A modeling approach to assess the hydrological response of small mediterranean catchments to the variability of soil characteristics in a context of extreme events

This paper presents a modeling study aiming at quantifying the possible impact of soil characteristics on the hydrological response of small ungauged catchments in a context of extreme events. The study focuses on the September 2002 event in the Gard region (South-Eastern France), which led to catastrophic flash-floods. The proposed modeling approach is able to take into account rainfall variability and soil profiles variability. Its spatial discretization is determined using Digital Elevation Model (DEM) and a soil map. The model computes infiltration, ponding and vertical soil water distribution, as well as river discharge. In order to be applicable to ungauged catchments, the model is set up without any calibration and the soil parameter specification is based on an existing soil database. The model verification is based on a regional evaluation using 17 estimated discharges obtained from an extensive post-flood investigation. Thus, this approach provides a spatial view of the hydrological response across a large range of scales. To perform the simulations, radar rainfall estimations are used at a 1 km<sup>2</sup> and 5 min resolution. To specify the soil hydraulic properties, two types of pedotransfer function (PTF) are compared. It is shown that the PTF including information about soil structure reflects better the spatial variability that can be encountered in the field. The study is focused on four small ungauged catchments of less than 10 km<sup>2</sup>, which experienced casualties. Simulated specific peak discharges are found to be in agreement with estimations from a post-event in situ investigation. Examining the dynamics of simulated infiltration and saturation degrees, two different behaviors are shown which correspond to different runoff production mechanisms that could be encountered within catchments of less than 10 km<sup>2</sup>. They produce simulated runoff coefficients that evolve in time and highlight the variability of the infiltration capacity of the various soil types. Therefore, we propose a cartography distinguishing between areas prone to saturation excess and areas prone only to infiltration excess mechanisms. The questions raised by this modeling study will be useful to improve field observations, aiming at better understanding runoff generation for these extreme events and examine the possibility for early warning, even in very small ungauged catchments

Appropriate model use for predicting elevations and inundation extent for extreme flood events

Flood risk assessment is generally studied using flood simulation models; however, flood risk managers often simplify the computational process; this is called a “simplification strategy”. This study investigates the appropriateness of the “simplification strategy” when used as a flood risk assessment tool for areas prone to flash flooding. The 2004 Boscastle, UK, flash flood was selected as a case study. Three different model structures were considered in this study, including: (1) a shock-capturing model, (2) a regular ADI-type flood model and (3) a diffusion wave model, i.e. a zero-inertia approach. The key findings from this paper strongly suggest that applying the “simplification strategy” is only appropriate for flood simulations with a mild slope and over relatively smooth terrains, whereas in areas susceptible to flash flooding (i.e. steep catchments), following this strategy can lead to significantly erroneous predictions of the main parameters—particularly the peak water levels and the inundation extent. For flood risk assessment of urban areas, where the emergence of flash flooding is possible, it is shown to be necessary to incorporate shock-capturing algorithms in the solution procedure, since these algorithms prevent the formation of spurious oscillations and provide a more realistic simulation of the flood levels

Scaling properties of heavy rainfall at short duration: A regional analysis

International audienceThe aim of this paper is to assess the scaling properties of heavy point rainfall with respect to duration. In the region of interest, the probability distribution tails of hourly to daily rainfall display log-log linearity. The log-log linearity of tails is a feature of fat-tailed distributions. The conservation of this property throughout the scales will be investigated in the framework of scale-invariant analysis. Evidence of the scaling of heavy rainfall is shown for one particularly long rainfall series through the conservation of the survival probability shape at durations in the range 1-24 h. An objective method is implemented to estimate the hyperbolic-tail parameters of rainfall distributions. This method is automatized and detects the lower bound above which the distributions exhibit power law tails and determines the power law exponent α using a maximum likelihood estimator. The application of unbiased estimation methods and scale-invariant properties for the estimation of the power law exponent provides a significant reduction of the intergage power law variability. This achievement is essential for a correct use of geostatistical approaches to interpolate the power law parameters at ungaged sites. The method is then applied to the rain gage network in the Cévennes-Vivarais region, a Mediterranean mountainous region located in southern France. The maps show thicker rainfall distribution tails in the flat area between the seashore and the foothill. It is shown that in a flat region closer to the Mediterranean Sea the rainfall distribution tails are hyperbolic and the power law exponent is quasi-constant with duration, whereas, over the mountain, the power law behavior is less defined. The physical reasons for such results and the consequences for the statistical modeling of heavy rainfall are then discussed, providing an innovative point of view for the comprehension of the rainfall extremes behavior at different temporal scales

3. - Comparaison d’estimations radar et pluviométriques de pluies horaires sur de petits bassins versants cévenoles

Faure D., Andrieu Hervé, Creutin J. D. 3. - Comparaison d’estimations radar et pluviométriques de pluies horaires sur de petits bassins versants cévenoles. In: Crues et inondations. 23èmes journées de l'hydraulique. Congrès de la Société Hydrotechnique de France. Nimes (France), 14-15-16 septembre 1994. Tome 2, 1994

The role of soil in the generation of urban runoff: development and evaluation of a 2D model

A two-dimensional numerical model is developed to determine the role of soil in the formation of urban catchment runoff. The model is based on a modeling unit, called the Urban Hydrological Element (UHE), which corresponds to the cross-section of an urban cadastral parcel. Water flow in the soil of a UHE is explicitly simulated with a finite element code for solving the Richards' equation. Two runoff components, dependent on soil behavior, are represented: runoff from natural surfaces and drainage of groundwater into the rainwater network. In an initial case study, the model is applied to a 4.7-ha suburban catchment. Simulated and observed runoff and soil water pressure heads show reasonable agreement. Soil appears to play a significant role in the formation of runoff at the scale of the small catchment under examination: its contribution represents an average of 14% of the total per-event runoff volume. Soil contribution is particularly important during rainfall events characterized by a shallow water table level, which explains a determinant part of the seasonal trend of catchment response