CS6B - Concurrent Session 06B: Evaluating Spillway Capacity with Changing Data– The Effects of a Statewide PMP Study on a Portfolio of Dams

With 30 high hazard dams spread throughout Pennsylvania, the Pennsylvania American Water Company seized the opportunity to re-evaluate the spillway capacity of their dams based on updated estimates of Probable Maximum Precipitation (PMP). Previous studies had evaluated spillway capacity based on the PMP from Hydrometerological Reports 51 and 52 and concluded that only 6 out of 30 of their high hazard dams passed greater than 50% of the PMF. Only 1 if the dams passed 100% of the PMF. Based on the broad finding that many of the dams required significant modifications to pass the spillway design flood, the Pennsylvania American Water Company embarked on a major dam modification program aimed at updating all their high hazard dams to passing the full PMF and meet current dam safety requirements. The rehabilitation program started in 2008 and is on-going with nearly half of the dams completed or in final design.

The 2019 Pennsylvania PMP study developed updated precipitation depths for various durations for the entire state. In conjunction with the study the Pennsylvania Department of Environmental Protection established application guidelines for using the precipitation depths to generate the PMF. The Pennsylvania American Water Company commissioned a study that included the re-analysis of the Probable Maximum Storm for the 30 high hazard dams as well as updated hydrologic analysis to establish the PMF based on the new application guidelines.

This presentation will explore the effects of the statewide PMP study on spillway capacity of their high hazard dams and the effects the statewide PMP study has had on the dam rehabilitation program.