The Omochumne-Hartnell Water District operates four flashboard dams on the lower Cosumnes River. These dams are raised during the summer to increase groundwater percolation.
One dam, Blodgett, was damaged in the 1997 flood and became inoperable. The flood created a four-foot drop below the dam, which prevented fish passage over the dam. Restoration of the dam called for repairing a breech on the left bank, filling the 20-foot-deep scour hole below the dam, and constructing a boulder weir fish ladder.
The 120-foot-long ladder was designed to have a drop of one foot at 20-foot intervals with holding pools between drops. The ladder required approximately 4,000 tons of aggregate material, ranging in size from sand and gravels to 3,000-pound boulders.
RBI staff coordinated all aspects of dam restoration design, preparation of construction bid documents, bid analysis and selection, and construction management. Project design included consulting with the California Department of Fish and Game for the design of the boulder weir fish passage structure to meet criteria for chinook salmon migration. RBI staff served as the technical lead for performing hydraulic analyses using a HEC-RAS model to evaluate channel scour forces and fish passage conditions at various flow levels and design options.
The reconstruction of Blodgett Dam provides a stable fish passage structure at flows as low as 10 cfs. The stability of this boulder structure (no concrete was used in construction) has been tested at flows as high as 32,000 cfs. The dam and boulder weir have withstood these flows with only minor maintenance requirements.