Lower Alameda Creek Fish Passage Improvements (Phase 1) 0.5 Mile Upstream of Mission Blvd., Alameda County, Zone No. 5 Project

The project is located within the Alameda Creek flood control channel (Zone 5, Line A), approximately 0.5 mile upstream of Mission Boulevard in Fremont. The Alameda County Flood Control and Water Conservation District (District) proposes to modify the upper concrete sill (Sill) at the mouth of Niles Canyon. The project is intended to improve fish passage, stabilize the channel, and restore a low flow, fish passable channel upstream and downstream of the Sill. The Sill was constructed in 1980 and repaired in 1983 to address streambed and bank erosion. It consists of a reinforced-concrete grade beam approximately 2 feet wide and 6 feet deep. When constructed, the beam extended 4 to 6 feet into the channel bed. The Sill spans approximately 125 feet across the channel, with approximately 10 feet embedded in the northern bank. Concrete bank slopes, retaining walls, and grouted rock line the southern bank immediately upstream and downstream of the Sill. Over time, degradation of the downstream channel bed has exposed the upper 2 feet of the Sill above the downstream water surface. During low flow conditions, the exposed portion creates an abrupt step in the channel bed and water surface that impedes fish passage and may also compromise the structural integrity of the Sill. This project will remove the water surface step by constructing a 25-foot-wide notch in the concrete Sill. This notch will improve fish passage and provide a more natural transition through the channel. The remaining portions of the Sill will be buttressed upstream and downstream with large, immobile boulders and rock matrix. Areas of channel bank erosion and undermining will be repaired by placing rock revetment to fill existing voids, stabilize the banks, and restore the integrity of the structure and channel transition. During construction, a temporary upstream cofferdam will be installed to isolate and divert flows around the work area. Channel flows will be routed around the work area either through an automatically actuated pump and pipeline system or by gravity. The cofferdams are anticipated to consist of gravel-filled structural barriers or inflatable dams, supplemented with sandbags if needed. Once the work is complete, all the temporary diversions and cofferdams will be removed, and normal channel flow will be restored. Large construction equipment will access the dewatered channel to repair undermined portions of the existing structure, modify the Sill, and place buttressing rock at the Sill and along the channel banks where needed. All work will occur within District property and easements. The completed project will improve low flow channel conditions and fish passage at the Sill, stabilize the existing flood control facility and adjacent channel banks, and support the safe upstream migration of steelhead, coho salmon, and Chinook salmon to spawning and rearing habitat.