Bed Load Influence on Scour at Submerged Weirs

Submerged weirs or sills are low head hydraulic structures that span the full width of the channel for limiting excessive bed degradation and for bed stabilization. However, in alluvial rivers their presence in the flow also results in a local scour phenomenon which may undermine the structures themselves. This paper presents an experimental study on the influence of bedload transport on the scouring process at submerged weirs under live-bed conditions. Different flow rates and weir heights were applied to the tests in this study. Bed profile measurements along the flume were recorded as a function of time using Seatek's Multiple Transducer Arrays (MTAs). Under live-bed scour conditions, scour downstream of submerged weirs develops very fast. The maximum scour depth downstream of the weir is attained within a very short time then fluctuates around a mean depth in response to migrating bed forms. The magnitude and frequency of fluctuations is strongly dependent on the bed features (patterns and transport rates) that propagate through the scour zone. Observations show a scour and fill process occurring immediately upstream of the weir. The scour hole at the upstream base of the weir develops as a dune trough approaches the weir, and reaches its maximum size when the dune trough arrives at the weir, then gradually fills as the next dune crest arrives. A prediction equation is proposed for scour at submerged weirs under live-bed scour conditions.