Chasing Storage during Flood Events in a Small, Multi-Jurisdictional Urban Watershed

Middletown, PA, receives piped stormwater runoff from >5 other entities and passes it through to the river. On July 23, 2017, a microburst storm dropped 5.3 inches of rainfall on the watershed in 2 hours, with 4.7 inches coming in the first hour, resulting in substantial flash flooding in non-FEMA-floodplain areas. Because the watershed contains multiple jurisdictions, flooding mitigation will require involvement by all watershed entities. The Penn State Harrisburg Water Resources Engineering capstone class developed a PCSWMM-based model of the watershed, using data on stormwater piping systems, detention systems, outfalls, and green infrastructure installations from the primary stormwater dischargers in the watershed. The model allowed the students to look for locations in the system where storage was available. The results to date show that opportunities for temporary storage exist in the system. In several locations, the piping system capacity and detention pond capacities were not fully used. The outlet structure sizing for the ponds was large and prevented temporary retention of substantial amounts of water (several ponds were never more than half full). Modeling of floodplain reconnection of two streams in the watershed also provided substantial storage that could mitigate flooding. Green infrastructure installations are being explored, but, as expected, the modeling highlights the minimal impact on this type of flooding. This year, the class is exploring real-time controls of pond outlet structures and piping storage in the upper watershed to provide extended detention. In addition, modeling is being used to evaluate the impact of the storm direction on the volume and duration of flooding. Preliminary results show delays in peak node depth and flow rates, depending on storm direction. The project highlights the need for a systems analysis of a watershed rather than a single-site development analysis. Each of the recent developments in this watershed was designed, built, and operated according to the federal and state guidance. However, it appears that the interaction of these devices increases flooding in the watershed, specifically due to the multiple detention facilities in the watershed that discharge their peaks at a similar time. The systems analysis also is able to identify where pipe storage is underused, even during extreme storm events.