Software Tools for the Changing Environment

Runoff Reduction Analysis Made Easy



To prevent harmful pollutants from stormwater runoff being discharged into a Municipal Separate Storm Sewer Systems (MS4s), operators must obtain a NPDES permit and develop a stormwater management program before any land disturbance activity can begin. The Virginia Department of Conservation & Recreation (DCR) and the Virginia Department of Environmental Quality (DEQ) have put in place the new Virginia Stormwater Management Program {VSMP) Permit Regulations which will become effective from July 2014. Localities are authorized to adopt their own stormwater management ordinances as long as these ordinances are based upon findings of local or regional comprehensive watershed management studies or findings developed through the implementation of a MS4 permit or are determined to be necessary to prevent any further degradation to water resources or to address specific existing water pollution. Some projects already in the pipeline are ‘grand-fathered’ from these requirements until July 2019. These regulations and ordinances will require a change in the mindset of the “permittee”. Unlike in the past when the pollutant removal efficiencies of “Traditional” stormwater management facilities (BMP) did not take into account the removal that occurred when the runoff volume is reduced, the Virginia Runoff Reduction Method (RRM) developed by the Center for Watershed Protection (CWP), the Chesapeake Stormwater Network (CSN), and the DCR recognize a BMPs' capacity to reduce the overall runoff volume as well as mass pollutant removal reflecting a “Mass Balance” Approach. In many existing urbanized areas, the infrastructure changes needed to retrofit existing stormwater drainage systems with structural BMPs – to provide for stormwater quality and quantity control – can both be difficult due to right-of-way issues and also be expensive. In these cases, low-impact BMPs and non-structural means for controlling stormwater can be put in place to reduce pollutant at source before more expensive structural BMPs are instituted. Ensoftec has developed SWMSoft, a database managed computer program, which simplifies the RRM by providing step-by-step procedures for calculating the RR volumes. The different types and design levels, riser assembly, outfall analysis, treatment train, and stormwater best management practice (BMP) routing are already built-in. SWMSoft which assists the designer to comply with runoff and pollutant reduction targets for each outfall and the hydrologic unit code (HUC) boundary. The program also calculates the peak flows for each outfall using the modified TR-55 Curve Numbers for the reduced runoffs. SWMSoft accounts for pollutant loads contributed by managed turf in addition to impervious cover. The designer can select the appropriate practices to treat turf areas and turf-intensive land uses. The emphasis is to provide low impact designs or environmental site designs that can achieve runoff and pollutant load reduction ahead of structural BMP’s. In most cases, the full range of practices can be considered as part of the treatment train. SWMSoft provides detailed designs for all these facilities excluding proprietary manufactured devices. Because SWMSoft is a managed database, permutations of various stand-alone BMP facilities and combinations of various facilities (treatment train) are straightforward procedures and iterative modifications of BMP facilities can be easily performed to provide optimized designs.


How to Cite: Saranthi, P. (2014) “Runoff Reduction Analysis Made Easy”, National Hydraulic Engineering Conference 2014. 1(2014).