Jean-Louis Boillat

Hydropeaking indicators for characterization of the Upper-Rhone River in Switzerland

River channelization and the construction of high-head storage schemes have been the basis of agricultural and socio-economic development in many alpine regions. One example is the Upper-Rhone River in Switzerland. The Upper-Rhone’s morphology changed considerably between 1863 and 1960 as a result of two major channelizations and, from 1950 on, the construction of a large number of high-head storage hydropower schemes in the catchment. These modifications have brought large benefits to the local population, at the cost, however, of substantial disturbances in aquatic and terrestrial ecosystems in and along the river. A primary factor behind these disturbances is the alteration of the natural flow regime, namely hydropeaking due to the operation of the high-head storage hydropower plants. For sustainable river-restoration projects on regulated rivers, scientists and engineers now widely accept the necessity of integrated management of the river. Different aspects such as river morphology, sediment management, water quality, temperature, and the naturally variable flow regime should be considered simultaneously. Mitigation of non-natural, sub-daily flow fluctuations due to hydropeaking is a crucial step in restoring natural flow regimes, but is especially challenging due to the economic constraints such mitigation places upon hydropower plants. With the goal of addressing this challenge, this paper proposes three indicators to describe the flow regime of rivers in alpine catchments with and without high-head storage hydropower plants. The indicators quantify: (1) the seasonal distribution and transfer of water, (2) sub-daily flow fluctuations, and (3) the intensity and frequency of flow changes. Indicators are evaluated in a case study of the Upper-Rhone River for pre- and post-impact situations, and the benefit of a multipurpose project reducing hydropeaking on hydrologic conditions is quantified. Furthermore, the paper explores the possibility of using these indicators to link aquatic and terrestrial ecosystem well being to their hydrology.

Hydraulic design of A-type Piano Key Weirs

Piano Key Weirs (PKWs) are an alternative to linear overflow structures, increasing the unit discharge for similar heads and spillway widths. Thus, they allow to operate reservoirs with elevated supply levels, thereby providing additional storage volume. As they are relatively novel structures, few design criteria are available. Hence, physical model tests of prototypes are required. This study describes comprehensive model tests on a sectional set-up of several A-type PKWs, in which the relevant parameters were systematically varied. Considering data of former studies, a general design equation relating to the head–discharge ratio is derived and discussed. The latter is mainly a function of the approach flow head, the developed crest length, the inlet key height, and the transverse width. To extend its application range, case study model tests were analysed to provide a design approach if reservoir approach flow instead of channel flow is considered.

Attractiveness of a lateral shelter in a channel as a refuge for juvenile brown trout during hydropeaking

Peak power production in hydroelectric storage power plants results in frequent and intense flow variations in the rivers downstream of the plants. Fish populations can be negatively impacted when subjected to these so-called hydropeaking phenomena. In researching mitigation solutions, shelters in the riverbanks of channelized rivers have been identified as a means of protecting fish from excessive flow velocities. These shelters were studied systematically using juvenile brown trout (Salmo trutta fario) in an experimental configuration in which a straight channel was equipped with a lateral embayment. The purpose of the experiments was to generate hydrodynamic hydropeaking conditions in the channel that are undesirable for juvenile trout, thereby causing them to enter the shelter. The flow velocity distribution in the intersection plane between the main channel and the lateral shelter was found to be a significant parameter for attracting fish to the shelter. The utilization rate of trout in the shelter was used as a performance indicator. Using a basic rectangular shelter configuration without forced water exchange between the shelter and the channel, the utilization rate was only 35xa0%. This rate was more than doubled by introducing a deviation groyne to force water exchange between the channel and the shelter. The position and orientation angle of this groyne were systematically varied to maximize the utilization rate. Maximum utilization rates approaching 90xa0% were obtained for an optimum configuration in which an island-type groyne was placed in the shelter. The results of the systematic channel tests showed the potential of the shelter to attract fish. Such a shelter could be used in channelized rivers both for morphological revitalization and to improve fish habitats. As a next step in this research, prototype shelters will be built on a natural river and monitored for 2–3xa0years under a hydropeaking flow regime.

Water-surface oscillations in channels with axi-symmetric 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.

