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. In December 1999, rainstorms induced thousands of landslides in such an area along the Cordillera de la Costa, Vargas, located north of Caracas, Venezuela; an area of dense human settlement located at the base of steep mountains. Flash floods and debris flows caused severe property destruction on alluvial fans at the mouths of the coastal mountain drainage network. Rainfall accumulation on December 2 and 3 totaled 293 mm and was followed by a major storm that dropped 911 mm of rain from December 14 through 16. More than 8000 individual residences and 700 apartment buildings were destroyed or damaged and roads, telephone, electricity, water, and sewage systems were severely disrupted [Salcedo, 2000]. Total economic losses are estimated at US$1.79 billion [Salcedo, 2000]. The debris flows and floods inundated coastal communities and resulted in a catastrophic death toll of as many as 19,000 people [USAID, 2000]. The landslides and flash floods also changed hill slopes, stream channels, and alluvial fan morphology. The alluvial fans along this Caribbean coastline are dynamic zones of high geomorphic activity. Because most of the coastal zone in Vargas consists of steep mountain fronts that rise abruptly from the Caribbean Sea—rising to elevations in excess of 2,600 m--the alluvial fans provide practically the only flat areas upon which to build. Rebuilding and reoccupation of these areas requires careful determination of hazard zones to avoid future loss of life and property.

A rare, high-magnitude storm in northern Venezuela in December 1999 triggered debris flows and flash floods, and caused one of the worst natural disasters in the recorded history of the Americas. Some 15,000 people were killed. The debris flows and floods inundated coastal communities on alluvial fans at the mouths of a coastal mountain drainage network and destroyed property estimated at more than $2 billion. Landslides were abundant and widespread on steep slopes within areas underlain by schist and gneiss from near the coast to slightly over the crest of the mountain range. Some hillsides were entirely denuded by single or coalescing failures, which formed massive debris flows in river channels flowing out onto densely populated alluvial fans at the coast. The massive amount of sediment derived from 24 watersheds along 50 km of the coast during the storm and deposited on alluvial fans and beaches has been estimated at 15 to 20 million m3. Sediment yield for the 1999 storm from the approximately 200 km2 drainage area of watersheds upstream of the alluvial fans was as much as 100,000 m3/km2. Rapid economic development in this dynamic geomorphic environment close to the capital city of Caracas, in combination with a severe rain storm, resulted in the death of approximately 5% of the population (300,000 total prior to the storm) in the northern Venezuelan state of Vargas.