58
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
Two postprocessing methods have been presented for the referencing of the
DiSC bathymetry. The first one is based on tidal gauge measurements and assumes
the homogeneity of water level for the whole area. For the second approach, the
radar depth maps are considered as proxy data, and the use of an independent
bathymetric data is necessary for the referencing. The offset referencing method
is more widely applicable because only gauge measurements or modeled water
level is required, but the error of the final bathymetric map depends on the actual
depth. The main characteristic of the regression method is the luck of a trend, but
the disadvantage is the need for echo soundings. The statistics of the two methods
proved that their consistencies are similar but the deviation of the offset method is
0.25 m smaller.
The 12 h averaged with the offset method bathymetry of DiSC has a mean error
on the order of 0.3 m or a mean relative error over the whole area of approximately
10%. The error depends mainly on the relation between the wavelength and the local
depth. It is increased in the deep areas where the wave field does not interact with
the sea bottom. In addition, the DiSC current field measurement has a comparable
variability of approximately 0.25 m/s with the state-of-the-art point measurements
such as ADCP.
By knowing the accuracy and the variability of the two products in this study,
the impact of a storm on the littoral bathymetry has been qualitatively identified
and quantitatively estimated, and current field measurements have been recorded in
a tidal inlet during stormy conditions and during the trespassing of an extreme lowatmospheric-pressure front. The bathymetric comparison proved that the 10 days
of storm changed the main characteristics of the geomorphological features by the
motion of vast amount of sediment for this short period. In shallower areas, the estimated difference of the bathymetry is more than 1 m, the near-shore geostructure
has propagated toward the north, the ship channel was widened, and a significant
quantity of sediment was deposited in it.
This is the very first time of acquiring time series of the current field in the littoral zone with a 0.5-h time step and 40-m spatial resolution. These unique data
permit the identification of the interaction between the current field and the geomorphological features such as the ship channel and the shoals. The direction of
the current is defined by the local bathymetry, which also has an impact on the
current velocity. The spatial gradient of the velocity field proved, during flooding approximately 50% difference between the velocity magnitude in the deeper
part of the channel and the shallow areas. In addition, due to the synoptic current
measurements, the formation of sea surface hydrodynamic features (e.g., eddies
during slack water) could be observed and studied. Furthermore, in this very specific case, the “inverted barometer” effect on the current field in a coastal area has
been observed.
In general, the present scientific investigation illustrates the potential of
ground-based remote sensing methods in the small-area changes in the coastal
environment caused by mesoscale forcing, because DiSC offers spatial and temporal information simultaneously, which is impossible to be obtained with typical
in situ measurements. The combination of the nautical radar with the DiSC algorithm is a mature and validated technology for the operational determination of the
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
Two postprocessing methods have been presented for the referencing of the
DiSC bathymetry. The first one is based on tidal gauge measurements and assumes
the homogeneity of water level for the whole area. For the second approach, the
radar depth maps are considered as proxy data, and the use of an independent
bathymetric data is necessary for the referencing. The offset referencing method
is more widely applicable because only gauge measurements or modeled water
level is required, but the error of the final bathymetric map depends on the actual
depth. The main characteristic of the regression method is the luck of a trend, but
the disadvantage is the need for echo soundings. The statistics of the two methods
proved that their consistencies are similar but the deviation of the offset method is
0.25 m smaller.
The 12 h averaged with the offset method bathymetry of DiSC has a mean error
on the order of 0.3 m or a mean relative error over the whole area of approximately
10%. The error depends mainly on the relation between the wavelength and the local
depth. It is increased in the deep areas where the wave field does not interact with
the sea bottom. In addition, the DiSC current field measurement has a comparable
variability of approximately 0.25 m/s with the state-of-the-art point measurements
such as ADCP.
By knowing the accuracy and the variability of the two products in this study,
the impact of a storm on the littoral bathymetry has been qualitatively identified
and quantitatively estimated, and current field measurements have been recorded in
a tidal inlet during stormy conditions and during the trespassing of an extreme lowatmospheric-pressure front. The bathymetric comparison proved that the 10 days
of storm changed the main characteristics of the geomorphological features by the
motion of vast amount of sediment for this short period. In shallower areas, the estimated difference of the bathymetry is more than 1 m, the near-shore geostructure
has propagated toward the north, the ship channel was widened, and a significant
quantity of sediment was deposited in it.
This is the very first time of acquiring time series of the current field in the littoral zone with a 0.5-h time step and 40-m spatial resolution. These unique data
permit the identification of the interaction between the current field and the geomorphological features such as the ship channel and the shoals. The direction of
the current is defined by the local bathymetry, which also has an impact on the
current velocity. The spatial gradient of the velocity field proved, during flooding approximately 50% difference between the velocity magnitude in the deeper
part of the channel and the shallow areas. In addition, due to the synoptic current
measurements, the formation of sea surface hydrodynamic features (e.g., eddies
during slack water) could be observed and studied. Furthermore, in this very specific case, the “inverted barometer” effect on the current field in a coastal area has
been observed.
In general, the present scientific investigation illustrates the potential of
ground-based remote sensing methods in the small-area changes in the coastal
environment caused by mesoscale forcing, because DiSC offers spatial and temporal information simultaneously, which is impossible to be obtained with typical
in situ measurements. The combination of the nautical radar with the DiSC algorithm is a mature and validated technology for the operational determination of the