Survey observations of untrenched pipelines and cables in areas with mobile seabed sometimes show a significant buildup of sediment (or sedimentation) either side of the pipe/cable, without any signs of pipeline or cable lowering. Whilst earlier research has explored these observations and recreated similar profiles in laboratory experiments, limited work has explored sedimentation in time-varying flow conditions. Motivated by this gap in knowledge, this paper presents results from a series of experiments conducted using the Large O-Tube testing facility at The University of Western Australia to investigate sedimentation-induced changes to pipeline/cable embedment in time-varying flows. Initially, a series of experiments were conducted with bi-directional currents having a constant flow intensity to measure changes in embedment due to sedimentation around a pipeline/cable. The resulting time series of embedment for each flow intensity was then parameterised in terms of an equilibrium embedment depth and a time scale, indicating that the equilibrium depth is not sensitive to flow intensity, whilst the time scale reduces with increased flow intensity. Next, a model was developed to predict sedimentation-induced changes to local embedment using a time-stepping scheme similar in concept to the ‘STEP’ model introduced for scour prediction. To validate this model, an additional experiment was undertaken in bidirectional flow with a time-varying flow intensity. Comparisons of the prediction with this additional experiment agree reasonably well, indicating that the model may be applied to predict sedimentation build-up in time varying currents.

sedimentation,laboratory experiment,pipeline/cable,embedment,time-varying flows,STEP model