Radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions

Abstract

The investigations carried out between 2002 and 2004 during six field experiments within the Operational Radar and Optical Mapping in monitoring hydrodynamic, morphodynamic and environmental parameters for coastal management (OROMA) project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters. The coastal monitoring radar of the GKSS Research Center, Geesthacht, Germany, is based on a Kelvin Hughes RSR 1000 X band (9.42 GHz) vertical (VV) polarized river radar and was mounted on board the research vessel Ludwig Prandtl during the experiments in the Lister Tief, a tidal inlet of the German Bight in the North Sea. The important progress realized in this investigation is the availability of calibrated X band radar data. Another central point of the study is to demonstrate the applicability of the quasi‐specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed. Radar data have been taken at very low grazing angles ≤2.6° of flood and ebb tide–oriented sand wave signatures at the sea surface during ebb tidal current phases. Current speeds perpendicular to the sand wave crest ≤0.6 m s−1 have been measured at wind speeds ≤4.5 m s−1 and water depths ≤25 m. The difference between the maximum measured and simulated normalized radar cross section (NRCS) modulation of the ebb tide–oriented sand wave is 27%. For the flood tide–oriented sand wave, a difference of 21% has been calculated. The difference between the minimum measured and simulated NRCS modulation of the ebb tide–oriented sand wave is 10%, and for the flood tide–oriented sand wave, a value of 43% has been derived. Phases of measured and simulated NRCS modulations correspond to asymmetric sand wave slopes. The results of the simulated NRCS modulation show the qualitative trend but do not always quantitatively match the measured NRCS modulation profiles because the quasi‐specular scattering theory at very low grazing angle is a first‐order theory.

Sun glitter imaging of submarine sand waves on the Taiwan Banks: Determination of the relaxation rate of short waves

“…Upstream of the sand wave crests, they were generally upward oriented, but downward oriented downstream. The maximal vertical current reached 0.2 m s −1 , which was stronger than, but in the same order as, those measured in the North Sea [Hennings et al, 2004;Hennings and Herbers, 2006].…”

“…[8] Since m r is a function of the wave number of short waves, the above measurements cannot be directly used in imaging models explained by the quasi-specular scattering theory, e.g., the Sun glitter imaging model and the shipborne X band [Hennings and Herbers, 2006] and Ka band [Hennings and Herbers, 2010] radar imaging models. The value of m r in a quasi-specular scattering model remains as a challenge.…”

A theory of the Ka band radar imaging mechanism of a submerged wreck and associated bed forms in the southern North Sea

“…In this study, the quasispecular scattering theory [Barrick, 1968] is applied for two different wind regimes. The NRCS modulation ds/s 0 caused by the disturbance of the surface current dU(x) due to marine sand waves based on quasi-specular scattering and a Gaussian distribution of scattering facets is given by [Hennings et al, 1994;Hennings and Herbers, 2006;Matthews et al, 2008]:…”

“…A transverse secondary circulation flow model is presented describing the interaction between the wreck/sand ribbon and the current field of an elliptical gyre caused by the unidirectional (tidal) current stream passing the wreck. It has been generally accepted by the scientific community that the radar imaging mechanism of underwater bottom topography can be physically and mathematically described as a process consisting of at least three steps [Alpers and Hennings, 1984;Hennings and Herbers, 2006]: (1) The interaction between the (tidal) current and the sea bottom topography causing variable current velocities and creating alternating divergent and convergent zones at the sea surface.…”

Surface Imprints of Water-Column Turbulence: A Case Study of Tidal Flow over an Estuarine Sill

“…Boils have been reported for relatively weak stratification in a shallow tidal sea (water depth about 45 m) [3,4]; in a 200-m-deep estuary [5]; over 25-m-deep sand waves [6]; and downstream of 4-m-deep sills [7,8]. Boils have also been reported in association with a headland-induced front [9,10] and with large-amplitude internal waves [11,12]; and boils can be expected to occur in other situations, as well, e.g., in association with upwelling of buoyant outflow water at the edge of an ice shelf.…”