Rivers are frequently controlled by weirs and low dams. The flow over these structures has a significant potential for scour even for comparatively low heads. Hence, stilling basins are commonly built to keep energy flows from scouring the streambed. However, the flow turbulence level within them is generally high and local bed scour can develop. This paper deals with temporal and spatial evolution of local scour downstream of low-head stilling basins. In this aim, experiments were carried out in a 1 m wide rectangular channel. Free overfall jets were caused by an ogee-crested spillway and plunged into a positive-step stilling basin. Additional runs were also carried out with an horizontal apron without endsill, as usually done in literature. Four different nearly-uniform sediments were used for the mobile bed among which lead spheres to deeply explore the dependence of local scour on the densimetric Froude number and gravel to better examine sediment grain size effect. Tests were of long durations mainly to get well pronounced scour pits and quasi-equilibrium conditions as well. Based on the experimental data, salient features of the bed morphology evolution are provided. Major effects of the endsill on the scour process are also highlighted.

Local scour progress downstream of low-head stilling basins

OLIVETO, Giuseppe;
2010

Abstract

Rivers are frequently controlled by weirs and low dams. The flow over these structures has a significant potential for scour even for comparatively low heads. Hence, stilling basins are commonly built to keep energy flows from scouring the streambed. However, the flow turbulence level within them is generally high and local bed scour can develop. This paper deals with temporal and spatial evolution of local scour downstream of low-head stilling basins. In this aim, experiments were carried out in a 1 m wide rectangular channel. Free overfall jets were caused by an ogee-crested spillway and plunged into a positive-step stilling basin. Additional runs were also carried out with an horizontal apron without endsill, as usually done in literature. Four different nearly-uniform sediments were used for the mobile bed among which lead spheres to deeply explore the dependence of local scour on the densimetric Froude number and gravel to better examine sediment grain size effect. Tests were of long durations mainly to get well pronounced scour pits and quasi-equilibrium conditions as well. Based on the experimental data, salient features of the bed morphology evolution are provided. Major effects of the endsill on the scour process are also highlighted.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11563/14345
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