The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the ...development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three‐dimensional migratory behaviour of downstream‐migrating silver eels. The eels were tracked as they approached a hydropower plant with a state‐of‐the‐art angled bar rack and full‐depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream‐moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full‐depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.
► A 3D RANS model was used to compute the suspended sediment distribution in a reservoir. ► The study includes 3D measurements of the sediment distribution using a LISST-SL device. ► The measurements ...were used to validate the numerical model. ► The morphological bed level changes during an entire operation year were simulated. ► The simulations showed good agreement with the measured deposition pattern.
The three-dimensional numerical model SSIIM was used to compute suspended sediment distribution and deposition pattern in a hydropower reservoir. The study also included three-dimensional measurements of suspended sediments in the reservoir using the LISST-SL instrument. The measurement device is based on a laser-diffraction method and measures concentrations and grain size distributions instantly. It was applied to 25 locations in the reservoir where vertical profiles were taken. The measurements and computed results were compared and reasonable agreement was found. In addition, computed bed elevation changes were compared with measured values in the conducted study. The results of the numerical model agree well with the bed levels taken by echo sounding.
The numerical model SSIIM solves the Reynolds-averaged Navier–Stokes equations in three dimensions and uses an adaptive grid which moves in accordance to changes in the water and bed levels. The suspended sediment transport is calculated by solving the convection–diffusion equation and the bed load transport by an empirical formula. The used implicit free-water surface algorithm provides the possibility of using large time step sizes, which makes a simulation of an operation year on a desktop PC possible.
Reservoirs in areas with a high sediment yield will without mitigation sooner or later be filled up with sediments, reducing the volume available for regulation for electric generation. Flushing of ...sediments is a management strategy used in many reservoirs in the world, with varying degree of success. The Angostura reservoir is a shallow reservoir located on an inundated river, making it extra vulnerable to sedimentation. It is estimated a yearly inflow of 1.5 million tons of sediment. Other reservoirs are located upstream, and the flushing of these leads to large quantities of sediment inflow in a short period of time. In September every year, the water level in Angostura is partially drawn down to route this sediment through the reservoir. In November, the water level is drawn down completely, and a full sediment flushing is performed. The RESCON model is a spreadsheet program designed to find a technically feasible sediment management strategy that maximizes the economical benefits of the project. Flushing is modeled with a simple algebraic model. SSIIM is a three-dimensional computational fluid dynamics program designed for hydraulic engineering. Sediment erosion, transport and sedimentation can be modeled in a complex reservoir geometry using an adaptive grid with a moveable bed. The main objective of this thesis is to test the performance of the RESCON and SSIIM models. The RESCON model was tested on the November flushing to see if it can reproduce the measured volume of flushed sediments. SSIIM was used for simulation of sediment concentration distribution in the reservoir, and to model the September flushing. The results are compared to measured values. The reservoir geometry is based on bathymetry data from September 2011. Inflow series are from logged values at the hydropower plant. Depth-averaged concentration values are used to present the longitudinal concentration distribution, and Hunter Rouse profiles to present the vertical distribution. Using known reservoir values and the recommended coefficients, the RESCON model was not able to get close to the actual volume of flushed sediments. Although the model has given promising results in other cases, the complex reservoir geometry in Angostura sets heavy restrictions on the flushing processes in the reservoir, which an algebraic model of this type cannot reproduce. The steady state simulation was successful in explaining previously unexplained variations in the concentration in the lateral and longitudinal direction of the reservoir. The simulation reproduces the longitudinal and vertical concentration distribution well. A bug in the implementation of the second-order scheme in SSIIM was discovered, which has later been fixed, giving more similar results for the first-order and second-order schemes. The time allocated for the flushing simulation was not enough to get satisfactory results. The erosion in the upstream end is modelled well, but the measured sedimentation in the downstream area is much larger than in the simulations. With more time for this simulation, it would have been possible to increase the quality of the results.
