Characterization of Sedimentation Trends in a Shallow Rectangular Reservoir

Shallow flows are related in Hydraulic Engineering to turbulent flows whose horizontal dimensions are considerably larger than the vertical one. As a result, they are usually characterized as two-dimensional or quasi-2D.

The shallow reservoirs, for which the aforementioned premises apply, tend to present low velocity flows, which make them vulnerable to a continuous sedimentation process. Thus, it is extremely important for the scientific community to expand knowledge on the hydrodynamic behavior and sediment deposition patterns observed in these reservoirs, due to their geometric and hydraulic characteristics.

The objective of this article is to characterize sedimentation trends observed experimentally in a shallow rectangular reservoir, due to its geometric characteristics and the flow rate imposed under steady flow regime.

For this purpose, a shallow rectangular reservoir with a flat bottom, 1.5 m long, 1.0 m wide and 0.15 m deep, was constructed in the Hydraulic Laboratory of the Instituto Superior Técnico of the University of Lisbon (Portugal). The inlet and outlet channels were 1.0 m long, 0.063 m wide and 0.15 m deep. Three alternative locations were provided for coupling the two channels within the reservoir: to the left (L-position), aligned with its longitudinal axis (C-position, center) and to the right (R-position). Different geometric combinations were thus evaluated: CC, LL, LC, LR and CR configurations.

The water flow rate in all the experiments was 1.00 l/s under steady flow regime. For sediment representation, granular bakelite (ρ=1.30 g/cm³; D₅₀=0.75 mm) was used with constant concentration of about 3.0 g/l of water, mixed to it upstream of the studied reservoir. The tests lasted about 6 to 9 hours, depending on the geometric configuration, and were interrupted after reaching the solid equilibrium of the upstream and downstream concentrations. A periodic bathymetric survey was carried out to follow the morphological evolution of the solid deposits and an evaluation of the final morphology was made at the end of the experiment. In addition, some points were selected for measuring depth velocities using a Vectrino Profiler probe. The LSPIV technique was used to evaluate the flow pattern observed in the reservoir before and after the sediment insertion.

It was found experimentally that the geometric configuration LR was the one that presented greater sediment retention, unlike the LL configuration, which resulted in a lower solid accumulation. It was observed a significant influence of the trajectory extension performed by the main jet.