39
Storm Impact on the Coastal Geomorphology and Current Field
presented (Alpers and Hennings 1984). According to this theory, the imaging is
attributed to surface effects induced by current variations over bottom topography. The current modulates the short-scale surface roughness, which is displayed
as difference in radar reflectivity. This approach has been discussed by many
researchers (Shuchman et al. 1985; Zimmerman 1985; Hennings 1990; Romeiser
et al. 1997; Vogelzang et al. 1997). In 1998, an overview of general spatial scales
of bed forms and ocean floor topography as a function of water depth by using different remote sensing radar systems was given by Hennings (1998). In 2000, based
on those approaches, a commercial system for the determination of the bathymetry
(Bathymetry Assessment System) was launched (Calkoen et al. 2001). In the frame of
the Operational Radar and Optical Mapping in monitoring hydrodynamic, morpho -
dynamic, and environmental parameters for coastal management—OROMA project
(Ziemer et al. 2004)—a similar algorithm for the radar imaging mechanism of the
seabed by analyzing very low grazing angle radar data was demonstrated (Hennings
and Herbers 2006).
3.2.2 aveRaged RadaR iMage Sequence
The averaging of image sequences is the evolution of the previously mentioned methodology. The introduction of ground-based remote sensing permitted the acquisition of image sequences of the sea surface; the hydrodynamic modulations (e.g.,
wave breaking or changes of the current regime due to bathymetric changes) were
imaged, and their spatial differences could be identified by averaging in time; the
general trend of the quantities could be determined by analyzing long time series
of the averaged images. First studies have been based on video image sequences
(Holman et al. 1993). This hardware–software combination video system is known
as Argus. Similar to video-based methods, time-averaged radar sequences are calibrated according to the underlying bathymetry (Wolff et al. 1999; Ruessink et al.
2002; Takewaka 2005; McNinch 2007), because in those methods, the backscatter
intensity (related mainly to the wave breakers) is important. The actual depth information refers mainly to the position of sand bars or other geomorphological structures. This property has been used successfully for the assimilation of radar data in
hydrodynamic modeling (van Dongeren et al. 2008). Recently, analysis of long time
series of averaged radar images sequences has been used for dune tracking in order
to quantify the bed-load transport (Davies 2009). In all these different approaches,
the meteorological and oceanographic conditions are taken into account for the calibration of the images.
3.2.3 inveRSion of the wave field PRoPagation
The determination of the bathymetry by the two previously presented methods is
beyond the interest of this chapter. Bathymetry can be estimated from fundamental
physical properties of waves propagating over an inhomogeneous bathymetry, as the
celerity of ocean waves is measureable in image sequences and is readily related to
the underlying depth through the dispersion relationship. Since WWII, the bathymetry in coastal environments has been estimated by utilizing ocean wave shoaling
Storm Impact on the Coastal Geomorphology and Current Field
presented (Alpers and Hennings 1984). According to this theory, the imaging is
attributed to surface effects induced by current variations over bottom topography. The current modulates the short-scale surface roughness, which is displayed
as difference in radar reflectivity. This approach has been discussed by many
researchers (Shuchman et al. 1985; Zimmerman 1985; Hennings 1990; Romeiser
et al. 1997; Vogelzang et al. 1997). In 1998, an overview of general spatial scales
of bed forms and ocean floor topography as a function of water depth by using different remote sensing radar systems was given by Hennings (1998). In 2000, based
on those approaches, a commercial system for the determination of the bathymetry
(Bathymetry Assessment System) was launched (Calkoen et al. 2001). In the frame of
the Operational Radar and Optical Mapping in monitoring hydrodynamic, morpho -
dynamic, and environmental parameters for coastal management—OROMA project
(Ziemer et al. 2004)—a similar algorithm for the radar imaging mechanism of the
seabed by analyzing very low grazing angle radar data was demonstrated (Hennings
and Herbers 2006).
3.2.2 aveRaged RadaR iMage Sequence
The averaging of image sequences is the evolution of the previously mentioned methodology. The introduction of ground-based remote sensing permitted the acquisition of image sequences of the sea surface; the hydrodynamic modulations (e.g.,
wave breaking or changes of the current regime due to bathymetric changes) were
imaged, and their spatial differences could be identified by averaging in time; the
general trend of the quantities could be determined by analyzing long time series
of the averaged images. First studies have been based on video image sequences
(Holman et al. 1993). This hardware–software combination video system is known
as Argus. Similar to video-based methods, time-averaged radar sequences are calibrated according to the underlying bathymetry (Wolff et al. 1999; Ruessink et al.
2002; Takewaka 2005; McNinch 2007), because in those methods, the backscatter
intensity (related mainly to the wave breakers) is important. The actual depth information refers mainly to the position of sand bars or other geomorphological structures. This property has been used successfully for the assimilation of radar data in
hydrodynamic modeling (van Dongeren et al. 2008). Recently, analysis of long time
series of averaged radar images sequences has been used for dune tracking in order
to quantify the bed-load transport (Davies 2009). In all these different approaches,
the meteorological and oceanographic conditions are taken into account for the calibration of the images.
3.2.3 inveRSion of the wave field PRoPagation
The determination of the bathymetry by the two previously presented methods is
beyond the interest of this chapter. Bathymetry can be estimated from fundamental
physical properties of waves propagating over an inhomogeneous bathymetry, as the
celerity of ocean waves is measureable in image sequences and is readily related to
the underlying depth through the dispersion relationship. Since WWII, the bathymetry in coastal environments has been estimated by utilizing ocean wave shoaling