Influence of bed ripples on scour-induced span elongation of pipelines/cables

This study presents results from laboratory experiments designed to investigate the scour propagation along a pipeline/cable under steady current. Most of previous research on this topic showed that the scour propagation has a primary and secondary stage, with the transition to the secondary rate believed to occur when the scour hole reaches some critical length. However, results from the present study show that in small-scale model testing, bed ripples have a direct influence on scour rate, causing a transition from a primary to secondary stage as they develop. This is demonstrated by comparing scour propagation rates before and after ripples develop upstream of the pipeline/cable, and after ripples are removed by flattening the upstream bed. It is found that bed ripples appear to have both an indirect and direct influence on scour propagation. Indirectly, ripples reduce near-bed velocity by altering the boundary layer, while directly they deflect flow upwards, sheltering the pipeline/cable and reducing flow into the scour front. Additionally, the sediment flux entering the scour front may be altered by the proximity of upstream ripples, potentially leading to temporary cessation of scour propagation. The findings from this work have implications both for interpreting scour results in earlier research and for designing future experimental models. • New laboratory experiments explore scour propagation along a pipe under steady current. • Two-stage scour propagation process reported in previous work is also captured in the present work. • The formation of bed ripples on a pre-flattened sandy bed plays a dominant role in the transition from the primary to the secondary propagation stage under live-bed conditions.