Glacial processing potentially impacting seabed dynamic as a key consideration for offshore renewable energy

Abstract

A global shift to renewable energy within energy systems is leading to an increase in offshore infrastructure. For sustainable development in offshore energy, knowledge of how mobile sediment can impact offshore infrastructure is required. Sediment waves represent the large-scale movement of mobile sediment, which can lead to the destabilisation of offshore infrastructure. This has led to research establishing the hydrodynamic and sedimentological forcings of sediment wave morphology and migration behaviour. Sediment waves are formed throughout the Celtic and Irish Seas (CISs) and display a range of morphological characteristics (spacings, height, degree of asymmetry) and migration rates. During the Last Glacial Maximum, glacial sediment was deposited across the CISs, which was then reworked during the Holocene transgression. This has caused the seabed stratigraphy to have a variable thickness in mobile sediment, and the seabed sediment to be of mixed composition with variable grain size and sorting. This research aims to examine if sediment wave morphology and migration rate are influenced 1. by limited sediment supply in areas where there is a limited thickness of mobile sediment and 2. potentially by the hiding exposure effect induced by the mixed sediment composition, causing increased or decreased immobility or mobility of certain grain size classes. Bathymetric data was used to characterise sediment wave morphology and migration behaviour (rate and direction) across the site and analyse their annual rate of change. Data taken from descriptive statistics derived from sediment grain size analysis and the multibeam echosounder backscatter grid was used to extrapolate the distribution of sediment grain size parameters of D50 values, D90 values, and the percent of fine gravel and medium sand across the site using simple linear regression models. Sediment grain size parameters were then used to determine sediment mobility and bedload transport. The influence of the hiding exposure effect was examined by finding where the Shields critical bed shear stress was increased or decreased for medium sand and fine gravel, as well as the mobility of these grain classes across the site. Spearman Rho’s correlation analysis showed that there was no statistical correlation between mobile sediment thickness and migration rate. However, an association between mobile sediment thickness and asymmetry was found but requires further analysis. No clear findings were determined for how the hiding exposure effect could influence sediment wave migration and morphology. It is suggested that the Spearman Rho’s correlation analysis be used as an indicator tool, in which the interpreted associations causality can be confirmed using methods such as flume experiments or modelling.

Date of Award	20 Aug 2024
Original language	English
Awarding Institution	Bangor University
Supervisor	Katrien Van Landeghem (Supervisor)