Silting of reservoirs
27-10-2021 | Posted by Joaquín Martí
When a river enters a reservoir, the incoming water brings along solid particles that will gradually settle as a function of their size and the local water velocity in the reservoir. With the passage of time, this process leads to the progressive silting of the reservoir with several undesirable effects, such as the reduction in the operational capacity of the reservoir and the increase in the forces exerted on the dam.
To prevent this from happening, dams usually have bottom drains that can be opened to discharge water and sediments. Those discharges must be conducted with care, as they entail potentially serious consequences for the ecology and morphology of the river downstream. Also, such discharges, which carry a considerable load of sediments, may damage the inlet and the conduit of the drain.
Photo: José Luis Casamor
This type of damage is precisely what was experienced in the bottom drain of a dam and, as a consequence, the drain had to be repaired, an opportunity that was used to redesign the inlet.
In support of this exercise, Principia modelled the water and sediments over a considerable part of the reservoir, including the modified inlet and the conduit. The purpose of the simulations was to establish the operability of the drain gate, which was practically covered with sediments, to determine the resulting profile of sediments in the bottom of the reservoir after a number of postulated discharges, and to minimise the damage produced in the inlet during the discharges.
Principia employed the CFD program XFlow, which uses the Lattice Boltzmann methodology for solving the flow of water and sediments leaving the reservoir through the bottom drain. XFlow features the highest fidelity Wall-Modeled Large Eddy Simulation (WMLES) approach to the turbulence modelling and provides a consistent local eddy-viscosity and near-wall behaviour. This was combined with an algorithm to represent the erosion and transport of sediments based on Shields’ approach.
As a result, we could offer predictions for the operation of the design alternatives being considered. It is not a simple problem to solve, but practically all our customers are perfectly capable of solving the simpler ones…