Arroyo Grande Creek Channel Flood Mitigation
Client: Coastal San Luis RCD, County of San Luis Obispo
In 1959 the lower three miles of the mainstem Arroyo Grande Creek were converted to a trapezoidal flood control channel to reduce flood risk to high value farm land. Sedimentation, vigorous riparian growth and changes in the watershed due to urbanization have altered the flood control channel to the point where it provides very little in the way of flood protection. In addition, environmental regulations that protect sensitive species have limited the ability of the County to perform channel maintenance work. In 2001 a high intensity storm event caused levee failure, resulting in millions of dollars in damage to adjacent farmland and residential properties. In response, the Coastal San Luis Resource Conservation District (RCD) commissioned Waterways staff, in conjunction with SH+G, to evaluate potential alternatives to improve flood protection along this reach of Arroyo Grande Creek. Identified alternatives included levee raising, environmentally appropriate vegetation and sediment management, and erosion control and flood detention projects in the upper watershed to reduce sedimentation in the flood control reach.
Habitat enhancement elements were also recommended to provide mitigation for project impacts. The alternatives were evaluated using a HEC-RAS model to determine the level of flood protection afforded each alternative and their respective costs of implementation. Following selection of a preferred alternative, Waterways has worked with SWCA on the regulatory permitting and environmental review phase of the project and are currently contracted with San Luis Obispo County to provide bid ready documents for the levee raise and sediment management components of the projects and provide construction-phase support. Waterways staff also prepared the long-term management plan for the flood control channel that included a comprehensive monitoring plan that addresses vegetation and accumulation of sediment in the channel.