Search

Shipwrecks are an integral part of our maritime archaeological landscape and are associated with diverse societal and cultural interests, yielding significant management challenges. Coupled hydrodynamic and geomorphological processes significantly impact the effective in situ preservation of these fragile sites. In this study, we assess sediment budget change and hydrodynamic triggers at metal‐hulled shipwrecks lost between 1875 and 1918, all located in the tidally dominated Irish Sea at depths between 26 and 84 m. This is conducted using time‐lapse, multibeam echosounder surveys at multiannual, annual, and weekly time steps, supported by sediment grain‐size analysis, modeled ocean currents, and shallow seismic data. Results indicate significant changes at all time steps for sites located in sand‐dominated environments, whereas the seabed around shipwrecks settled in multimodal sediments shows virtually no change outside of measurement errors (±30 cm). Variability in geomorphic change is attributed to local environmental factors, including bed shear stress, sediment supply, and spatial barriers to scour. We demonstrate that individual wrecks in similar shelf sea regions can be in very different equilibrium states, which has critical implications for the in situ management of underwater cultural heritage. [ABSTRACT FROM AUTHOR]

The large number of historic metal-hulled shipwrecks on the seabed is a major consideration for the marine environment, heritage management and spatial planning. Their stability is driven by linked hydro- and sediment-dynamics, which in turn influence chemical corrosion and biological encrustation. The dynamism at underwater sites is frequently expressed in elaborate patterns of depositional and erosional features developed due to seabed scour. These settings are complex, due to the variable morphologies of the wrecks, and diverse types of seabed geology and geomorphology. Not only are the flow patterns and geomorphic changes at shipwreck sites not fully understood, but how these influence the wreck structures remains under-researched. Here we combine high-resolution multibeam echosounder, ocean current and sediment data with 3D Computational Fluid Dynamics (CFD) to investigate interrelations between hydro- and sediment-dynamics and the deterioration of two complex, fully submerged tidally-influenced shipwrecks. Intricate patterns of wake and horseshoe vortices are observed, and modelled wall shear stresses predict geomorphic changes recorded in 4-year and one-week interval difference models. Moreover, substantial damage is detected on the wrecks, 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. • The sheer number of metal shipwrecks on the seabed creates perplexing management challenges. • Their structures significantly influence, and are influenced by, the local hydro- and sediment-dynamic settings. • Computational fluid dynamics and difference modelling using bathymetric data provide new insights into wreck deterioration. • Intricate patterns of simulated flows and wall shear stresses correspond to the areas of sediment erosion and deposition. [ABSTRACT FROM AUTHOR]

Abstract: Data from time-lapse single-beam and multi-beam bathymetric surveys are used to assess the rate and scale of morphological change at shipwreck sites caused by anthropogenic forcing. The technique is illustrated by characterizing changes in wreck site morphology over two sites located on the east coast of Ireland – the Drogheda Boat site on the River Boyne and the Arklow Bank site in the Irish Sea. Results indicate that repeated bathymetric surveys accurately and rapidly capture morphological change, with time-lapse accretion–erosion models indicating possible erosion rates of up to 0.04m per day and accretion rates of 0.36m per day at dynamic riverine and marine sites in response to external triggers. These data are valuable in understanding site formation processes at fully submerged archaeological sites and in the derivation of high-resolution site formation models. [ABSTRACT FROM AUTHOR]

Abstract: Mombasa Island, settled for almost 2000years, played a central role in the development of the central seaboard of East Africa, emerging by the 16th century as a centre of sustained port dominance. Marine geoarchaeological surveys were conducted in Mombasa Harbour in 2001 (i) to quantify the nature and extent of the submerged archaeological resource, (ii) to investigate site formation and preservation potential of historic shipwrecks, and (iii) to explore the natural evolution of the Old Port and adjacent basins in the context of hydrodynamic and sedimentary processes. Backscatter and single-beam echo-sounder data were acquired, interpreted and diver-truthed using surface-demand and SCUBA systems. It is proposed that sustained scour processes, resultant from the interaction of ebb and flood tidal currents with bedrock ridges, control the bathymetric evolution of Mombasa Harbour, negating requirement for mechanical dredging. The submerged archaeological resource in the area comprises fish traps and shipwrecks. Physical processes govern shipwreck site formation, with scour and bed-migration dominant, ultimately leading to structural instability and site degradation. The wreck of Santo António de Tanná (1697) has been subject to gravitational collapse and mass down-slope movement of material, concentrating wreck structure and material along the portside stern third of the vessel. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: The backscatter responses of submerged archaeological material remain poorly understood. To address this, a control experiment was conducted in Belfast Lough during July and August 2001. A selection of material and targets was arranged on the seabed in varying orientations, in an attempt to replicate the situation found on a range of archaeological sites where material is exposed on the seafloor. Repeat side-scan sonar surveys of the control array were conducted at variable ranges and fish heights to examine the backscatter responses and resolution of three commercially available side-scan sonar systems. Results indicate that the electro-acoustic specification of the pulse length and the beam angle of the acoustic sources govern the theoretical resolution of side-scan data. Backscatter data from the control experiment indicate both organic (wood and leather) and inorganic (metals, ceramics, glass and varying aggregate and ballast grades) natural and man-made materials can be imaged using side-scan sonar and a wide variety of archaeological material types and concentrations may be differentiated by their backscatter responses. [Copyright &y& Elsevier]