The role of scour in shipwreck site formation processes and the preservation of wreck-associated scour signatures in the sedimentary record – evidence from seabed and sub-surface data

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Abstract

Fully submerged shipwreck sites act as open systems, with the exchange of material (sediment, water, organic and inorganic objects) and energy (wave, tidal, storm) across system boundaries. Formation processes at wreck sites are driven by some combination of chemical, biological and physical processes, with physical processes dominant in initial phases of site formation. Scouring and associated depositional patterns that form in response to hydrodynamic forcing are commonly the dominant physical processes acting at shipwreck sites. Scour is initiated by the introduction of a shipwreck to the seafloor, leading to increases in flow velocity and turbulent intensity around the structure. Near-field and far-field scour pits form at wreck sites due to the interaction of horseshoe and lee wake vortices with the mobile substrate. The morphology of resultant scour signatures are controlled by the orientation of the wreck structure in relation to the prevailing hydrodynamic regime, the morphology and size of the wreck and individual site components, the hydrodynamic regime (currents, waves or combined waves and currents), bathymetry and the geology of the site (seafloor and sub-surface conditions). Individual objects or artefacts may act as nuclei to promote scour at a local scale. Under high-energy conditions, groups of artefacts and/or disarticulated structural components emanating from a wreck may compound natural scour processes by rolling or sliding. Under suitable environmental conditions, wreck-associated scour features can be preserved in the sedimentary record.

Introduction

Scour occurs at the seafloor when sediment is eroded from an area in response to forcing by waves and currents [52]. Commonly, scour is initiated by either the migration or change in morphology of bedforms [36], [52] or by the intentional (e.g. coastal engineering) or accidental (e.g. shipwreck) introduction of an object to the seafloor. Marine structures (intentional or accidental) are vulnerable to erosion due to scouring by waves and tidal currents, and scour processes can ultimately lead to the complete failure and collapse of structures on the seafloor [36], [52]. Scour signatures are widely reported in marine sciences, and their development and importance in short- and long-term site evolution in coastal engineering and seabed development [7], [9], [13], [45], [29], [34], [36], [37], [39], [41], [46], [52], glacial and geomorphological research [15], [30], [32], [33], [49], [50], mine burial and detection [14], [35], biology [10] and archaeology [1], [2], [3], [6], [8], [19], [22], [24], [27], [42], [43], [44], [48], [51] are commonly described in the scientific literature.

In archaeological investigations, scour signatures are reported from nearshore shallow-water submerged wreck sites [1], [6], [22], [24], [27], [42], [43], [48], [51] to deep-water archaeological sites on the continental shelf [2], [3], [19], [44]. Scour signatures are reported from intact and scattered wreck sites [1], [6], [22], [24], [27], [42], [43], [48], [51] and from individual artefacts and artefact scatters [2], [3], [19] emanating from wrecks. Although frequently reported and often described in a qualitative (or at best semi-quantitative) manner, there is a distinct paucity of quantitative discussion in the archaeological literature on the role of scour in shipwreck site formation processes. In maritime archaeology, the recent focus on site formation theory [12], [23], [25], [48], [51] and an acceptance that physical process dominate site formation in the early stages [48] suggests that a greater understanding of scour processes and associated depositional and erosional processes at wreck sites are vital.

The intended aims of this paper are to review the current knowledge of scour at fully submerged structures under steady and oscillatory flow regimes, to integrate this knowledge in terms of shipwreck sites and to outline the role of scour in wreck site formation from a number of case studies, highlighting scour development and the subsequent preservation of scour features in the sedimentary record.

Section snippets

Flow regimes at fully submerged structures

A comprehensive discussion of scour processes (and associated hydro- and sediment-dynamics) is beyond the remit and scope of this paper. This section consequently outlines the general principles of scour and introduces terminology relevant to this study only. Comprehensive overviews detailing the principles of scour at marine structures are presented in books by Whitehouse [52] and Soulsby [36] and in review papers by Sumer and Fredsoe [38] and Sumer et al. [39].

In general terms, the

Scour and wreck site formation

Site formation processes at wreck sites are driven by some combination of chemical, biological and physical processes, with physical processes dominant in initial phases of site formation [47], [48]. The majority of publications on site formation outline general connections between wrecks and their physical environment [12], [25], [27], [48], [51], but many fail to effectively link the physical attributes of the wreck site with the processes controlling wreck formation [48]. Many authors

Scour under steady and oscillatory flow conditions

In coastal and shelf seas, both waves and currents play important roles in sediment dynamics [36]. Therefore, at shipwreck sites, scour processes resulting from both uni- and bi-directional steady flows (e.g. tidal currents) and oscillatory flows (e.g. waves) are important considerations. Furthermore, many inshore shipwreck sites occur in environments where the hydrodynamic regime is characterised by a combination of tidal current and wave processes, thereby increasing the complexity of

Discussion

The introduction of a shipwreck to the seafloor leads to an increase in flow velocity and turbulent intensity, often culminating in scouring of seafloor sediment and destabilisation of the superstructure. Positive and negative feedbacks, operating between physical, chemical and biological processes in the water column and sediment pile, are dependent upon the complex erosion and accretion characteristics of wreck sites. Due to the dynamic and mobile nature of submerged wreck sites and artefact

Conclusions

The conclusions drawn from this research are outlined below:

  • (a)

    Scouring at wreck sites is controlled by the orientation of the wreck structure relative to the prevailing hydrodynamic regime, the morphology and size of the wreck and individual site components, the category of hydrodynamic regime (currents, waves or combined waves and currents), bathymetry and the geomorphological and geological environment of the site (seafloor and sub-surface conditions). Sub-surface geology may act as a control

Acknowledgements

Thanks to innumerable colleagues for protracted scour-related discussions over the past 10 years. Thanks to Airtricity (Ireland), Titan Environmental Surveys and Donal Boland for permission to use the raw multi-beam bathymetric and side-scan sonar data from the Arklow Bank wreck site and thanks to Enviroscan, Inc for permission to use the side-scan data in Fig. 2c. Finally, I wish to thank the four anonymous reviewers whose comments greatly improved the content of the manuscript.

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