Conference paper

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 components

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