California Alluvial Fan Task Force County of San Diego Alluvial Fan Flooding and Risk Reduction Sara Agahi, PE, CFM January 4, 2008
Outline of Presentation Show location of San Diego County Alluvial Fan Area of Concern: Borrego Springs Illustrate Historical Flooding Events Describe County s Risk Reduction Efforts Regulatory Requirements Criteria for Alluvial Fan Development Summary
Looking southwest to de Anza Golf Club (notice Borrego Palm, Fire, and Hellhole Canyons). Looking southeast from Henderson Canyon to de Anza Golf Club.
Borrego Palm Canyon 8/16/79 Average annual precipitation in Borrego: 3-6 Average annual precipitation along upstream ridgeline: up to 25 Flash Flood
Historical Flooding in Ocotillo, Hurricane Kathleen, 1976.
De Anza Flooding (view from apex of Borrego Palm Canyon)
Agua Caliente Airport (south of Borrego) and Agua Caliente Creek Example of sedimentation from alluvial fan flooding Agua Caliente Airport (south of Borrego) and Agua Caliente Creek
De Anza Golf Course flooding
Risk of damage to infrastructure
Gabion fences located between Henderson and Borrego Palm Canyons. Divert flow from Borrego Palm Canyon. Examples of non-county risk reduction projects Built in 1960s or 70s. Who built these fences? Anyone maintaining them?
County of San Diego Efforts to Reduce Risk 1968, US Congress established National Flood Insurance Program (NFIP) administered by FEMA. July 1972, Board of Supervisors adopted General Plan for Flood Control Improvements, Borrego Valley, prepared by County Dept of Sanitation & Flood Control. December 1983, Board adopted County Flood Damage Prevention Ordinance #8334, Section 811.101, which establishes flood protection criteria for construction of structures in flood prone areas. 1987 FEMA produces FIRMS for this area. July 1989, Board adopted Alluvial Fan Flood Management Report, Guidelines for non-structural Flood Protection and Flood Insurance Applications, prepared by DPW, created Borrego Valley Alluvial Fan Hazard Map. October 1989, Board accepted Boyle Engineering report, Borrego Valley Flood Management Report, which specifically deals with flood protection on alluvial fans in Borrego Springs.
Borrego Valley Alluvial Fan Flood Hazard Map Shows maximum expected flood depth and velocity Shows predicted risk to potential land owners
County of San Diego Guidelines for Risk Reduction for Structures in Borrego Springs Elevation Criteria Elevate lowest floor (including basement) above highest adjacent grade to at least as high as the number specified on the FIRM (Zone AO SFHA, sheet-type flow, high velocities). Elevate mechanical and utility equipment over depth of flooding. Open Space Criteria Maintain at least 50% of lot as open (unobstructed) space and provide adequate drainage paths around structures. Design Criteria Position structures perpendicular to fan flow so flood flows will not be diverted and damage other properties.
Borrego Valley Flood Management Report also includes design guidance to help reduce risk for those who build on the alluvial fan.
Examples of elevated houses (notice the open space on lots) Structures that incorporate risk reduction into their designs
House on piers
Flood wall
Summary History of extreme flood events in San Diego County alluvial fan areas. Demand to develop here because of lovely environment, pleasant climate, relatively cheap land prices. County committed to allowing legal development while protecting public safety.
Final Thought Even if your house is high and dry during a flood, consider: Roads Utilities Emergency Response Post flood clean up
Thank You Sara Agahi, PE, CFM Program Manager Watershed Protection Program County of San Diego (858) 694-2665 Sara.Agahi@sdcounty.ca.gov
Rams Hill Flood flows are intercepted upstream from development Channel at bottom of development redirects flow back to original alluvial fan Channels divert flow around development Requires dedicated channel maintenance funding source