Project Overview

The Bureau of Reclamation is preparing an environmental assessment for the Conconully Safety of Dams Modification project. Conconully Dam, located in northcentral Washington, is owned by the Bureau of Reclamation and operated by the Okanogan Irrigation District. While Conconully Dam is performing as designed, the Bureau of Reclamation has determined that the risk of seismic failure exists at the dam. Investigations conducted under the Bureau of Reclamation’s Safety of Dams program determined that the dam is potentially at risk of seismic failure associated with liquefaction of embankment and foundation materials, or when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. This risk could result in dam failure, which poses unacceptable risk levels to people, property, and the environment. This project is authorized under the Safety of Dams Act (Public Law 95-578).

Purpose and Need

The purpose of the proposed project is to improve public safety by reducing risk associated with liquefaction of embankment and foundation materials; maintain water deliveries and flows for irrigators and endangered species; and continue to provide recreational benefits and fish and wildlife habitat.

The need for the proposed action is to comply with the Bureau of Reclamation's Safety of Dams’ requirements and bring static and hydrologic risks at Conconully Dam below public protection guidelines, improve control of water flows through the outlet works, and increase the reliability and longevity of the dam and water conveyance system.

Background

Conconully Dam and Reservoir are located on Salmon Creek, approximately 17 miles north of Omak, Washington. As a component of the Okanogan Project, the reservoir provides irrigation water to downstream irrigators and recreation opportunities out of the town of Conconully north of the dam. Conconully Reservoir has a storage capacity of approximately 13,000 acre-feet and a reservoir water surface area of 450 acres at full pool.

Conconully Dam was constructed using hydraulic fill methods between 1907 and 1910 at the confluence of the north and west forks of Salmon Creek. The hydraulic fill construction technique used during the construction of the dam consisted of selectively placing soil or other materials using a stream of water. The resulting puddled-core embankment is currently known to be susceptible to liquefaction and increased risk of failure in the event of seismic activity.

Evaluations of the risk of seismic failure associated with liquefaction of the dam’s embankment and foundation materials began in the 1990s. The geotechnical and risk analyses indicated that existing risks were above public protection guideline values and justified additional work to better define or understand the risks. Subsequent geotechnical evaluations, field explorations, and risk analyses demonstrated that the embankment requires modifications to reduce the seismic risks to the dam.

The Bureau of Reclamation completed a corrective action study to evaluate alternatives that would reduce seismic risks to Conconully Dam. The corrective action study developed two structural modification alternatives, a no action alternative, a reservoir restriction alternative, and a dam breach alternative. The two structural modification alternatives include an excavate and replace alternative and a deep soil mixing alternative. The excavate and replace alternative proposes to excavate and replace foundation materials susceptible to liquefaction. The deep soil mixing alternative proposes the use of deep soil mixing reinforcement columns to improve foundation materials. Both structural modification alternatives would construct a stability berm on the specified foundation type on the downstream face of the dam. 

After evaluating the corrective action study alternatives, the Bureau of Reclamation selected the deep soil mixing alternative as the proposed action to be evaluated in the EA.

Proposed Action

The Bureau of Reclamation proposes to reduce the risk of dam failure by constructing a stability berm on a foundation of deep soil mixing reinforcement columns on the downstream face of Conconully Dam. To provide improved operational functionality and compatibility with the proposed stability berm, the Bureau of Reclamation also proposes to upgrade the outlet works by extending, lining, and reinforcing water conveyance components and installing contemporary water control systems and facilities (such as, multiple state-of-the-art valves and valve house). These modifications would be performed primarily downstream (dry or stream side) of the dam, with limited activities upstream (wet or reservoir side) of the dam. Fill material would be excavated from a borrow area on the left bank of the reservoir.

Project Area Overview Map

Deep Soil Mixing Foundation and Stability Berm

Deep soil mixing is a ground improvement technique that augments the characteristics of weak soils by mechanically mixing them with a binder to a target depth. The proposed rows of soil-cement—referred to as soilcrete—overlapping columns increase the soil’s strength and rigidity. This creates a foundation that improves the structural integrity of the embankment which reduces the risk of seismically induced dam failure and meets public protection guidelines.

Construction of the stability berm would start after installation of the deep soil mixing columns. The berm will be constructed with a mixture of silt, sand, gravel, and cobbles. The majority of the miscellaneous compacted backfill materials will be sourced from the borrow area. The crest elevation of the stability berm will be approximately 1 foot higher than the top of the current dam embankment. When completed, the crest width of the proposed berm and existing dam will be approximately 180 feet and include access roads to the spillway and outlet works valve house (Figure 1).

Outlet Works and Valve House

To provide improved operational functionality and compatibility with the proposed stability berm, the Bureau of Reclamation also proposes to upgrade the outlet works by extending, lining, and reinforcing water conveyance components and installing contemporary water control systems and facilities (Figure 2). A new reinforced concrete building (i.e., valve house) near the toe of the stability berm would accommodate multiple state-of-the-art valves to facilitate a range of water discharges for specific purposes—emergency reservoir, irrigation, base flow, and low reservoir elevation releases (Figure 3).

Next Station: NEPA and Public Involvement

For more information on the Conconully Safety of Dams Modification project EA, please visit the project website or contact Jason Sutter, Bureau of Reclamation NEPA lead at bor-sha-consod@usbr.gov.

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