A Hydrology Model for Mimicking Pre and Post Development Runoff Volumes
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The Energy Independence and Security Act, passed by Congress in 2007, included a section commonly referred to as EISA 438. This section of the Act requires any new development or redevelopment of federal facilities, involving more than 5,000 square feet, to retain on-site a sufficient volume of rainfall to mimic the pre-development hydrology. The retained rainfall must be dissipated by means of infiltration into the ground, evapo-transpiration, or reuse. It cannot be directly discharged into a storm drain or surface water. Therefore, detention basins are not an appropriated best management practice (BMP) for complying with this regulation, since they have direct discharges. The Federal Environmental Protection Agency (EPA) published a guidance document for complying with the regulation entitled, “Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act”. This guidance document presents two options for estimating the retention capacity for designing best management practices to comply with this regulation. Option one is based on a statistical analysis that requires 95% of the average annual rainfall be retained and dissipated. The problem with this option is that it fails to consider soil infiltration or changes in ground cover, which are the primary factor influencing the runoff volume. This option is appropriate for site planning estimates, but not BMP design. Option two puts forward the concept of modeling the pre-development and post-development hydrology to estimate the appropriate storage capacity. Although Option two suggests a continuous simulation model, the method presented in Appendix A of this guidance document is an analysis for a single 24-hour rainfall event. The single event analysis has the problem that it fails to consider the dissipation rate, and subsequently it fails to consider the actual storage volume available for the next rainfall. Described in the following paragraphs is the “Retention Volume Simulation Model” (RVSM). The RVSM is a continuous simulation, volume based hydrology RVSM, appropriate for estimating the retention capacity for BMPs to comply with new EISA 438 regulation.