Black Butte Dam Erosion Repairs

The City of Santa Clara operates a hydroelectric power generating facility at Black Butte Reservoir near Orland, CA.

When the facility was constructed, a re-regulation basin was created below the dam spillway. The re-regulation basin elevated the water surface downstream of the spillway, putting it in contact with unprotected banks. During high flow releases from the dam’s “flip-bucket” spillway, a jet of water develops that can  reach up to 40 ft above the pond elevation before re-entering, causing turbulence and waves when it does. The turbulence and waves from higher flow releases eroded the banks of the basin, jeopardizing infrastructure and access to the site.

 

Phase 1: cbec performed an evaluation of hydraulic conditions in the reservoir to develop recommendations for erosion repairs. A topographic (survey-grade RTK GPS) and bathymetric (boat-based single-beam sonar) survey was performed to characterize the surface of the re-regulation basin and its banks. cbec determined that due to the free surface created by the flip-bucket spillway, a 3D CFD model would be required to evaluate hydraulic conditions in the basin. cbec developed a 3D hydraulic model of existing basin conditions using FLOW 3D software to capture the momentum of the jet and the turbulence from its impingment on the re-regulation basin water surface, and to evaluate the resulting flow velocities and shear stresses on the basin bed and banks. cbec then provided recommendations for erosion repair and protection based on spatial distribution of those values and existing conditions, documented in a basis of design report.

 

Phase 2: Based on cbec’s recommendations in Phase 1, GEI prepared 60% erosion repair designs. cbec reviewed the plans and provided design comments and recommendations on the 60% design. cbec updated the 3D hydraulic model, incoroporating GEI’s revised 60% repair designs to evaluate the integrity of the design. Revised design recommendations were provided based on the updated model.

Waterbody / Watershed

Stony Creek