Alluvial fans in urban and rural areas are sites of episodic, rainfall-induced natural hazards [Garner 1959; Campbell, 1975; Wieczorek et al., 2001]. Debris flows, hyper-concentrated flows, and flash floods that occur episodically in these alluvial fan environments place many communities at high risk during intense and prolonged rainfall. Although scientists have become better able to define areas of high natural hazard, population expansion and development pressures in such areas have put more people at risk than ever before. Recognition of the magnitude and distribution of debris-flow and flash-flood hazards is therefore a critically important area of natural hazard research.