Uncertainties of a 1D Hydraulic Model with Levee Breaches: The Benchmark Garonne

Abstract

In a fluvial environment, the main role of levees is to canalise water downstream of rivers and to reduce the risk of flooding in 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 Reole (Sect. 2). The model is introduced and compared to two other hydraulic models used in the benchmark (Sect. 3). Two sensitivity analyses with respect to sets of hydraulic parameters and levee breach parameters are carried out (Sect. 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 (Sect. 5). Historical evidence shows that a significant number of breaches occurred since 1875.