Assessing Dynamic Seabed Change on the Scottish Continental Shelf

Abstract

The dynamics of sediment bedforms mobilised by modern day coastal fluid mechanics can reduce the lifespan of offshore construction by modifying the foundational stability for offshore projects such as wind turbines and subsea cables. Dunes represent wave-like bedforms that range in size and mobility. They are generated by the mobility of sediment forced by the hydrodynamic regime. Geophysical analysis of dune morpho dynamics on the Scottish continental shelf is essential to facilitate the predicted expansion of offshore renewables in this area. This thesis therefore aims to investigate dune mobility on the Scottish continental shelf that is driven by energetic hydrodynamics which may be prospected for their offshore renewable energy potential. The case-study approach combines numerical modelling of the hydrodynamics with dune observations from timeseries bathymetric surveys. This approach aims to provide details on site-specific influences of non-tidal hydrodynamics on bedform formation and mobility. A numerically modelled prediction map of dune dynamics was provided in chapter 3 (first results chapter) validated by observations of dune heights at the North Channel to the Irish Sea, western UK. Including the effects of surface waves in the hydrodynamic model increased the temporal variability of model domain-averaged dune heights by 329%. In chapter 4, dune migration followed the sediment transport pathways at the Pentland Firth reflecting complex residual flow dynamics. However, the residual flow was found to vary when forced by time-varying non-tidal hydrodynamic drivers (e.g., wind). Increasing the magnitude of non-tidal hydrodynamic forcing resulted in both local increases and decreases of local sediment transport depending on the site and the redirection of the local currents by wind stress at he surface. The greatest simulated sediment flux by bedload transport (2.5x10-5 m3 s-1) of all the case studies occurred at the west approach to the Gulf of Corryvreckan (chapter 5). However, comparing this with the other results of the case studies, the sediment volume flux by dune migration did not scale proportionally with the sediment transport simulated in numerical models. No volume flux by dune migration of the very large dunes (with maximum height of 13m) at the Gulf of Corryvreckan was detected during the 10-year survey interval. Instead, greater volume flux by bedform migration was found in more non-stationary systems (e.g., the Falls of Lora, chapter 6), where non-tidal forcing had greater influence on the hydrodynamics (e.g., rainfall effects). Therefore, dune mobility appears dependent on an active sediment transport regime but also appears to be driven by the degree of morphological disequilibrium between the dune and the time-averaged sediment transport. This result is useful to inform planning of offshore installations in macrotidal seabed environments that are already highly modified by non-stationary non-tidal hydrodynamic drivers (e.g., surface waves). Changes to the non-tidal drivers due to future climate change will likely have a site-specific effect on the dune mechanics, seabed stability, and consequently the suitability of the seabed for offshore construction.

Date of Award	5 Aug 2023
Original language	English
Awarding Institution	University of the Highlands and Islands
Sponsors	Scottish and Southern Energy (SSE)
Supervisor	John Howe (Supervisor), Chris Allen (Supervisor) & Richard Bates (Supervisor)