Abstract
Rainfall during the winter of 1992–1993 reached 195 percent of normal and caused landslides and mudflow/debris flows reminiscent of previous wet years when the rainfall exceeded 140 percent, the apparent threshold for damaging flooding and slope failures. Winters with rainfall exceeding this threshold have occurred at predictable frequencies and are expected to continue. Annual losses average approximately $200,000,000 from slope/ground failure and flooding in California for years with more rain than 140 percent of normal. Slope movements can be reasonably predicted when successful and sufficient subsurface exploration and materials testing is combined with the proper application of existing knowledge. However, the actions of humans still cause many failures through ignorance of physical processes, faulty reasoning, and rejection of current technology. Prudent and diligent engineering geology coupled with good geotechnical engineering practiced during the past three decades have helped protect sites and tracts from slope movement and flood damage, but failures are still occurring. A responsible effort by building officials in review of reports and enforcement of existing grading codes and ordinances can help reduce losses by 90 to 95 percent.
