Josep Dolz

The thermal structure of Sau Reservoir (NE: Spain): a simulation approach

Abstract In this study, a 1D model of reservoir hydrodynamics DYRESM has been applied to Sau Reservoir, a river valley reservoir in the North-Eastern Spain. Simulation is undertaken for 3 years (1995–1997). Meteorological input data measured at the dam are only available from May of 1997. In this case the simulation results fit measured temperatures very well. In the remaining periods, some meteorological data (radiation, wind and rainfall) were obtained from two nearby stations. Simulated temperature distribution in 1996 is close to the observed one. In 1995, however, the simulated result is far from the observed data. Inflows , outflow and local meteorological events such as storms and gusts of wind seem to be responsible for the differences. By changing some parameters, the effects of flow, light extinction coefficient and outlet elevation on thermal stratification are investigated. Simulations demonstrate that the inflow with high temperature is the main factor controlling the thermal structure in Sau Reservoir and demonstrate that the effect of residence time on thermal stratification is manifested mainly by the changes in the depth of thermocline.

Characterization of the nonaerated flow region in a stepped spillway by PIV

The development of the roller-compacted concrete (RCC) as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewed interest in stepped spillways. Previous research has focused on studying the air-water flow down the stepped chute with the objective of obtaining better design guidelines. The nonaerated flow region enlarges as the flow rate increases, and there is a lack of knowledge on the hydraulic performance of stepped spillways at high velocities that undermines its use in fear of cavitation damage. In the present, study the developing flow region in a stepped channel with a slope 1v:0.8h is characterized using a particle image velocimetry technique. An expression for the growth of the boundary layer thickness is proposed based on the streamwise distance from the channel crest and the roughness height. The local flow resistance coefficient is calculated by application of the von Karman integral momentum equation. The shear strain, vorticity, and swirling strength maps obtained from the mean velocity gradient tensor are presented. Also, the fluctuating velocity field is assessed. The turbulent kinetic energy map indicates the region near the pseudobottom (imaginary line joining two adjacent step edges) as the most active in terms of Reynolds stresses. The turbulence was found to be very intense with maximum levels of turbulence intensity from 0.40 to 0.65 measured near the pseudobottom. Finally, the quadrant analysis of the velocity fluctuations suggests the presence of strong outflows of fluid from the cavities as well as inflows into the cavities. It is conjectured that the mass transfer/exchange between cavities and main stream, play an important role in the high levels of turbulent energy observed.

Developing flow region and pressure fluctuations on steeply sloping stepped spillways.

The hydrodynamic pressure field is important for the design and safety of steeply sloping stepped spillways, which are typically designed for considerably lower maximum specific discharges than smooth spillways. The hydraulic performance of stepped spillways at high velocities may compromise its use due to major concern with safety against cavitation damage. Hydraulic model investigations were conducted in different large-size stepped chutes to characterize the nonaerated flow region which is potentially prone to cavitation damage and the pressure field acting on the step faces. The clear water depths and energy dissipation in the developing flow region are described in terms of integral measures of the turbulent boundary layer. Expressions for the location of and the flow depth at the inception point of air entrainment are derived. Pressure distributions on the horizontal and vertical faces of the step along the spillway are presented. Measurements indicated a different behavior of the pressure field in the aerated and nonaerated flow region. The mean and fluctuating pressure coefficients along the spillway are approximated by a regression function. The vertical face near the outer step edge close to the inception point of air entrainment is identified as a critical region for predicting cavitation inception in flow over stepped spillways. From the analysis of the pressure fluctuations in that region a maximum velocity of 15 m/s is proposed as a criterion to avoid extreme negative pressures in typical prototype steeply sloping stepped spillways, eventually leading to the occurrence of cavitation in the nonaerated flow.

Genetic programming and standardization in water temperature modelling

An application of Genetic Programming (an evolutionary computational tool) without and with standardization data is presented with the aim of modeling the behavior of the water temperature in a river in terms of meteorological variables that are easily measured, to explore their explanatory power and to emphasize the utility of the standardization of variables in order to reduce the effect of those with large variance. Recorded data corresponding to the water temperature behavior at the Ebro River, Spain, are used as analysis case, showing a performance improvement on the developed model when data are standardized. This improvement is reflected in a reduction of the mean square error. Finally, the models obtained in this document were applied to estimate the water temperature in 2004, in order to provide evidence about their applicability to forecasting purposes.

Mixing dynamics at the confluence of two large rivers undergoing weak density variations

Simulations of tracer experiments conducted with a three-dimensional primitive-equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers (Ebro and Segre), with distinct physical and chemical properties, at their confluence, upstream of a meandering reservoir (Ribarroja reservoir). Mixing rates downstream of the confluence are subject to hourly scale oscillations, driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence. Even though density contrasts are low—at most O(10−1) kg m−3 difference among sources—and almost negligible from a dynamic point of view—compared with inertial forces—they are important for mixing. Mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40% larger than those occurring under neutrally buoyant conditions. The buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing (distortion of the mixing layer). For strong density contrasts, though, when the contact area between the streams becomes nearly horizontal, larger density differences between streams will lead to weaker mixing rates, as a result of the stabilizing effect of vertical density gradients.

