The Event
Heavy rains fell in December 1999 along the north-central coast of Venezuela, culminating in a period of extreme intensity from the 14th to the 16th of December. Starting around 8 PM local time (AST) on December 15, runoff entered channels and rushed towards the sea, picking up and depositing sediments on its way. Generally after this first wave of flooding, from the coast to just past the crest of the Sierra de Avila, these rains triggered thousands of shallow landslides that stripped soil and rock off of the landscape and sent them slipping down the mountainside. Additional water liquefied these landslides into debris flows, which are granular flows in which water mixes with high concentrations of rock and mud. The first eyewitness accounts of debris flows were from 8:30 PM on the 15th, and the final debris flows were reported between 8 and 9 AM on December 16. Many catchments released multiple debris flows, some of which carried large boulders and tree trunks onto the alluvial fan deltas. Starting between 7 and 9 AM on the 16th and continuing until late that afternoon, a new wave of floods occurred. These floodwaters were less concentrated in sediment and were therefore able to entrain new material and incise new channels into the flood and debris flow deposits from the previous days.
The debris flows moved rapidly, and many of them were highly destructive. Based on the maximum sizes of boulders measured in the flood deposits and the amount by which the flow on the outside of a bend was higher than that on the inside, geologists estimate the flow velocities to range from 3.3 to 14.5 meters per second. These rapid, bouldery flows resulted in much of the observed destruction.
In addition to these debris flows, flash floods carrying extremely high sediment loads were very dangerous. Together the flash floods and debris flows destroyed hundreds of houses, bridges, and other structures. They incised new channels to depths of several meters into every alluvial fan delta on the Vargas state coastline, and they blanketed these fans with sediment.
Read more about this topic: Vargas Tragedy
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