Which hydrological activity in weathered bedrock during flash floods? The case of Mediterranean catchments

"Flash flood" term refers to flood with a high specific discharge resulting from storm events of strong intensity. The steepness of numerous small catchments (of a few km2) contributes to a short response time, and the generation of runoff can suddenly turn into devastating floods. In these cases, the discharge seems to be controlled by fast flow transfers. Overland flows explain most of the peak discharge, but subsurface flows can also represent a significant contribution, particularly during the recession limb. Owing to the short time scales, subsurface flows seem to occur into shallow soil horizons. Therefore, hydrological models often use soil characteristics of the upper horizons from BD-sols databases. Unfortunately, experimental data on subsurface flow are not available at the studied space scale (about 100km2). However, many studies (Garambois, 2012; Vannier et al, 2013; Sayama et al, 2011) show that the storage capacities of upper horizons cannot explain alone the water balance during flash floods. As a matter of fact, effective soil storage capacities on Mediterranean catchments often largely exceed the storage capacities of the upper soil layers described in the BD-sols (Vannier et al., 2013). This study implements two methods to assess the actuality of a hydrological dynamic into weathered rock layers during flash floods. These are applied on two past decades observations of flash flood over Mediterranean catchments with different geological characteristics. The first one consists in assessing water storage capacities on weathered rock from observed events data. The second one consists in using a distributed hydrological model dedicated to flash flood events (Roux et al., 2011) to estimate the flow and storage in the deeper soil horizons. Results show that flows through weathered bedrock layers contribute significantly to the flood hydrograph and more particularly on catchments with granitic bedrock. As an example, on the Ardèche catchment, the observed water storage of some events is up to twice the storage capacity of the upper soil layers. Hydrological simulations also estimate an average contribution of 30 % of the deeper layers to the total flood water volume. As a conclusion, the study emphasizes the impact of the weathered bedrock on the hydrological dynamic and shifts future researches toward a more realistic representation of the subsurface flows