Dynamic bedforms, such as sand waves, may pose a threat to offshore energy infrastructure. Due to their impact on seabed mobility, dredging is often required to reduce risks, for example, to avoid cable exposure. However, commonly used data-driven methods cannot predict the evolution of partially dredged sand waves. Therefore, we apply a high-resolution Delft3D FM model to reproduce the measured morphological evolution of a dredged cable trench through sand waves and quantify the 3D interactions between the dredged area and surrounding sand waves. Our simulations confirm that sand wave regrowth is driven by tide-averaged vertical circulation. Remarkably, these circulation cells persist even over a flat trench bed because of the influence of adjacent nondredged sand waves. Trench infill results from gravitational modification of bed-load transport over the trench side slopes, causing amplification, damping, and deflection of sediment transport. Because infill volume rates depend on slope steepness, sedimentation varies over time as slopes flatten and spatially with the surrounding sand-wave bathymetry. In contrast to side-slope steepness, trench width has little effect on infill volume rates. Temporal variations in the trench infill clearly follow the spring-neap cycle. Although storm-induced currents did not significantly increase trench infill rates, they cause preferential infilling on one side by altering tide-induced transport asymmetry. The model reproduces the multiyear sediment balance over the trench well, with default model settings, showing its potential predictive capabilities. These process-based insights help explain differences in recovery timescales between environments and trench designs. ... Read more

Sand wave is one of the large-scale seabed morphological features with a typical wave height of metres and a wavelength of hundreds of metres. Its mobile nature causes a nuisance to offshore infrastructure and marine transport. Pre-sweeping is a construction procedure commonly involved in submarine utilities installation to dredge existing sand waves but recovery of sand waves is usually observed after dredging. The underlying processes of sand wave recovery remain relatively underexplored. Given the potential implications for improved dredging planning, this thesis aims to investigate the drivers of sand wave recovery and examine the modelling performance of Delft3D FM on the recovery process through a case study. The numerical model is the first 3D morphological model specifically for simulating sand wave recovery after pre-sweeping. With the incorporation of DelftBlue and MORFAC, computational efficiency has been significantly enhanced. The results show that the transverse slope effect and the oblique current flow supply sediment into the dredged area causing sand wave recovery. This study also elaborates on the two stages in the recovery: an adaption period referring to the morphological adaption of the dredged profile in response to the forcing and a subsequent regeneration period referring to the growth of a feature developed during the adaption period. Although the precise estimation of the recovery duration by Delft3D FM remains challenging, this pioneering research provides an explanation on how tidal flow drives the recovery and reveals the accomplishment of Delft3D FM in modelling the phenomenon. Further exploration of this subject, such as the impact due to storm and the influence of trench configuration, is suggested to further enrich our understanding of sand wave recovery. ... Read more