Salmon Creek School Storm Damage Repair Project

Salmon Creek School Storm Damage Repair Project

The project involves two sites located approximately 400-feet apart from one another on a reach of Salmon Creek that were damaged by the January 2006 storm event. An access bridge on the north side of the property was damaged as a result of high energy flows which scoured the concrete abutments and caused a large tree to fall on the bridge deck. The failed bridge will be removed from the site as well as the concrete abutments at the top of the banks. This work will conducted from top of banks, eliminiating the need for equipment to enter the streambed. A 12-foot wide pre-manufactured bridge deck spanning 75-feet will be placed on piles placed from the top of the stream channel bank on both the west and east ends of the deck. The bottom of hte bridge deck will be placed 4.95 feet above the calculated 100-year water surface elevation. Because the construction practices will occur outside of the stream during a low stream flow, a diversion of the channel flow or dewatering efforts will not be required. Clearing the project reach of native salmonids and implementing fish screens above and below the project reach will not be required. In addition to the bridge replacement, an existing failed elliptical concrete culvert located 100-feet west of the bridge crossing will be replaced with a HPDE culvert. The culvert provides roadside drainage conveyance underneath the access road. A rock apron placed at the outlet will provide flow energy dissipation for scour prevention. Also, another culvert at the entrance of the access road and underneath Bohemian Highway will be replaced by the Sonoma County Department of Transportation and Public Works in order to prevent erosion from undermining the construction of the new bridge. The new culvert will be a 48" x 40 foot HDPE pipe and will be 1/2 foot deeper than current pipe bottom. Vegetation removal will be minimal and some trimming will be necessary to access the pipe. Rock slope protection, likely 12-inch engineer's rock, may be placed at the outlet. The area surrounding the pipe will be backfilled with cement/sand slurry. The second project site is located 400-feet downstream from the access bridge and includes the restoration of approximately 50 feet of failed stream bank on the east side of the stream channel. The project includes re-grading the slope, installation of rip rap along the 50-foot channel toe, and bioengineering practices such as live willow wall and biodegradable, erosion control disturbed areas. Temporary sand bags may be preferable to direct continuous downstream stream flow away from bank during construction. Since construction will take place out of the stream, during a low stream flow, channel dewatering, clearing of native salmonids and fish screens will not be necessary.

Sonoma