Turbine wear caused by hydro-abrasion due to suspended sediment in the turbine water has a negative impact on the power production and revenue of hydropower schemes. Efficiency of desanding facilities that reduce suspended sediment load ahead of turbines is key to limiting hydro-abrasion. Existing facilities built according to common design approaches often show lack of performance, in particular under non-ideal site-specific inlet and outlet conditions. Consequently, a new design concept that allows for the optimized design of desanding facilities at hydropower schemes has been developed based on prototype field measurements and CFD modeling. The effects of facility layout and components, such as tranquilizing racks, transition zone and outflow weir and related design parameters on the flow field and involved particle settling have been investigated in a comprehensive numerical model study. Hence, length adjustment terms were deduced from resulting changes in trapping efficiency, which allow for the estimation of the required basin length and an optimized facility layout. Compared to design approaches considering linear settling trajectories of sediment particles, the proposed procedure leads to longer settling basins in general and thus to increased trapping efficiencies of desanding facilities. Furthermore, the findings about the effect of specific facility components may allow for the optimisation of existing desanding facilities in an economical way.

At medium- and high-head hydropower schemes, desanding facilities are deployed to reduce the sediment load and particle size in the turbine water and thus to reduce hydro-abrasion at steel parts in the turbine water system and at hydraulic machinery. The intensity of hydro-abrasion is directly linked with the trapping efficiency of the desanding facilities. An extended design concept for desanding facilities was developed based on a combination of field measurements and CFD simulations. In particular, the consideration of inlet- and outlet flow conditions of the desander basin allows for optimisation of existing desanders and robust design of new facilities.

Desanding facilities are located between the water intake and the penstock of medium and high-head hydroelectric power plants to avoid the entrance of suspended sediments into the penstock and therefore to reduce hydro-abrasion at turbine parts. This paper focusses on the selection and description of measurement instrumentation to characterize flow and sediment flux at desanding facilities with high spatial and temporal resolution. The selection criteria for the instrumentation are highlighted and the capabilities and limitations of the devices and techniques are described. The modular design of mounting system carrying the instrumentation allows for measurement under several structural constraints. The chosen concept for data acquisition in terms of measurement grid, measurement duration and sampling frequency provides high resolution data of flow quantities and sediment flux. For reliable quantification of sediment fluxes, in-line measurements of turbidity are correlated with suspended sediment concentration obtained by water sampling, also considering density and temperature. Selected findings from the field measurement campaign at two sites in Switzerland are presented and discussed. The employed experimental setup proved to be appropriate and reliable to characterize the flow field and sediment fluxes in desanding facilities at high resolutions.

This paper deals with flow field and sediment flux measurements at alpine desanding facilities. 3D flow velocities and turbidity were recorded and water samples were taken at three alpine desanding facilities. The samples were evaluated regarding suspended sediment concentration (SSC) and particle size distribution (PSD) in the laboratory. SSC was correlated with turbidity and reliable correlations were found for two facilities. The applied instrumentation and methods proved to be appropriate to assess flow field and sediment fluxes. The results show that the flow field is inhomogeneous in large parts of the basins and that the presence of tranquilizing racks has a strong influence on the flow. PSD revealed a refinement of the mean particle size in streamwise direction. The mass-related trapping efficiency of the desanding facilities was estimated based on calculated sediment fluxes and compared to two different trapping efficiency definitions. The results are briefly compared with a current design guideline.

The use of hydrodynamic numerical 3D models in engineering companies gets more and more popular, mainly driven by the progress in the field of computer technology. Nevertheless, the model application is still demanding and requires sound experience. Many challenges may have to be faced in the calibration and validation process, such as the limited availability of reference data or the choice of a turbulence model suitable for the problem at hand. In this regard, some aspects will be discussed herein and illustrated by two practical examples: channel flow with transition to a settling basin and the improvement of turbine approach flow by optimization of geometry.

In diesem Beitrag wird die Messung von Strömungsfeld und suspendierten Sedimenten an drei schweizerischen Entsandern von Wasserkraftanlagen vorgestellt. Die untersuchten Anlagen weisen verschiedene Geometrien sowie Zuund Auslaufbedingungen auf. Die Messungen wurden derart konzipiert, dass Fliessgeschwindigkeiten und Schwebstoffkonzentration (SSC) in den Absetzbecken der Anlagen aufgezeichnet bzw. charakterisiert werden konnten. Die Messtechnik umfasste Acoustic Doppler Velocimeter (ADV) und Trübungssonden. Für die zeitlich und örtlich auf die ADV-Messungen abgestimmte Wasserprobenentnahme an zahlreichen Beckenpositionen wurde eine mobile Pumpe eingesetzt. Von den Wasserproben wurde im Labor die gravimetrische SSC und Korngrössenverteilung der suspendierten Partikel bestimmt. Es wurden lineare Korrelationen zwischen Trübung und Schwebstoffkonzentration erstellt. Zur Charakterisierung anlagenspezifischer Absetzmuster wurden ergänzende Proben der abgesetzten Sedimente entnommen und ebenfalls hinsichtlich Korngrössenverteilung untersucht.