HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Distributed under a Creative Commons Attribution-NonCommercial-NoDerivatives| 4.0 International License
In a fluvial environment, the main role of levees is to canalize water downstream of rivers and to reduce the risk of flooding to nearby areas. Levee failure can be either structural or hydraulic. Structural failure occurs where a breach in a flood defence system leads to the inundation of the protected area whereas hydraulic failure refers to flooding before the designed protection level is attained and without prior damage to the flood defence system. Nowadays, hydrodynamic modelling codes are able to perform hydraulic failure such as overflowing by means of any appropriate weir equation, however, only a few allow to simulate structural failure. HEC-RAS can do both and enables to model levee breaches with a simple but flexible parametric module. The aim of our study is to evaluate the capacity of a 1D hydraulic model to represent levees breaches and subsequent flooding. To do so, a 1D storage area model is built with HEC-RAS and calibrated using data provided by the "Benchmark Garonne" project initiated by EDF. The study case is based on the 1981 historical flood event of the Garonne River between Tonneins and La Réole (section 2). The model is introduced and compared to two other hydraulic models used in the benchmark (section 3). Two sensitivity analyses with respect to sets of hydraulic parameters and levee breach parameters are carried out (section 4). Results expressed as maximum water levels show that the main channel roughness coefficient and the final breach width are the most influencing model parameters, respectively. Levee breaches appear to be a non-negligible source of uncertainty in hydraulic modelling, comparable to uncertainties arising from model structure or model calibration. In order to improve our modelling approach, a ground survey and a literature survey is conducted to collect data about the breaches that occurred in the study area, in particular during the 1981 flood (section 5). Historical evidence shows that a significant number of breaches occurred since 1875.
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
The Holocene sedimentary record of the flood plain of the Saint-Ciers-Sur-Gironde marsh was examined on sediment cores from the right bank of the Gironde estuary with regard to the evolution of the marsh and its potential to preserve high-energy deposits. Sedimentological, geochemical, geophysical and micropaleontological methods were applied. Radiocarbon ages in the central part of the investigation area reach back to 7,971 ± 44.5 BP. The sediments of the Saint-Ciers-sur-Gironde marsh reflect the condi- tions of a Holocene estuarine salt marsh before human activity and draining. The interpretation of the core data showed that the study area includes different facies during development from the pre-Holocene to recent times: tidal mudflats in the northwest at Mortagne and Beaumont, and a fluvial facies develop- ing into an estuarine facies in the southeast near Camp and Saint-Ciers-Sur-Gironde. The last stage is the formation of the saltmarsh. The changes in grain size reflect different transport mechanisms during the development from a fluvial environment to the recent marsh. First gravel, then sand and finally muddy sediments were deposited in the estuary and finally in the marsh area.. Below the salt marsh deposits in Mortagne-Sur-Gironde, there is some evidence of deposits from energy-rich events in tidal mudflats. The southern and central part, in which estuarine clays were deposited, was probably most of the time outside the range of storms. On the marshland surface, erosion, pedogenesis and bioturbation processes destroy storm relevant layers in a very short time.
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