Modeling sediment pulses on an armored channel

Dams trap sediments in the upstream reservoir, thus the continuity in sediment transport along the river is interrupted. Several negative aspects related to the lack of sediment supply were observed downstream many dams and the artificial replenishment of sediment was applied as a method to restore natural morphological conditions. The replenishment technique may be seen as a pulse of sediments added to the channel. Once the pulse-like volume of sediments is added into the reach, it is seen like an alteration of the channel geometry producing cross section narrowing and flow acceleration lengthwise the river. The river geometry modification together with the water flow allow erosion and transportation of the replenished material. In this work, the response of the flow to the material added in the channel is studied by means of the 2D shallow water equations model in combination with the Exner equation for the sediment continuity equation. Computational outcomes are compared with experimental data obtained from several replenishment configurations studied in the laboratory. It is observed how the bed fining provokes a larger spread of the material without aggradation within the armored channel. Kantoush et al. (2010). They showed that sediment deposits are eroded by a combination of lateral erosion on the placed sediment toe by hydraulic forces and subsequent mass failure by gravitational mechanism due to over-steepening of the banks. After mass failure, the collapsed mass is transported further downstream. This mass of sediments is responsible for the bed fining which afterwards, it causes a rapid transport of the replenishment material (Battisacco et al. 2015, Bösch et al. 2016). These results are obtained by a series of laboratory experiments performed at the Laboratory of Hydraulic Constructions at École Polytechnique Fédérale de Lausanne (EPFL), in Switzerland. Furthermore, the same tests are reproduced numerically in this work by using the 2D shallow water equations and the Exner equation. The spatial and temporal evolution of the hydrodynamic and morphodynamic models is solved by a synchronous treatment (Aricò et al. 2008), since there is a strong interaction between the flow and the replenishment material. The set of equations is computed by means of an explicit Finite Volume numerical scheme (Juez et al. 2014) where the numerical stability is controlled dynamically by an augmented CFL condition. The paper is structured as follows: first the methodology to obtain the experimental data is described. Then the next two sections are devoted to briefly outline the mathematical model and the numerical scheme considered. The following section presents the results obtained with this study and their later discussion. Finally, the conclusions of the work are summarized.