Study On Local Scour Around Spur Dikes On Sand Beds Of Yellow River By Model Experiment

Spur dike is a kind of hydraulic structure which is widely used in river training, channel improvement and highway subgrade protection. Previous research methods were mostly based on small-scale flume experiments, and without the consideration of the similar relationship with the natural river, it is often difficult to accord with the fact when the results were applied to practical projects, especially under the conditions of the sandy river bed.In systematic retrospect of previous researches, according to the generalization of the flow conditions of the sandy riverbed in alluvial river of Yellow River main stream, riverbed boundary and the diameter of the bed sand; selecting a dangerous anti-flood projects nearby main stream as the geometric boundary condition of the prototype channel, pseudochark as model sand; making use of the similar condition of the movable bed model, a normal movable bed model was designed and established. Then, six groups of tests were set up to study the spur dike local scour problems in different discharge per unit width. With the new method of measuring surface velocity and the local topography of scour holes, surface flow field near spur dikes, water level changes, velocity distribution, the development of local scour, sediment armoring, local scour depth, scour hole range and other aspects were observed.This paper has reached the following main conclusions: (1) Water flow contraction is mainly caused by deflected current flow along dam axis. High-velocity distribution in surface current does not correspond to the deepest point of scour hole. When water flows around the spur dike, the flow velocity at different depths will partially increase and the increase range in the middle part and bottom will be greater than surface, making vertical velocity distribution in dike head and downstream nearby homogeneous. (2) As the local scour develops, the top of the slope of the local scour hole continues silting and heightening. Local scour depth accompanies by the increase of discharge towards per unit width or flow velocity in front of the dike, and this relation is not influenced by dike length, flip angle, dike location and so on. Graded tracking observation on the surface sediment of the bottom shows obvious armoring phenomenon, which has inhibiting action on the development of local scour. (3) When the local scour depth formula in being is used for spur dikes, the calculated value is generally larger than the measured one. In practical projects, the dike head near by mainstream or not and spur dikes shielded by each other have significant effect on scour depth. In addition, an empirical relation for the calculation of local scour depth of spur dikes is established according to the tested points.