Investigating the role of geology in the hydrological response of Mediterranean catchments prone to flash-floods: Regional modelling study and process understanding

Abstract

s u m m a r yIn this study, a regional distributed hydrological model is used to perform long-term and flash-flood event simulations, over the Cévennes-Vivarais region (south of France). The objective is to improve our understanding on the role played by geology on the hydrological processes of catchments during two past flash-flood events. This modelling work is based on Vannier et al. (''Regional estimation of catchment-scale soil properties by means of streamflow recession analysis for use in distributed hydrological models", Hydrological Processes, 2014), where streamflow recessions are analysed to estimate the thickness and hydraulic conductivity of weathered rock layers, depending on the geological nature of catchments. Weathered rock layers are thus implemented into the hydrological model CVN-p, and the contribution of these layers is assessed during flash-flood events simulations as well as during interevent periods. The model is used without any calibration, to test hypotheses on the active hydrological processes. The results point out two different hydrological behaviours, depending on the geology: on crystalline rocks (granite and gneiss), the addition of a weathered rock layer considerably improves the simulated discharges, during flash-flood events as well as during recession periods, and makes the model able to remarkably reproduce the observed streamflow dynamics. For other geologies (schists especially), the benefits are real, but not sufficient to properly simulate the observed streamflow dynamics. These results probably underline the existence of poorly known processes (flow paths, non-linear spilling process) associated with the planar structure of schisty rocks. On a methodological point of view, this study proposes a simple way to account for the additional storage associated with each geological entity, through the addition of a weathered porous rock layer situated below the traditionallyconsidered upper soil horizons, and shows its applicability and benefits for the simulation of flash flood events at the regional scale.

MobRISK: a model for assessing the exposure of road users to flash flood events

“…3.2, a probability of submersion is assigned to every road cut by combining the flooding susceptibility level of the road section and the return period of stream discharge in the river section. The CVN (Cevenne) distributed hydrological model (Vannier et al, 2016;Branger et al, 2010;Viallet et al, 2006) is used to compute the discharge at the 738 road cuts identified in the Alès case study in hourly time steps for the 2002 flash flood. The CVN model is especially developed for simulating hydrological responses in flash flood events in the Cévennes region (south of France).…”

Groundwater management of a highly dynamic karst by assessing baseflow and quickflow with a rainfall-discharge model (Dardennes springs, SE France)

“…The high dynamics of karst systems have a great impact on flood hazard and drinking groundwater management. First, flash floods in streams are correlated to run-off on urban (Miller et al, 2014) or impervious geological surfaces, and increase in the Mediterranean environment due to the extreme Mediterranean rain-type (Merz and Bloschl, 2003;Nied et al, 2014) and karst contribution (Maréchal et al, 2008;Vannier et al, 2016). Second, the volume of groundwater discharged during floods represents a high recharge rate.…”

Mapping topsoil field-saturated hydraulic conductivity from point measurements using different methods

“…An interesting perspective of this work would be the use of both Kf s maps in uncalibrated distributed hydrological models (see Vannier et al, 2016) to see if discharge simulation is improved at the regional scale.…”

“…This map was compared to the results obtained by Manus et al (2009) and Vannier et al (2016) using the Rawls and Brakensiek (1985) pedotransfer functions (RB85) over the area. For this mapping, these authors used a soil data base from the IGCS (Inventaire Gestion et Conservation des Sols, https://www.gissol.fr/tag/igcs) program, associated with a vector map of soil cartographic units at the 1/250000 scale (pedology map in the following discussion).…”

“…In this region, several hydrological models were proposed for flash flood simulation and/or forecasting, such as models derived from TOPMODEL (Saulnier and Le Lay, 2009;Vincendon et al, 2016), SCS based models (e.g. Gaume and Bouvier, 2004;Laganier et al, 2014;Naulin et al, 2013), models based on the Richards equation (Vannier et al, 2016) or infiltration excess runoff generation , modeling approaches including infiltration excess and sub-surface flow such as the MARINE model (Garambois et al, 2013;Roux et al, 2011). All of these models require mapping of the variation in saturated hydraulic conductivity at regional scale.…”