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, six 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.