The reservoir at Howard Hanson Dam has reached an elevation of 1,167 feet above sea level, allowing engineers, geologists and other scientists to run a battery of tests to gather more data on the right abutment drainage improvements that are completed and as well as repairs still underway.
At the end of testing, the U.S. Army Corps of Engineers will share findings with national experts and will determine prior to November if the dam can return to its authorized flood risk reduction operation, according to a Friday media release from the Seattle division of the corps.
The corps is measuring seepage rates through the abutment, functionality of the new drains, as well as conducting other investigations during the next couple of weeks. Crews have installed additional drains and wells at the dam.
The results of that data analysis will help determine paths for further interim measures, as well as any potential long-term repairs, to reduce the flood risk in the highly populated cities of Auburn, Kent, Renton and Tukwila and unincorporated King County.
Problems with water storage behind the dam were discovered by the corps when a 10-foot-wide depression formed on the embankment next to the dam after heavy rain in January 2009. The corps stored a record amount of water in the reservoir during that storm to prevent flooding.
“As construction on the drainage improvement work continues, all indications are that the new measures will perform as intended,” said Rob Romocki, the Seattle District’s dam safety program manager. “Once the current improvement work has been fully completed, we will evaluate the effectiveness of those measures and determine if we will be able to hold a flood pool as the dam was originally designed. We need to look at all the data from this summer’s conservation pool and analyze what we see before a decision can be made on how to safely operate the dam next flood season.”
The corps received $44 million from the federal government for the Hanson Dam repairs. About $30 million of that has been spent and the remaining funds will cover additional repairs to complete in an effort to handle a 1 in 10,000-year flood. That work includes installation of booms to block debris from entering the dam’s spillway. Officials would only use the spillway if a flood is expected to exceed the design capacity of the dam.
While the reservoir level is being held steady throughout this testing, outflows from the dam will track reservoir inflows. As of Friday, inflows and outflows are roughly 1,200 cubic feet per second. However, flows can change in a matter of hours, dependent upon weather, so caution is advised. Flows downstream of the dam are expected to remain well below flood stage.
More details and history about Howard Hanson Dam and the right abutment issue can be found at http://www.nws.usace.army.mil.
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