Residual relief modelling: digital elevation enhancement for shipwreck site characterisation

Abstract

Scour processes play a critical role in the preservation status of submerged historic shipwrecks. Erosion of sediment leads to enhanced exposure of archaeological sites to physical, chemical and biological processes. Current methods for identifying erosional and depositional features at wreck sites are based primarily on visual interpretation of data, which is labour-intensive and entirely subjective. The increasing availability of high-resolution multibeam echosounder-derived digital elevation models (DEMs) of historic wreck sites allows for an entirely new level of detailed interrogation and analyses of the geomorphological features associated with these. In this study, we present a residual relief modelling method for the semi-automated extraction of such depositional and erosional features at wreck sites. Relief modelling is supplemented with a breakpoint classification approach, with final separation supported by DEM visualisation enhancement techniques. We applied the method to three World War I shipwreck sites and evaluated it against traditional manual vectorisation techniques. The results suggest that the semi-automated modelling method is robust, time-effective and capable of quantifying the products of scour processes with increased objectivity. Our method holds great potential for the objective characterisation of erosional and depositional patterns and processes at wreck sites, which have important implications for site formation studies and in situ preservation of underwater cultural heritage.

Spatial and temporal variability in geomorphic change at tidally influenced shipwreck sites: The use of time‐lapse multibeam data for the assessment of site formation processes

“…Retaining some pixels within the detection threshold highlights trawl marks running across the site, including one trawl mark directly intersecting the shipwreck (Figure 7b). As their linear appearance somewhat resembles striping artifacts normally occurring within the same DoD threshold, an additional examination of a multidirectional hillshade surface of the 2019 survey's DEM was performed (Majcher et al, 2020). The trawl marks are clearly manifested in the resultant hillshade raster (Figure 7c).…”

“…For example, at the SS Santa Maria site located off the north coast of Ireland, a significant volume of material has been eroded from around the bow of the wreck (Figure 5b and Table 2), and it is the only site where no deposition of sediment is recorded. The complex bathymetry of the site makes it difficult to differentiate shipwreck‐related geomorphological features from those developed naturally, even after applying a residual relief modeling approach to site characterization (Majcher et al, 2020). In contrast, high‐resolution MBES data from the nearby SS Lugano site display no erosion, despite predictions from amplified shear stress calculations.…”

“…As manual delineation through vectorization is highly subjective, the residual relief modeling approach of Majcher et al (2020) was used to separate the wreck marks from the surrounding geomorphology. The methodology relies on a high‐pass filtered DEM (Majcher et al, 2020; Walbridge et al, 2018; Wessel, 1998), combined with a breakpoint classifier. Separate high‐pass filtered moving mean kernels were chosen to delineate erosional and depositional signatures.…”

Geoscience Solutions for Sustainable Offshore Wind Development

“…Unexploded ordnance (UXO), shipwrecks, shallow gas and boulder submerged in sediments, need to be identified prior to turbine foundation installation. Multibeam bathymetry data are acquired to identify shipwrecks in high resolution (Majcher et al, 2020), and can be combined with magnetic gradiometer surveys to identify UXOs (Clare et al, 2012;Liingaard et al, 2012). Shallow gas hazards can be identified on sub-bottom and seismic profiles, but the extent of shallow gas can be hard to detect.…”

Exploration and reconstruction of a medieval harbour using hydroacoustics, 3‐D shallow seismic and underwater photogrammetry: A case study from Puck, southern Baltic Sea

“…Currently, multibeam echosounder (MBES) surveying is used quite widely in underwater archaeology, especially in the exploration of wrecks (e.g., Majcher et al, 2020;Plets et al, 2011) and large archaeological sites (Passaro et al, 2013). Comparison of MBES datasets from different periods allows the tracking of temporal changes, like sediment changes in the tidal inlet of the Chioggia site in the Venice Lagoon, Italy (Janowski et al, 2020).…”