The use of distributed hydrological models for the Gard 2002 flash flood event: Analysis of associated hydrological processes

Summary This paper presents a detailed analysis of the September 8-9, 2002 flash flood event in the Gard region (southern France) using two distributed hydrological models: CVN built within the LIQUID® hydrological platform and MARINE. The models differ in terms of spatial discretization, infiltration and water redistribution representation, and river flow transfer. MARINE can also account for subsurface lateral flow. Both models are set up using the same available information, namely a DEM and a pedology map. They are forced with high resolution radar rainfall data over a set of 18 sub-catchments ranging from 2.5 to 99 km2 and are run without calibration. To begin with, models simulations are assessed against post field estimates of the time of peak and the maximum peak discharge showing a fair agreement for both models. The results are then discussed in terms of flow dynamics, runoff coefficients and soil saturation dynamics. The contribution of the subsurface lateral flow is also quantified using the MARINE model. This analysis highlights that rainfall remains the first controlling factor of flash flood dynamics. High rainfall peak intensities are very influential of the maximum peak discharge for both models, but especially for the CVN model which has a simplified overland flow transfer. The river bed roughness also influences the peak intensity and time. Soil spatial representation is shown to have a significant role on runoff coefficients and on the spatial variability of saturation dynamics. Simulated soil saturation is found to be strongly related with soil depth and initial storage deficit maps, due to a full saturation of most of the area at the end of the event. When activated, the signature of subsurface lateral flow is also visible in the spatial patterns of soil saturation with higher values concentrating along the river network. However, the data currently available do not allow the assessment of both patterns. The paper concludes with a set of recommendations for enhancing field observations in order to progress in process understanding and gather a larger set of data to improve the realism of distributed models

Etudes structurales et fonctionnelles de l'activateur du plasminogène du venin de serpent Trimeresurus stejnegeri

PARIS-Museum Hist.Naturelle (751052304) / SudocSudocFranceF

Etude du rôle de la géologie sur la réponse hydrologique de bassins versants méditerranéens, sujets aux crues rapides : modélisation régionale et compréhension des processus

International audienceIn 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 inter-event 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 traditionally-considered upper soil horizons, and shows its applicability and benefits for the simulation of flash flood events at the regional scale

Estimation régionale des propriétés de sols à l'échelle du bassin versant à l'aide d'analyse de courbes de récession de débits, pour une utilisation en modélisation hydrologique distribuée

International audienc

Vers une modélisation distribuée régionale pour l'évaluation du risque de crue éclair

International audienceFlash floods result from the combination of meteorological and hydrological conditions. Recognition of the coupled meteorological / hydrological nature of flash floods is now evident in interpretative studies and in the development of predictive models. There is a real need for research to improve the understanding of the major atmospheric and hydrologic factors leading to extreme flood events, especially those affecting small to medium ungauged basins

Simulation hydrologique des crues rapides à l'échelle régionale : signatures spatiales de la vulnérabilité à l'inondation des routes

International audienceThis work contributes to the evaluation of the dynamics of the human exposure during flash-flood events in the Mediterranean region. Understanding why and how the commuters modify their daily mobility in the Cévennes - Vivarais area (France) is the long-term objective of the study. To reach this objective, the methodology relies on three steps: i) evaluation of daily travel patterns, ii) reconstitution of road flooding events in the region based on hydrological simulation at regional scale in order to capture the time evolution and the intensity of flood and iii) identification of the daily fluctuation of the exposition according to road flooding scenarios and the time evolution of mobility patterns. This work deals with the second step. To do that, the physically based and non-calibrated hydrological model CVN (Vannier, 2013) is implemented to retrieve the hydrological signature of past flash-flood events in Southern France. Four past events are analyzed (September 2002; September 2005 (split in 2 different events); October 2008). Since the regional scale is investigated, the scales of the studied catchments range from few km2 to few hundreds of km2 where many catchments are ungauged. The evaluation is based on a multi-scale approach using complementary observations coming from post-flood experiments (for small and/or ungaugged catchments) and operational hydrological network (for larger catchments). The scales of risk (time and location of the road flooding) are also compared to observed data of road cuts. The discussion aims at improving our understanding on the hydrological processes associated with road flooding vulnerability. We specifically analyze runoff coefficient and the ratio between surface and groundwater flows at regional scale. The results show that on the overall, the three regional simulations provide good scores for the probability of detection and false alarms concerning road flooding (1600 points are analyzed for the whole region). Our evaluation procedure provides new insights on the active hydrological processes at small scales (catchments area < 10 km2) since these small scales, distributed over the whole region, are analyzed through road cuts data and post-flood field investigations. As shown in Vannier (2013), the signature of the altered geological layer is significant on the simulated discharges. For catchments under schisty geology, the simulated discharge, whatever the catchment size, is usually overestimated

