1__Project_Overview_Conconully_Dam_Modification_1.pdf

United States Department of the Interior

BUREAU OF RECLAMATION

1150 North Curtis Road Boise, ID 83706-1234

Project Overview Conconully Safety of Dams Modification

The Conconully Safety of Dams (SOD) Modification Project represents a major infrastructure and public-safety investment aimed at reducing unacceptable seismic risk associated with Conconully Dam, a hydraulic-fill embankment located on Salmon Creek in north-central Washington. Numerous studies, risk analyses, and technical reviews consistently show that the dam, in its current configuration, could undergo excessive deformation during a significant earthquake. This behavior could compromise the structural integrity of the embankment, threaten downstream communities, and jeopardize water deliveries for the Okanogan Project. Because a single adverse seismic event could result in catastrophic failure, the dam has been prioritized for a comprehensive safety modification under Reclamation’s Dam Safety Program. The project aims not only to reduce seismic risks to acceptable levels but also to ensure long-term operational reliability.

The main engineering feature of the proposed modifications is the deep soil mixing (DSM) treatment that will be installed across the downstream foundation. The DSM grid at Conconully will extend to depths of roughly sixty feet in some locations, reflecting the depth and variability of the liquefiable layers.

In addition to DSM, a large downstream stability berm will be constructed to increase the resisting forces acting on the embankment during a major earthquake. This berm will be composed of engineered fill materials coordinated with filter and drainage layers that support both seepage control and seismic performance. Multiple drains will be constructed in conjunction with the stability berm to capture water within the berm. The berm is intended to limit both the magnitude and rate of post-earthquake deformation, ensuring that even under a strong seismic event, the dam will maintain its structural integrity. General fill for the stability berm will be obtained from an on-site borrow area that will allow this material to be hauled to the dam without leaving Reclamation property or requiring travel on public roads. Aggregate materials for filter and drainage layers are anticipated to be supplied from commercial sources and hauled to the site.

Another major component of the modification project involves improvements to the outlet works system. Construction activities will include:

• steel lining in the existing outlet works

• grout encasement of steel lining and steel bypass piping

• replacement of slide gates in the existing gate chamber

• extension of the outlet works via steel lined, concrete encased conduit to accommodate the new stability berm

• new downstream control valves

• outlet works discharge channel

The existing control house and above grade elements of the gatehouse will be demolished and replaced with new structures. New downstream control valves will be housed in a concrete valve vault. Completion of the outlet works improvements will require substantial rock excavation and stabilization that will have to be completed without the aid of explosives.

A range of limitations and constraints will shape the overall implementation strategy. The dam must continue to operate throughout the modification period, maintaining reservoir levels that support irrigation deliveries while providing safe working conditions for DSM, embankment construction, and outlet works upgrades. Both irrigation deliveries and minimum non-irrigation season flows in Salmon Creek must continue uninterrupted during construction. Pumps, siphons, and interim seasonal use of the outlet works may be employed by the contractor to meet these requirements. Reservoir drawdown schedules cannot compromise water supply reliability, so windows for low-water construction may be limited. To prevent uncontrolled releases over the spillway during construction, Reclamation may require the contractor to make the outlet works available for interim operation in response to snowpack and inflow conditions. This will require close coordination between Reclamation and the contractor during the late winter and early spring period.

Construction of the noted improvements will require filling existing wetlands and a portion of [EB2.1] Salmon Creek. Mitigation for these impacts is being provided through an aquatic and wetland habitat restoration project just downstream of the new outlet works discharge channel and extending approximately 2,600 feet downstream. Construction of this restoration work will be handled and contracted separately by Reclamation and is anticipated to begin as the dam modification contractor is in the final stages of construction.

Revegetation of the site after construction will be performed by Reclamation through a contract with the U.S. Forest Service. The dam modification contractor will not be required to perform revegetation of disturbed areas.

While subject to change, current project planning anticipates the following key project milestones:

• Industry Day: October 2026: Details to be announced via SAM.gov posting

• Estimated Solicitation Announcement: Q3 Fiscal Year 2027