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Karst/river interactions during Mediterranean flash-floods: Case of the Coulazou river with the karst system of the Thau lagoon (South France)

Bailly-Comte Vincent

This poster deals with the influence of a karst aquifer on the propagation of flash-floods in an ephemeral Mediterranean river. Near Montpellier (South France), Jurassic limestones of the karst system of the Thau lagoon outcrop widely on the "Causse d'Aumelas". In this large and arid calcareous area, the allogenic Coulazou River is in contact with the karst aquifer. This ephemeral river interacts with the karst aquifer through numerous karst features along the riverbed like sinkholes or estavelles (karst features acting as sinkhole or as spring according to the hydrological conditions).
The Coulazou River starts from an upstream watershed where intense Mediterranean rainfalls on clay formation generate flash-floods. The aim of this study is to understand how the flood peaks are routed along the river and what the role of the karst aquifer is.
Three approaches are proposed:
i) An hydrologic model based on a Digital Terrain Model shows the inability of a standard rainfall-runoff model to reproduce the recorded flood hydrograph at the exit of the karst system. These results were interpreted as an initial karst system recharge followed by significant contribution to surface flow that the model not accounts for.
ii) The analysis of the groundwater hydrodynamic confirms these results: the karst system first absorbs part of the rainy event, which induces a general water table rising within the aquifer, and then contributes to surface flow.
iii) Time series analyses on discharge at the entrance and the exit of the karst system show that the flood energy decreases (surface water storage) or increases (surface water release or groundwater contribution). For each flood event a convolution model based on linear assumptions is used to represent the transfer functions of the discharge time series between the entrance and the exit of the karst system. Their evolutions are indicative of the dynamics of karst storage, release or contribution to surface waters.