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

Migrating bedforms in alluvial rivers can exacerbate scour damage instream structures, leading to severe structure failures, which can cause large-scale and long-term issues with fluvial morphology and environment. This study investigates the interaction between scour fluctuation at instream structures and bedform migration based on large datasets from flume experiments. Both typical 2-D (submerged weirs) and 3-D (uniform and complex piers) structures are considered. Two components constitute real-time live-bed scour depth: the dynamic scour amplification ▵ds caused by bedforms, and the local flow-induced baseline scour depth dsNB without bedform (i.e. denoted as NB). ▵ds is the subtraction of mean of maximum live-bed scour depths and dsNB. Results show that, for 2-D structures, the baseline scour depth is negligible (dsNB→0), while the live-bed scour is mainly caused by the approaching bedform. The dynamic scour amplification normalized by the approaching bedform height (▵ds/Hb) varies between 0.5∼2 depending on the upstream angle. Specifically, ▵ds/Hb decreases with the increase of upstream. For uniform 3-D structures, the local flow could cause a considerable baseline scour depth, and ▵ds/Hb varies between 0.25∼0.5 and decreases with an increase in the flow capacity to deform and damp the approaching bedforms. For complex 3-D structures (e.g. complex piers), the flow pressurisation beneath the structural components (e.g. pile-caps) could magnify ▵ds/Hb to near 1. Summarising experimental data shows that ▵ds/Hb is inversely correlated with ds_NB/y0, where y0 is flow depth. Finally, new equations are proposed for estimating the dynamic scour amplification at various instream structures with design rules recommended. This study, by the first time, enables the understanding of the time-sensitive scour amplification at multiple instream structures from a non-static perspective.