Fine sediment routing in a cascade of alpine reservoirs: Influence of the inlet angle on settling of fine sediments

Reservoir sedimentation is, at present, one of the main concerns in the operational management of dams. The resulting reduction of storage capacity has negative impacts on the hydropower production, flood protection, and availability of water for irrigation and human consumption. Previous studies suggest that the most efficient and sustainable measures to cope with sediment settling consist in providing continuity to the sediment flux from the tail to downstream of the reservoir, avoiding sedimentation. In pumped-storage hydropower schemes, the cyclic flow exchange between two reservoirs can contribute significantly to inhibit sediment settling, by maintaining or increasing the turbulence levels in the vicinity of the water inlets/outlets. This study aims firstly at exploring the interaction between hydrodynamics and sedimentation processes in a reservoir with a given initial concentration. Secondly, this study aims at analyzing the evolution of an initial sediment concentration in a reservoir, under the influence of a horizontal jet-like inflow for different angles of incidence and discharges. For those purposes, numerical simulations were conducted for an alpine reservoir (Räterichsboden) by means of Ansys CFX. The results will allow characterizing the interaction between the jet-like inflow and the sedimentation processes.

Preventing fine sediment settling with jet-like inflows and intake-like outflows in reservoirs

Samuel Luke Vorlet

Reservoir sedimentation is one of the main issue in the operational management of dams. The resulting filling up of reservoirs has negative impacts on hydropower production, irrigation, flood protection and drinking water supply. New economical and sustainable solutions for sediment management in reservoirs have to be found. The latest concept suggests that the most efficient and sustainable measures consist to maintain sediment flows continuity from the tail to downstream of the reservoir. In pumped-storage plants, an adequate location and combination of inlets and outlets should generate a sediment routing through the reservoir avoiding sedimentation. This study aims to describe how the hydrodynamic of the reservoir induced by a jet-like inflow and an intake-like outflow influences the sedimentation rate of fine sediment contained within the reservoir. Moreover, the influence of various parameters, such as the relative position of the jets, the concentration of sediment entering the reservoir and the jet velocity, and their impact on sedimentation process and sediment deposition pattern are investigated. At this aim, numerical simulations were performed on a Computational Fluid Dynamic software (ANSYSCFX). The results will allow to characterize the hydrodynamic in the reservoir and its influence on sedimentation process

2018

Experimental study on the influence of the geometry of shallow reservoirs on flow patterns and sedimentation by suspended sediments

Sameh Ahmad Kantoush

The worst enemy of sustainable use of reservoirs is sedimentation. Often the main silting process is the result of settling down of suspended sediments. In shallow reservoirs the flow pattern and the sediment deposition processes are strongly influenced by the reservoir geometry. The trap efficiency of a shallow reservoir depends on the characteristics of the inflowing sediments and the retention time of the water in the reservoir, which in turn are controlled by the reservoir geometry. With the purpose of controlling the sedimentation in shallow reservoirs, the effects of the geometry on flow pattern and deposition processes were investigated with systematic physical experiments and numerical simulations. This allowed identifying ideal off-stream reservoir geometries, which can minimize or maximize the settlement of suspended sediments. The objective was also to gain deeper insight into the physical processes of sedimentation in shallow reservoirs governed by suspended sediments. The systematic experimental investigations were carried out in a 6m long, and 4m wide and 0.3m deep shallow basin. The influence of the shallow reservoir geometry was studied for the first time by varying the width, the length, and the expansion angle of the basin in the experiments for clear water and with suspended sediment. In total 11 different reservoirs geometries and 4 water depths were analyzed. During the tests several parameters were measured, as 2D surface velocities, 3D velocity profiles, thickness of deposited sediments, and sediment concentration at the inflow and outflow. Crushed walnut shells with a median grain size, d50, of 50 µm, and a density of 1500 kg/m3 were used to simulate the suspended sediments. After having reached a stable flow pattern with the clear water, velocity measurements were performed. In a second phase, the evolution of the flow and deposition patterns under suspended sediment inflow were investigated. Tests were carried out with durations from 1.5 hours up to 18 hours, in order to follow the morphological evolution. In order to investigate the efficiency of flushing, flushing operations at normal water level as well as with drawdown were examined after the sedimentation tests. Numerical simulations of the laboratory basin were performed and compared with experimental results. The purpose was to assess the sensitivity of the results on different flow and sediment parameters and different turbulence closure schemes. The experimental investigation of the flow and sediment behavior in axi-symmetric geometries with different shapes provides further information on the evolution of the flow pattern and the sediment deposition. Beside the expansion ratio and form ratio of the basin the flow regime was classified by the geometry shape factor SK and inlet Froude number Frin. The geometry shape factor, defined as a function of wetted perimeter, total reservoir surface area, aspect ratio and expansion density ratio, was used to compare and analyze the experimental results obtained from the different investigated basin geometries. The clear water experiments investigations revealed under what geometrical conditions the flow changes from symmetrical to asymmetrical behavior. The length of the basin has a strong influence on the change of the flow field. The water depth also significantly affects on the type of jet and vortex structure forming in the basin. On the other, hand the sediment deposition pattern was strongly influenced by the jet type and the flow behavior changed with increasing deposits. The prediction of sediment deposition is linked to the prediction of flow behavior. Furthermore it is very sensitive to the basin geometry and the boundary conditions of inflow and outflow. Some tests were performed until the sediment released at the outlet was equal to the sediments entering at inlet into the basin that means a quasi equilibrium was reached. Flushing at normal water level allows only a relatively small part of the deposited sediment to be evacuated. As expected, drawdown flushing is much more effective and a significant amount of sediment deposits could be flushed out of the basin. Regarding the flow pattern in shallow basins empirical relationships for the estimation of the reattachment length of gyres and the normalized residence time as a function of the geometry shape factor, SK, were established. Also empirical equations for the prediction of the jet flow type and velocity magnitude depending on the basin geometry could be formed. Finally empirical equations for the prediction of sedimentation index, silting ratio, trap efficiency and relative deposition thickness, as well as normalized residence time and flushing efficiency could be found. The numerical simulations revealed that the observed asymmetry in flow and deposition patterns can be explained by the sensitivity of the flow regarding geometry and boundary conditions. The influence of the length and width of the basin on the flow pattern can be predicted in good agreement with experiments by these simulations. Some recommendations are given for the design procedure of a new shallow reservoir in view of minimizing the sedimentation due to suspended sediment. The deposited sediment volume can be efficiently minimized by an optimal designed reservoir geometry.

