Emergency repairs restore Coyote Percolation Dam and improve fish passage

After strong flows in Coyote Creek last winter damaged an in-channel dam, emergency repairs completed over the fall have restored the Coyote Percolation Dam to keep our water supply safe and reliable. The Santa Clara Valley Water District facility is located on Coyote Creek near Metcalf Road in South San José and serves to divert water from the creek to replenish the groundwater aquifer in the area.

The damage occurred during the same storm that caused Coyote Creek to overtop its banks in several San José neighborhoods last February. The storm flows damaged the dam and adjacent stream banks. Due to the severity and frequency of storms last winter, the full extent of damage was not revealed until June, when creek flows receded enough to allow staff to visually and physically evaluate the dam.

The rushing water had eroded the dam apron, a concrete structure that protects the downstream side of the dam. When in operation, steel panels are secured to the concrete foundation (adjacent to the apron) to create a dam to divert water. However, these panels are removed when we anticipate Anderson Reservoir may spill.

Over the summer, when we were able to make a full assessment, we determined the dam and its apron could not weather another winter of high flows. If the concrete structure gave out during a storm, it could cause concrete chunks, and large amounts of sediment and debris to be transported downstream, creating blockages and increasing flood risks along the creek.

There was also impact to fish passage through the area. The high flows had washed out a channel that directed fish to an existing fish ladder. The fish ladder helps fish migrate and pass through the barriers that the dam creates. The steps, or rungs, consist of ascending pools that allow the fish to leap and rest in each pool until they reach the top of the ladder. Crews created a new series of pools to help fish reach the bottom step of the existing fish ladder.

On Aug. 22, 2017 the board of directors declared the dam condition an emergency, allowing the water district to quickly hire a contractor and begin repair work immediately by means of an abbreviated procurement process, as permitted by state code during emergencies.Work began on Aug. 26 and was completed by Oct. 15, 2017, in time for winter. A new concrete apron was installed, and fish pools adjacent to the fish ladder were repaired and enhanced to improve passage. The damaged stream banks were repaired and buttressed with boulders. The approximate cost of the repairs was $670,000.

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