Howard A. Hanson Dam
Encyclopedia
Howard A. Hanson Dam is an earthen embankment dam
on the Green River
, 21 miles (34 km) 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.
infestation destroyed most of the crops in 1890. Since then, the area mainly consisted of cattle and berry farms but farmers had to cope with yearly floods. These floods would lead to log jams which redirected the Green and other rivers. Sometimes farmers would illicitly clear log jams with dynamite
which led to the flooding of other farms. To deal with the matter, the Associated Improvement Clubs of South King County was formed in 1926. The association created several dikes and repaired river courses but they would soon realize more help was needed.
It was not until 1936 that community leaders, citizens and with additional effort by Congress
, the U.S. Army Corps of Engineers (USACE), Seattle District began to search for a suitable flood control project site on the Green River. The selection was delayed by the Second World War and finally in 1949, Eagle Gorge was recommended to Congress as a site for a dam
and reservoir
project.
In 1955, funds were appropriated for the project; Congress gave $37 million, Washington State $1.5 million and King County provided $500,000. Construction on the dam which included the relocation of 13 miles (21 km) of the Northern Pacific
Railway began in February 1959. During construction in December 1959, the last devastating flood occurred on the Green River; damaging homes and removing topsoil
from farms. Despite this, the dam was completed almost five months ahead of time on Christmas
Day, 1961. The completion of the dam ended a 70-year era of flooding in the Green River Valley and by 1996, the dam had prevented an estimated $694 million in flood damages.
The dam was originally named the Eagle Gorge Dam but was renamed by Congress in 1958 to the Howard A. Hanson Dam. Hanson led civic and government groups in an effort to construct the dam. He was largely instrumental in obtaining funding for the dam. Hanson died in 1957 before construction began.
on its left side capable of discharging overflow at 107000 cuft/s. The spillway is controlled by two 45 x 30 ft (15 x 6 m) Tainter gate
s while an outlet tunnel at the base of the dam is controlled by two 10 x 12 ft (3 x 4 m) Tainter gates.
The dam serves its flood control purpose during the rainy season collecting and then controlling the release of flood waters through the base outlet tunnel. This process occurs as often as necessary and in the event of an extreme flood, the spillway can be used. However, spillway use has never been required. In March, the dam serves to conserve water and during the summer, releases specific amounts to assist in fish migration and spawning along with enhancing sports fishing. During the winter, the reservoir is kept near empty. The dam is located within the Tacoma watershed and public access is not allowed, creating controversy among recreation proponents for some time. Water released from the dam is diverted into a water purification
facility 3 miles (5 km) downstream for civil consumption.
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 does not believe the dam is 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 reduced the chance of flooding from a 1 in 3 to a 1 in 25 chance during 2010.
Although the level of the reservoir is restricted, there are still increased chances for flooding downstream. In the event of a major flood, increased releases in order to maintain proper levels may cause water to over-top downstream levees. Sandbags have and are being distributed throughout the Green River Valley as the USACE continues to inspect, monitor and repair the dam.
Embankment dam
An embankment dam is a massive artificial water barrier. It is typically created by the emplacement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock. It has a semi-permanent waterproof natural covering for its surface, and a dense, waterproof...
on the Green River
Green River (Washington)
The Green River is a long river in the state of Washington in the United States, arising on the western slopes of the Cascade Mountains south of I-90....
, 21 miles (34 km) east of Auburn, Washington
Auburn, Washington
-Parks:Auburn has an extensive system of parks, open space and urban trails comprising 29 developed parks, 5 undeveloped sites under planning, 2 skate parks, 2 water roatary parks, and over of trails , and almost of open space for passive and active recreation.-Environmental Park:The Auburn...
. The dam was completed in 1961 and its primary purpose is flood control
Flood control
In communications, flood control is a feature of many communication protocols designed to prevent overwhelming of a destination receiver. Such controls can be implemented either in software or in hardware, and will often request that the message be resent after the receiver has finished...
along with water supply for Tacoma, Washington
Tacoma, Washington
Tacoma is a mid-sized urban port city and the county seat of Pierce County, Washington, United States. The city is on Washington's Puget Sound, southwest of Seattle, northeast of the state capital, Olympia, and northwest of Mount Rainier National Park. The population was 198,397, according to...
.
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.
