Flood control failure: San Lorenzo River, California

The flood control project on the San Lorenzo river, California, completed in 1959, was designed to contain a 100-year flood. Due to heavy siltation, it is now estimated that it will contain only a 25-30 year flood. The authors discuss the effects of altering natural channel gradients and the reasons for high erosion rates. They conclude that the principal errors in the original design were gradient alteration, inaccurate flood flow velocities and thus incorrect size of channel. Suggestions for the future include dredging, erosion control, constructing dams, raising levees and bridges and widening the river channel. They also discuss how flood control works encourage development in flood plains. A flood occurred in January 1982 which exceeded the 100-year storm.