March 12, 2001
Tolt Dam passes post-quake inspection
by Valley View staff
Seattle Public Utilities officials have announced that the Tolt Dam, water intake, Tolt Reservoir and other facilities showed no damage after the recent earthquake.
In a prepared statement, SPU officials said that immediately after the earthquake, the Seattle Public Utilities Tolt maintenance crew inspected the dam. The Tolt warning system, which controls the sirens in Carnation, had no damage. However, because the earthquake occurred one hour before the normal Wednesday noonday test, Seattle Public Utilities decided not to sound the usual siren, fearing it might cause panic among residents in Carnation.
The Tolt Project is located in the South Fork Tolt River Watershed, a few miles east of Duvall. It consists of three earth embankment dams and associated structures.
The highest is the Tolt Dam, a 200-foot-high earth embankment dam completed in 1961. This dam has a spillway to release water during periods of high reservoir levels. The water for water supply is piped about five miles downstream to the regulating basin. Failure of the Tolt Dam could impact the town of Carnation and residents along the Tolt River Road and therefore a new Tolt Dam Failure Warning System was installed in 1993, to replace an aging system. The warning system gives Carnation 1.1 hours to perform evacuation before the Tolt River would overtop its banks in the highly improbable failure of the Tolt Dam.
Several years ago the project was evaluated for earthquake reliability. It was found that the south dam at the regulating basin needed strengthening. That work was completed in 1995. The intake tower was strengthened in 1998. The intake's failure would not affect the dam, but would affect SPU's ability to deliver water to the region, the statement concluded.