Meeting Stormwater Regulatory Requirements Using Continuous Simulation Approaches

The use of Green Infrastructure (GI) and its addition to stormwater regulatory requirements became widespread with the publication of the USEPA National Pollutant Discharge Elimination System requirements in 2000 (EPA, 2000). In most areas of the United States, this this was addressed with the addition of a volume management criteria to address water quality, recharge, and streambank geomorphology, while maintaining past peak flow requirements. For example, in Pennsylvania, the pre-construction water volume cannot be increased, and water quality cannot be degraded for new development for all storms up to the 2-year/24-hour event (PADEP, 2006). The peak flow restrictions for the 2-, 10-, 50- and 100-year storm remain as well (Pennsylvania Code, 2022). Most applicants have used NRCS techniques based upon the design storm approach (NRCS, 2007). While GI was embraced and brought to the design of stormwater systems, the design storm approach was not rethought. This too often results in the need for a hybrid of two separate stormwater systems - one for small, frequent storms (targeting water quality and geomorphologic concerns) and one for large, infrequent events (such as 50, 100 year storms). The use of continuous simulation was proposed by the USEPA as far back as the 1970's for water quality design (USEPA, 1979). Continuous simulation was proposed as superior to the design storm approach even for peak flow. A major challenge of the mission to use continuous simulation at that time was the lack of available rainfall and climate data with fine intervals. Design storms were intended for designs of structures whose failure would cause catastrophic consequences. The stormwater best management practice design guide from the USEPA expressed concern as to the accumulation effect of peak flows and to the extended peak flow erosion a streambanks (Clar et al., 2004). Pennsylvania and other states combine these two regulations requiring both the pre-construction volume and peak rate to be met. Unfortunately, the design storm approach for long storm events (such as 24-hour durations) artificially concentrate most rainfall volume over a small time interval which occurs at the center of the storm. The statistics used to develop design storm distributions do not separate the high-intensity, small volume thunderstorm from that of the low-intensity, long duration frontal system. Thus, the statistics that create the high-intensity center peak are more consistent with the intensity of short duration thunderstorm than that of a 24-hour rainfall event. An example, a recorded 24-hour storm event is compared the design storm of the same rainfall volume and duration (Figure 1). The graph shows a distinct difference between the design storm and the recorded rainfall event in terms of peak intensities.Vegetated GI design, that utilize infiltration and evapotranspiration, is strongly influenced by the short-duration, high-intensity peaks and as such temporary storage is needed. The additional storage is rarely used but required by the design storm approach. This prejudices the selection of GI towards lesser green solutions (such as underground detention basins), which require more space and typically more maintenance. It is hypothesized that a site tailored to the soil, vegetation, and local rainfall patterns would be more resilient, more appropriate, and more feasible especially in urban areas (Traver and Ebrahimian, 2017). Villanova University was approached by the Commonwealth of Pennsylvania to develop an update to the Stormwater management Manual, which enabled the researchers to implement what has been learned worldwide since the previous manual was published in 2006. Besides the introduction of climate change adaptations, the researchers are proposing parallel paths (continuous simulation or design storm approach) for meeting the 2-year/24-hour volume and water quality requirement as well as for peak flow (using continuous simulation/storm of record approach or design storm approach). Results from model studies used to validate these approaches will be presented. References Environmental Protection Agency (EPA). 2000. 'Proposed Reissuance of National Pollutant Discharge Elimination System (NPDES) Storm Water Multi-Sector General Permit for Industrial Activities.' Federal Register. Accessed January 17, 2023. National Resource Conservation Service (NRCS). (2007). Chapter 7 Hydrologic Soil Groups. Pennsylvania Department of Environmental Protection (PADEP). (2006). Pennsylvania Stormwater Best Management Practices Manual. Pennsylvania Code. 2022. '25 Pa. Code § 102.8. PCSM requirements.' Accessed January 17, 2023. http://www.pacodeandbulletin.gov/Display/pacode?file=/secure/pacode/data/025/chapter102/s102.8.html&d=reduce. United States Environmental Protection Agency (USEPA). 1979. '1978 Needs Survey: Continuous Stormwater Pollution Simulation System - Users Manual.' Office of Water Programs Operations. Clar, M., B. J. Barfield, and T. O'Connor. 2004. Stormwater Best Management Practices Design Guide Volume 1 - General Considerations. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-04/121. Traver, R. G., and Ebrahimian, A. (2017). 'Dynamic design of green stormwater infrastructure.' Frontiers of Environmental Science & Engineering, 11(4), 15. United States Environmental Protection Agency (USEPA). (2016). Summary of State Post Construction Stormwater Standards. Washington State Department of Ecology. (2012). Stormwater Management Manual for Western Washington (Publication Number 12-10-030).