Channel Degradation Due to Gravel Mining: Application of Geomorphic Analysis and Sediment Transport Modeling Approaches

The San Benito River has experienced approximately 4.6 meters of degradation in the past 30 years. Bank failures are threatening water treatment facilities, and downcutting exposed a buried sewer line. The river was studied using both qualitative and quantitative methods. The primary variable effecting the river appears to be gravel and sand mining, which occurs in the river bed, banks, and terraces. A geomorphic analysis indicates that the river is in a state of disequilibrium. The river is a braided system with slopes smaller than normally anticipated. The thalweg of the SBR is lower than its tributaries. Processes indicating active bank erosion include undercutting, mass-movements, and soil tension cracks. A HEC-6T model was calibrated using conditions observed during the geomorphic analysis. The model predicts up to 2 meters of scour associated with the movement of a headcut at a proposed mine site. When combined with historic degradation rates, the channel is predicted to downcut at least 6.4 to 7.6 meters in the next 30 years.