When it comes to assessing the impact of low-impact development (LID) and green infrastructure (GI) practices, site development teams are frequently asked to determine the percentile of rainfall events that can be managed by the site development solution.
For instance, the LEEDv4.1 Rainwater Management credit asks project teams to retain on-site the runoff from the developed site for, at minimum, the 80th percentile of regional or local rainfall events. The credit further rewards teams that can manage the 85th or 90th percentile of rain events.
Similarly, SITESv2 requires that site development projects retain the precipitation volume from the 60th percentile precipitation event, with an additional credit that awards points for retaining or treating the 80th, 90th, and 95th percentile precipitation event on-site.
So, how is the percentile precipitation event determined? Methodologies may vary and there are various resources available in the marketplace today to help assist project teams with such calculations, but a best practice aligned with LEED and SITES is to conduct the following four steps.