If Harvey-like storm hits Pittsburgh, head for the hills
Most people probably didn’t notice, but Hurricane Harvey hit Pittsburgh over the weekend.
The remnants of the storm that dumped more than 50 inches of rain in parts of southeastern Texas couldn’t even manage to squeeze out a quarter-inch of water over southwestern Pennsylvania.
That may come as a relief to those who remember the devastating impact on the region from the leftovers of Hurricane Ivan in 2004 or Hurricane Agnes in 1972.
The city’s rivers crested at 31 feet during Hurricane Ivan — 6 feet above the 25-foot flood stage — from about 6 inches of rain. During Hurricane Agnes, they crested at 35.82 feet, nearly 11 feet above flood stage. That was caused by 4 to 8 inches of rain over several days.
“Now imagine if we had received 30 inches of rain over four days,” said Jon Nese, a professor in Penn State University’s meteorology department. “I’m not a hydrologist … but my gut tells me if that much rain fell across Pittsburgh in four days, it would far
exceed anything that has ever been experienced in the city.”
While the absurd, and unofficial, rain total of 51.88 inches was recorded in Texas during Hurricane Harvey, Mr. Nese said, Harris County, where Houston is located, received an average of 35 inches from the storm. Harris County was one of the areas hit hardest by the storm and continues to deal with flooding as well as a humanitarian crisis.
That kind of rainfall would cause unthinkable damage in Pittsburgh, although it’s virtually impossible to get a storm of that magnitude here.
The St. Patrick’s Day flood in 1936 was the worst flood in the city’s recorded history. The rivers that day crested at 46 feet — 21 feet above flood stage — from 8 to 10 inches of rain combined with snow melt. According to the National Weather Service, up to 15 feet of water covered the Golden Triangle.
It’s hard to say what 50 inches of rain would look like in Pittsburgh. Flood totals generated from a certain amount of rain the area has received in the past cannot be simply multiplied because the water would not rise evenly. The water would flow farther upstream and into valleys that haven’t been flooded before, or, at least, not flooded as badly.
“That has to do with our topography and elevation,” said Joe Palko, a senior hydrologist with the National Weather Service in Pittsburgh. “Around here, you have hills and valleys and [water] just concentrates.”
Water from the storm in Texas rose relatively slowly because the land there is fairly flat. In Pittsburgh, water would be funneled to low-lying areas and cause dangerous flash flooding.
If a storm dumped 50 inches of rain here, the Hill District, Polish Hill and higher portions of the North Side could remain above floodwaters, Mr. Palko said, but Downtown would be in trouble.
Mr. Palko estimated that even after the water spreads out, the Golden Triangle would be covered by 25 feet of water and the rivers would crest at 60 feet.
According to statistics from the weather service, the fountain at Point State Park would be enveloped quickly by water, as would roads and railroads along the perimeter of Downtown, the North Shore and South Side. Point State Park would be submerged to the overpass, followed by the playing surfaces at Heinz Field and PNC Park. The floor level of Rivers Casino would soon be underwater, and water would continue to rise. Downtown would flood all the way to PPG Paints Arena.
The water would make its way to places that regularly flood, such as Etna, Carnegie and down the Ohio River to Beaver, and up the Monongahela River to Elizabeth. Waterand sewage treatment plants along the river would be inundated. Industry and transportation in and along the rivers would be impacted.
The 16 flood control reservoirs in and around Pittsburgh can hold only about 3½ to 6 inches of runoff, said Megan Gottlieb, a hydraulic engineer with the U.S. Army Corps of Engineers. She explained that if a storm is predicted to drop more than 6 inches of rain, the reservoirs likely would be set to spillway flow, meaning water is released downstream.
“Spillway flow in our reservoirs isn’t necessarily a bad thing,” Ms. Gottlieb said. “It’s what the dam is designed to do so that it’s not over top, but it’s uncontrolled flow.”
That’s what would happen if a storm brought more than 6 inches of rain, but what about 50 inches? It would be unprecedented and difficult to speculate on, she said. “I cannot give you a high-water mark that 50 inches would do, but it would most certainly cripple us,” Ms. Gottlieb said.
Of course, that much water would cripple any city.
Mr. Nese put it this way: The entire surface of Pennsylvania would be covered by about 32 inches of water if that much rain fell across the state.
“There are estimates that 25 trillion gallons of water fell in southeast Texas and Louisiana — 25 trillion gallons,” he said. “I calculate, just as a local benchmark, that is 20 percent of the volume of Lake Erie. To essentially take 20 percent of the water out of Lake Erie and then drop it as rain on southeast Texas and Louisiana. That is what happened, which to me is pretty remarkable.”