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This work focuses on the experimental classification of flow patterns in rectangular shallow reservoirs, including symmetric flows without any reattachment point to asymmetric flows with one reattachment point, two reattachment points, or two reattachment points and one detachment point. The median position and the natural variability of the reattachment lengths 2 of asymmetric flows were measured for forty geometric and hydraulic conditions. The effects of dimensionless flow depth, Froude number, lateral expansion ratio and dimensionless length on the median reattachment lengths were analyzed. A number of regression equations were proposed. For "high" dimensionless flow depths and a Froude number of 0.20, a shape parameter was proposed for predicting the transition between symmetric and asymmetric flows. The results of this study are useful knowledge for improving current methods to predict the trapping efficiency and the preferential regions of deposition in reservoirs.

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In this study, the capability of a two-dimensional shallow-water numerical model to simulate the symmetric and asymmetric flows that can take place in rectangular shallow reservoirs with different lateral expansion ratios and dimensionless lengths is investigated. Numerically, the main difficulty is to properly reproduce the transition between symmetric and asymmetric flows. For a large lateral expansion ratio, the use of two protocols of simulation highlighted a high sensitivity of the simulated flow pattern to the initial condition. Comparison between simulated results and experimental data showed a good agreement for the critical shape parameter (combination of the lateral expansion ratio and the dimensionless length) between symmetric and asymmetric flows. A good agreement was also found for the value of the shorter reattachment length of asymmetric flows. For small lateral expansion ratios, the agreement was not so good. The model was used for even larger lateral expansion ratios in order to numerically extend the experimental dataset. This predictive work showed that the shape parameter, whose expression was only based on experiments carried out for small lateral expansion ratios, was also relevant for larger values. Moreover, the predicted values of the shorter reattachment length were also consistent with a regression only based on experimental results.

6This work involved the experimental investigation of flow pattern, preferential regions of 7 deposition and trap efficiency as a function of the length of rectangular shallow reservoirs. 8Four flow patterns were identified (from longer to shorter reservoirs): an asymmetric flow 9 with two reattachment points, an asymmetric flow with one reattachment point, an unstable 10 flow, and a symmetric flow without any reattachment point. Using dye visualizations, the 11 median value and the temporal variability of the reattachment lengths were precisely 12 measured for the asymmetric flows. For each stable flow, sediment tests with plastic particles 13 were carried out. The regions of deposition on the bed of the reservoir were clearly a function 14 of the flow pattern. The transition from an asymmetric flow pattern to a symmetric flow 15 pattern was responsible for an abrupt decrease of the trap efficiency; a number of regression 16 laws were discussed to take it into account. 17 18

The purpose of this study is to develop and validate a numerical tool for evaluating the performance of a settling basin regarding the trapping of suspended matter. The Euler-Lagrange approach was chosen to model the flow and sediment transport. The numerical model developed relies on the open source library OpenFOAM, enhanced with new particle/wall interaction conditions to limit sediment deposition in zones with favourable hydrodynamic conditions (shear stress, turbulent kinetic energy). In particular, a new relation is proposed for calculating the turbulent kinetic energy threshold as a function of the properties of each particle (diameter and density). The numerical model is compared to three experimental datasets taken from the literature and collected for scale models of basins. The comparison of the numerical and experimental results permits concluding on the model's capacity to predict the trapping of particles in a settling basin with an absolute error in the region of 5% when the sediment depositions occur over the entire bed. In the case of sediment depositions localised in preferential zones, their distribution is reproduced well by the model and trapping efficiency is evaluated with an absolute error in the region of 10% (excluding cases of particles with very low density).

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