Analysts: Levees Work In Some Areas, Worsen Others
WILKES- BARRE, Pa. (AP) – The levee system running along the Susquehanna River through much of the Wyoming Valley did its job – protecting property and people in its greatest test since the system was built in response to the 1972 Agnes Flood.
But the levees also did something else that levees do: they made waters higher and flooding worse in unprotected areas upstream and downstream.
People in flooded, low-lying levels of West Pittston reported levels higher than those during Agnes. That's no surprise to engineers and scientists familiar with flooding and natural disasters.
“When you make an alteration to a drainage system you affect someone somewhere else,” said Stephen A. Nelson, the chair of the Department of Earth & Environmental Sciences at Tulane University and an expert in natural disasters.
That alteration could be a new parking lot in what had been a Tunkhannock field, or a subdivision replacing forested land in Dallas Township, both of which will speed up rainfall's journey to the river. An alteration could be something more dramatic: a stateof the-art levee system like the one protecting portions of the Wyoming Valley.
A levee system forces the swollen river into a narrower channel, Nelson said, creating the side effect of a backup, which raises levels upstream. In the levee, the flowing water has less friction and more force behind it, so the river gains velocity. Downstream, the pent up water is freed from the girdle of the levee, and pours forward and outward, like a pitcher spilled on a table.
Experts have been rethinking levees, said Christopher P. Konrad, a research hydrologist for the U.S. Geological Survey. In some places, land is being condemned and levees set farther back, allowing more room for the river and effectively reducing flood levels. Developers are beginning to look more critically at land use in flood plains. Rather than building an expensive levee to protect a housing development or shopping center, riverbanks are being left alone, and the land used for farming or as transportation corridor, uses that could tolerate occasional flooding, Konrad said.
After the Wyoming Valley levee was raised an additional three to five feet in 2003, the U.S. Army Corps of Engineers said the modified levee would protect from an Agnes-level flood, estimated to be a once every 345-year event. On Friday morning, the Susquehanna River in Wilkes-Barre crested at 42.66 feet, about 1.5 feet higher than Agnes.
How could a 345-year flood happen twice in a half century?
Part of the answer may be sprawl and development in outlying areas that drain into the Susquehanna River. Wetlands and forests act as a sponge, soaking up and consuming water. Every rooftop, roadway, driveway, parking lot and piece of concrete robs the land beneath it from its absorbency. Rather than being retained in the ground, the runoff water is swiftly carried to gorged rivers and streams.
But Konrad said he’s not sure if development made a difference in Northeastern Pennsylvania's recent flooding. While homes, roads and patios make the ground underneath impervious to water, saturated ground can be just as impervious. For that reason, development is more likely to promote flash flooding.
Dave Bollinger, outreach coordinator for the Federal Emergency Management Agency, said weather is getting more extreme, more often.
Somewhere between the upstream rural run-off and the state-of-the-art levee system are communities such as Duryea and West Pittston _ places underwater Friday. For them, the flood prompted by the one-two punch of tropical storms Irene and Lee brought inevitable comparisons to the Agnes Flood.
AccuWeather meteorologist Dave Dombek, who grew up in the Dallas area and remembers Agnes, sees similarities.
In both the Agnes and Irene/Lee events, above-average rain was falling several weeks previously. Closer to the event, rain fell daily. The soil, super-saturated, could hold no more water. Areas far upstream, in the Northern Tier of Pennsylvania and New York state saw even more rain, which all drained through the Susquehanna River.
After the waters recede, the agency will assess the levees to see whether the river incurred any serious damage.