Impact on Bioretention Design of the Calculation Method for the 95th Percentile Rain Event

Abstract

The new federal facilities' guidance for stormwater management Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act set the goal of retaining the 95th percentile storm event on site and managing it through low impact development techniques. The procedure outlined in this technical guidance describes that the 95th percentile rain event can be calculated based on daily rainfall records, typically records based on rain from midnight to midnight. Other researchers and guidance documents have defined a storm event as the total depth of rainfall measured with any time period where there has not been a break in rainfall of a prespecified time. Typically, this inter-event period between defining separate rain events has been 6 hours. Using this definition, multiple rain events can occur within one day or single rain event may occur over several days. Using rain data from approximately 20 National Weather Service rain gauges (minimum of 23 years of record), the 95th percentile calculation was performed both using the Federal Guidance and using a 6-hour interevent in Pennsylvania. As expected, the 95 th percentile event was larger when the storm event was calculated using a 6-hour interevent size since many storms, such as Tropical Storm Agnes, occur over several days. These data are being used, in conjunction with traditional design requirements of reducing the 2-year storm peak rate and volume to pre-development conditions, to design bioretention systems. WinSLAMM and SWMM 5.0 will be used then to assess, using continuous simulation, the percent capture of the bioretention system over the period of record. If the results can show that these systems capture the same percentage of annual runoff regardless of the design technique, it may be simpler to define the design runoff peak rate and volume based on a rainfall event size that can be readily extracted from NOAA data. This will eliminate the subjectivity of defining pre-development conditions. This paper will focus on the differences in calculating the 95 th percentile rain event.