northern Venezuela debris flow 1999

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Debris flows in northern Venezuela
When: December 1999
Combination of climatological and geomorphological factors.
Between the 8th19th of December a slow moving cold front deposited 914mm of rain in
the costal mountains. This triggered landslides, flash flood and debris flows.
3 features of the regions geomorphology also contributed in this region of the Northern
Venezuela the Cordillera de la Costa runs parallel to the coast. the Summit ridge
20002700m high, is just 10km from the sea as a result the seawardfacing slops are
very sleep runoff is rapid and streams have high energy
2nd the igneous rocks that form the mountains are mantled in easily eroded clay
(phyllite) which feeds large amounts of sediment to debris flows.
3rd widespread deforestation has further increased runoff and erosion.
Depends on the size of the debris flows and the population at risk. Boulders
up to 5m in diameter were transported to the coast and 2million m3 of
debris was added to coastal alluvial fans.
So much debris was transported from the mountains that in places the
coastline was extended seawards by over 250m.
Distribution of population along the coast meant that thousands were
exposed to risk. Because the mountains drop abruptly to the sea there is
little flat land for settlement.
Alluvial fans formed when rivers emerged from the mountains onto the
narrow costal plan, offered some of the few sites for settlement. Recent year
rapid population growth 6.3% in Vargas state between 1990 and 2001, has
concentrated urban development on the alluvial fans. E.g. Caraballeda.
o Lack of preparedness and any strategy for disaster management increased
the vulnerability of the coastal population of Vargas to debris flow and
landslide hazards.
o Early signs of forthcoming debris flow hazards are given by the
accumulation of sediments in stream channels in the mountains. Only when
sediment accumulations are large is the risk from debris flow significant.
o Priority should have been the removal of slums built on steep slopes near
the coast. President of Venezuela announced shortly after the disaster that
victims of the floods would be resettled away from the coast. But given
that many people live in the costal slums the logic of this policy is
questionable. 100,000 people whose homes have been destroyed were
temporarily evacuated.
o The canyon plains of the uria and Cerro Grande Rivers will not be suitable
for settlements unless check dams are built in the mountains to hold back
debris flows and floods. Flood channels could be constricted on some
alluvial fans to allow the passage of large scale debris flows. Other
responses designed to mitigate the impact of debris flows include

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Environmental Social Economic
Habitats destroyed Deaths range from Total damaged estimated at
15,00030,000 $1.9 billion
Affected a total of 214,000
people of whom 44,000
became refugees.
20,000 houses destroyed
and 40,000 damaged.…read more


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