Condit Dam, White Salmon River
Condit Dam was located 3.3 miles upstream from the confluence of the White Salmon and Columbia Rivers in southwestern Washington. Water impounded by the dam formed Northwestern Lake and was used to generate hydroelectric power for the surrounding region. The dam was constructed between 1911 and 1913 and was operated by PacifiCorp beginning 1947. In 2011, PacifiCorp chose to remove the dam rather than add fish passage capability required under a new federal dam license.
Decommissioning and removal of the dam was initiated by draining Northwestern Lake through a drill-and-blast tunnel under Condit Dam. The work consisted of excavating a nominal 15 ft by 15 ft adit through the concrete dam; drilling and loading a live lake tap round near the rear face of the dam; and blasting the tap round to drain the existing lake. During the lake tap event approximately 750 acre-ft of water was drained into the White Salmon River downstream of the dam and into the Columbia River, helping to wash the accumulated sediment from the reservoir. Removal of the dam and restoration of the White Salmon River then followed.
Lachel provided project management as well as design and construction services for the Kiewit Infrastructure Group during the tunnel and lake tap portion of the decommissioning work. The blast holes, explosive selection and loading, and the initiation system for the final lake tap round were designed by Lachel to ensure a successful breach through the dam into the lake. Lachel was also responsible for the design of the thickness of the final blast round to ensure that the concrete left in place prior to the tap round was adequate to resist the water pressure and provide safety for the personnel working in the tunnel. This information was used during the tap round design to assure that the full, final round was blasted through the upstream face of the dam. Lachel also provided vibration monitoring services to ensure compliance with project specifications and installed seismographs at multiple locations (including nearby residences, a water supply facility for the City of White Salmon, and the dam itself) to monitor and document the vibrations produced by blasting operations.