Local erosion over a submerged intake in an alluvial channel
Maclean, A. G.
PublisherUniversity of Aberdeen
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This study was concerned with the submerged bottom type of river intake, which consists of a filter located in the river bed through which water can be pumped. Under certain circumstances, bed erosion will occur as a result of the abstraction process. Attention was concentrated on the problem of estimating the bed shear stress, and the resulting depth of scour, in the vicinity of the intake. A study of the literature on local scour in alluvial channels was undertaken. This showed that in any approach to a local scour problem it was important to obtain an adequate description of the modification of the flow field giving rise to the scour. Several different turbulence models were studied to assess their suitability for application to the intake problem. The mean flow momentum equation was judged to provide the most suitable approach, and a model was developed which was based on the hypothesis that the increase in shear stress at the bed was directly related to the momentum given up by the abstracted fluid as it entered the bed. Measurements of the flow field modification associated with a suction zone were obtained by hot-wire anemometry conducted in a wind tunnel. Shear stress measurements over a model intake in a laboratory flume were made by observing the combinations of suction and flume flow which induced threshold conditions for sand grains, placed on an impermeable disc in the suction zone, whose threshold shear stress in uniform flow was known. Bed shear stress was also deduced from estimated rates of scour based on bed profile measurements made at different stages in the development of a scour hole. The latter method was judged to give a more accurate measure of shear stress, and the measurements accorded reasonably well with values predicted by the mathematical model.