EPFL2008

Influence of in- and outflow sequences on flow patterns and suspended sediment behavior in reservoirs

Michael Müller

Reservoir sedimentation and the resulting storage losses impact reliability, efficiency, safety and thus the sustainability of the hydropower schemes. Beside traditional storage hydropower plants, also pumped-storage facilities are affected. Flexible turbine and pump operations between two reservoirs allow demand-depending electricity production and absorption. Thus, they play a dominant role in peak load energy production as well as grid regulation. The storage volumes are influenced by continuous in- and outflow cycles. Their impact on sediment settling processes has not been addressed in research yet. Inspired by the cyclic, bidirectional water exchange of pumped-storage plants, a novel method for sediment removal from reservoirs was investigated in the present study. After high sediment yield into the reservoir, fine sediment should remain in suspension in front of the water intake due to the pumped-storage induced turbulence. Thus, the settling process may be delayed and sediment be evacuated by the power or flushing tunnels. In prototype measurements and laboratory experiments the influence of pumped-storage operations on flow patterns as well as the settling rates and sediment balance in two interconnected reservoirs has been studied. The two approaches were completed by numerical simulations. In the lower reservoir of a pumped-storage hydropower plant in the Swiss Alps, flow velocities in front of the intake/outlet were measured by Acoustic Doppler Current Profilers (ADCP). The devices with independent energy supply were implemented on the reservoir bottom and sampled 2D flow velocity profiles over several weeks. The measurements showed only local influence of pumping (outflowing water) near the intake, whereas turbine operations (inflowing water) induce large eddy flow fields in the reservoir. Depending on lake topography, patterns with backflow can appear. A frequency analysis of the discharge and flow velocity series indicated a corresponding main period between the flow velocity profiles and the in- and outflow cycles of 1 day. The ANSYS-CFD simulated flow fields corresponded to the in situ flow patterns. The computed turbulent kinetic energy input due to turbine operation was some 25 times higher than the natural input by wind-forcing. In the upstream part of the power shaft of the same hydropower scheme, a turbidity probe was installed for monitoring reasons. Over a period of eight months, sediment concentration of the operated water was continuously measured. An autonomous remote data acquisition and transfer system may be helpful for real-time monitoring by the hydropower operators. The measurements showed seasonal change of sediment concentration in the power system. In winter, high reservoir levels and ice-cover reduced sediment content, whereas in spring, snowmelt and low reservoir levels increased sediment yield. Short-term variations of sediment concentration up to 16% correlated with the in- and outflow cycles, especially for low reservoir filling. During the sampling period, about 45’000 t of fine sediment were moved between the two lakes. However, the sediment balance due to pumped-storage operation remained equilibrated. Fine sediment was kept in suspension in front of the intake/outlet structure by the operation induced flow patterns. Pumped-storage facilities would only increase sedimentation in case of unequal sediment concentrations in the two reservoirs. The impact of magnitude and frequency of the in- and outflow cycles on flow field and fine sediment settling behavior was studied in laboratory experiments. Between two interconnected rectangular basins, sediment-laden water was pumped back and forth according to regular in- and outflow cycles. In both volumes, turbidity was continuously measured. In the main basin, Ultrasonic Velocity Profilers (UVP) allowed 2D flow velocity measurements and the analysis of the developing inflowing jet. In a numerical model, time for achieving steady state conditions in the reservoir was defined. This discharge dependent duration indicated the frequency of the initial in- and outflow cycles. A reference test in stagnant water conditions defined the sediment settling curve without operation, corresponding to a basis of comparison for the investigated test configurations. During five in- and outflow cycles of various magnitude and frequency, the evolution of suspended sediment ratio SSR was measured and compared to the reference case. Sediment influx and release (influx sediment rate ISR and evacuated sediment rate ESR) and thus the sediment balance SB of the system could be defined. In a first stage, 60% of the initially suspended sediment settled independently from magnitude of the in- and outflow cycles. High cycle frequency applied during this phase led to considerably higher SSR by the end of the test sequence. Low discharges increased particle suspension by 10 to 40% compared to conditions without operation, whereas high discharges increased concentration by 50 to 80%, depending on cycle frequency. With an intake/outlet structure close to the bottom or to the reservoir surface, 20% higher sediment concentrations could be achieved than in case of intermediate position. Sediment balance is only marginally impacted by in- and outflow cycles. In situ as well as laboratory investigations highlight the fact that pumped-storage operations influence the flow patterns in reservoirs, especially in the near intake area. The induced turbulence keeps fine sediment in suspension, leading to an extensive sediment exchange between the storage volumes. The sediment balance remains unaffected, as long as the sediment concentrations in the reservoirs are similar.