The Stormwater Program manages activities that potentially result in polluted runoff from urban areas discharging into the New York City (NYC) water supply in both the east-of-Hudson (EOH) and west-of-Hudson (WOH) watersheds. The pollutants of concern in stormwater primarily consist of fine sediment (turbidity), phosphorus and nitrogen (nutrients), and microbial pathogens. Other pollutants in stormwater runoff include oil and grease, floatables, dissolved metals, and toxic organics (Debo and Reese, 2002; Smullen et al., 1999).
Implementation of the Stormwater Program of the Watershed Protection Program predates issuance of the 1997 filtration avoidance determination (FAD) because federal, state, and New York City Department of Environmental Protection (NYC DEP) stormwater regulations were promulgated in the early 1990s to control pollutants discharging from municipal drainage systems. These regulations require treatment of stormwater runoff from new urban development as well as sediment-laden runoff from disturbed land at construction sites. The Stormwater Program is a mature program consisting of (1) the mandatory federal/state stormwater regulations for new development, and (2) the Stormwater Retrofit Program specifically targeting NYC’s water supply watershed as obligated by the FAD.
Stormwater treatment is accomplished through the installation of stormwater control measures (SCMs) such as detention, retention, and bioretention ponds and grassy swales, as well as more recent green infrastructure, which incorporates SCMs into landscaped urban developments including rain gardens and green roofs (NRC, 2009; NYC DEP, 2010a). During construction activities, erosion prevention and sediment control (EPSC) devices such as silt fences, rock check dams, and hay are commonly used to treat construction site runoff. Acceptable SCMs and EPSC devices are found in standard design guidance documents provided by the New York State Department of Environmental Conservation (NYS DEC), such as the New York State Stormwater Management Design Manual (NYS DEC, 2015) and New York State Standards and Specifications for Erosion and Sediment Control (Blue Book, NYS DEC, 2016). In addition, NYC DEP has created a similar design document for standard practices, Guidelines for the Design and Construction of Stormwater Management Systems (NYC DEP, 2012). Estimated reductions in pollutants per SCM or EPSC device are based on published review data summarized in the National Pollutant Removal Database for Stormwater Treatment Practices (Winer, 2000, 2007) and the International Stormwater BMP Database (Clary et al., 2017).
Stormwater Programs for New Development
As discussed in Chapter 3, stormwater discharges are regulated by the 1987 Clean Water Act (CWA) and 1990 rules codified in 40 C.F.R. § 122.26. Under the CWA, the authority to issue state pollutant discharge elimination system (SPDES) permits that cover stormwater discharges is delegated to the NYS DEC.
Phase I stormwater regulations must be complied with by municipalities of over 100,000 people, of which there are none either EOH or WOH. Phase II (small) municipal separate storm sewer systems (MS4s) requiring SPDES permits include community areas with populations greater than 10,000 with a population density greater than 1,000 per square mile, and populations greater than 50,000. In May 2010, NYS DEC expanded the permitting requirements to include community areas (town lines) contiguous with existing designated small MS4s where sensitive waters receive stormwater runoff. For Phase II MS4s, the SPDES permits are “general” in that multiple dischargers with similar pollutant discharge types and operations are issued as a common single permit.
The two general SPDES permits issued by NYS DEC and relevant to protecting the NYC water supply are GP-0-15-003 and GP-0-15-002—permits for Phase II MS4s and construction site activities, respectively. As part of the SPDES GP-0-15-003 permit, MS4s must reduce pollutants in stormwater discharges to the maximum extent practicable through six minimum control measures. They are: (1) public education and outreach program, (2) public participation program, (3) illicit discharge detection and elimination program, (4) construction site stormwater runoff control program, (5) post-construction stormwater maintenance plan, and (6) pollution prevention and good housekeeping for municipal operations and facilities. There are 22 Phase II MS4 permittees located in the EOH watershed (Figure 9-1) and none in the WOH watershed. Most EOH MS4s are located in the Croton watershed, but some are located around the Kensico Reservoir: Harrison, Mount Pleasant, New Castle, and North Castle.
SPDES permits are also required for construction activities disturbing more than one acre, and permittees must develop an approved stormwater pollution prevention plan (SWPPP) prior to land disturbance.
