15 research outputs found
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 ā¤ 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
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 currentāshort 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
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
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 āOperational Radar and Optical Mapping in monitoring hydrodynamic,
morphodynamic and environmental parameters for coastal management (OROMA)ā
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 ā¤2.6Ā° of flood and ebb tideāoriented sand wave signatures at the sea surface during ebb tidal current phases. Current speeds perpendicular to the sand wave crest ā¤0.6 m sā1 have been measured at wind speeds ā¤4.5 m sā1 and water depths ā¤25 m. The difference between the maximum measured and simulated normalized radar cross section (NRCS) modulation of the ebb tideāoriented sand wave is 27%. For the flood tideāoriented sand wave, a difference of 21% has been calculated. The difference between the minimum measured and simulated NRCS modulation of the ebb tideāoriented sand wave is 10%, and for the flood tideāoriented 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
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
Comparison and characteristics of oceanographic in situ measurements and simulations above submerged sand waves in a tidal inlet
Ocean color and its transparency are related to turbidity caused by substances in water like
organic and inorganic material. One of the essential climate variables (ECV) is ocean color.
However, this implies the correct interpretation of observed water quality parameters.
Acoustic Doppler Current Profiler (ADCP) data of the three-dimensional current-field, echo
intensity, modulation of suspended sediment concentration (SSC), and related water levels
and wind velocities have been analyzed as a function of water depth above submerged
asymmetric 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 intensities and calculated
SSC modulations in the water column depend strongly on wind and current velocity. Bursts of
vertical current component and echo intensity are triggered by sand waves itself as well as by
superimposed megaripples due to current wave interaction at high current 1.0 m s-1 and
wind speeds 10.0 m s-1, preferably of opposite directions, measured at high spatial
resolution. The magnitude of currents and SSC modulations during ebb and flood tidal current
phases are only weakly time dependent, whereas the local magnitudes of these parameters are
variable in space above the sand waves. Intense ejections caused by tidal current velocity
transport higher SSC near the bottom boundary layer at the sand waves superimposed by
megaripples towards the free water surface. Such typical upwelling mechanism above sand
waves creates distinct SSC signatures of remote sensing data visible in air- and space-borne
optical imagery. Hydrodynamic parameters such as dynamic buoyancy density, total energy
density and action density due to semi-diurnal M2 tide motion which are associated with sand
waves are investigated and analyzed. Results and characteristics of simulated hydrodynamic
parameters in coastal waters above sand waves are presented. It is shown that ADCP
measurements are to be consistent with simulations based on the applied theory
Observed suspended sediment dynamics during a tidal cycle above submerged asymmetric compound sand waves
The data from Acoustic Doppler Current Profiler (ADCP) of the three-dimensional current-field, echo intensity, modulation of Suspended Sediment Concentration (SSC), and related water levels and wind velocities have been analyzed as a function of water depth above submerged asymmetric 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 intensities and calculated SSC modulations in the water column depend strongly on wind and current velocity. Bursts of vertical current component and echo intensity are triggered by sand waves itself as well as by superimposed megaripples due to current wave interaction at high current 1.0 ms-1 and wind speeds 10.0 ms-1, preferably of opposite directions, measured at high spatial resolution. The magnitude of currents and SSC modulations during ebb and flood tidal current phases are only weakly time dependent, whereas the local magnitudes of these parameters are variable in space above the sand waves. Some hydrodynamic parameters are further investigated and analyzed, showing a consistence of ADCP measurements in the applied theory