page_title: Howard A. Hanson Dam
section_title: Flooding concerns
hierarchical_section_title: Howard A. Hanson Dam / Flooding concerns
caption_attribution_description: English: Image created by and courtesy of U.S. Army Corps of Engineers. Exact date not listed.
context_page_description: Howard A. Hanson Dam is an earthen embankment dam on the Green River, 21 miles east of Auburn, Washington. The dam was completed in 1961 and its primary purpose is flood control along with water supply for Tacoma, Washington.
  After the dam's reservoir reached record levels in January 2009, the U.S. Army Corps of Engineers discovered seepage in the right abutment of the dam, leading to efforts to mitigate a potential flood. Improved drainage works were completed in October 2011 and although studies are still underway, the dam was declared safe to maintain its maximum reservoir level.
context_section_description: In January 2009, 15 inches (380 mm) of rainfall within 24 hours peaked water inflows behind the reservoir at 30,500 cubic feet per second (860 m³/s), raising the reservoir level to a record high of 1,189 feet (362 m) above sea level. In comparison, the maximum authorized storage level is 1,206 feet (368 m) above sea level and the normal summer level is 1,167 feet (356 m). The Howard A. Hanson dam had controlled and prevented a flood that would have caused an estimated $4 billion in damages.
  However, soon after the flood the USACE discovered two depressions on the right abutment of the dam, increased water levels in groundwater monitoring wells and the appearance of sediment-laden water entering the abutment drainage tunnel. The USACE did not believe the dam was at immediate risk of failing but assessed increased danger to downstream communities until the problem was resolved. The USACE began to lower and restrict the elevation of the dam's reservoir. In addition, by November 2009, the USACE constructed an interim seepage barrier along with improving the drainage of the right abutment. These actions were estimated to reduce the chance of flooding from a 1 in 3 to a 1 in 25 chance during 2010.
  The level of the reservoir was restricted due to increased chances for flooding downstream. In the event of a major flood, increased releases may cause water to over-top downstream levees. Sandbags were distributed throughout the Green River Valley as the USACE continued to inspect, monitor and repair the dam. After improvements to the dam's drainage works were completed in October 2011, the dam was declared able to hold its maximum storage level. Previously in the summer, the reservoir reached normal summer levels. A full study of the dam is still underway and is expected to be published in 2013.
caption: Howard A. Hanson Dam USACE annotated aerial photo
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