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The threat of future flooding in the Wyoming Valley has been upped.
Why?
The historic Susquehanna River flooding that occurred here four years ago this week sparked a fresh study of water — the amount coming into the Wyoming Valley and where it goes once it gets here.
Information about topography, past flooding events, stream gauge readings and other hydrology and hydraulic data were loaded into a computer modeling program that has become more advanced since the last analysis in 2003, officials said.
The findings:
• Climate change has led to more frequent, severe storms. Four of the eight highest Wyoming Valley floods since 1936 have occurred in the past 20 years.
• Paving and deforestation from land development have reduced the sponge-like ability of this northern watershed to absorb water, creating more runoff
• Changes to the river channel — sediment, tree growth, bridges — have reduced how much water is contained within the Susquehanna River bank during peak flow periods.
The U.S. Army Corps of Engineer focused heavily on the 10,000-square-mile watershed that drains into the local stretch of the Susquehanna — a swath that stretches west and north, tickling Wellsboro in Pennsylvania and extending into the Finger Lakes and Catskills in New York.
“Every drop of water in this area theoretically comes down to Wilkes-Barre, and this watershed is massive — massive,” said Christopher Belleman, an engineer and executive director of the Luzerne County Flood Protection Authority that oversees the Wyoming Valley levee.
“When I show people the size of this watershed, their jaws drop,” he said. “The next maps will show more properties are at risk. That’s why it’s important to get people out of the flood zone, especially if they’re not levee-protected.”
100-year flood
The government translates its hypotheses through a standard known as a “base flood elevation,” or the estimated height water is projected to rise during a “base flood.”
Commonly known as a “100-year flood,” a base flood has a 1-percent chance of happening or being exceeded in any given year.
The base flood elevation is used for a myriad of purposes that have a major impact in flood-prone communities: which mortgaged properties require flood insurance, how much property owners will pay for insurance and which high-risk zones require tougher restrictions or limitations on construction.
The new study concluded the base flood elevation has increased by 0.7 feet to 4 feet in some locations along the Susquehanna in the 100-mile stretch from the Luzerne County line in Exeter Township south to Sunbury, Belleman said.
The Federal Emergency Management Agency (FEMA) is expected to implement new flood maps incorporating the updated analysis within the next few years. The current maps were implemented in November 2012.
“Everything in the past that was built for old flood levels is kind of obsolete now because now we have a higher threat of more water coming down,” Belleman said.
Levee rating
Properties protected by the Wyoming Valley levee aren’t immune from the revised flood risk.
The federal government has determined the Wilkes-Barre/Hanover Township and Plymouth stretches of the levee won’t meet accreditation requirements because some sections no longer have a required precautionary 3-foot back-up buffer on top — known as a “freeboard” — with the new projections, Belleman said.
Accreditation means the federal government is confident a levee will provide adequate base flood risk reduction for insurance and building requirements.
The analysis showed the freeboard is now closer to 2.3 feet in some areas of both non-accredited levee stretches.
The Kingston-to-Exeter levee stretch will remain accredited because it’s on higher ground, Belleman said.
The owners of properties around the insufficient freeboard may face higher flood insurance premiums as a result, but not as high as properties that have no levee protection, he said.
The impact the new data will have on the flood insurance for properties along the Susquehanna — levee-protected and not — won’t be finalized until the next set of maps takes effect in a couple of years, Belleman said.
What to expect
The Wyoming Valley levee-raising completed in the early 2000s, which cost around $175 million, was based on water modeling analysis completed in the mid-1980s, Belleman said.
He doesn’t foresee another boost of the levee height in his lifetime.
“I don’t think there’s a lot of political will to have another levee-raising, and someone would have to pay the local sponsor share, which would be in the millions. Nobody wants to take on that debt,” he said.
It’s not as simple as dumping more dirt atop the levee stretches of concern because levees must be widened as the height increases to keep them stable, requiring additional property acquisition, he said.
The federal government has been emphasizing the levee is officially called the “Wyoming Valley Flood Risk Project,” not “protection project,” so the public isn’t lulled into the perception the system can hold back everything that may be thrown at it in the future, Belleman said.
“You can’t build these things for every conceivable flood event, so you select a design level which you think will take care of most events,” Belleman said.
Still, the levee held up and was not overtopped when the Susquehanna reached a record 42.6 feet during Tropical Storm Lee in September 2011, saving an estimated 65,000 properties, he said. It was a close call because the levee was designed for 41 feet, with an additional 3-foot freeboard.
The Army Corps estimates nearly $8 billion in flood damages have been averted by the levee system since 1968, and the federal government continues to give the authority high marks for maintaining it.
“We’re very fortunate to have this in our area,” Belleman said. “A lot of communities would kill to have this system,” Belleman said.