Scour evaluation for the Folsom Dam stilling basin

Folsom Dam is located on the American River within the northeastern limits of the City of Folsom, California, USA. It was designed in the late 1940's and constructed between 1948 and 1956. Increased development through the downstream region (Sacramento) has necessitated improved flood protection including an increase in the release capacity of the dam. With an increase in release capacity follows a concern that the rock foundation downstream of the dam will scour resulting in damage to the spillway and stilling basin. Kleinfelder, Inc. performed a scour investigation that included detailed geomechanical mapping and statistical evaluation of the rock foundation downstream of the dam. We cataloged the geomechanical information using geographical information systems, and computed the Required Stream Power (RSP) to scour the rock foundation following the procedures outlined in Annandale (1995). The RSP was compared to estimates of Available Stream Power (ASP) predicted by the US Bureau of Reclamation. When the ASP exceeds RSP, it is anticipated that the rock will scour. We found that isolated areas of weak and sheared rock may scour during the discharge conditions modeled and provided recommendations including the use of rock reinforcement and lean-mix dental concrete at isolated zones of the rock foundation. The paper presents a summary and the results of these evaluations. 14 pp., 12 figures.