Characterization of process-oriented hydrologic model behavior with temporal sensitivity analysis for flash floods in Mediterranean catchments

This paper presents a detailed analysis of 10 flash flood events in the Mediterranean region using the distributed hydrological model MARINE. Characterizing catchment response during flash flood events may provide new and valuable insight into the dynamics involved for extreme catchment response and their dependency on physiographic properties and flood severity. The main objective of this study is to analyze flash-flood-dedicated hydrologic model sensitivity with a new approach in hydrology, allowing model outputs variance decomposition for temporal patterns of parameter sensitivity analysis. Such approaches enable ranking of uncertainty sources for nonlinear and nonmonotonic mappings with a low computational cost. Hydrologic model and sensitivity analysis are used as learning tools on a large flash flood dataset. With Nash performances above 0.73 on average for this extended set of 10 validation events, the five sensitive parameters of MARINE process-oriented distributed model are analyzed. This contribution shows that soil depth explains more than 80% of model output variance when most hydrographs are peaking. Moreover, the lateral subsurface transfer is responsible for 80% of model variance for some catchment-flood events’ hydrographs during slow-declining limbs. The unexplained variance of model output representing interactions between parameters reveals to be very low during modeled flood peaks and informs that model parsimonious parameterization is appropriate to tackle the problem of flash floods. Interactions observed after model initialization or rainfall intensity peaks incite to improve water partition representation between flow components and initialization itself. This paper gives a practical framework for application of this method to other models, landscapes and climatic conditions, potentially helping to improve processes understanding and representation

EVALUATION DE SYSTEMES DE PRODUCTION INNOVANTS INSCRITS EN AGRICULTURE DURABLE : LE CAS DES SYSTEMES BOVINS HERBAGERS DU HAUT-BOCAGE POITEVIN

N° ISBN - 978-2-7380-1284-5International audienceSince 1990, farmers of Poitou's Haut-Bocage (center West of France) have set up cattle grassland systems based upon André Pochon‘s Britain example and largely based on grazing of temporary grass-clover pastures. Adapted by farmers' groups to the specific pedoclimatic conditions of this region, these innovative systems create high value added by important cost savings, in complete contrast to the main agricultural development in the region, aiming at increasing work productivity thanks to the growth of production per hectare and per worker. The effects of grassland systems implementation in this region are appraised by using the concept of production system and the methods of project's assessment (comparison of a grassland scenario and a counterfactual scenario), from standpoint of both farmers and the whole national economy. The results of the differential between this two scenarios point out that farmers benefit from higher incomes and less working load and that, between 1990 and 2009, this implementation allowed net national income increase, 50% more farmers' job creation and a better labour remuneration in all the sectors. These less subsidized cattle grassland systems, characterized by a higher social and environmental viability, also allowed to cut down public expenditures

Geophysical investigation of landslides : a review

International audienceIn the last two decades, shallow geophysics has considerably evolved with the emergence of 2D spatial imaging, then 3D spatial imaging and now 4D time and space imaging. These techniques allow the study of the spatial and temporal variations of geological structures. This paper aims at presenting a current state-of-the-art on the application of surface geophysical methods to landslide characterization and focuses on recent papers (after 1990) published in peer-reviewed International Journals. Until recently, geophysical techniques have been relatively little used for the reconnaissance of landslides for at least two main reasons. The first one is that geophysical methods provide images in terms of physical parameters which are not directly linked to the geological and mechanical properties required by geologists and engineers. The second reason shown through this study probably comes from a tendency among a part of the geophysicists to overestimate the quality and reliability of the results. This paper gave the opportunity to review recent applications of the main geophysical techniques to landslide characterisation, showing both their interest and their limits. We also emphasized the geophysical image characteristics (resolution, penetration depth) which have to be provided for assessing their reliability, as well as the absolute requirements to combine geophysical methods and to calibrate them with existing geological and geotechnical data. We hope that this paper will contribute to fill the gaps between communities and to strength of using appropriate geophysical methods for landslide investigation

A physically-based parsimonious hydrological model for flash floods in Mediterranean catchments

