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). ------- 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 ------- 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) ------- 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 ------- 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. ------- |