Impact des caractéristiques des sols sur la réponse hydrologique dans un contexte d'épisodes extrêmes en zone méditerranéenne

The study deals with the September 2002 event in Southern France which was responsible for severe flash floods in the Gard region. We used a modelling approach to quantify the impact of the variability of soil properties on runoff generation. The study allowed to localize various generation mechanisms (infiltration excess and saturation excess) and provides insight on runoff occurence within small ungauged catchments

Modélisation hydrométéorologique des crues éclair - le cas de l'épisode de 2002 dans le Gard (France)

International audienceFlash floods are rare events (in Europe, one ore two flash flood per year have dramatic consequences) and they are ill documented (each country has its own investigation and archiving rules frequently separating the meteorological and hydrological aspects). Flash floods result from the concatenation of meteorological and hydrological conditions. Recognition of the coupled meteorological/hydrological nature of flash floods is now evident in interpretative studies and in the development of predictive models. There is a real need for research that will improve the understanding of the major atmospheric and hydrologic factors leading to extreme flood event, especially those affecting small medium ungauged basins. This paper proposes few advances in our understanding of the hydrometeorological processes and their associated modeling requirements that may be useful to introduce within operational forecasting chain

Liens entre observations in-situ et modèles pour améliorer la compréhension de la dynamique de la zone critique

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAU [ADD1_IRSTEA]Hydrosystèmes et risques naturelsInternational audienceOne of the main objectives of the French OZCAR (Critical Zone Observatories: Research and Applications) Research Infrastructure (RI) is a better understanding of the terrestrial compartments of the Earth System and an integrated representation of the critical zone coupled water, energy and matter cycles, including biogeochemical cycles; covering various spatial and temporal scales and incorporating the heterogeneity of the critical zone and human influence on its environment. OZCAR CZOs offers the opportunity to document all those coupled processes in selected sites/catchments and to progress in process understanding and modeling. However, as revealed by a survey, disseminated in the OZCAR-RI community, the modeling activity is currently fragmented amongst the disciplines. The challenge is thus to increase the level of integration of Critical Zone models able to address OZCAR scientific challenges and provide feedbacks to the society on the status of its environment. To this aim, strong interactions between data science and modeling approaches are necessary. Such an approach is already widely used by the atmosphere community to provide, through data assimilation, reanalyses of the state of the atmosphere and of the components of the water cycle at the global scale. Such approach could be developed for the Critical Zone, providing valuable and novel information for any research or planning activities; and feedbacks on the necessary variables to be measured in CZOs. The presentation will give an overview of the modeling approaches developed independently at the CZO scale within the OZCAR community. This state of the art highlights a strong complementarity between these models in terms of i) compartments of the critical zones concerned by the model; ii) investigated time and space scales, iii) how the CZOs observation data are used in the modeling chain and iii) modeling objectives addressed so far by the community. Based on this state of art, we will discuss the future challenges that will be addressed in OZCAR, in synergy with international initiatives, in order to propose novel integrated modeling approaches of the dynamics of the critical zone. This implies performing coupling between processes described by models running at their own space and time scales and coming from different disciplines; strengthening the links between databases, GIS layers and models; and developing data assimilation tools able to integrate all the available knowledge