Propagation of surge waves in channels with large-scale bank roughness

In open channels, a sudden rise in water elevation generates a positive surge. Positive surges are commonly observed in man-made channels (Bazin 1865, Treske 1994) and a natural occurrence is the tidal bore in macro-tidal estuaries (Tricker 1965, Chanson 2011a). The positive surge may propagate upstream or downstream (Fig. D1). It is a rapidly-varied flow and the flow properties immediately upstream and downstream of the front must satisfy the continuity and momentum principles (Rouse 1938, Liggett 1994). The authors investigated positive surge waves in a long channel with a range of sidewall configuration. Their configuration corresponded to a downstream surge configuration (Fig. D1, right). The contribution is a welcome addition to the literature on rapidly-varied unsteady open channel flows. In this discussion, it is shown that the effects of boundary friction were previously documented, and a recent investigation provided some insight into the energy dissipation induced by large-scale sidewall roughness

The use of soft shore protection measures in shallow lakes: Research methodology and case study

AbstractShore protection in lakes is an issue of major importance in Switzerland where several big lakes in plains suffer from a pronounced bank erosion. For the moment, in shallow lakes, soft and biotechnical protection measures proved their reliability. Unfortunately, the scientific basis for the design of such techniques does not exist in some cases or not appropriate enough in order to have an optimized effect. Therefore, the aim of an on-going research project is to study, on the basis of physical and numerical modeling, the impact of such measures on the shores regarding bank erosion, and to establish the main basis for their dimensioning. A 2-D numerical model was used to simulate the eroded beach of Préverenges on the North coast of Lake Geneva. Hence, this case study allowed a better understanding of the numerical capacities of the program by modelling wave effect on bedload sediment transport and shore erosion as well as wind role in the generation of littoral currents

Le dimensionnement hydraulique

Le comportement hydraulique des réseaux d’égouts est généralement de type gravitaire et par essence non-stationnaire, car tributaire des cycles de consommation et du caractère aléatoire des apports naturels. Toutefois, les conditions rencontrées, liées en particulier à des sections prismatiques à rugosité constante ainsi qu’à une variation lente du débit, sont favorables à un dimensionnement stationnaire. Cette approche simplifiée offre l’avantage d’imposer un contrôle systématique du réseau en tenant compte des particularités locales sur l’écoulement. Le dimensionnement des canalisations vise à garantir un écoulement à surface libre, avec une circulation d’air suffisante pour éviter leur mise en charge. Les débits à considérer sont préalablement définis par le Plan Général d’Evacuation des Eaux (PGEE). Ils concernent la capacité maximale requise d’une part et le débit de temps sec pour lequel la formation de dépôts doit être évitée. Le présent article passe en revue les méthodes classiques du dimensionnement hydraulique, dans le respect des consignes de la Norme SIA 190 (2000). Il se concentre ensuite sur les conditions particulières d’écoulement susceptibles de réduire la capacité théorique d’une canalisation. Les effets liés à la turbulence de l’écoulement et à l’entrainement d’air sont considérés et des méthodes sont proposées pour les prendre en compte dans le calcul. Finalement deux exemples de dimensionnement préliminaire sont présentés pour illustrer la démarche. Le premier concerne une canalisation à faible pente, le second traite le cas d’une forte pente

Flood forecast uncertainty and alert decision. application to the alpine Rhone river catchment / Incertitude des prévisions de crue et décision d’alerte. Application au bassin du Rhône Alpin

The main goal of the 3rd Rhône Correction project in Switzerland is to improve the flood protection in the Upper Rhone River basin. In this framework, the MINERVE project contributes to the flow control during flood events by preventive turbine and bottom outlet operations from the numerous storage power plants existing in the watershed. For this purpose, a semi-distributed hydrological model was developed for the Upper Rhone River basin. It is currently operational for a real-time flood forecast in the Rhone Valley. It simulates the snow and glacier melt, soil infiltration and run-off processes, flood routing in rivers and reservoirs as well as hydropower scheme operations. For the computation of flood prediction, the numerical meteorological forecast models COSMO-2, COSMO-7 and the probabilistic COSMO-LEPS, delivered by MeteoSwiss, have been assimilated. The forecasts were used as input for the hydrological simulation of three historical flood periods and the results are analyzed and discussed. Furthermore, a warning report providing flood warnings has been also developed. It gives the evolution of the hydrological situation at control points in the catchment area. Finally, it provides three levels warnings during a flood situation depending on respective critical discharge thresholds

