GEOMORPHIC EFFECTS AND SEDIMENT FLUX OF LARGE DEBRIS FLOWS AND FLASH FLOODS, NORTHERN VENEZUELA, 1999

In December 1999, an unusually severe series of rain storms near the Venezuelan capital of Caracas triggered landslides, flash floods, and debris flows that caused one of the worst natural disasters in the recorded history of the Americas. Much of the loss of life (15,000 killed) and property (~ US$2 billion) occurred where debris flows and floods rapidly inundated coastal communities built on alluvial fans. Sediment carried by these flows originated largely from landslides in steep catchments underlain by schist and gneiss on the north side of a coastal mountain range. Soil from some hillsides was entirely stripped by individual or coalescing failures. The large, rapid delivery of runoff and sediment resulted in floods, debris flows, and hyperconcentrated flows, with average volumetric sediment concentrations in floods and hyperconcentrated flows of 35-40%. An approximate sediment budget from 24 watersheds along 50 km of coastline indicates that 15 to 20 million cubic meters of sediment were deposited on alluvial fans and beaches during the storm. This corresponds to ~100,000 cubic meters per square kilometer of sediment (or 170,000 tonnes per square kilometer by mass, assuming a bulk density of 1.7 tonnes per cubic meter) from the ~200 square kilometer watershed upstream of the fans. This is equivalent to 0.1 m of average surface lowering in the watersheds during this single event. This impressive number illustrates that this series of storms produced one of the highest sediment yields ever documented for rainfall-triggered geomorphic activity. The sudden delivery of this material as floods and debris flows onto urbanized alluvial fans killed ~5% of the population in the northern Venezuelan State of Vargas.