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- Conconully Safety of Dams
Questions and Answers
Who owns and operates Conconully Dam?
The Bureau of Reclamation owns Conconully Dam, and the Okanagan Irrigation District operates the facility. The dam is a hydraulic earth-fill structure located in the Okanogan Highlands in Okanogan County, Washington—a mountainous district bordered by the Columbia Plateau to the south and the Cascade Mountains to the west. The dam is a key storage feature for the Okanagan Project, which provides irrigation, flows for fish and opportunities for recreation.
Original construction of the dam was completed in 1910 and utilized engineering techniques appropriate for that era. It was 70 feet high and contained 359,000 cubic yards of fill. In 1920, the dam was raised 2.5 feet, increasing the capacity to 13,000 acre-feet. Between 1968 and 1969, the crest of the dam was repaired with new embankment materials and riprap. The original open-chute concrete spillway was replaced with a concrete-baffled apron spillway to carry a capacity of 11,580 cubic feet per second.
An earth-fill dam (also known as an earthen dam) continues to be the most common type of dam, principally because its construction involves using materials obtained from earth and rock materials obtained from borrow areas located near the dam site.
Why is Reclamation conducting a study of Conconully Dam?
Results of Reclamation’s studies and analyses show the dam and foundation have a potential to liquefy during high, infrequent, magnitude earthquakes. Reclamation is considering a modification to address any safety concerns.
Liquefaction takes place when loosely packed, saturated soil loses its strength in response to strong ground shaking. Liquefaction occurring beneath/in the dam may cause settlement and possible breach of the dam.
Why is it necessary to modify the dam?
Conconully Dam is operating well and is performing as designed. However, the goal of Reclamation’s Safety of Dams program is long-term stability of dams to protect lives and property. The performance of the dam during a potential seismic loading condition needs to be improved to meet modern design standards.
Does the State of Washington have earthquakes?
There are faults in Washington that may result in earthquakes. The Cascadia Subduction Zone is located approximately 180 miles from Conconully Dam and has the potential to result in a large earthquake, in addition to more local faults.
Why are you conducting work around the dam?
Reclamation is conducting geological investigations. This information will be used to analyze which alternatives would be best to modify the existing dam and to address dam safety concerns regarding infrequently occurring earthquakes.
If you decide to pursue a modification, what is the cost of the project?
Reclamation is in the early phases of looking at options for Conconully Dam. Currently the project is in the pre-design phase and will transition to final design in 2023. During the pre-design phase, two structural alternatives were selected for further evaluation. We are evaluating several options; the cost of modifying the dam is unknown.
What is the schedule for selecting an alternative and when would it be implemented?
Reclamation anticipates performing final design for a preferred alternative in 2023. We anticipate work to begin in 2027; however, that schedule may change depending on funding and other dam safety projects.
How long will construction take?
The length of time for construction will vary depending on the selected alternative; however, we will do as much as we can to limit impacts. Once Reclamation begins construction, the goal is to complete the work within two years.
Under the Safety of Dams Act, costs are shared among the Bureau of Reclamation, associated water users and the Okanogan Irrigation District.
Will the work at the dam impact my property upstream/downstream/on shoreline?
Details of the exact work and impacts will be forthcoming when they are understood. Reclamation makes every effort possible to ensure there is clear and open communication with the public.
How do I voice my opinion/concerns about this project?
Reclamation is committed to remaining open and transparent as we make progress toward addressing concerns at Conconully Dam. We anticipate holding a public meeting in the future to allow the public an opportunity to provide input.
How will earthquake warnings be issued?
Warnings are issued and handled through the Okanagan County Emergency Management office. They have various means of communication, ranging from text alerts to notification from emergency responders.
Should I leave my house if an earthquake occurs?
If you are below a dam, protect yourself—know your flood zones and evacuation routes. In the event of an earthquake, do not wait, take action. Warnings are issued and handled through the Okanagan County Emergency Management office. Follow all directions and information from their office. If an earthquake that causes damage to Conconully Dam, proceed to high ground.
What are you doing now to improve dam safety?
Reclamation is working on interim risk reduction measures to improve safety. Reclamation will provide updates to the Conconully Safety of Dams webpage when available. We anticipate holding a public meeting in the future.
Contact
Keenan Arnold
Regional Safety of Dams Coordinator
208-378-5246
karnold@usbr.gov
Columbia–Pacific Northwest Region
Bureau of Reclamation
1150 N Curtis Road
Boise, Idaho 83706-1234
