Investigating geomorphic and structural change at historic wreck sites using CFD and difference modelling

Abstract

Shipwrecks not only provide unique opportunities to investigate relatively unaltered archaeological deposits, but they are also linked to environmental, heritage management and marine spatial planning issues. Physical processes, comprising linked hydro- and sediment-dynamics, can determine the pathways of evolution of shipwreck sites. For example, water flowing around a submerged wreck may cause scour, and together with changes in local geomorphology, can greatly influence the preservation state of the wreck. This geomorphic change can be exacerbated further by anthropogenic activities. This complex interplay between hydro-dynamics, geomorphic change, anthropopressure and structural deterioration remains poorly understood. Consequently, monitoring and management strategies capable of conserving this vast and finite submerged resource remain inefficient.

This thesis aims to extend our knowledge of the influence of combined sediment- and hydro-dynamic processes on the in-situ preservation of fully submerged, historic wreck sites. This study uses multibeam echosounder (MBES) data of exceptional, centimetric resolution collected over ten metal-hulled shipwrecks standing proud on the seabed in the Irish Sea between water depths of 26 and 84 m. Additional seabed and sub-seabed geological and hydrodynamic data are derived from sediment grabs, shallow-seismics and an oceanographic model. This unique combination of datasets is used in three complementary studies corresponding to the research objectives set out
in the thesis.

Residual relief modelling (RRM) is tested for the objective mapping and delineation of wreck-related scour features, enabling their quantitative description. RRM relies on high-pass filtering of digital elevation model (DEM) data and is enhanced with breakpoint classification, with final separation of the features of interest supported by DEM visualisation enhancement techniques. The results indicate that the semi-automated workflow is time-efficient and quantifies the products of scour with improved objectivity when compared to manual digitisation. Development of the method has important implications for site formation studies and decision-making in in-situ preservation planning.

The temporal and spatial scales at which sediment erosion and deposition occur around wrecks remain largely unknown. Analysis of timelapse MBES survey data successfully captures true site extents and allows for the investigation of weekly, annual, multi-annual and decadal seabed changes, significantly expanding the number of case-studies reported for shipwreck site scour. Results of difference modelling show major changes at all time intervals for sites located in sand‐dominated environments, whereas the seabed around wrecks settled in multimodal sediments shows virtually no geomorphic change outside of vertical measurement uncertainty (±30 cm). Quantification of local environmental processes and factors, including bed shear stress, sediment supply, and spatial barriers to scour explains variability in geomorphic change and aids predictions of future site dynamics. The study demonstrates that individual wrecks in similar shelf sea regions can be in very different equilibrium states with their environment, which has critical implications for their in-situ management.

Computational fluid dynamics (CFD) is used in conjunction with high-resolution MBES data-derived 3D wreck models to simulate tidal flows at two contrasting sites. Intricate patterns of wake and horseshoe vortices are observed, and modelled wall shear stresses successfully explain geomorphic change recorded in 4-year and one-week interval difference models. Moreover, substantial damage is detected on the wreck structures, correlated with areas of elevated wall shear stress and pressure in CFD simulations. The combined approach aids site management and provides analogies for offshore engineering.

It is anticipated that new knowledge realised in this study on the multi-method characterization of physical site formation processes at shipwreck sites will aid decision making in underwater heritage management and marine spatial planning.

Date of Award	Feb 2022
Original language	English
Sponsors	Vice Chanchellor's Research Scholarship
Supervisor	Rory Quinn (Supervisor), Ruth Plets (Supervisor) & Chris Mc Gonigle (Supervisor)