SEATTLE – The Oroville Dam has some things in common with the Howard Hanson dam in Washington state and some major differences.
Portions of both dams are earthfill embankments, and both have seen trouble. In January 2009, the Howard Hanson dam on the Green River held back a record amount of water following major storms.
The dam was opened in 1961, primarily to provide flood control for the Green River Valley running through much of southern King County. For decades, the Green River would flood. Howard Hanson brought an end to that. It also provides drinking water to the City of Tacoma.
After that record-high pool of water, the Army Corps of Engineers, which operates the dam, found a problem. Two depressions formed in the earthen part of the dam. That earthen area termed the “right abutment” is up against the fixed concrete portion of the dam. The concrete portion includes gates leading to a spillway that did not have to be opened. Those depressions indicated that the integrity of the rock and dirt that makes up the right abutment might be compromised not only by the weight of the water that seeps through it, but that seepage was washing out fine silts and sediments that help hold the earthen part of the dam together.
What followed was three years of construction work to make sure the dam would not fail, which included injecting the earthen part of the dam with cement-like material known as grout. The dam also had pumps installed and major upgrades to its monitoring equipment.
The Army Corps of Engineers said it might have to release water down the Green River until that work was completed. Because the Green River runs through cities like Auburn, Kent, and Renton, large sandbags and other retention devices were installed to prevent flood waters from reaching homes and industries that have replaced much of the farmland that was in the area before the dam was built.
After the fixes, the sandbags were removed in 2012.
Unlike the Oroville Dam, concerns over Howard Hanson never reached the point of triggering evacuations. While there were worries about the dam, it did not fail.
But Howard Hanson is also much smaller than the Oroville Dam, which holds back 22 times the amount of water and catches and retains water from 3,607 square miles. Howard Hanson catches water from 220 square miles.