Optimization of the flood protection effect of a hydropower multi-reservoir system

The use of existing hydroelectricity multi-reservoir systems for flood protection may be an efficient approach in many catchment areas. The assessment of the protection potential offered by the hydropower plants during floods requires a comprehensive analysis of the catchment area, including the simulation of flood scenarios. A methodology for the optimization of turbine and bottom outlet operations of multi-reservoir systems during floods is presented. Based on a theoretical catchment configuration, the most relevant parameters in view of reducing of the peak flows in rivers located downstream of the reservoirs are analysed in a systematic way with the help of hydrological forecasts. The influence of the drained areas, the installed turbine capacities, the emergency rules and the location of the reservoirs on the flood control in such a complex catchment area is highlighted. The optimization approach is applied in the Upper Rhone River basin in Switzerland, where 10 major hydropower schemes with large reservoirs are located. The results regarding peak flow reduction at the catchment outlet are in good agreement with the theoretical values obtained with the developed methodology.

Flow Resistance Caused by Large-Scale Bank Roughness in a Channel

Systematic experimental investigations have been performed under steady flow conditions in a channel whose banks are equipped with large-scale rectangular roughness elements. The purpose was to determine the flow resistance owing to such macrorough banks. The practical motivation of the study is to see how morphological restoration of banks in channelized rivers, such as lateral cavities, influences the steady flow characteristics. The experiments performed in 40 different geometrical configurations revealed various two-dimensional flow characteristics in the bank cavities created by the roughness elements compared to the prismatic reference channel. 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 because of the disturbance of the bank geometry inducing large-scale depressions. The additional flow resistance attributable to macroroughness has been related to the forms of the banks. By separating the observed flow behavior into a normal recirculating, a reattachment and a square-grooved flow type, macrorough flow-resistance formulas according two different approaches could be developed that are in good agreement with the laboratory experiments

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

Abstract Shore 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 Preverenges 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.

Hydrological modelling of the Zambezi River Basin taking into account floodplain behaviour by a modified reservoir approach

ABSTRACT Floodplains are regions of great interest for environmental assessment as they constitute important ecological reserves and contribute efficiently to natural flood attenuation. However, the implementation of a model describing the basic hydrological behaviour of floodplains is not an easy task due to the complexity of the processes included. Although several attempts have been made to simulate floodplain effects in global rainfall-runoff models, no satisfactory routines have been developed yet. In this study, an adapted version of the Soil and Water Assessment Tool (2009) reservoir model is proposed and applied to the Zambezi Basin at daily time step with the intention of adequately modelling floodplain behaviour. The model separates the outflow of the reservoir simulating the floodplain into main channel flow and flow over the floodplain area. The improved solution was compared with the original model regarding its potential to simulate observed discharges in terms of volume ratio, the Nash–Sutcliffe coefficient and hydrograph plots. These evaluation criteria attest, for both calibration and validation periods, that the modified model is superior to the original one for simulating the discharge downstream of large floodplains. A sensitivity analysis is carried out at two geographical levels: at the outlet of a floodplain and at the outlet of the entire basin. The results show that upper flow parameters are more sensitive than base flow parameters.

Semi-empirical model for channel bed evolution due to lateral discharge withdrawal

Side weirs and overtopable levees are widely used to increase flood routing along a channel or river. The lateral loss of water reduces the sediment transport capacity leading to the formation of a local sediment deposit close to the overflow. Since the extent of the morphological bed changes is not known a priori, the design discharge is increased by this flow–sediment transport interaction in an uncontrolled way. Based on a systematic flume study, a semi-empirical model to predict the three-dimensional bed evolution of the aggraded channel reach in the vicinity of the overflow is developed. The shape of the deposit is modelled by an adapted Maxwell-type distribution function. The main input parameters of the model are expressed in terms of dimensionless parameters accounting for main channel and side overflow geometry as well as flow and sediment transport characteristics. The application of the empirical model in numerical flow calculations predicts 90% of the measured overflow.

Modeling of Sediment Management for the Lavey Run-of-River HPP in Switzerland

Reservoir sedimentation hinders the operation of the Lavey run-of-river hydropower plant (HPP) on the Rhone River in Switzerland. Deposits upstream of the gated weir and the lateral water intake reduce the flood release capacity and entrain sediments into the power tunnel. Past flushing operations of the relatively wide and curved reservoir have been inefficient. To improve sediment management, the enhanced scheme Lavey+ with an additional water intake and a training wall for improving flushing was set up. The performance of the enhancement project was tested on a physical model. For its calibration, sediment transport, deposition, and flushing of the present scheme were investigated and compared with prototype measurements. The enhanced scheme was then analyzed in detail to define the flushing discharge and duration, and define the gate operation to ensure maximal erosion of deposits with minimal water loss