Scale effects in flow dislodgement of rocks from rock weirs

Rock weirs are low-head hydraulic structures commonly used in river training. The flow over rock weirs can dislodge rocks to cause structural failure, but studies investigating the scale effects in this phenomenon are limited. Two flume tests (i.e. large-scale and small-scale flume tests) with fixed bed were conducted and combined as a scale series to investigate this topic. The process and limit of rock dislodgement from the rock weir in each flume are compared, and the causes for the scale effect are discussed. The results show that: (a) for partially submerged conditions, the over-flow regime corresponding to the incipient rock dislodgement varies with flume scale, leading to different rock dislodgement processes; (b) the limit representing the incipient or a few amounts of rock dislodgement agrees well for different flume scales, while the limit related to weir failure in a large-scale flume is lower than that in a small-scale flume; (c) the scale effect of rock dislodgement from rock weirs is attributed to the fact that geometrically similar rock weirs often exhibit dissimilar void ratios. To minimise the scale effect in flow-induced rock dislodgement of rock weirs, effective control of the similarity in weir void ratio needs to be considered.