Adequate sediment handling at high-head hydropower plants to increase scheme efficiency - Design optimization of Alpine desanding facilities
Paschmann, Christopher Sequeira Fernandes João Nuno Vetsch, David Florian
Operating high-head hydroelectric power plants under Alpine conditions may expose facility components to hydro abrasion due to mineral suspended sediments in the turbine water. Particularly, turbines can be affected by wear, leading to a considerable efficiency decline affiliated to power and financial losses. Therefore, 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 show that the settling efficiency of existent desanding facilities often is below expectations, frequently due to shortcomings of the geometrical design. Thus, the geometric optimization of existing and proposed facilities is of major importance.
The project’s objective is to develop an enhanced guideline for the design of 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 is systematically investigated by means of a hybrid approach, modeling flow and settling processes by numerical simulations based on precedent field experiments.
The project is funded within the scope of the National Research Programme NRP 70 “Energy Turnaround” by the Swiss National Science Foundation. It started at the beginning of 2015 and will be completed by the end of 2017.
Bühlmann, Marius Vetsch, David Florian Boes, Robert
Concrete dam related long-term processes, such as valley deformation, concrete ageing, alkali aggregate reaction and changes in seepage flow can lead to damage or even failure of the structure. The consequences of a failure are serious. Thus, dam monitoring is essential to recognise abnormal behaviour at early stage. The safety assessment can be done by a comparison between observed and predicted behaviour indicators (e.g. radial pendulum displacement at crest level). The predicted value can be calculated using a model based on input variables such as environmental conditions (e.g. water level, temperatures). Basically, there are two modelling approaches; the deterministic method links the behaviour of the structure and input variables on the basis of physical laws, and the statistical method links it by regression analysis. For statistical models, multiple linear regression models with a least square approach are commonly used for approximation.
The application of common statistical modelling approaches is not straightforward. For a behaviour analysis, a regressor model equation that approximatively describes the relationship between the environmental conditions and the behaviour of the structure has to be defined. The model accuracy and the prediction capability particularly depend on the chosen model equation. The most difficult part is to choose functions which consider the effect of temperature on the displacement. There are various approaches in the literature. Many of them work well for some dams, whereas they may lead to physically meaningless results for other dams. The consideration of the thermal inertia seems to be important, because a change in temperature acts with a certain delay onto the structure.
Firstly, established procedures for dam behaviour analysis and models from literature considering the influence of temperature will be studied and evaluated. Secondly, new approaches considering the temperature influence shall be developed and tested. The focus of the current research project lies on statistical models predicting the crest displacement on concrete dams. The investigation is based on measurement data from different concrete dams in Switzerland.
Potential for future hydropower plants in Switzerland: A systematic analysis in the periglacial environment
Ehrbar, Daniel Schmocker Lukas Vetsch, David Florian Boes, Robert
The expected glacier retreat in the coming decades due to atmospheric warming will offer new perspectives for the construction of reservoirs and hydropower plants in newly formed landscapes in front or just beyond the outer limits of glaciers (periglacial environment). However, the immediate proximity to the glacial environment will pose challenges in terms of construction, operation and maintenance, as the temporal evolution of glacier runoff and sediment transport have to be considered.
Especially reservoir sedimentation is of major importance in the periglacial environment. Several reservoirs in the Swiss Alps already face severe sedimentation rates, whereas others are observing accelerating sedimentation processes that are possibly linked to climate change. So far, the governing processes of reservoir sedimentation in highly glaciated catchments are not fully understood. In order to predict future reservoir sedimentation and guarantee sustainable reservoir management, additional research is needed.
This research project will focus on the reconstruction of past and future sediment yield using current measurements of sedimentation volumes in the reservoir. An extensive measurement campaign will provide information about suspended sediment concentrations and the spatial and temporal variation of grain diameters and fractions within the reservoir. Amongst water and sediment samples, new measuring techniques such as Acoustic Doppler Current Profilers (ADCP) and Laser In-Situ Scattering Transmissometry (LISST) will be used. This data can be linked to the evolution of sediment transport in the catchment caused by glacier meltdown.
The obtained data set will allow to model sedimentation processes in the reservoir and forecast the reservoir sedimentation for the next 80 years. Furthermore, they allow to quantify the possible energy production of the hydropower plant and its contribution to the energy turnaround.
This project is funded within the scope of the National Research Programme NRP 70 “Energy Turnaround” by the Swiss National Science Foundation (SNSF). It started in January 2015 and will be completed in December 2017.
Numerical modelling of instream hydraulic structures
Vanzo, Davide Vetsch, David
Hydraulic structures such as weirs or gates may restrict river sediment continuity. Depending on the interaction with the flow field, such structures can trigger aggradation or erosion processes which could reduce the functionality and the stability of the structure itself. Modelling such processes with a numerical tool is not trivial and requires the implementation of robust and effective numerical solutions based on the empirical relations commonly used to design hydraulic structures.
Another challenging task is the modelling the behaviour of culverts and bridges in presence of sediment transport. In case of adverse hydro-morphological conditions (e.g. extreme floods) these structures can switch from open flow to pressurized flow, with sudden and potentially dangerous reduction of both sediment and liquid discharge capacity.
Furthermore, the presence of lateral structures such as groynes or side weirs may also affect the main channel sediment transport hence also such structures require a proper numerical discretization.
In this context we investigate and develop appropriate and robust numerical solutions that ensure sediment continuity when modelling different hydraulic structures.