Radar imaging mechanism of underwater wrecks in coastal waters with strong tidal currents

Predicted climate change and related sea-level rise will strongly modify hydro-, sediment- and morphodynamics in coastal zones worldwide. Varying tidal currents close to the sea bed cause sediment motion around wrecks leading to formation of scour holes and sand ribbons. Scouring can destabilize the position and shape of wrecks. Changing water depths above the shallowest parts of wrecks due to such sediment displacements can be crucial for the safety of ship navigation. For that reason, many wreck positions must be routinely re-surveyed. Therefore, basic research is still necessary to achieve new insights of wave- and current-induced sand transport in the boundary layer of the sea bed covered by wrecks and sand ribbons. These investigations are also important for assimilation of predictive coastal morphological models.
This workshop contribution will focuses on the role of active microwave remote sensing potentials for studying radar signatures at the water surface caused by submarine wrecks. The radar imaging mechanism of underwater wrecks is investigated applying the quasi-specular scattering theory of shallow sea bottom topography considering both, the gravity as well as the capillary wave ranges. Multi-beam echo sounder images of wreck marks like sand ribbons arising close to the wreck of the German motor vessel (M.V.) Birkenfels are analyzed. M.V. Birkenfels with a 6974 gross register tons (GRT) and a 156.1 length over all (LOA) sunk on 7 April 1966 after a collision with the German M.V. Marie Luise Bolten (15395 GRT) about one nautical mile southerly of Noord Hinder lightship in the southern North Sea. The formation of wreck marks at the sea bed and the manifestation of radar signatures at the water surface are caused by a vortex pair or helical flow cells triggered by unidirectional tidal current flow interacting with the wreck. The calculated strain rate of the imaging theory has the same order of magnitude as those obtained for other bedforms such as marine sand waves. This implies that the responsible hydrodynamic interaction produce radar signatures of submarine wrecks and make them visible at the sea surface. Wreck marks manifesting as sand ribbons at the sea bed as well as radar signatures of wrecks at the water surface are corresponding indicators of the local tidal current direction.