Howard A. Hanson Dam
| Howard A. Hanson Dam (Howard A. Hanson Reservoir / Pool) |
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| Coordinates | 47 ° 16 '40 " N , 121 ° 47' 15" W | ||||||
| Data on the structure | |||||||
| Construction time: | 1959-1961 | ||||||
| Height of the barrier structure : | 72 m | ||||||
| Crown length: | 206 m | ||||||
| Crown width: | 7 m | ||||||
| Base width: | 292 m | ||||||
| Operator: | US Army Corps of Engineers | ||||||
| Data on the reservoir | |||||||
| Total storage space : | 131 000 000 m³ | ||||||
The Howard A. Hanson Dam is an earth dam on the Green River in King County in the US state of Washington , 34 km east of Auburn . The dam was completed in 1961; its primary purposes are flood control and water supply for Tacoma .
After the reservoir reached record heights in 2009, the US Army Corps of Engineers discovered a seepage point at the right end of the dam, leading to efforts to avoid a potential flood. Work to improve the drainage was completed in October 2011 and although investigations are still being made, the dam has been declared safe to hold the maximum storage volume.
history
The Green River Valley was settled by whites in the 1850s and developed into a significant agricultural area until an aphid plague destroyed the large hop plantations in 1890 . Since then, the area has mainly consisted of dairy cattle and berry farms, but the farmers had to cope with floods every year. These floods also resulted in damming by transported logs that diverted the Green River and other rivers. Occasionally the farmers cleared such dams with dynamite , but this led to the flooding of other farms. To deal with this situation, the Associated Improvement Club of South King County was formed in 1926. The company built several culverts and corrected rivers, but soon reached their limits and asked for help.
It wasn't until 1936 that communities and citizens began looking for a suitable location for a flood protection project on the Green River, with additional support from the US Congress , the US Army Corps of Engineers (USACE), and the Seattle District. The selection was delayed by World War II ; finally, Eagle Gorge was proposed to Congress in 1949 as the site for a dam and reservoir project.
In 1955, funds were approved for the project; US $ 37 million from Congress, US $ 1.5 million from Washington State and US $ 500,000 from King County. Construction on the dam, which included the laying of 21 kilometers of the Northern Pacific Railway , began in February 1959. In December 1959 - while construction was still in progress - the last devastating flood occurred on the Green River; it damaged residential buildings and carried topsoil from the farms. Regardless, the dam was completed five months ahead of schedule for Christmas 1961. Completion ends a 70-year era of flood events in the Green River Valley and by 1996 the dam prevented an estimated $ 694 million in flood damage.
The dam was originally called Eagle Gorge Dam and was renamed Howard A. Hanson Dam by Congress in 1958. Hanson led civil and government groups to build the dam. He was instrumental in the successful financing of the dam, but died in 1957 before construction began.
features
The Howard A. Hanson Dam is an earth dam with a length of 205 meters and a height of 70 meters. The foot of the dam is 290 m wide and blocks the Eagle Gorge, a gorge with almost vertical rock walls. The lake dammed by the dam collects the water from a 570 km² catchment area and is itself 11 km long. The dam contains a flood relief made of concrete on its left side , which has a capacity of up to 3,000 m³ / s. The flood relief is controlled by two 15 m × 6 m segment weirs , while the outlet tunnel at the base of the dam is controlled by two 3 m × 4 m segment weirs.
The dam is used for flood protection during the rainy season by collecting the water and then allowing it to flow off in a controlled manner through the outlet tunnel. This process runs as often as necessary; the relief is used in extreme floods. However, this has never been necessary before. From March, the dam is used to store water and during the summer certain amounts of water are drained to support fish migrations and spawning as well as fishing enthusiasts. During the winter, the reservoir is kept almost empty. The dam lies within the Tacoma water catchment area. There is no public access, which from time to time leads to controversy with proponents of recreational use. The water diverted from the reservoir is fed to a waterworks for civil use 5 km downstream.
Flood hazards
In January 2009, rainfall of 380 millimeters within 24 hours caused an influx of 860 cubic meters per second and raised the lake level to a record level of 362 meters above sea level. For comparison: the maximum permitted congestion level is 368 meters and the normal summer level is 356 meters above sea level. The Howard A. Hanson Dam controlled and prevented a flood with an estimated damage amount of US $ 4 billion.
However, the USACE soon discovered two depressions at the right end of the dam, increased levels in the groundwater levels and the occurrence of sediment- laden water at the drainage of the dam end. The USACE did not believe there was an imminent risk of dam rupture, but identified an increased risk for downstream locations until the problem was resolved. The USACE began to lower and limit the congestion destination . In addition, a temporary seepage barrier was built by November 2009 and the drainage at the right end was improved at the same time. These actions should reduce the risk of dam rupture from a 1 in 3 to a 1:25 chance during 2010.
The target of the lake was restricted because the risk of flooding below the dam had increased. In the event of a flood of the century, increased runoff would flood the dikes downstream. Sandbags were distributed throughout the Green River Valley as USACE continued to inspect, monitor, and repair the dam. After the improvements to the dam's drainage were completed in October 2011, it was declared that the dam was able to maintain its maximum retention target. In the summer of 2012, the dam reached its normal summer congestion destination for the first time. A full survey of the dam should have been published in 2013.
See also
Web links
- USACE - Howard A. Hanson Dam - Official Website (English)
- History of the Howard A. Hanson Dam at HistoryLink
Individual evidence
- ↑ a b c d e f g h USACE - Howard A. Hanson main page ( Memento of the original from June 18, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ History Link - Howard A. Hanson History
- ↑ Water seeping through Howard Hanson Dam is picking up speed - Seattle Times - January 24, 2009 ( Memento of the original from June 27, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Engineers continue testing, gathering data at Howard Hanson Dam - June 1, 2009 ( Memento of the original from June 19, 2009 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ USACE Howard A. Hanson Dam FAQ ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ^ Fact Sheet - Howard A. Hanson Dam . US Army Corps of Engineers. September 2011. Retrieved on March 16, 2012. ( Page no longer available , search in web archives )