History and construction
The Green River Valley was settled in the 1850s and became a notable farming area until an aphidAphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...
infestation destroyed most of the crops in 1890. Since then, the area mainly consisted of cattle and berry farms but farmers had to cope with yearly floods. These floods would lead to log jams which redirected the Green and other rivers. Sometimes farmers would illicitly clear log jams with dynamite
Dynamite
Dynamite is an explosive material based on nitroglycerin, initially using diatomaceous earth , or another absorbent substance such as powdered shells, clay, sawdust, or wood pulp. Dynamites using organic materials such as sawdust are less stable and such use has been generally discontinued...
which led to the flooding of other farms. To deal with the matter, the Associated Improvement Clubs of South King County was formed in 1926. The association created several dikes and repaired river courses but they would soon realize more help was needed.
It was not until 1936 that community leaders, citizens and with additional effort by Congress
United States Congress
The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....
, the U.S. Army Corps of Engineers (USACE), Seattle District began to search for a suitable flood control project site on the Green River. The selection was delayed by the Second World War and finally in 1949, Eagle Gorge was recommended to Congress as a site for a dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
and reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
project.
In 1955, funds were appropriated for the project; Congress gave $37 million, Washington State $1.5 million and King County provided $500,000. Construction on the dam which included the relocation of 13 miles (21 km) of the Northern Pacific
Northern Pacific Railway
The Northern Pacific Railway was a railway that operated in the west along the Canadian border of the United States. Construction began in 1870 and the main line opened all the way from the Great Lakes to the Pacific when former president Ulysses S. Grant drove in the final "golden spike" in...
Railway began in February 1959. During construction in December 1959, the last devastating flood occurred on the Green River; damaging homes and removing topsoil
Topsoil
Topsoil is the upper, outermost layer of soil, usually the top to . It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.-Importance:...
from farms. Despite this, the dam was completed almost five months ahead of time on Christmas
Christmas
Christmas or Christmas Day is an annual holiday generally celebrated on December 25 by billions of people around the world. It is a Christian feast that commemorates the birth of Jesus Christ, liturgically closing the Advent season and initiating the season of Christmastide, which lasts twelve days...
Day, 1961. The completion of the dam ended a 70-year era of flooding in the Green River Valley and by 1996, the dam had prevented an estimated $694 million in flood damages.
The dam was originally named the Eagle Gorge Dam but was renamed by Congress in 1958 to the Howard A. Hanson Dam. Hanson led civic and government groups in an effort to construct the dam. He was largely instrumental in obtaining funding for the dam. Hanson died in 1957 before construction began.
Dam characteristics
The Howard A. Hanson Dam is an earthen embankment structure at a length of 675 ft (205 m) and height of 235 ft (70 m). The base of the dam is 960 ft (290 m) wide and sits within Eagle Gorge, a ravine with nearly vertical rock walls. The reservoir withheld by the dam collects water from 220 square miles (570 m2) of drainage area 7 miles (11 km) long. The dam structure contains a concrete spillwaySpillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...
on its left side capable of discharging overflow at 107000 cuft/s. The spillway is controlled by two 45 x 30 ft (15 x 6 m) Tainter gate
Tainter gate
The Tainter gate is a type of radial arm floodgate used in dams and canal locks to control water flow. It is named for Wisconsin structural engineer Jeremiah Burnham Tainter....
s while an outlet tunnel at the base of the dam is controlled by two 10 x 12 ft (3 x 4 m) Tainter gates.
The dam serves its flood control purpose during the rainy season collecting and then controlling the release of flood waters through the base outlet tunnel. This process occurs as often as necessary and in the event of an extreme flood, the spillway can be used. However, spillway use has never been required. In March, the dam serves to conserve water and during the summer, releases specific amounts to assist in fish migration and spawning along with enhancing sports fishing. During the winter, the reservoir is kept near empty. The dam is located within the Tacoma watershed and public access is not allowed, creating controversy among recreation proponents for some time. Water released from the dam is diverted into a water purification
Water purification
Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...
facility 3 miles (5 km) downstream for civil consumption.
Flooding concerns
In January 2009, 15 inches (380 mm) of rainfall within 24 hours peaked water inflows behind the reservoir at 30500 ft3/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 ft (367 m) above sea level and the normal summer level is 1,167 feet (355 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
Abutment
An abutment is, generally, the point where two structures or objects meet. This word comes from the verb abut, which means adjoin or having common boundary. An abutment is an engineering term that describes a structure located at the ends of a bridge, where the bridge slab adjoins the approaching...
of the dam, increased water levels in groundwater monitoring wells and the appearance of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
-laden water entering the abutment drainage tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
. The USACE does not believe the dam is 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 reduced the chance of flooding from a 1 in 3 to a 1 in 25 chance during 2010.
Although the level of the reservoir is restricted, there are still increased chances for flooding downstream. In the event of a major flood, increased releases in order to maintain proper levels may cause water to over-top downstream levees. Sandbags have and are being distributed throughout the Green River Valley as the USACE continues to inspect, monitor and repair the dam.