Erosion modeling over a steep slope: application to a dike overtopping test case

The most violent floods are due to the failure of embankments, such as dams or levees. In case of dike overtopping, the erosion over the steep downstream slope is one of the main processes leading to the breaching. This study analyses the ability of numerical models to simulate this process. Various sediment transport formulations are presented and compared. A special attention is paid to the ways to account for the slope effects in these expressions. These sediment transport equations are included in two different one dimensional numerical models, based on the assumptions of a clear water layer and of a sediment-water mixture layer respectively. These two models are applied on a new dike overtopping experimental test-case, representing a small-scale sand dike with a sand layer downstream of the dike. The numerical results are compared to the experimental measurements, with a special attention paid on the sediment transport formulation, on the steep slope correction factor, and on the choice of the numerical model

One-dimensional finite-volume modeling of the flow and morphological processes during the 1996 Lake Ha!Ha! dyke break event

The major flood following the Lake Ha!Ha! dyke-break in 1996 in Québec (Canada) induced important morphological changes in the valley. For instance, massive erosion was witnessed along with significant valley enlargements. Another interesting observation was the creation of an avulsion, i.e. a new river bed. Due to the very complex and irregular topography of the valley, flow simulations in that area constitute a real challenge for numerical models. Indeed, the river presents very steep slopes at some locations, or very abrupt changes in cross-section shape. The presence of rocky outcrops creates another difficulty because of its impact on the erosion process. The focus of the research is to simulate the Lake Ha!Ha! dyke-break event using a one-dimensional finite-volume scheme solving the Saint-Venant-Exner equations in a decoupled way. First, the analysis is conducted from the pure hydrodynamical point of view allowing the characteristics of the flow to be computed. Then, these characteristics are used to determine the sediment transport and, finally, the morphological changes. Even though the model uses a one-dimensional framework, bank erosion and bank failure processes are also accounted for. Comparisons between the computed final topography and field measurements are provided, along with a discussion about the quality of the results and the key issues for future research

DAM-BREAK FLOW IN A CHANNEL WITH A 90° BEND AND MOBILE BED: EXPERIMENTAL AND NUMERICAL SIMULATIONS

Predicting the morphological changes induced by fast transient flows such as flash floods or dam-break flows is still a challenging task, especially when sediment transport is considered. In order to validate numerical simulation tools designed for such flow conditions, reliable experimental data is needed. In this paper, dam-break flow experiments over a mobile bed made of coarse uniform sediments are presented. The initial conditions consist of a constant water-depth in the upstream reservoir and a saturated sediment bed downstream. Due to the strong reflection of the flow in the bend, complex morphological patterns can be observed, with significant erosion and deposition. Measurements were obtained using non-intrusive devices to capture the free-surface evolution as well as the bed morphological evolution. The free-surface evolution was measured using ultrasonic probes at several locations. The bed morphological evolution was measured using digital imaging devices and a laser-sheet to isolate given cross-sections of the flow on the images. Such a technique proved its efficiency in slower flow conditions, but its application to the present case with a fast flow involving significant sediment transport is challenging. These measurements are then compared to numerical simulations performed using a finite-volume scheme with a lateralized HLLC scheme for the flux predictions. This scheme solves in a coupled way the system composed by the shallow-water equations and the Exner equation for the bed morphology, closed by the Meyer-Peter and Müller formula for sediment transport. Finally, a discussion of the numerical results is proposed, through the comparisons with the experimental data