Hydro-abrasive turbine wear caused by sediments in the water has negative effects on hydroelectric power generation. Desanding facilities are adequate countermeasures, but often do not meet the design requirements. An improved design concept was developed using both numerical simulations and field experiments to limit scale effects with fine sediment modelling.

Operating medium and high-head hydroelectric power plants under alpine conditions may expose facility components to hydro-abrasion due to mineral suspended sediments in the water. Particularly turbines can be affected by wear, leading to a considerable efficiency decrease affiliated to power and financial losses. Therefore, medium and high-head hydroelectric power plants are commonly equipped with desanding facilities to reduce the amount of suspended sediments.

Nowadays, climate change causing glacier meltdown entails increasing sediment yield from glaciated catchment areas into alpine waters. Additionally, experiences showed that the settling efficiency of current desanding facilities is below expectations, mainly due to shortcomings of the geometrical design. Thus, the geometric optimization of existing and proposed facilities is of major importance.

The overarching objective of the project was to develop enhanced design guidelines for desanding facilities to improve the settling efficiency, putting an emphasis on the effects of various geometrical parameters as well as different headwork arrangements. For this purpose, the optimization potential was systematically investigated by means of a composite approach, modeling flow and settling processes by numerical simulations based on precedent field experiments (Fig. 1). The following results were obtained:

High-quality data at a high temporal and spatial resolution were obtained at the investigated prototype facilities, corroborating the hypothesis that the flow field in desanding facilities is rather inhomogeneous over a large part of the total basin length despite the use of tranquilizing racks. The CFD software enabled to simulate the complex 3D flow and sediment processes. The parametric study allowed to quantify the effects of the various parameters investigated, especially with regard to the approach flow conditions and transition zone geometry. Among others, the significant effect of both asymmetric and supercritical approach flow could be shown, and the mitigation effect of tranquilizing racks to homogenize the flow became clear. In general, to assure settling of particles of the critical size, desanding basins or chambers need to be significantly longer than resulting from the application of today’s design approaches, thus corroborating the hypothesis that a large number of existing facilities do not work properly. The findings resulted in a new design concept for desanding facilities, which was further elaborated in a step-by-step design guideline be applied by design engineers.

Figure 1: Bearing system for measurement instrumentation used at investigated desanding facilities (left); contour plot of simulated flow velocity magnitude at one of the investigated desanding facilities (right)

This project was part of the Swiss Competence Center for Energy research – Supply of Electricity (SCCER-SoE). It was financially supported by the Swiss National Science Foundation (NRP70, No. 153861) and technically supported by the Swiss Federal Railways, EnAlpin and Gommerkraftwerke AG. The monograph can be downloaded from the group’s webpage.

The ESC member involved in this project is Prof. Robert M. Boes, head of the Laboratory of Hydraulics, Hydrology and Glaciology (VAW). VAW performs research in the areas of hydraulic engineering, river engineering and glaciology.