Discharge coefficients calculation for the Vaugris dam

Context

In order to automatically regulate hydroelectric schemes, systems rely on level measurements at certain characteristic points of the reservoirs (control points), as well as on flows at the structures (dams and plants). Sufficiently accurate flow data are therefore essential for effective overall control of the hydroelectric chain.

The dam studied here features two types of discharge device: flap gates, which allow water to flow over the dam, and bottom gates, which allow water to flow under the dam. The fundamental role of these elements is to control flooding by maintaining the dam’s elevation below its stability limit during these episodes. Their main function is therefore to ensure the safety of the structure. Their design incorporates safety coefficients to ensure that flood flows are largely evacuated. However, there are other reasons why we need to know the flow rate of these devices as accurately as possible:

• The first is legislation, which imposes an instream flow on natural watercourses to maintain fauna, flora and agricultural uses.
• The second is the economic stakes for the operator, since what is discharged by the spillways is not turbined, and therefore represents a loss of income.

To achieve the best possible prediction, operators use physical models. However, these models are built at scales that do not always capture all phenomena.

Objective

In order to overcome the limitations of physical models, linked to the problem of similitude, a numerical model of the dam to be studied is created. This numerical model serves a dual purpose:

• Simulate the flow on the physical model, at a scale of 1/35, and “calibrate” the numerical model by comparison with the results of experimental measurements.
• Run numerical simulations on a scale of 1, quantify deviations from the physical model and refine knowledge of actual flow rates.

Simulation and results

In this context, numerical simulation helps to better understand these phenomena. The first step is to recalibrate the results and compare them with the experimental measurements obtained from the physical models, in order to validate the numerical model. The second step is to simulate the structure on a full-scale numerical model, under operating conditions and without introducing any similitude bias. In general terms, CFD modeling can be used to calculate flow rates for the various safety components of a dam: segment gates, flap gates, labyrinth, circular or piano key weirs, etc.

Dam flow modeling is one of OptiFluides’ main fields of activity. This type of hybrid modeling combines the advantages of physical models (such as the representation of all 3D phenomena in similitude) with those of three-dimensional numerical simulation (such as the determination and visualization of velocity and pressure fields over large domains and at any point in space). These results have enabled us to gain a better understanding of the physical phenomena involved, and of the flows involved, and thus to better regulate and control the hydroelectric structures on the Rhône.

These results were published at a conference organized by the Comité Français des Barrages et des Réservoirs (French Committee for Dams and Reservoirs).