A spatially distributed hydrological model, dedicated to flood simulation, is developed on the basis of physical process representation (infiltration, overland flow, channel routing). Estimation of model parameters requires data concerning topography, soil properties, vegetation and land use. Four parameters are calibrated for the entire catchment using one flood event. Model sensitivity to individual parameters is assessed using Monte-Carlo simulations. Results of this sensitivity analysis with a criterion based on the Nash efficiency coefficient and the error of peak time and runoff are used to calibrate the model. This procedure is tested on the Gardon d'Anduze catchment, located in the Mediterranean zone of southern France. A first validation is conducted using three flood events with different hydrometeorological characteristics. This sensitivity analysis along with validation tests illustrates the predictive capability of the model and points out the possible improvements on the model's structure and parameterization for flash flood forecasting, especially in ungauged basins. Concerning the model structure, results show that water transfer through the subsurface zone also contributes to the hydrograph response to an extreme event, especially during the recession period. Maps of soil saturation emphasize the impact of rainfall and soil properties variability on these dynamics. Adding a subsurface flow component in the simulation also greatly impacts the spatial distribution of soil saturation and shows the importance of the drainage network. Measures of such distributed variables would help discriminating between different possible model structures

Data assimilation method for real-time flash flood forecasting using a physically based distributed model

The MARINE model (Roux et al, 2011) is a physically based distributed model dedicated to real time flash flood forecasting on small to medium catchments. The infiltration capacity is evaluated by the Green and Ampt equation and the surface runoff calculation is divided into two parts: the land surface flow and the flow in the drainage network both based on kinematic wave hypothesis. In order to take into account rainfall spatial-temporal variability as well as the various behaviours of soil types among the catchment, the model is spatially distributed, which can also help to understand the flood driving processes. The model integrates remote sensing data such as the land coverage map with spatial resolution adapted to hydrological scales. Minimal data requirements for the model are: the Digital Elevation Model describing catchment topography and the location and description of the drainage network. Moreover some parameters are not directly measurable and need to be calibrated. Most of the sources of uncertainties can be propagated thanks to variational method (Castaings et al, 2009) and finally help to determine time dependent uncertainty intervals. This study also investigates the methodology developed for real-time flash flood forecasting using the MARINE model and data assimilation techniques. According to prior sensitivity analyses and calibrations, parameters values were determined as constants or initial guess. Then a data assimilation method called the adjoint state method is used to update some of the most sensitive parameters to improve accuracy of discharges predictions. The forecast errors are evaluated as a function of lead time and discussed from an operational point of view. Multiple strategies in term of updatable parameters set, length of time window, parameters bounds and observation threshold used to trigger the assimilation method are discussed regarding accuracy, robustness and real-time feasibility

On the assimilation of SWOT type data into 2D shallow-water models

In river hydraulics, assimilation of water level measurements at gauging stations is well controlled, while assimilation of images is still delicate. In the present talk, we address the richness of satellite mapped information to constrain a 2D shallow-water model, but also related difficulties. 2D shallow models may be necessary for small scale modelling in particular for low-water and flood plain flows. Since in both cases, the dynamics of the wet dry front is essential, one has to elaborate robust and accurate solvers. In this contribution we introduce robust second order, stable finite volume scheme [CoMaMoViDaLa]. Comparisons of real like tests cases with more classical solvers highlight the importance of an accurate flood plain modelling. A preliminary inverse study is presented in a flood plain flow case, [LaMo] [HoLaMoPu]. As a first step, a 0th order data processing model improves observation operator and produces more reliable water level derived from rough measurements [PuRa]. Then, both model and flow behaviours can be better understood thanks to variational sensitivities based on a gradient computation and adjoint equations. It can reveal several difficulties that a model designer has to tackle. Next, a 4D-Var data assimilation algorithm used with spatialized data leads to improved model calibration and potentially leads to identify river discharges. All the algorithms are implemented into DassFlow software (Fortran, MPI, adjoint) [Da]. All these results and experiments (accurate wet-dry front dynamics, sensitivities analysis, identification of discharges and calibration of model) are currently performed in view to use data from the future SWOT mission

