disasters

Map showing susceptibility to earthquake-induced landsliding, San Juan Metropolitan Area, Puerto Rico

Santiago Marilyn, Map showing susceptibility to earthquake-induced landsliding, San Juan Metropolitan Area, Puerto Rico, 2 plate.em>

Abstract: 
Analysis of slope angle and rock type using a geographic information system indicates that about 66 percent of the San Juan metropolitan area (SJMA) has low to no susceptibility to earthquake-induced landslides. This is at least partly due to the fact that 45 percent of the SJMA is constructed on slopes of 3 degrees or less, which are too gentle for landslides to occur. The areas with the highest susceptibility to earthquake-induced landslides account for 6 percent of the surface area. Almost one-quarter (23 percent) of the SJMA is moderately susceptible to earthquake-induced landslides. These areas are mainly in the southern portions of the SJMA where housing development pressures are currently high because of land availability and the esthetics of greenery and hillside views. The combination of new development and moderate earthquakeinduced landslide susceptibility may indicate that the southern portions of the SJMA may be at greatest risk.

NATURAL HAZARDS ON ALLUVIAL FANS: THE VENEZUELA DEBRIS FLOW AND FLASH FLOOD DISASTER

Larsen, M.C., Wieczorek, G.F., Eaton, L.S., Morgan, B.A., Torres-Sierra, H., 2001,
Natural Hazards on Alluvial Fans: the Venezuela debris-flow and flash-flood disaster: U.S. Geological Survey Fact Sheet, FS 103-01, 4 p.

Abstract: 
In December 1999, rainstorms induced thousands of landslides along the Cordillera de la Costa, Vargas, northern Venezuela. Rainfall on December 2-3 totaled 200 millimeters (8 inches) and was followed by a major storm (911 millimeters, or 36 inches) on December 14 through 16. Debris flows and flash floods on alluvial fans inundated coastal communities, caused severe property destruction, and resulted in a death toll estimated at 19,000 people. Because most of the coastal zone in Vargas consists of steep mountain fronts that rise abruptly from the Caribbean Sea, the alluvial fans are the only areas where slopes are not too steep to build. Rebuilding and reoccupation of these areas requires careful determination of potential hazard zones to avoid future loss of life and property.

THE RAINFALL-TRIGGERED LANDSLIDE AND FLASH FLOOD DISASTER IN NORTHERN VENEZUELA, DECEMBER 1999

Larsen M.C., Wieczorek G.F., Eaton L.S., Torres-Sierra H. (2001) – The rainfall-triggered landslide and flash-flood disaster in northern Venezuela, December 1999. Proceedings of the Seventh Federal Interagency Sedimentation Conference, Reno, NV, IV, 9-16.

Abstract: 
A combination of climatologic, geologic, and demographic factors makes the Caribbean coast of Venezuela in the state of Vargas highly susceptible to episodic debris flows and flash floods. An extremely steep, tectonically active mountain front forms the boundary with a tropical sea. Easterly tradewinds can force moist air masses upslope and precipitate large rainfall volumes, creating conditions for high-magnitude debris flows and flash floods. The population of several hundred thousand people that reside at the base of the mountains is inevitably vulnerable to hydrologic disasters that seem to recur once or twice per century. The flash flood-debris flow process combination is highly destructive in populated areas. Without careful planning of human settlements, the impacts of these types of disasters are likely to increase in the future.

Venezuela debris-flow and flash-flood disaster of 1999 studied

Larsen, M.C., Wieczorek, G.F., Eaton, L.S., Morgan, B.A., Torres-Sierra, H., 2001, Venezuela debris-flow and flash-flood disaster of 1999 studied: EOS, Transactions: American Geophysical Union, v. 82, no. 47, p. 572-573.

Abstract: 
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.

LANDSLIDE HAZARDS ASSOCIATED WITH FLASH-FLOODS, WITH EXAMPLES FROM THE DECEMBER, 1999 DISASTER IN VENEZUELA

Larsen, M. C., M. T. Va´squez Conde, and R. A. Clark, Landslide
hazards associated with flash-floods, with examples from the December,
1999 disaster in Venezuela, in Coping with Flash Floods,
edited by E. Gruntfest and J. Handmer, NATO ASI Ser., in press,
2000.

Abstract: 
Landslides and flash floods commonly occur together in response to intense and prolonged rainfall. Although these phenomena may be viewed by the popular media as distinct events, rainfall-triggered landslides and flash floods are part of a continuum of processes that includes debris flows, hyperconcentrated flows, and streamflow. This combination of processes has proven to be highly destructive in populated areas. Without careful planning of human settlements, the impacts of these types of disasters are likely to increase in the future. 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.”

Geomorphic effects of large debris flows and flash floods, northern Venezuela, 1999

Larsen, M.C. and Wieczorek, G.F., 2006. Geomorphic effects
of large debris flows and flash floods, northern Venezuela,
1999, Z. Geomorph. N.F., Suppl 145:147-175.

Abstract: 
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.
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