Exploring Storm Intensities and the Implications on Green Stormwater Infrastructure Design

The peak intensity that occurs during a storm event can drive the performance of a green stormwater infrastructure (GSI), which may or may not align with the expected performance of the GSI. Both the peak intensity volume and where it occurs within an event are found to influence the GSI response. The design criteria set the expectation of how well a GSI will manage stormwater within a watershed. The Villanova University bioinfiltration rain garden (BRG) has been monitored since 2003, providing a long hydrological data record that is used to study local and observed rainfall patterns in comparison to design criteria to understand the impact of storm intensity on GSI performance. Intensities for all the storms recorded at the site were assessed at different timesteps and compared to the intensities typically used by the design storm approach in meeting regulatory criteria. There were 1482 storm events analysed and for all timesteps, the values commonly used for meeting design regulations were seen to be well above what was observed at the BRG, with 98% of the storms occurring below these values. Out of the 1482 storms, only 46 storms (3%) had effective durations longer than 10 h and no storm observed had an effective duration longer than 22 h, yet their peak intensities were still below the peak intensity associated with design regulations. This finding highlights the difference in the duration these sites are designed to manage (typically 24 h), in comparison to the ones experienced by the systems. The peak intensity analysis done at the different timesteps shows that for the storms recorded at the BRG, the intensities vary with changing time intervals and events. Of all the assessed events, only two events recorded larger intensities than the regionally specified NOAA C design storm, demonstrating the skewness of the approach. There was no trend in peak rain intensities over the 20-year rainfall record. This study concludes that due to their dynamic performance, vegetated GSI have a natural resilience to a variety of precipitation patterns and climate changes that may be compromised when designing to a static value set through design storms.