Experimental investigation of bed-load and suspended-load transport over weirs

Abstract

An experimental investigation was undertaken to model the transport of bed-load and suspended-load sediment over steep slopes such as dikes and weirs. Six tests were performed using a vertical wall weir and a sloped (1 : 4) wall weir. Measurements were made of bed-load and suspended-load transport rates upstream and downstream of the test section. It was found that upstream of a vertical weir a strong down-flow vortex forms. This vortex creates a scour hole at the base of the weir and entrains sediment entering the region so that all of the sediment is transported in suspension over the weir. Downstream of the weir a strong reverse vortex traps a proportion of the material in suspension and creates a deposition zone. The sediment supply and flow rate control the temporal development of this deposit. For a sloped weir the down-flow vortex is eliminated at the upstream weir face. However the strong acceleration of the flow moving up the slope is sufficient to entrain all of the bed-load material so again, all sediment is transported over the weir in suspension. No sediment deposition was noted for the region upstream of the weir, although ripples were able to move partway up the slope before being completely entrained. Downstream, deposition in the deceleration zone occurred as expected. The results for both weir shapes differ from the expected sediment depositional patterns. In the field deposition has been noted on the upstream slope of dikes after flood events. Possible reasons for the difference between field and experimental observations are discussed.