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Background:
Following a record-high water level behind Howard Hanson Dam in January 2009, the U.S. Army Corps of Engineers, Seattle District, became concerned after discovery of two depressions on the right abutment, increased water levels in groundwater monitoring wells and the appearance of sediment-laden water entering the abutment drainage tunnel. These concerns compelled the Corps to limit the dam’s operational capacity, in order to not put it at further risk. While the dam itself was not in immediate danger of failing, there was increased risk to downstream communities until seepage concerns with the right abutment were resolved. The Corps of Engineers immediately placed restrictions on flood storage and established an aggressive monitoring program, in addition to other risk reduction measures. Reduced flood storage capacity resulted in more frequent and larger volume releases during flood events, which increased the probability that levees in the lower valley could be overtopped. Work completed reduced this risk and included constructing a seepage barrier (grout curtain) and improvements to drainage in the abutment. Extensive coordination with local communities occurred to aid emergency preparedness. In September 2011, District Engineer Bruce Estok confidently announced the Corps could safely store water to the summer conservation pool level of 1,167 feet above sea level (48 percent of full) during the 2011-2012 flood season.

Situation:
The announcement came after investing $40 million and two years to complete repairs. Drainage improvement work finished in September 2011. Drains through the area of highest concern, known as the short path seepage area, were completed and tested. The reservoir at Howard Hanson Dam reached a summer elevation of 1,167 feet above sea level in June 2011, allowing engineers, geologists and other scientists to run a battery of tests to gather more data on the completed right abutment drainage improvements. The Army Corps of Engineers determined in September 2011 that Howard Hanson Dam could safely store a flood pool elevation of 1,067 during the 2011-2012 flood season. The rainfall and flow rate during the February 2012 flooding landed in the top ten events in Howard Hanson Dam’s history, but many may not have realized the potential impact because the dam functioned according to design. Without Howard Hanson Dam holding back flows, estimated flows on the Green River would have been near 22,000 cubic feet per second, nearly double the river’s maximum design flow of 12,000 cubic feet per second measured in Auburn, Wash. After spending $40 million in repairs, Seattle District engineers and scientists have concluded that the Corps can operate the dam to hold its design full pool – an elevation of 1,206 feet above sea level – with low risk to the dam. The Corps is also making good progress on a detailed dam safety study to ensure all significant and credible risks have been evaluated. The study is scheduled to be completed by the end of 2013.

Key Points:

  • Public safety is the Army Corps’ number one priority. 
  •  Howard Hanson Dam is being operated to hold its design pool, an elevation of 1,206 feet above sea level, with low risk to the dam. 
  •  Engineering investigations and completed actions since 2009 greatly improve our understanding of the conditions at the dam and increase our confidence in the structure.
  • All indications are that the new measures will work as intended to the dam’s full flood storage design capacity. However, until the dam experiences a flood above summer pool level (1,167 feet), there is not 100 percent certainty of how the new measures will perform. The Corps has a detailed monitoring plan in place to enable dam operators to have real-time information about how the dam is performing in order to make any necessary changes to the planned operation.
  • The Corps will continue to monitor the dam to ensure effectiveness of the corrective measures, especially during high flow events. Depending upon the results of the ongoing dam safety study, additional measures may need to be implemented to further address identified risk concerns.
  • The return of full operational capacity of Howard Hanson Dam, in partnership with a functioning levee system downstream does not eliminate all risks of flooding. The dam and levees only reduce the risk of flooding. The Corps is working with local communities to prepare for the risk of flooding to downstream areas using the best available information.

Additional Background Information:
Howard Hanson Dam is located on the upper reach of the Green-Duwamish River in King County, 63 river miles above the mouth. It is in the city of Tacoma’s municipal watershed 35 road miles east of Tacoma, 6 miles upstream from Palmer. This project is protected from public access.

The Howard Hanson Dam serves multiple purposes by providing flood risk reduction, municipal water supply and summer and fall low flow augmentation for fish. Flood risk reduction in the Green-Duwamish River Basin is accomplished by capturing excessive water runoff from the upper drainage area of the river and releasing the water under controlled conditions. After the end of the annual winter flood season, water is gradually stored in the reservoir beginning about March 1 for municipal water supply and for conservation (low flow augmentation) purposes.

Flood damage prevented by Howard Hanson Dam from the January 2009 flood is estimated at about $4.6 billion.

The dam is an earth and rockfill structure with inclined core drain and filters. Outlet works on the left bank consist of an approach channel, an intake structure providing upstream control, a 19-foot diameter horseshoe concrete-lined tunnel, a stilling basin, and an auxiliary 48-inch diameter bypass pipe. A gated spillway on the left abutment with two 45 by 30-foot tainter gates permits reservoir storage to elevation 1,206 without utilizing the spillway for discharge. The paved spillway chute is 656 feet long.

The Corps of Engineers constructed a seepage barrier in November 2009 to reduce seepage and improved the drainage of the right abutment by installing drains that more effectively direct seepage into the drainage tunnel. Testing showed that the work controlled seepage more effectively.

Along with controlling seepage in the right abutment by installing additional vertical and horizontal filtered drains, the Corps recently completed projects to increase confidence that the dam can safely operate during extreme flood events (i.e., pools above elevation 1,206 feet that involve use of the spillway). These measures, completed in 2012, included the installation of additional log booms to prevent debris from blocking the spillway and placing additional rock along the upstream face of the dam to protect it against erosion from fast-moving water in the event the spillway is used.


abutment dam flood Green River Howard Hanson risk