IWRA Proceedings

AbstractIn December 1999, thousands of landslides were triggered by heavy rainfalls along the north coastal range of Venezuela. These landslides generated large debris flows and flash floods in about 24 streams that washed away many towns settled on the alluvial fans, along 50 km of the coastal zone, killing an estimate of 15.000 people and destroying properties esti-mated at more than $2 billion in the State of Vargas. The disaster was caused by man’s un-regulated occupation and urban development on the alluvial fans, canyons and slopes of the coastal range, without any control works upstream for debris flow mitigation. Starting in the year 2000, government authorities initiated different strategies for management of the debris flow risk. Structural protection works do not guarantee a full safety for the population. An in-tegrated approach that takes into account structural and non-structural measures for flood mitigation is described in this paper. Some of these measures are presently being implemented in the Sate of Vargas, Venezuela. Government agencies through CORPOVARGAS with local and foreign funds, have initi- ated an intensive program to canalize the water courses in the alluvial fans, and to build sedi-ment control dams in the canyons of the streams affected by the torrential flows of 1999. At present, 35 dams have been completed (23 closed dams and 12 open dams). Field observations indicate that approximately 50% of the closed dams are completely filled up with sediment, due to the large sediment yield capacity of the basins. The Institute of Fluid Mechanics has developed hazard maps to show the hazard level on alluvial fans. A methodology has been proposed to delineate hazard maps due to mud and de-bris flow events, based on the application of simulation models (FLO-2D) combined with Geographic Information Systems (GIS). The methodology was applied in twenty three sites in the Caracas and Vargas State region in Venezuela. The hazard maps for the Vargas region are now used by planners to design emergency plans and new land use policies. The methodology is also being expanded to other flood hazard regions in Venezuela. Other non-structural measures include monitoring meteorological and hydrological vari-ables in the watersheds, and the implementation of warning systems and contingency plans. During the years 2006 and 2007, a monitoring network of 23 rainfall and flow stations have been instrumented in three basins (Tacagua, La Zorra and Mamo) in order to protect the town of Catia La Mar, with a population of 100.000 people. This project, funded by the European Union and CORPOVARGAS, is the first effort to implement an early warning system in the area affected by the 1999 event. The project required the development and instrumentation of forecasting models and rainfall thresholds. An integrated strategy including structural and non-structural measures for management the risk of debris flows is currently under way in the Vargas State of Venezuela. Further efforts have to be made in creating land use regulations and enforcing the law to prevent reoccupation of areas subjected to high level of hazard.