0
13%
25%
44%
59%
71%
78%
89%
94%
16%
30%
51%
66%
76%
82%
91%
95%
20%
36%
58%
73%
81%
87%
94%
97%
24%
42%
66%
79%
87%
91%
96%
98%
33%
54%
78%
88%
93%
96%
99%
100%
55%
77%
93%
98%
99%
100%
100%
100%
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 =1-81/100 = 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^^ed
= 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.
Eq. 4 Added DCIAblor,
etention-new IA
= 0.5ac*0.25 = 0.13ac
*(
Eq. 5 Reduced DCIAporouspavement = 1 ac "(1-0.25) = 0.75 ac
Reduced DCIA^^d = 3.0 ac *(!-!.0) = 0 ac
Reduced DCIAblo.exlstmg IA = 0.7 ac *(1-0.25) = 0.53 ac
Step 5. Sum DCIA for entire site.
Eq. 6 Total Proj ect Added DCIA = 0.13 ac + 0 acnew managed IA
= 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 (netgain of 4.5 ac)
DCIA = 50 acbaseime +0.9 acproject l + 0.13 ac pr0ject2- 1-28 acproject2
= 49.75 ac DCIA (net reduction of 0.25 ac)
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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 each community 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.
Checklist of What to Expect EPA to Provide
EPA will provide all regulated MS4 communities in
Massachusetts with the following information:
• Delineation of subbasin boundaries.
• Baseline estimates of IA for each subbasin in your
regulated area in tabular and GIS formats (i.e., as an
impervious area map layer).
• Baseline estimate of DCIA for each subbasin (tabular
format).
• DCIA calculation and tracking spreadsheet.
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 adding 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).
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 when using traditional
detention ponds. Your community can help reduce total
IA and DCIA by:
• Adopting LID design requirements for new
development projects.
• Providing for LID Site Design Credits per MassDEP' s
Stormwater Management Standards.
* 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.
Are we Required to Follow This Protocol?
Permittees are encouraged to refine DCIA baseline
estimates where local data is more accurate; however the
general methodology for calculating annual change in
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DCIA should be applied. Deviations from the
methodology are subject to review by EPA and must be
described in the annual report.
Where Can I go for More Information?
For more information regarding the new permit
requirements for Massachusetts and for the North
Coastal Small MS4s specifically, go to
www.epa.gov/ne/npdes/stormwater/index.html and
www.epa.gov/ne/npdes/stormwater/draft mane sms4gp.html.
respectively. 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
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
Chesapeake Stormwater Network. 2009. CSN Technical
Bulletin No. 4: Technical Support for the Bay-wide
Runoff Reduction Method Version 2.0.
www.chesapeakestormwater.net/documents/research-
files/CSN20TB20No.2042020BaYwide20Runoff20Reduc
tion20Methodl.pdf
EPA, 2010. Stormwater BMP Performance Analysis.
www.epa.gov/regionl/npdes/stormwater/assets/pdfs/BMP
-Performance-Analvsis-Report.pdf
Schueler, T. 2009. Guidance for meeting NPDES Permit
Requirement in Montgomery County, MD
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. Can. J. Fish Aquatic Sci. 65 1250-1264.
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