Responses of live-bed scour at instream structures to fluvial bedform migration

Migrating bedforms in alluvial rivers can exacerbate scour damage at instream structures and lead to structural failure. This study investigates the interaction between scour at instream structures and bedform migration. Both two-dimensional (submerged weirs) and three-dimensional (uniform and complex piers) structures are considered. Two components constitute live-bed scour depth: the dynamic scour amplification Δd_s caused by bedforms and the local flow-induced baseline scour depth d_sNB. Results show that, for 2D structures, the baseline scour depth is negligible, while the live-bed scour is mainly caused by approaching bedforms. The dynamic scour amplification normalised by the approaching bedform height (Δd_s/H_b) decreases with the decrease of upstream angle and varies between 0.5 and 2. For uniform 3D structures, the local flow could cause a considerable baseline scour depth, and Δd_s/H_b varies between 0.25 and 0.5 and decreases with the increase in the flow capacity to damp bedforms. For complex 3D structures, flow pressurisation beneath the pile-cap magnifies Δd_s/H_b to near 1. In general, Δd_s/H_b is inversely correlated with d_{s_NB}/y₀, where y₀ is flow depth. New equations are proposed for estimating the dynamic scour amplification at various instream structures with design rules recommended.