Analysis of pressures on a stepped spillway

The pressure profiles on the horizontal and vertical step faces of a stepped spillway in the fully developed skimming flow region were measured. These observations allow to provide generalized equations that enable designers to analyze the amount and the location of negative pressures along the steps. The step locations subjected to negative pressure are also discussed including an analysis of the duration of the negative pressures. On the horizontal step faces, negative pressure were only observed on their upstream half for dimensionless discharges larger than dc/h = 1.3. On the vertical step faces, only the region close to the adjacent horizontal face was not subjected to negative pressure

Envisat/ASAR Images for the Calibration of Wind Drag Action in the Doñana Wetlands 2D Hydrodynamic Model

Donana National Park wetlands are located in southwest Spain, on the right bank of the Guadalquivir River, near the Atlantic Ocean coast. The wetlands dry out completely every summer and progressively flood again throughout the fall and winter seasons. Given the flatness of Donana’s topography, the wind drag action can induce the flooding or emergence of extensive areas, detectable in remote sensing images. Envisat/ASAR scenes acquired before and during strong and persistent wind episodes enabled the spatial delineation of the wind-induced water displacement. A two-dimensional hydrodynamic model of Donana wetlands was built in 2006 with the aim to predict the effect of proposed hydrologic restoration actions within Donana’s basin. In this work, on-site wind records and concurrent ASAR scenes are used for the calibration of the wind-drag modeling by assessing different formulations. Results show a good adjustment between the modeled and observed wind drag effect. Displacements of up to 2 km in the wind direction are satisfactorily reproduced by the hydrodynamic model, while including an atmospheric stability parameter led to no significant improvement of the results. Such evidence will contribute to a more accurate simulation of hypothetic or design scenarios, when no information is available for the atmospheric stability assessment.

Aportaciones de la limnología a la gestión de embalses

This article describes some of the contributions of limnology to reservoir management according to a literature review and the authors’ experience. After explaining the main challenges that reservoir managers have to face, some examples of management measures are given and the need to take into account the different management scales are underlined, whether temporal (multiannual, annual, daily) or spatial (catchment, river reach, reservoir or reservoir chain). The available methods for the study of reservoir ecological status are also reviewed.

Source term treatment of SWEs using the surface gradient upwind method

The Authors’ works, who have designed well-balanced 1D schemes for the Saint-Venant equations on irregular geometries, are of interest to develop efficient numerical methods. In recent years, most efforts have focused on improving the performance of 2D models, so that 1D schemes were forgotten to a certain degree, as evidenced by the large number of references of this discussion dealing with 2D models. However, 1D schemes, either by themselves or in combination with 2D schemes, still have advantages relative to lower computational cost and reduced information need. The Discussers would like to highlight two considerations: the first being related to the formulation, whereas the second to the verification.

Application of a Two-Dimensional Water Quality Model (CE-QUAL-W2) to the Thermal Impact Assessment of a Pumped-Storage Hydropower Plant Project in a Mountainous Reservoir (Matalavilla, Sil River, Spain)

Water temperature is surveyed and simulated in a small mountainous reservoir (Matalavilla, NW Spain) in order to assess the thermal and hydrodynamic effects of a projected pumped-storage hydroelectric plant in the water body and downstream on the Sil River. Detailed field measurements were taken from June 2014 to November 2015 including meteorology, radiation balance over the water surface, water temperature of tributaries and thermal vertical profiles inside the reservoir. Such database provides information to analyse the current thermal and hydrodynamic behaviour of the system as well as to calibrate and validate the two-dimensional water quality model CE-QUAL-W2 for the simulation of the future pumped-storage plant scenario. The model satisfactorily reproduces the intensity and temporal occurrence of the thermal processes observed in the reservoir, with a mean error below 0.05 °C (root mean square error below 1.1 °C) at a daily temporal scale and 1 m vertical spatial scale. Currently, in the absence of the projected pumped-storage plant, Matalavilla Reservoir is subjected to an important flow regulation for hydroelectric generation purposes, which influences its thermal structure. When the water column is stratified (summer, according to its annual monomictic thermal cycle), it presents two well-defined thermoclines: one generated from the atmosphere–water interface, whose location is related to the water surface position and whose stability is highly sensitive to the inflows regime; and another one related to the level of the reservoir water outlet, located in depth next to the dam and fairly stable most of the year. In the presence of the projected pumped-storage hydroelectric plant, the model results predict the homogenisation and heating of intermediate depths of the water column and mixing about one month earlier. Water outflow from the reservoir to the downstream Sil River would be slightly warmer, up to a maximum temperature increase of 1.5 °C under the ordinary interbasin transfer management.