Though municipalities in the WOH watershed do not have federal/state MS4 permits to contend with, stormwater runoff is regulated by NYC DEP Watershed Rules and Regulations (WR&R). In 1997, NYC DEP promulgated WR&R to regulate activities that could potentially contaminate the water supply and its sources, including activities associated with construction of impervious surfaces and stormwater runoff (NYC DEP, 2010b). These regulations, amended in 2010 to better support the 2007 FAD, prohibit construction of an impervious surface within 100 feet of a watercourse or wetland, or within 300 feet of a reservoir, reservoir stem, or controlled lake. An exception to this rule in the WOH watershed is in a village, hamlet, village extension, or area zoned for commercial or industrial uses, where construction of new impervious surfaces is allowed with approval of a SWPPP. The same rule applies in the EOH watershed but for designated main street areas or village centers. Additional site development criteria for SWPPPs are described in the WR&R. WR&R §18-39 also requires an Individual Residential Stormwater Permit (IRSP) for construction of new residences (including driveways) not located in a subdivision, and located within 100 feet of a perennial stream or wetland, or within 50 feet of an intermittent stream. A plan, similar to a SWPPP, is required and approved by NYC DEP. IRSP requirements are applicable in both the EOH and WOH watersheds.
The stormwater programs for new development are managed jointly by NYC DEP and the East-of-Hudson Watershed Corporation (EOHWC) with MS4s in the EOH watershed, and by NYC DEP and the Catskill Watershed Corporation (CWC) in the WOH watershed. In the EOH watershed, each MS4 funds its stormwater program to meet the federal and state regulations. In the WOH watershed and under the 1997 Memorandum of Agreement (MOA), NYC DEP established two future stormwater cost-sharing programs: (1) the WOH Future Stormwater Control Program funded by NYC DEP and administered by CWC, which reimburses 100 percent of eligible implementation costs for municipalities and large businesses and 50 percent of the costs for small businesses; and (2) a program administered by NYC DEP that reimburses 100 percent of eligible costs to low-income single-home owners and 50 percent to small businesses. Both programs pay for the incremental costs of SCMs for new construction incurred to comply with NYC DEP regulations, including the costs associated with SWPPP and IRSP development. Permit reviews are conducted by NYC DEP and CWC. A process is currently under way to merge the two programs into a single program administered by CWC. Since 1997, the CWC Future Stormwater Control Program has funded 87 new stormwater projects, while the NYC DEP-administered program has separately funded 70 additional projects in the WOH watershed (NYC DEP, 2016). The Future Stormwater Control Program is open to all applicants and there is no apparent project prioritization.
Stormwater Program Retrofits
Because many EOH and WOH urban areas were developed prior to promulgation of the stormwater regulations, improvements to stormwater drainage infrastructure, termed retrofits, have been needed to reduce pollutant loadings to meet FAD requirements. Retrofits have been constructed through three distinct NYC DEP-managed programs: (1) an EOH program through in-house and contracted work, (2) a grant program through a contract with the EOHWC, and (3) a WOH program through a contract with the CWC.
Early FADs required stormwater retrofit installation around Kensico Reservoir to be implemented by NYC DEP, which resulted in the completion of 47 projects including bank stabilization, drainage improvements, outlet stilling basins, detention basins, and sand filters (NYC DEP, 2010c, 2016). Project prioritization was based on initial reconnaissance, the Kensico Action Plan in 2007, and a modeling study for phosphorus reductions utilizing outputs from a combination of three water quality models (NYC DEP, 2009). The models were the Watershed Treatment Model (WTM; Caraco, 2002), the Source Loading and Management Model for Windows (WinSLAMM; Pitt and Voorhees, 2002), and ArcView Generalized Watershed Loading Function (AVGWLF; Evans et al., 2002). To estimate typical phosphorus loading, the modeling was performed utilizing either site-specific or watershed-wide analyses. For site-specific estimates, 100-acre developed areas were modeled utilizing WTM and WinSLAMM, accounting for site export of phosphorus from impervious surfaces, contributions from pervious areas such as turf grass, landscape areas subject to fertilizer application, and SCM treatment. WTM provided variable phosphorous outputs depending on the selected SCM. In watershed-wide
analyses, two major load sources, septic systems and urban runoff load, were evaluated utilizing AVGWLF. Load reductions were applied by assuming a percentage reduction in surface runoff load estimates.
To date, stormwater retrofits around the Kensico Reservoir are largely completed. Outside the drainage area around Kensico Reservoir, the EOHWC supports MS4 retrofit projects, prioritizing projects based on the highest ratio of estimated phosphorus reduction to project cost. Within the EOH watersheds in four FAD basins, 80 stormwater retrofits have been completed. Starting in 2010, the total phosphorus reduction goal set by the EOHWC is 459 kg/yr for every five-year period, and it has achieved a level of 440 kg/yr, slightly less than their goal (NYC DEP, 2016).
