This publication provides information on the risks of alluvial fan flooding. "Flood hazards in the American West are often greatly underestimated due to the dry conditions, lack of rainfall and absence of defined watercourses. Ironically, western floods are quite severe and powerful, exhibiting unpredictable flow paths and high velocities that usually occur with little advance warning time. These floods can cause considerable erosion in some areas while depositing large amounts of sediment and debris in others. With rapid growth continuing throughout the West (particularly in many major metropolitan areas) hillside building sites have become more popular as the supply of prime developable land becomes depleted. This has resulted in an increasing amount of development occurring in floodplain areas called alluvial fans."
Federal Emergency Management Agency: http://www.fema.gov/