What Should Be the 95th Percentile Rainfall Event Depths?

Based on Section438 of the Energy Independence and Security Act of 2007, the U.S. Environmental Protection Agency (USEPA) published a report providing technical guidance that recommends using the 95th percentile rainfall event for federal development or redevelopment projects to preserve or restore predevelopment hydrology of the site. In this study, the 95th percentile rainfall depths using daily rainfall data were computed following the USEPA guidelines from 18 selected weather stations. The purpose of this study is to investigate whether the 95th percentile daily rainfall depths truly represent the 95th percentile event rainfall depths. For analysis, the 95th percentile event rainfall depths were computed using hourly rainfall data for storm events separated by minimum interevent dry periods of 6, 8, 12, 18, 24, 48, and 72-h and compared with the 95th percentile daily rainfall depths. The 95th percentile event rainfall depths calculated using minimum interevent dry periods of 6-72h are on average 11-100% greater than the 95th percentile daily rainfall depths recorded at 18 selected weather stations.