BRV Debris Flows 1999

In December 1999 Vargas province in  the Bolivarian Republic of Venezuela was by many debris flows, made of sand, gravel, boulders and trees, with a consistency similar to concrete.

Boulders up  5m diameter were transported down to the coast. 2 million^3 of debris appeared in coastal alluvial fans, which extended of the coast by up  250m into what previously the sea.

Causes

Between the 8th to the 19th of December 1999, a cold front across the BRV deposited 914mm of rain on mountainous coastal regions. In that area, the Cordillera de la Costa runs alongside the coastline. The highest is just 10km away  from shoreline. The seaward slopes thus very steep. Runoff of water is very rapid and the steams high energy because of this.

The mountains themselves heavily weathered. They covered in clay, which very weathered away. The clay feeds of sedimentation downstream.

Widespread deforestation has reduced numbers of trees hugely. Roots are less able to fulfil purposes; therefore interception is reduced, and an even greater addition of slope instability.

Because the mountains drop suddenly, there is not much free available for settlement, and alluvial have become popular for settlement. A 6.3% population increase between 1990 and 2001 gas concentrated urban development closely. These coastal developments hit hardest by debris . The fans themselves are caused by continuous flooding events, so a disaster  bound to at some point.

Impacts

Estimates of death toll range  15,000 to 30,000 deaths. 214,000 people effected. 44,000 people refugees.

20,000 homes destroyed with a further 40,000 damaged. Many single storey homes entirely buried. Towns in Carmen de Uria were even swept away.

Hazard mitigation

The people entirely unaware of the risk and thus there was no preparation in case such an event occurred. Flows can be somewhat predicted based on accumulation of sediment in mountain waters, as a debris cannot without sediment present.

Removal of slums on slopes should have been a priority. The president announced shortly afterwards that victims be re-settled away from the coast, but this was questionable as many  people  chose to live on the coast in order to avoid  the struggles of life in the interior of the country. Afterwards, 100,000 people whose homes had n destroyed were relocated to neighbouring  regions.

Many plains will not be suitable for human settlement in the future unless check dams can be built along rivers in  area. Flood channels have been constructed on the alluvial fans. Monitoring and early warning of exceptional rainfall runoff events had suggested, as have land controls in mountain catchment areas, and alignment of towns to match the path debris flows.

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