In the WOH watershed, the CWC administers the Stormwater Retrofit Program established as part of the 1997 MOA. This program funds projects in areas of concentrated impervious surfaces. Projects include expanding undersized drainage systems or parts of systems (i.e., SCMs and/or conveyance infrastructure), and installing new outlet structures to improve SCM performance, particularly to achieve greater reductions in fine sediment. The focus by NYC DEP and CWC has been on treating runoff from street drainage and highway maintenance activities. NYC DEP and CWC jointly review grant proposals from local jurisdictions, where project prioritization and selection are based on a set of scoring criteria (NYC DEP, 2019): (1) size of drainage area served, (2) the amount of pollutant removal (including total suspended solids, total phosphorus, biological oxygen demand, and fecal coliform) to be accomplished by the completed SCM, (3) the amount of existing impervious surfaces in the drainage area, (4) a cost/benefit analysis of the project, (5) the anticipated costs of annual operation and maintenance, (6) the life expectancy of SCMs to be installed, and (7) other considerations such as basin restrictions and availability of other funding. No modeling effort has been applied in the WOH watershed for prioritization of stormwater retrofit projects.
Between 1993 and 2019, NYC DEP provided approximately $25 million for EOH stormwater retrofits. Funds provided to EOHWC have now been fully expended. NYC DEP will pay an additional $22 million to the EOHWC to continue constructing stormwater retrofits in basins in the EOH watershed over the next five to six years, equating to approximately $3.5 million per year.
In the WOH watershed, NYC DEP expenditures for the Future Stormwater Control Program have totaled $36.4 million to date since the 1997 MOA (NYC DEP, 2019). During the same period, NYC DEP paid $30.2 million for the Stormwater Retrofit Program. Annual future funds for the Stormwater Program WOH will total $1.3 million ($0.5 million for reimbursements of new SCMs, $0.5 million for stormwater retrofit capital costs, and $0.3 million for stormwater retrofit operation and maintenance costs).
The Stormwater Program has the potential to accomplish many goals embedded in the MOA, including (1) reductions in urban runoff pollutant loading to receiving waters, (2) reduction in catchment runoff peak flows and volumes to prevent channel and streambank erosion, (3) improvements in water quality and ecosystem health within the reservoirs, with an emphasis on reduced fine sediment (turbidity) and total phosphorus loadings, (4) reductions in flood risk in low-lying areas of the watershed, and (5) enhancements of environmental aesthetics such as green space, which promotes community vitality. Yet, achievement of many of these goals is not reflected in the current Stormwater Program metrics.
Currently, the metrics of success of the Stormwater Program are based on program expenditures, the number of permits processed, and the number of SCMs installed. For new construction, program evaluation consists of reporting the number of SPDES general construction permits and IRSPs issued with approved SWPPPs, the number of new stormwater projects with successfully constructed SCMs and EPSC devices, and the annual funding expenditures by NYC DEP. In the EOH watershed, MS4 communities oversee construction of stormwater projects for new developments. EOHWC reports to NYC DEP on program expenditures, and MS4
communities report to the NYS DEC according to their SPDES permit requirements. For stormwater retrofit projects funded by NYC DEP in both the EOH and WOH watersheds, evaluation is enumerated by project numbers and annual program expenditures.
NYC DEP does not specifically conduct water quality, field-based performance monitoring or watershed modeling to assess the water quality benefits of the Stormwater Program. Regulations do not require event-based performance monitoring of constructed SCM and EPSC devices. Water quality data reported in FAD Annual Reports and Assessments (e.g., NYC DEP, 2016, 2018) only include analyses of samples collected from streams, lakes, and reservoirs; these data are not correlated with specific SCM performance and pollutant removals through specifically located collection points. Even in EOH basins where there are stormwater total phosphorus reduction targets per total maximum daily load (TMDL) requirement,1 it is unclear how effective SCMs have been because their performance is not directly measured.
