Hennings, Ingo and Herbers, Dagmar (2010) Radar imaging mechanism of the Birkenfels wreck in the southern North Sea. [Paper] In: Hydro 2010. , 02.-05.11.2010, Rostock-Warnemünde . Extended abstracts / HYDRO 2010 .

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Abstract

Varying tidal currents close to the sea bed cause sediment motion around wrecks leading to
formation of scour holes and sand ribbons. Consequently, scouring destabilizes 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. In 2008 changes occurred at 12 % of 259
investigated wrecks in German sea areas compared with the last survey. Especially lower
water depths above wrecks have been measured. 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. Here, it will be focused on the role of active
microwave remote sensing potentials for studying radar signatures at the water surface caused
by submerged wrecks. The Ka band radar imaging mechanism of the submerged wreck/sand
ribbon of the motor vessel (M/V) Birkenfels in the southern North Sea is investigated by
applying the quasi-specular scattering theory and considering the capillary as well as the
gravity wave ranges of the wave energy density spectrum. Multi-beam echo sounder images
of the Birkenfels wreck and associated sand ribbons as well as other available environmental
in situ data have been analyzed. The formation of sand ribbons at the sea bed and the
manifestation of its radar signatures at the water surface are caused by an elliptical vortex or
helical flow cell triggered by unidirectional tidal current flow interacting with the wreck. The
difference of simulated and measured normalized radar cross section (NRCS) modulation as a
function of the space variable is less than 31.6%. Results are presented for NRCS simulations
dependent on position for different effective incidence angles, unidirectional current speeds,
wind speeds, and relaxation rates.

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