Radar imaging mechanism of marine sand waves at very low grazing angle illumination

The investigations carried out between 2002-2004 during several field experiments within the Op-erational radar and optical mapping in monitoring hydrodynamic, morphodynamic and environ-mental parameters for coastal management project (OROMA) aimed to improve the effectiveness of new monitoring technologies such as shipborne imaging radars in coastal waters. The coastal monitoring radar of the GKSS Research Centre, Geesthacht, Germany, is based on a Kelvin Hughes RSR 1000 X-band (9.42 GHz) 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 the-ory for the radar imaging mechanism of the sea bed. It is shown that specular point scattering con-tributes significantly to the normalized radar cross section (NRCS) modulation due to marine sand waves. According to the theory quasi-specular scattering can be applied for wind speeds Uw ≤ 8 m s-1. Measured and simulated NRCS modulations caused by flood and ebb tide oriented marine sand waves have been compared and agree fairly wel

Radar imaging mechanism of the Birkenfels wreck in the southern North Sea

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

Evolution of secondary cellular circulation flow above submarine bedforms imaged by remote sensing techniques

Normalized radar cross section (NRCS) modulation and acoustic Doppler current profiler (ADCP) measurements above submarine sand ribbons and sand waves are presented. The two study areas are located in the Southern Bight of the North Sea at the Birkenfels wreck and in the sand wave field of the Lister Tief in the German Bight of the North Sea. These measurements reveal the developments of secondary cellular circulations in tidally induced coastal sea areas. Secondary circulation cells can develop perpendicular as well as parallel to the direction of the dominant tidal current flow. Circulation cells developed perpendicular to the direction of the dominant tidal current flow are associated with marine sand ribbons manifested near an underwater wreck. Secondary circulation cells within the water column observed parallel to the direction of the dominant tidal current flow have been initiated during flood and ebb tidal current phases associated with submarine sand waves. These two types of cellular circulations must obey the Hamiltonian principle of classical mechanics. The current–short surface wave interaction is described by the action balance or radiation balance-equation based on weak hydrodynamic interaction theory. The calculated current gradient or strain rate of the applied imaging theory has the same order of magnitude for both bedforms such as marine sand ribbons and sand waves, respectively

Oceanographic phenomena caused by interaction of currents and bottom topography - historical notes and theoretical modelling

Over the last few centuries, different structures and colours of the ocean surface have fascinated sailors, fishermen, discoverers, and scientists. Variations of oceanographic parameters caused by the interaction of water currents with bottom topography are summarized based on measurements at different locations of the oceans. Eleven observation areas are presented here which reflect, in particular, the origin from identification, by just watching oceanographic phenomena in relation to the sea bed, until present day theoretical modelling. It is shown that marine remote sensing data are essential for the understanding of different imaging mechanisms in the electromagnetic spectrum. An up to date theory on the radar imaging mechanism of submarine sand waves applying quasi-specular scattering is outlined explaining surface current and bottom topography interaction

New perspectives on indigenous navigation tradition

Stick charts are a significant part of the Micronesian Marshallese navigation tradition in the Pacific Ocean. The islanders navigated without instruments just by observing, among others, oceanic phenomena such as swells, currents, and roughness characteristics of the sea surface. For a long time, the explanation of the various sticks of such latticework remained secret and something of a mystery and was only obtained by oral transmission under great difficulties. Old and new interpretations of stick charts are compared and presented. Signatures of different swells manifested on stick charts were identified and proved by satellite remote sensing data. Current research on indigenous navigation is growing to conserve such unique tradition in the Pacific and especially in the sea area of the Marshall Islands. For scientific research on indigenous navigation knowledge collaboration between natural scientists and ethnologists is necessary

First results of the OROMA experiment in the Lister Tief of the German Bight in the North Sea, EARSeL Proceedings

The objective of the project entitled “Operational Radar and Optical Mapping in monitoring hydrodynamic, morphodynamic and environmental parameters for coastal management (OROMA)” within the Fifth Framework Programme of the European Commission (EC) is to improve the effectiveness of monitoring technologies in coastal waters. The Research Vessel (R.V.) Ludwig Prandtl of the GKSS research centre was equipped with special sensors and instruments to measure the position of the ship, the water depth, the salinity, the water temperature, the current speed and direction, the modulation characteristics of short-wave energies, and relevant air-sea interaction parameters due to the presence of submarine sand waves. The first experiment of the OROMA project on 5-16 August 2002 took place in the Lister Tief, a tidal inlet of the German Bight in the North Sea. The seabed morphology of the Lister Tief reveals a complex configuration of different bedforms which is four-dimensional in space and time. A significant upward orientated component uvert of the three-dimensional current velocity field was observed. Marked vertically so-called waterspouts of uvert above the crests of sand waves have been measured by the Acoustic Doppler Current Profiler (ADCP) as straight lines. They cause water upwelling with turbulence patterns at the water surface affecting the Normalized Radar Cross Section (NRCS) modulation. A first impression of expected NRCS modulation signatures of sea bottom topography detected by the GKSS shipborne X-band radar are presented as an uncalibrated composite of five single sea clutter images acquired in the Lister Tief on 22 November 1990

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

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

Suspended sediment dynamics above submerged compound sand waves observed during a tidal cycle

Detailed Acoustic Doppler Current Profiler (ADCP) data of the three-dimensional current-field, echo intensity, modulation of suspended sediment concentration (SSC), and related water level and wind velocity have been analyzed as a function of water depth over asymmetric submerged compound sand waves during a tidal cycle in the Lister Tief of the German Bight in the North Sea. Signatures of vertical current component, echo intensity and calculated SSC modulation in the water column depend strongly on wind and horizontal current speed as well as on wind and horizontal current direction, respectively. Bursts of vertical current component and echo intensity are triggered by the sand waves itself and also by superimposed megaripples due to current/wave interaction