Dam Safety

Grant PUD to evaluate potential erosion of embankment at Wanapum hydro project

Grant PUD to evaluate potential erosion of embankment at Wanapum hydro project

Commissioners with the Grant County Public Utility District in Washington State discussed a proposed 10-year, $15 million contract with engineering firm Gannett Fleming to examine the interior of an embankment at its 1,092 MW Wanapum hydro project for potential erosion.

Wanapum Dam, on the Columbia River, is a concrete structure. Overall the project, completed in 1963, consists of left and right earth embankments, left and right bank fish ladders, a concrete gravity spillway, a 10-unit reinforced concrete powerhouse, and a future unit intake section.

The embankment in question connects the concrete portion of the dam to the shoreline. The embankment is made of earth and rocks, with a silty sand core that limits the amount of water that passes through. The core is surrounded by layers of coarser-grained soils and protective rock.

The most recent in-depth dam safety inspection by independent consultants, in 2021, concluded that internal erosion, if severe enough, could lead to failure of the embankment. The analysis will show Grant PUD if any repairs are needed to keep the structure sound and safe.

Evaluation of the embankment for potential internal erosion and seismic stability is a federal requirement. The evaluation will begin with existing data and use a phased approach, which could include drilling bore holes.

Engineering Manager Becca Simpson told commissioners the embankment was built using the best practices available in the early 1960s, when the dam was built. The proposed analysis will determine if repairs are needed and their cost.

Four companies bid on the contract for the Wanapum Dam embankment. Gannett Fleming, which has done extensive work for Grant PUD, scored the highest.

Commissioners will vote on the proposed contract at the Aug. 23 meeting.