Modeling of flood in urban areas with a porosity model: Directional effects

The porosity model is a simplified model of the propagation of floods through urban areas. It is based on the idea that a city can be assimilated to a large-scale porous medium where the pores would be the streets and the solid matrix the buildings. The urban medium is anisotropic because of the presence of larger and wider avenues creating preferential paths. Here the anisotropy is taken into account via an eddy head loss term. A new expression for this term taking into account anisotropic effects is proposed, including a drag tensor and a directional parameter. Experiments were conducted in laboratory on a simplified city layout of 5×5 buildings, with different streets width, and placed either aligned either skewed with the main flow direction. The head loss expression is calibrated and then validated against these experimental data. © 2012 Taylor and Francis Group

Flow in skewed compound channels with rough floodplains

This paper presents an experimental investigation of the flow in a skewed compound channel with rough floodplains. In the investigated geometry, the main channel is prismatic; the left floodplain width reduces from 400 to 0mm on a 6m length; while the right floodplain enlarges from 0 to 400mm on the same distance. Contrarily to previous investigation on non-prismatic geometries found in literature, where both main channel and floodplains always had smooth bed boundaries, the floodplains have a roughness value twice the main channel one. The measurements show that the left floodplain width reduction forces slow water flow to enter into the main channel, shifting the maximum velocity filament to the right; while main channel fast water flows into the right floodplain. Comparison with smooth boundaries skewed channel measurements shows that the increased floodplain flow resistance in the rough bed case reduces the velocity diffusion on the receiving floodplain, at least at low relative flow depth. Lastly, the Independent Subsection Method (ISM) is applied to both smooth and skewed channel cases, and proves to estimate satisfactorily the flow resistance and the discharge distribution evolution along the channel

Sediment transport models to simulate erosion of overtopped earth-dikes

An accurate simulation of the erosion of an earth-dike by overtopping is directly linked to the choice of the sediment transport model. The model should account for rather high sediment transport on the steep downstream slope. This study presents two different numerical approaches to simulate the overtopping of a river embankment. The first approach is based on the classical coupled Saint-Venant-Exner equations, as-suming one layer of pure water over a movable bed that can undergo erosion or deposition. Several empirical formulations are used to estimate the sediment transport capacity. The second approach assumes a fluid layer composed of a mixture of water and sediment, characterised by a depth-averaged density, for which the bed deformation rate is modelled as a function of both the erosion and deposition rates. These two models are compared with laboratory experiments on a one-dimensional dike failure due to overtopping, for which both the temporal water and sediment level evolutions were measured