Local erosion over a submerged intake in an alluvial channel
This study was concerned with the submerged bottom type of river intake, which consists, essentially, of a filter located in the river bed, through which water can be pumped. This type of intake has been used recently, for example, in salmon rivers, to minimise disruption of fish movement. The problem of assessing the change in bed shear stress over the intake was studied with a view to estimating the depth of the scour hole, which was known to be a feature associated with abstraction. The literature on local scour in alluvial channels was studied, several different types of scour being covered by the survey. Although most of the methods described were limited in application to a certain type of scour, some general principles were observed. In particular, it became clear that 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 assessed before a decision was made on the approach to be adopted to the problem. The more complex models were considered unsuitable, mainly because excessive attention to detail in the flow field would be unwarranted when other aspects of the problem, such as the suction force on the grains and the relationship between grain movement and boundary shear stress, were not known with sufficient accuracy. In addition, there was some uncertainty in the modifications to such models that would be required for their application to the intake problem. The extended law of the wall for suction flows was rejected because it is valid only for suction velocities up to about 1% of the external flow velocity, whereas the suction velocity in the intake flows studied ranged from 2% to 10% of the mean flume velocity. Solutions based on the mean flow momentum equation were studied, and a model was developed which was based on the hypothesis that the increase in shear stress at the bed was related directly to the momentum given up by the abstracted fluid at the bed. This model gave significantly improved predictions compared with a previous model, in respect of both the magnitude and pattern of shear stress distribution in the suction zone. It also had an advantage over the earlier model in that the results did not depend significantly on the assumption of a hydrostatic, or any other, pressure distribution in the main flow. Measurements of the modification of the velocity field associated with abstraction were made over a suction zone in a wind tunnel. The velocity profiles at a number of different points along the suction zone were measured using a hot-wire anemometer and these provided confirmation that the profiles assumed for the purposes of calculation gave an adequate description of the flow. 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. The measurements of shear stress using this method gave results which were between five and ten times lower than the predicted values, although they were comparable to estimates of shear stress based on the near-bottom velocities measured in the wind tunnel. It was deduced that the absence of suction at the disc itself was responsible for this large difference, since tests with dye showed that the disc did not interfere noticeably with the main flow. Measurements were also made of the bed profile over the model intake in the flume at various stages in the development of the scour hole, and the bed shear stress estimated from these measurements. These estimates showed a reasonable correlation with the predictions of the new mathematical model, but it was clear that there was a need for a better understanding, especially with regard to the interpretation of the experimental data. This study has provided a means of estimating the boundary shear stress associated with abstraction, and the resulting grain dislodgement rate and scour. These estimates are approximate in nature, but ways of obtaining improved predictions have been indicated, and these should provide a sound basis for an extension of this work to the study of three-dimensional scour which is important in the application of the results to intakes in practice.