Overall, stormwater pollutant load reductions are assumed to occur from SCM or EPSC project implementation using performance-based design standards established by the Center for Watershed Protection in the early 2000s. These performance standards were based on a comprehensive review of the available literature relative to typical pollutant loads in stormwater runoff and the performance of control practices (Winer, 2000). According to their guidance documents (NYC DEP, 2016, 2018), NYC DEP, along with NYS DEC and many other state agencies, adopted general design criteria for detention and retention basins (ponds), underground tanks, and grassy swales assuming these SCMs are capable of 80 percent removal of total suspended solids and 40 percent removal of total phosphorus. Since the initial data were collected, SCM performance data have been compiled in the International Stormwater BMP Database, including other SCMs such as bioretention, porous pavement, and low-impact development (Clary et al., 2017). As an example of the data from Clary et al. (2017), influent and effluent concentrations for total suspended solids and total phosphorus for 12 different SCMs, compiled from many studies internationally, are shown in Figure 9-2. The adoption of performance standards from guidance manuals for most of the SCMs (such as in NYC DEP, 2012) is an approach to stormwater management typical of many local and state programs nationally. Evident from Figure 9-2 is the wide range of pollutant removals (influent minus effluent concentrations) for the different SCMs. As expected, the removals of total suspended solids are generally greater than those for total phosphorus since only a fraction of the phosphorus is in the solids; in a few cases (e.g., bioretention, grass swale), the data show that effluent total phosphorus is, on average, greater than the influent. It is well established that SCM performance depends on the media and plants used in the SCM (Mangangka et al., 2015; Skorobogatov et al., 2020), on the infiltration capacity of the underlying soils, and on maintenance.
Although the Stormwater Program is largely administered according to federal and state regulations, a number of activities would improve the Stormwater Program. These efforts could include confirming SCM performance standards through storm event-based monitoring, and water quality assessments made at the catchment scale through targeted monitoring and modeling. Annual inspection and maintenance practices are important to ensuring that SCMs continue to perform at maximum efficiency. In addition, attention should be given to the possible effects of climate change on SCM performance so that existing SCM design criteria can be modified as needed. These efforts are discussed in more detail below.
Storm Event-based SCM Performance Monitoring
SCM performance is highly variable and differs among the various stormwater infrastructure types (as shown in Figure 9-2). Yet the extent to which this variability occurs in the NYC watershed is undocumented in the absence of performance monitoring of individual SCMs. Differences in regions, seasons, and land uses have been found to lead to differences in SCM performance (Hamel et al., 2013; Roseen et al., 2009; Selvakumar and Borst, 2006). A more comprehensive assessment of SCM performance would consider a range of flows
1 Within the Croton system, stormwater SPDES permits discharging to Lake Carmel and Palmer Lake specify total phosphorus reduction targets to meet designated TMDLs.
beyond the SCM design inflow of the 10-year, 24-hour runoff event, and include extreme events that may be observed in the future under climate change (see section below). Performance-based monitoring is impractical for all constructed SCMs; however, performance-based monitoring of a select number of the most commonly installed SCMs is a reasonable goal and would allow NYC DEP to evaluate, using a mass balance approach, whether installed SCMs meet the pollutant reductions stated in the design guidance. The longer that event-based monitoring at an SCM is sustained (e.g., over multiple years), the better one can ascertain both SCM performance efficiency, particularly for nutrient removal, and whether SCM maintenance is adequate.
Catchment-scale Water Quality Assessments
In addition to understanding the individual performance of commonly constructed SCMs, an understanding of SCM effectiveness at a collective, broader catchment scale could greatly inform the Stormwater Program. Interpretation of monitoring data at the catchment scale can be complicated, depending on the catchment size and land use characteristics. Generally, increased catchment size leads to multiple pollutant sources, both point and nonpoint, making analysis more difficult. However, special studies in a few selected small catchments (about 100-250 acres in drainage area size) could provide valuable information on program effectiveness and provide data to support a modeling effort.
The existing water quality monitoring program (discussed in Chapter 12) consists of many sampling locations throughout the water supply watersheds. However, locations were not chosen to assess the effectiveness of the Stormwater Program and therefore are probably not optimally positioned for this purpose. As a special study, a review of existing sampling locations and water quality data could be conducted to identify possible sites that could be used to assess the collective performance of SCMs in a small catchment. For example, if existing data were available from locations both upstream and downstream of an urban center, these data, together with a history of SCM installation, could be statistically analyzed to assess the significance of pollutant reductions from SCMs. Such a study would require a “control” catchment with no SCMs.