Multiconfiguration GPR measurements for geometric fracture characterization in limestone cliffs (Alps)

Until now, geophysical methods have been rarely used to investigate vertical limestone cliffs, mainly due to the extreme conditions for data acquisition. Nevertheless, these techniques are the only available methods which could provide information on the internal state or a rock mass in terms of discontinuities, which play a major role in rock-fall hazards. In this case study, detailed GPR measurements were carried out on a test site with different acquisition configurations deployed on vertical cliff faces. Conventional 2D profiles, common midpoints (CMP) and transmission data were acquired to evaluate the potential of radar waves to improve the characterization of the geometry and properties of the main discontinuities (fractures) within the massif. The results show that the 3D geometry of fractures, which is a crucial parameter for stability assessment, can be retrieved by combining vertical and horizontal profiles performed along the cliff. CMP profiles acquired along the cliff allow a velocity profile to be obtained as a function of depth. Finally, transmission experiments, which generate complex radargrams, have provided valuable and quantitative information on the rock mass, through the modelling of the waves generated. On the other hand, a velocity tomography obtained from the first arrivals travelling through the rock mass from the transmitters to the receivers, shows an image of the investigated zone with a poor resolution

Borehole seismoelectric logging using a shear-wave source: Possible application to CO2 disposal?

International audienceThe behaviour of CO2 deposition sites-and their surroundings-during and after carbon dioxide injection has been matter of study for several years, and several geophysical prospection techniques like surface and crosshole seismics, geoelectrics, controlled source electromagnetics among others, have been applied to characterize the behaviour of the gas in the reservoirs. Until now, Seismolectromagnetic wave conversions occuring in poroelastic media via electrokinetic coupling have not been tested for this purpose. In this work, by means of numerical experiments using Pride's equations-extended to deal with partial saturations-we show that the seismoelectric and seismomagnetic interface responses (IR) generated at boundaries of a layer containing carbon dioxide are sensitive to its CO2 content. Further, modeling shear wave sources in surface to borehole seismoelectric layouts and employing two different models for the saturation dependence of the electrokinetic coefficient, we observe that the IR are sensitive to CO2 saturations ranging between 10% and 90%, and that the CO2 saturation at which the IR maxima are reached depends on the aforementioned models. Moreover, the IR are still sensitive to different CO2 saturations for a sealed CO2 reservoir covered by a clay layer. These results, which should be complemented by the analysis of the IR absolute amplitude, could lead, once confirmed on the field, to a new monitoring tool complementing existing ones

Méthodes de régionalisation pour un modèle pluie-débit distribué et à base physique dédié aux crues éclair

Cette étude s’intéresse aux méthodes de régionalisation pour des jeux de paramètres d’un modèle pluie‑débit distribué et à base physique, dédié aux crues éclair. Les performances du modèle MARINE sont testées sur un total de 117 évènements de crues éclair survenues sur des bassins versants de l’arc méditerranéen français. Etant donnée la relative rareté des enregistrements de crues éclair, ce jeu de données représente un échantillon conséquent des paysages et de l’hydrologie des régions allant du piémont pyrénéen à la Provence en passant par les Cévennes et le Vivarais. Des approches de régionalisation basées sur la proximité géographique ou les similarités physiographiques sont testées avec plusieurs combinaisons de descripteurs. Des résultats encourageants sont obtenus avec les deux méthodes de similarités physiographiques basées sur deux ou trois bassins donneurs. Une perte de performance de 10% en régionalisation par rapport à la calibration/validation est relevée pour ces méthodes. Pour 13 bassins versants sur 16, au moins un évènement est simulé avec de bonnes performances. Cette étude met en avant l’importance des informations hydrologiques contenues dans les évènements de calibration disponibles sur site ou sur les bassins donneurs. De plus les techniques de régionalisation produisent de meilleures performances sur les bassins présentant un comportement hydrologique apparemment plus régulier. Le paramètre le plus sensible du modèle MARINE, CZ, contrôlant le volume de sol et ainsi le bilan en eau, est plutôt bien contraint par les approches de régionalisation par similarité grâce aux descripteurs du socle rocheux