The Effect of Record Length and Duration on Continuous Modeling to Design Stormwater Control Measures

Abstract

Currently, Stormwater Control Measures (SCMs) are typically designed using synthetic design storms which lack consideration of the variability of rainfall patterns, focuses solely on the period of the storm while excluding the effect of infiltration during the storm, as well as excluding the removal of soil water storage through infiltration and evapotranspiration during interevent periods. The stormwater community has new tools – like long-term, readily-available rainfall records and continuous simulation models – to design SCMs based on a more realistic performance rather than synthetic event conditions. However, a framework is needed to define how to model an SCM designed via continuous simulation to meet single-event volume requirement set by Pennsylvania regulation. This research aims to recommend a length and duration of precipitation and climate input (i.e., how many years of record and how recent) and statistical analysis method (partial duration and annual maximum) to design an SCM using a continuous modeling (EPA’s Stormwater Management Model; SWMM) approach in various locations throughout Pennsylvania (Erie, Lehigh Valley, Philadelphia, and Pittsburgh). The volume requirement in PA is to manage the net change (from pre- to post construction conditions) in the 2-year/24-hour storm event which is proposed to be met using a percent exceedance ranking of daily runoff volume from the continuous simulation (i.e., daily 50% exceedance equal to the 2-year/24-hour storm event). The most conservative SCM footprint sized to meet the daily 50% exceedance is used to inform the recommended precipitation and climate record length in a continuous simulation model. Both partial duration and annual maximum statistics are evaluated in conjunction with various record lengths, durations, and locations in Pennsylvania using SWMM. Results indicate that use of the partial duration statistical method in conjunction with the most recent 10-year rainfall record provides a conservatively sized SCM footprint for all PA locations.