SOCWA Coastal Treatment Plan Access Bridge Protection

SOCWA Coastal Treatment Plan Access Bridge Protection

The California Department of Fish and Game is executing a Lake and Streambed Alteration Agreement number 1600-2004-0152-R2 pursuant to Section 1602 of the Fish and Game Code to the project applicant, Mr. Brian Peck, SOCWA. The applicant proposes to alter the streambed and banks through the proposed protection of the existing SOCWA Coastal Treatment Plant Access Bridge which spans Aliso Creek. The proposed project would consist of the construction of a sloping grouted stone grade control structure protecting the invert beneath the access road bridge over Aliso Creek. The grade control structure, which has a crest elevation at the upstream end of 8.8 m, slopes downstream at a two percent grade for 18.5 m. At the upstream end of the structure, a sloping grouted stone cutoff wall will extend to a depth of 2.4 m at a slope of 2H:IV. This upstream cutoff wall will serve to prevent undermining of the invert protection and also to protect in place the sewer outfall crossing. At the downstream end a similar cutoff wall would be constructed to the same depth at a slope of 3H:IV. The thickness of the entire grouted stone structure would be 0.6 meter. Dumped stone will be placed at the downstream end of the structure to provide scour protection. The existing grouted stone protection on the side slopes beneath the bridge would be removed and replaced with a 0.61 m bridge, the grouted stone would wrap around the abutments to protect the structure against large flood events. This portion of the structure would be backfilled to allow for planting of natural bank vegetation in the area upstream of the bridge. A 30.4 cm layer of riprap will be placed over this fill to protect it against smaller flood events. A topsoil layer of 30.4 cm will be placed over the riprap to facilitate the planting of new bank vegetation. To allow for continuity of fish migration upstream, a fish passage would be incorporated into the design. The fish passage would consist of a low flow center channel with a width of 2.4 m and a maximum slope of 10 percent. Large stones would be staggered in the center channel to dissipate energy and to create small eddies and pools for the fish to rest in their passage upstream. The entire grouted stone structure would be sloped to drain toward the center channel to ensure a constant flow of water for fish passage.

Laguna Niguel

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