A temporal study to determine SCM effectiveness would be to monitor water quality over the long term at a few selected locations within small catchments. Sample collection would occur before and after installation of new SCMs or SCM retrofits. A trend analysis would attempt to relate changes in water quality to the size and timing of SCM projects. For this analysis to be effective, several new SCM projects would need to be installed in a substantial portion of the watershed over the study period. Hence, such special studies most likely would have to occur in the EOH region where the percentage of urban land is higher than WOH. However, study outcomes and a better understanding of SCM effectiveness would be applicable in urban centers in both EOH and WOH regions. A trend analysis could also provide valuable information to assess the potential impacts of climate change on SCM performance.
Use of Watershed Models in the Stormwater Program
Among MS4 communities nationally, the Storm Water Management Model (SWMM) developed by the U.S. Environmental Protection Agency (EPA) is the most commonly used for watershed-scale planning and small catchment hydrological design. SWMM is a dynamic watershed model used for planning, analysis, and design related to stormwater runoff, combined and sanitary sewers, and other drainage infrastructure systems in urban areas (Rossman, 2015). SWMM can spatially combine SCMs within a catchment, including detention, bioretention, grassy swales, green infrastructure, and other practices. SWMM is used to determine SCM effectiveness to meet stormwater management objectives such as reducing runoff through infiltration and retention. With appropriately collected water quality and flow data, a calibrated and verified SWMM in selected catchments could be applied to unmonitored catchments to estimate pollutant loading reductions from SCMs. Outcomes of targeted monitoring and modeling would also assist in prioritizing locations to install SCMs within urban areas. This suggested modeling approach is more quantitative than the current practice of using tables with SCM pollutant load reductions generated from ranges reported in a national database (i.e., Clary et
al., 2017). The use of SWMM could be expanded by coupling it with the model SUSTAIN to spatially optimize the number of SCMs needed to achieve target loadings within a watershed (EPA, 2009). SUSTAIN coupled with SWMM also allows for an economic cost-benefit analysis developing costs per achievable levels of pollutant reductions with SCMs (Jayasooriya and Ng, 2014; Read et al., 2019).
As noted above in the Program Description section, NYC DEP used WTM, WinSLAMM, and AVGWLF models to prioritize SCM retrofits around the Kensico Reservoir area. WinSLAMM is more appropriately used as a site design tool, whereas WTM and AVGWLF models are catchment-scale models to estimate pollutant loading reductions from SCM implementation. SWMM is a dynamic model with greater planning capabilities than WTM and AVGWLF, with SWMM able to examine SCM placement and pollutant reductions in modeled catchments. The basic study design used for the Kensico Reservoir modeling effort could be applied in designated areas with a high population density or commercial/industrial activity.
Potential Impacts to Stormwater Program from Climate Change
As discussed in Chapter 4, the northeastern United States is expected to experience shorter winters, higher annual average air temperatures, and more frequent extreme heat and precipitation events n the future (Porter et al., 2015; Pradhanang et al., 2013). Extreme climate events have already begun to increase during the warm seasons but not the cool season (November through May) (Matonse and Frei, 2013). Using the Soil and Water Assessment Tool (SWAT) model, Pradhanang et al. (2013) predicted that future streamflow temporal patterns would be flashier, in part from earlier snowmelt, reduced snowpack, and rain on snow, increasing discharge magnitude and advancing the seasonal timing of peak flows in the winter and early spring.
Limited research and modeling have been conducted on the effects of climate change on the performance of SCMs (Hathaway et al., 2014). However, it is clear that revisions of design criteria for new and retrofitted SCMs will be needed to take into account hydrological changes. One question that needs to be addressed is “are there plans to update the ‘design storm’ magnitude-frequency for SCMs implemented by MS4 communities?” Research suggests that climate changes are already impacting NYC stormwater facilities. For instance, Catalano de Sousa et al. (2016) investigated the performance of a bioretention facility in Queens, New York, during the periods of Hurricane Irene and Superstorm Sandy and found that the facility attenuated a significant amount of runoff, although bioretention performance deteriorated with increased storm duration, rainfall volume, and peak-hour intensity, due to an inadequately designed inlet structure. Further investigation of SCM inlet structural stability and pollutant removal performance during extreme events should be investigated with the use of hydraulic models, that is, WinSLAMM and SWMM. Models can provide predictions on pollutant mass removals to assess whether current design standards need to be updated if the frequency of extreme events increases, as well as information to assess risks and costs. SCM implementation policy must weigh the extra cost of SCM construction to accommodate larger SCM flows with the estimated increased costs associated with more frequent extreme storm events.