Fish behavior during hydropeaking in a channel equipped with a lateral shelter

In the framework of a research project focusing on mitigation measures for hydropeaking, a lateral embayment at the channel bank is studied as a fish refuge. Systematic experiments with different refuge configurations were carried out. The basic configuration is rectangular with a length of 2 m and a width of 1.2 m installed at the right bank of a 12 m long and 1.2 m wide flume supplied with freshwater from a natural river. In order to trigger water exchange between the flume and the rectangular refuge a wall acting like a groyne was installed inside the refuge protruding slightly in the main channel. Position, inclination and protrusion rate of this groyne were varied systematically in order to obtain an optimal water exchange and the best attractiveness of the shelter for fishes during hydropeaking. Each configuration was tested three times with juvenile wild brown trout (Salmo trutta fario) (0+ and 1+), with 2 different groups of 10 and the combined group of 20 brown trout. They where exposed during 3 hours each time to a hydropeaking flow of 220 l/s in the main channel. During every test, the movements of the fish were recorded continuously by video camera and their positions were observed every 20 minutes. 6 series of 20 fishes were used for 36 sequences corresponding to the 12 configurations tested. For each configuration the analysis of the fish positions gave a global frequentation rate as well as the favorite staying places in the shelter. Some in- and outgoing fish trajectories were obtained by the treatment of video pictures. A particular focus was given to the interface section between the refuge and the main channel in order to relate the spatial distribution and the frequency of fish passage from up- and downstream into the shelter. In order to link the swimming trajectories of the trout with the flow conditions, systematic measurement of the velocity field was performed using UVP technique. The flow velocities were analysed in several horizontal and vertical transects across the refuge and flume. Comparing the velocity patterns with the fish trajectories, the attractiveness of different configurations of fish refuges could be analyzed. The tests reveal that a very basic refuge configuration, with low water exchange between shelter and channel, is not interesting for fish. When forcing a water exchange by introducing a deviation groyne into the shelter, its frequentation can be increased significantly. The fish can easily detect the refuge by the exchange flux when searching its way upstream. The refuge attractiveness can be optimized by testing different groyne orientations, creating an expanded velocity field close to the exit and the entrance. Important is a high velocity field leaving the refuge at its lower end but also a backwater zone near the groyne. The high velocity field attracts the fish and the close backwater zone allows the fish to enter the shelter. For the best configuration, more than 80% of the fish found the refuge by swimming mainly from downstream, 20 minutes after the beginning of hydropeaking

Flow exchange between a channel and a rectangular embayment equipped with a diverting structure

In the framework of a research project focusing on mitigation measures for hydropeaking, a lateral embayment in riverbank is studied as a fish shelter. With the goal to find attractive configurations of shelter, systematic experiments with wild juvenile brown trout (Salmo trutta fario) were carried out in a flume supplied with freshwater from a natural river. The experimental is equipped with a rectangular lateral embayment. In order to follow the fish trajectories, their movements were recorded continuously by video camera during every test, and their positions were periodically observed. In order to link the swimming trajectories of the trout with the flow conditions, 2D simulations were computed to obtain the diverted discharge in the shelter, and systematic UVP measurement of the velocity field was performed. The flow velocities were analysed in the vertical interface between the shelter and the flume. Comparing the velocity patterns with the fish trajectories, the attractiveness of different configurations of fish shelters could be analyzed. The first tests reveal that a very basic shelter configuration, with low water exchange between shelter and channel, is not interesting for fish. When forcing a water exchange By introducing a deviation groyne into the shelter with the aim to force the water exchange, the frequentation rate can be increased significantly. The fish can easily detect the refuge by the exchange flux when searching its way upstream. The shelter attractiveness was optimized by testing different groyne orientations, in order to create an expanded velocity field close to the exit and entrance sections. Important is a high velocity field leaving the refuge at its lower end but also a backwater zone near the groyne. The high velocity field attracts the fish and the close backwater zone allows the fish to enter the shelter. For the best configuration, more than 80% of the fish found the shelter by swimming mainly from downstream, during the next 20 minutes after the beginning of hydropeaking

Influence of macro-rough banks on steady flow in a channel

High-head storage hydropower plants operate their turbines during periods of high energy demand. The starting and stopping of turbines results in rapid fluctuations of discharge and water levels in rivers called hydropeaking, which are unfavorable from an ecological point of view. Morphological measures might help to reduce the fluctuations by increasing the natural retention capacity of rivers. With this practical background, the experimental investigations presented in this paper focused on the determination of the flow resistance under steady flow conditions caused by large scale roughness elements at the channel banks, namely rectangular cavities (depressions). The experiments conducted in 41 different geometrical configurations showed various two dimensional flow characteristics in the cavities. The overall head-loss of the flow is governed by the existence of different phenomena such as vertical mixing layers, wake-zones, recirculation gyres, coherent structures and skin friction. The analysis of the experiments for steady flow conditions showed that the flow resistance is significantly increased in the macrorough configurations due to the disturbance of the bank geometry. Three different approaches have been considered relating the additional flow resistance due to macro-roughness to the forms of the banks. By separating the observed flow conditions into a square grooved, a reattachment and a normal recirculating flow type, the developed macro-rough flow resistance formulas are in good agreement with the laboratory experiments. Furthermore, water body oscillations have been observed in axi-symmetric macro-rough configurations. They lead to water-surface oscillations and transverse velocity component

Water-surface oscillations in channels with axi-symetric cavities

Transverse or longitudinal movements of a water body are observed for flows along cavities, river embayments, groyne fields or harbours. They are significant for certain flow conditions and geometrical properties. To study the effect of large-scale roughness on banks, 36 geometries of axi-symmetric, rectangular cavities were investigated in a laboratory flume under subcritical, turbulent free surface flow conditions. Significant movements of the water body were detected. The frequency of these periodic movements, identified by level and velocity observations, is in agreement with the natural frequency of the water body in a rectangular basin assuming the first-order mode of sloshing. Major movements of the water body, which lead to significant and periodic oscillations of the water surface, are avoided by excluding Strouhal numbers near 0.42 and 0.84. For low aspect cavity ratios, the periodic water-surface oscillations are insignificant if the flow reattaches to the sidewalls of the widened channel reach