PENNSYLVANIA (WHTM) — Fifty years ago, remnants of Hurricane Agnes dumped rain on Pennsylvania. In late June 1972, the Susquehanna River crested 16 feet over the flood stage in Harrisburg. At the time, it was the costliest natural disaster in U.S. history, and it took the lives of dozens of Pennsylvanians.
According to sources like NASA, models predict that hurricanes will involve more intense rainfall in the future due to climate change. While hurricanes may not impact Central Pennsylvania as frequently as coastal areas, experts predict that wetter weather may become more common in the commonwealth.
While atmospheric changes can affect rainfall and subsequent flooding in Pennsylvania, conditions on the ground can also alter the impacts of rainfall, both for better and for worse.
Floods are caused by large amounts of runoff flowing quickly into streams and rivers, the United States Geological Survey explains. Development that creates more impervious surfaces — surfaces like pavement that can’t absorb water — increases runoff to nearby waterways.
“Removing vegetation and soil, grading the land surface, and constructing drainage networks increase runoff to streams from rainfall and snowmelt. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams,” a fact sheet from the USGS says.
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Planning for stormwater runoff is something that PennDOT Hydrology and Hydraulics unit section manager Nick Vivian considers.
“If you have a nice beautiful section of forest with a bunch of vegetation, and it rains, that vegetation captures that runoff and kind of holds it there. It doesn’t really get to go anywhere, it doesn’t concentrate or flow,” Vivian said.
“If you go in and cut those trees down, and you put a nice impervious surface where water can’t penetrate through, and very quickly a storm, even if it’s just a tenth of an inch, it hits that pavement, can’t penetrate into the ground, nothing really holds it there, and it just wants to go, follow the slope of the land. It very quickly concentrates. When it concentrates, it causes shear stresses to whatever ground there, and then if it hits soil or an area that’s not really reinforced or vegetated, it begins to tear the land apart,” Vivian explained.
In short, Vivian said, “Impervious area is directly proportional to runoff and flooding.”
So with increased development, might the impacts of another storm like Agnes be worse? Ben Pratt, water resources engineer with the Susquehanna River Basin Commission, says not necessarily.
“At some point, a lot of rain is just a lot of rain,” Pratt said. “We certainly need to be cognizant of the impact of development, but in terms of the scale of a storm like Agnes or Tropical Storm Lee or any hurricane or storm that’s going to provide that much rainfall, you’re going to have catastrophic flooding.”
In fact, some projects undertaken since Agnes hit Pennsylvania can actually help mitigate flooding during significant rainfall events. For example, additional flood control reservoirs were constructed in the Susquehanna River basin after Agnes, Pratt explained, and they would prevent nearby areas like Corning or Elmira that were hit hard in 1972 from being impacted by another storm the size of Agnes.
Levees can also minimize flooding. Pratt noted that a levee raised after Agnes at Wilkes-Barre helped keep the city mostly dry when Lee hit in 2011, and the Susquehanna River crested higher during Lee than during Agnes in that area.
It takes quite a bit of water to cause the Susquehanna River to flood, but localized flooding can be a different story. Take 2021’s Ida, for example, which flooded many smaller creeks and streams around the Midstate but, at least at Harrisburg, did not cause the Susquehanna to spill over its banks.
Localized flooding could possibly be more impacted than wide-scale flooding by the installation of more impervious surfaces.
“The hydrologic effects of urban development often are greatest in small stream basins where, prior to development, much of the precipitation falling on the basin would have become subsurface flow, recharging aquifers or discharging to the stream network further downstream. Moreover, urban development can completely transform the landscape in a small stream basin, unlike in larger river basins where areas with natural vegetation and soil are likely to be retained,” the USGS fact sheet says.
Development standards attempt to address some of the risks of runoff through stormwater management strategies. Engineered solutions like basins to capture stormwater runoff help mimic the natural hydrologic processes that would be going on in undeveloped spaces, Vivian explained, mitigating the impacts unchecked runoff might have on the environment and other communities and structures.
One potential issue, Vivian pointed out, is that current standards for stormwater management in Pennsylvania were established in 1978 and haven’t been updated in years, and as Pennsylvania’s weather changes, those standards may be lagging behind.
“What we’ve seen, especially through our department and through some of the programs and monitoring we do: that rainfall has changed through the years. You used to get a steady storm where you would get a specific amount of inches of rain over a very long duration of time — 24 hours, 48 hours, a week. Now, what we’re seeing is that you’re getting 3 inches or 4 inches in an hour, which is profound, and it’s causing significant damage, significant issues that we as engineers now have to react to and solve,” Vivian said.
Vivian said that people often comment to him that they are seeing flooding in places that have never flooded before, due at least in part to climate change-fueled shifts in weather patterns. These short, intense periods of rain are called microbursts, he said.
Rich Heineman, stormwater section chief in the Bureau of Operations at PennDOT, compares these short rainstorms to turning a firehose on in a yard.
“If you put a sprinkler out in your yard for three weeks, it’ll get saturated, and eventually the water will run to that lowest spot slowly, it could surge further down in the watershed. What Nick was saying with the microbursts we’re seeing, it’s like turning on a fire hose in your yard. It’s going to fill up real quick and not flood those areas that we’ve seen in the past,” which — no longer in the sprinkler analogy — tend to be lower areas in floodplains, Heineman explained.
The Pennsylvania Department of Environmental Protection’s 2021 Climate Change Impacts Assessment predicts that the state will experience “less frequent but heavier rain events” in the future. abc27 meteorologist Brett Thackara compiled data that shows that between 1888 and 2022, half of the dates with the greatest one-, two-, and three-day precipitation totals in the Harrisburg area happened after 2010.
Heineman hopes that people can shift to a more proactive mentality to prevent flooding from changing conditions. Because of costs and the infrequency of major rainfall events, though, he and Vivian say people can’t necessarily engineer their way out of every possible severe scenario.
For one thing, while people managing new development can include stormwater controls in their designs, retrofitting older development with new controls is much harder and more expensive, Heineman noted. In urban environments, there may not even be room to add tools like stormwater control basins.
On top on that, controlling for intense storms that have a 1-in-100 or -400 or -500 chance of happening in a given year may not be the most practical.
“It’s tough because when you get those larger events, the higher intensity events, we can’t build our way out of it because otherwise there would be no development and it would only be stormwater controls,” Heineman said, “so it’s that balance of that risk and probability of what size event is going to happen.”
“Development happens,” said Pratt from the SRBC. “As a society we need to develop. We need places to live, we need places to work, we need places to manufacture, so that’s going to happen. I think we need to be — and I’m not suggesting we’re not — we need to be thoughtful about how that particular development does relate to downstream interests and then put proper controls in place to control the impacts of that development.”
“We’re trying to do our part not just to protect our infrastructure — roadways, bridges — but when we do that, it’s trying to do our best for our neighboring properties, not passing that problem further along,” Heineman said.
So at a certain point, significant rainfall will cause significant flooding, Pratt said, but on a local scale, there are things that can be done to minimize negative impacts of development that increases impervious surface area.