Large populations live on or near alluvial fans in locations such as Los Angeles, California, Salt Lake City, Utah, Denver, Colorado, and lesser known areas such as Sarno, Italy, and Vargas, Venezuela. In time scales spanning thousands of years, alluvial fans are dynamic zones of high geomorphic activity. Debris flows and flash floods occur episodically in these alluvial fan environments, and place many communities at high risk during intense and prolonged rainfall.

In December 1999, rainstorms induced thousands of landslides along the Cordillera de la Costa, Vargas, Venezuela. Continuous rainfall during the first 2 weeks of December was followed by a major storm on December 14 through 16. Rainfall for this period totaled nearly 1,200 mm. Debris flows and flash floods on alluvial fans inundated coastal communities, caused severe property destruction, and resulted in a catastrophic death toll of as many as 19,000 people.

This example from Venezuela shows the potential for extreme loss of life and property damage where a large population occupies an alluvial fan. The possibility for an event of comparable magnitude exists in other parts of the world where extensive development has encroached on alluvial fans. Without careful planning of human settlements, the impacts and the numbers of these types of disasters are likely to increase in the future. By building communities and other infrastructure on alluvial fans, dramatic natural hydrologic processes have been changed into major lethal events. As stated by the Secretary General of the United Nations, Kofi Annan, “The term ‘natural disaster’ has become an increasingly anachronistic misnomer. In reality, human behavior transforms natural hazards into what should really be called unnatural disasters.”