CHALLENGES IN COASTAL HAZARD MITIGATION IN CALIFORNIA

Coastal California is subject to intense development pressure. Most coastal development occurs on coastal bluff tops or at beach level. The California Coastal Act requires assurance that such development will be stable for its design life. The primary hazard for bluff top development is coastal erosion (usually landslides or topples), whereas hazards for beach-level development are primarily flooding and wave impact. Sea level rise (SLR) exacerbates both hazards and impact assessments used by the California Coastal Commission (CCC).

Several recently developed tools help with such assessments. The Southern California Beach Processes Study (Scripps Institute of Oceanography) makes available LIDAR-derived beach and bluff profile data for southern California. These data will be useful in assessing future shoreline change. Wave uprush studies generally include the amount of SLR expected over the project’s design life. Given the uncertainty about future SLR, assigning a particular value is difficult. The California Climate Action Team has established 1.4 m/100yr for planning purposes, and the Pacific Institute has released a set of hazard maps illustrating the flooding to be expected from a 100-yr storm given that amount of SLR. Although these are useful planning tools, the CCC staff is supplementing them with a sensitivity analysis approach, testing vulnerability of projects at various rates of SLR.

Coastal bluff erosion presents a thornier problem. Recognizing that higher sea levels can only increase coastal erosion, the CCC has generally relied on the highest historic long-term average erosion rates when evaluating future bluff retreat. The USGS National Assessment of Shoreline Change provides databases of historic erosion rates. More recently, Phillip Williams and Associates has released estimates of increased coastal erosion to be expected under 1.0 and 1.4m/100yr scenarios, basing their hazard zones on proportional increases in historic rates resulting from greater water elevations, for longer times, at the base of bluffs. Such tools provide a regional context in which CCC staff can place site-specific studies undertaken in support of new development. In implementing the Coastal Act, the CCC strives to balance safety, resource protection, and property rights on a case by case basis.