Several articles reviewing the use of SCMs suggest that green infrastructure and low-impact development can be effective tools for climate change adaptation in urban environments (EPA, 2019; Giese et al., 2019). These newer SCM approaches are based on the fundamental concept that the reduction of impervious surfaces and the concomitant creation of infiltration pathways creates resiliency in urban hydrological systems. In contrast to older SCM designs, these approaches rely on spatial landscape planning to provide greater effectiveness for runoff volume reduction, pollutant load reductions, and channel protection flows, and to provide for greater program cost effectiveness (Matthews et al., 2015). Matthews et al. (2015) suggests that reducing institutional barriers to comprehensive planning and implementation of green infrastructure and low-impact development could lead to improved watershed-scale outcomes. Within the WOH watershed, the level of planning appropriate to a larger city may not be needed; however, some modeling and planning that sets site priorities for green infrastructure and low-impact development would be informative.
Community Vitality in the Stormwater Program
The Stormwater Program is expected to provide community vitality benefits by contributing to the enhancement and preservation of water quality, reductions of peak flows in streams, and, in some locations, reduction in flooding occurrences. These aspects are particularly important to maintaining the Catskills as an outdoor recreational destination (hiking, mountain biking, and skiing). However, evidence of these benefits, such as reduced flooding potential from SCMs, has yet to be gathered, for example, through a hydrologic modeling effort. In contrast, the Stormwater Program may also impose burdens on small watershed communities that come with the regulatory requirements of implementing the federal, state, and NYC DEP stormwater regulations. These burdens include multiagency redundancy of the SPDES and IRSP permit review and the time to process permits.
The NYC DEP is effectively managing the Stormwater Program as required by federal and state regulations, and has supplemented these regulations with additional measures in their WR&R§18-39 to meet FAD obligations. NYC DEP’s implementation of the Stormwater Program uses standard design criteria, commonly used nationally by MS4s and acceptable to federal and state environmental protection agencies. However, there are several opportunities for NYC DEP to enhance their evaluation of the Stormwater Program such that it more directly determines program impacts on water quality. In general, from an administrative perspective, it would also be good to review the multiagency permitting process to reduce any regulatory redundancies and improve assistance to local users for developing SWPPP and ISRP documents.
Performance-based monitoring of some of the most commonly installed stormwater control measures (SCMs) would allow the New York City Department of Environmental Protection (NYC DEP) to evaluate whether installed SCMs meet the pollutant reductions stated in design guidance. SCMs selected for study should be those used most often by developers and consulting design engineers. The study design consists of flow-weighted measurements of influent and effluent pollutants (e.g., phosphorus, nitrate, total suspended solids, and pathogens) for selected SCMs. The longer that monitoring at an SCM is sustained (e.g., over multiple years), the better one can ascertain both SCM performance efficiency, particularly for nutrient removal, and maintenance issues.
The New York City Department of Environmental Protection should take steps to measure the mass loading reductions from SCMs in a small catchment. A determination of whether any existing monitoring locations and data could be leveraged for this purpose should be undertaken. In the absence of sufficient data, a special study could be conducted using paired upstream and downstream locations around an urban center with a history of SCM installation to estimate the reduction in pollutant loads from SCM implementation. The study design could use paired catchments of similar drainage area size, one with SCMs and another without. Another study design would involve collecting water quality data over an extended period, with sample collections before and after installation of SCM retrofits and construction of new SCMs, in order to complete a statistically valid trend analysis.
Water quality data collected at new catchment locations should be used to calibrate watershed models to support the Stormwater Program. Prioritization should be for the EOH watershed and Kensico Reservoir area. The EPA’s SWMM is recommended because it can incorporate SCMs into its modeling framework. Outcomes of targeted monitoring and modeling would also assist in prioritizing locations to install SCM retrofits within urban areas. In addition, with the use of SWMM, pollutant load reductions can be estimated in a catchment from the SCMs, which could be used to better quantify the effectiveness of the Stormwater Program.
Current design standards for stormwater control measures should be evaluated for treatment efficiencies and structural stability based on projected changes in climate, particularly precipitation intensity and duration. Emphasis should be placed on assessing the inlet structure design for passage of high flows to prevent hydraulic short-circuiting and protect structural stability. Hydraulic design of SCMs can be evaluated using models to assess whether the inlet structures can accommodate the discharge from more frequent extreme precipitation events and whether treatment targets can be met.
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