DEP Updating Probable Maximum Precipitation Events To Better Define High-Hazard Dams

When residents of Smethport, PA, awoke to a rainy day on July 7, 1942, they probably did not expect to be witnessing history, or setting dam safety policy decades later.

The small town in McKean County in Pennsylvania’s northern tier would soon be an entry in the history books, with the most rain ever recorded in a 12-hr period: a drenching 34 inches of rain in just 12 hours, with more than 28 inches of it falling in just 3 hours.

This deadly storm event would go on to create a benchmark used today to determine if a dam would withstand the maximum amount of water that could fall in an area – and run downstream into a dam – without failing or being overtopped.

“The Smethport storm is a really good baseline when thinking about how much rain can fall in a single event,” said Jonathan Conville, Acting Chief of Dam Safety. “Even though we use more recent data than that storm, it is still one of the most extreme cases of rainfall over a short period, so it’s a good reference point.”

Dams are evaluated as to whether they can withstand the Probable Maximum Precipitation (PMP) event for a given area.

The PMP is determined by examining the weather patterns, previous storms, and other factors that could affect how much water might end up in a reservoir behind a dam – and how much would be needed for that dam to either fill up completely or otherwise fail.

This data is important, as underestimating the PMP could lead to hazardous flooding that could threaten the lives and property of people living downstream.

DEP is now conducting a study to update the PMP thresholds for Pennsylvania. This study will examine seasonal factors, geography, and other factors to identify PMP thresholds for dams across the state.

Currently more than 80 “high-hazard” dams have less than 50 percent of required spillway capacity, and could fill up and overtop if one of these maximum events were to occur. A "high-hazard dam" refers to the fact that if they failed there would be catastrophic effects downstream. This is not a statement on the condition of the dam.

Fortunately, the PMP is an outlier figure – one that would require historic rainfalls, so it is unlikely that any of those 80 dams are in any danger today.

The last PMP estimates are based on information from 1978 and 1982 – long before computer modeling was widely available.

“We want to have these updated PMP estimates to accurately analyze each dam and ensure Pennsylvania dams are safe.” said Conville.

For more information on dam safety, visit DEP’s Dam Safety webpage.

(Photos: Scenes of McKean County flooding in 1942, Painted Hills Genealogy Society.)
(Reprinted from DEP’s November issue of News & Views. Click Here to sign up for your own copy.)