Regulating land use and development in areas subject to subsidence and earth fissures: An update, Riverside County, California

Land subsidence and fissuring have been well documented in Riverside County, a large, diverse, and growing county in southern California. Prior to about 8 years ago, county personnel, developers, and the geotechnical community considered the impact of subsidence on development to be small. Most land affected by subsidence and fissuring was either agricultural land or open space. Owing to recent subsidence and fissuring in developing areas of Riverside County that previously were considered free of this hazard, subsidence has become the subject of regulatory review by the county. Geotechnical consultants are now required to evaluate the potential for subsidence and fissuring during preliminary site investigations. In specific cases where subsidence and fissuring were discovered during development of an urbanizing area, the county board of supervisors has adopted a local "subsidence report zone" by resolution. Three of these zones have now been adopted. The intent of a subsidence report zone is to preclude the development of new human occupancy structures astride known or potential earth fissures. Planning for structures located in areas subject to potential subsidence and fissuring within a subsidence report zone must include appropriate mitigation design measures. By using sound geologic and geotechnical criteria for zone adoption and consistent review and clearance guidelines for zone implementation, the county has allowed development to proceed in an orderly and timely manner without compromising public health and safety.