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Seattle District posts potential flood scenario maps online

The U.S. Army Corps of Engineers, Seattle District, announced Tuesday it was making flood mapping of the Green River Valley available for public use.

The mapping has been highly sought after since the corps raised concerns about the stability of a key structure - an earthen abutment - at the Howard Hanson Dam.

The maps show four possible scenarios, based on flows in the Green River, out of a myriad of potential conditions.

The maps represent a range of flow scenarios that the corps evaluated for planning purposes, to understand the consequences of types of operations at the dam, which could be necessary due to reduced operational capacity.

There is no probability associated with these flows.

The scenarios are based upon levels of response that may be needed, and can be useful for good planning by the civic authorities and public alike.

“We moved to get this information out in a more convenient way as quickly as possible, realizing that public knowledge of the potential flooding consequences of the reduced operational capacity of the Howard Hanson Dam contained within the mapping was critical to the public we protect and serve,” said Rob Romocki, the district’s dam safety program manager.

“The entire team working on the Howard Hanson Dam takes this challenge personally and professionally,” said Lt. Col. James Rollins, deputy district commander for Howard Hanson Dam. “We have good data for the public and can explain the impacts so proper planning can be developed.”

The Seattle District will continue evaluating reservoir operations at Howard Hanson Dam as interim risk reduction work is completed and tested. However, at this time the corps is saying it is unsafe to operate the dam at full capacity.

A major storm in January 2009 damaged an earthen abutment adjacent to the dam. That damage has reduced the effectiveness of the dam's ability to hold back water above a certain level, and increased the flood risk to the Green River Valley. The corps is installing a grout curtain along the abutment and improving a drainage tunnel as short-term fixes, but the agency also is designing a permanent repair – a subterranean concrete cut-off wall.

While the Corps intends to complete the design of the repair in one year, it is not known how long construction will take, considering the complexity of the problem. So, until that fix is in place the flood risk will be increased for the communities along the Green River.

Because these scenarios do not represent or limit the range of actual flows that may be released, the Corps cautions the public to be prepared even if a scenario map indicates that they may not actually have flood waters inundating their property. Furthermore, impacts in regards to utilities, transportation, etc. should also be taken into consideration in planning for any emergency contingency.

Since January, the Corps’ Seattle District has been working in partnership with King County and the cities in the Green River Valley to warn residents and businesses of the increased risk for downstream flooding due to decreased water-holding capacity at Howard Hanson Dam.

Information on flood preparedness may be found at: http://www.kingcounty.gov/safety/prepare/FloodPlan_GRiverBasin.aspx

Updates regarding the dam may be found at: www.nws.usace.army.mil

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