Neural network modeling and geochemical water analyses to understand and forecast karst and non-karst part of flash floods (case study on the &amp;lt;i&amp;gt;Lez&amp;lt;/i&amp;gt; river, Southern France)

T Darras,V Borrell Estupina,A Johannet,S Van-Exter,B Vayssade,L Kong-A-Siou,F Raynaud,S Pistre

doi:10.5194/piahs-369-43-2015

Abstract

Abstract. Flash floods forecasting in the Mediterranean area is a major economic and societal issue. Specifically, considering karst basins, heterogeneous structure and nonlinear behaviour make the flash flood forecasting very difficult. In this context, this work proposes a methodology to estimate the contribution from karst and non-karst components using toolbox including neural networks and various hydrological methods. The chosen case study is the flash flooding of the Lez river, known for his complex behaviour and huge stakes, at the gauge station of Lavallette, upstream of Montpellier (400 000 inhabitants). After application of the proposed methodology, discharge at the station of Lavallette is spited between hydrographs of karst flood and surface runoff, for the two events of 2014. Generalizing the method to future events will allow designing forecasting models specifically for karst and surface flood increasing by this way the reliability of the forecasts.

Highlights

Flash floods are intense and sudden hydrologic responses of small basins to huge rainfall events
Floods occurring on mixed karst and non-karst basins are very difficult to forecast because of the lack of information about the underlying processes and the different dynamics of the both floods
For this reason we proposed, in this work, a methodology able to estimate both the karst part and the nonkarst part of the flood as separate hydrographs

Summary

Introduction

Flash floods are intense and sudden hydrologic responses of small basins to huge rainfall events. In case of mixed, karst/non-karst basins, the role of the karst part of the basin is generally unknown In this context, the aim of this study is to improve understanding of karst hydrosystems behaviour during flash flooding. As generally karst basins boundaries are not superposed to those of the surface watershed and have different dynamics, it seems interesting to estimate two different floods: the surface flood and the underground flood, both being blended in rivers To this end, and as it is generally impossible to achieve these measurement on flash floods downstream of the basin, because of the intensity of flood, we propose in this study a methodology which allows to propagate measurements made on or near karst springs (lower discharge) to build discharge at the outlet of the considered basin. Floods coming from different parts of the watershed are propagated towards the outlet using hydrologic models, and the model of mixture

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