Assesing alluvial fan activity

Alluvial fan in La GuinguetaThe study area is a widening of a glacial valley in the Pyrenees. During the Holocene, the valley filled up with lacustrine and fluvial sediments behind the moraines. Various alluvial fans mound on the floodplain that form a risc for the villages and infrastructure on and close to the fans. Besides, what already happenend in the past, a debris-flow event can possibly cut off the main stream in the valley. This would flood the upstream town and might cause a sudden breakthrough of the dam with its consequences downstream.

In the past there have been several severe floods in the area; the latest one in 1982 when heavy rainfall and debris-flows caused the flooding of a village situated on one of the alluvial fans (La Guingueta). Besides, the alluvial fan reached the main stream in the valley and temporarily cut it off.

The aim of the study is to delineate hazard risk maps of the floodplain regarding the variable dynamics and activity of the different alluvial fans. Therefore, a field survey of the active source and sink areas and a recollection of the historical data and marks will be carried out. The gathered information will support the parameterization of a 2-dimensional hydrodynamic model. This model accounts for hyperconcentrated flows and should give us the tool to study the possible scenarios.

The analysis will give the responsible Planning Authority indications on reducing the hazard risks in the area through prevention measures on the alluvial fans. Secondly it will identify potential sites on which possible urban developments are viable.

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