51
Storm Impact on the Coastal Geomorphology and Current Field
from south to north was boosted by the storm. Near the shore, on the northeastern
side, the underwater spit embayed by the isoline at 7 m has been propagated during
the storm and, similarly, the geomorphological feature on the northwestern side. In
the west, during the first period, there is a shoal (approximately 3 m deep) that was
eroded approximately 2 m during the storm. The mean difference of the sediment
volume during the two periods is approximately –220,000 m 3 . This result lies within
the error bounds of the method or the mean error of the offset method with a value of
0.24 m, which corresponds to a volume of 295,000 m 3 . Despite this, there is a clear
change in the patterns of the geomorphological structures, which proves the motion
of the sediment.
3.5.3 cuRRent field MonitoRing duRing the StoRM
During period C, the wave field was monitored by a radar for 45 h at 30-min intervals; the current field has been extracted using DiSC with a 40-m spatial resolution,
and it covered an area of 3 km 2 . The time series of the current field shows that, in
shallow areas (over the shoal at the northwest side of the area and close to the shore),
there are often missing values, mainly during the ebb phase and the low-wave conditions. This is due to the limitations of the method. Due to the breaking waves, it is
impossible to be inverted for the determination neither by bathymetry nor current.
In the central shipping channel, there are continuous measurements. The extracted
current velocities are integrated over the wave height, because they have been calculated by the Doppler shift of the current on the waves. As the wave height varies
between 1 and 3 m and the area of investigation is relatively shallow (mean depth is
approximately 8 m), the DiSC current field could be considered as a depth-integrated
current field measurement.
Figures 3.10 and 3.11 illustrate the current field measurements during the trespassing of the low-pressure front and during the stabilization of the air pressure,
respectively. The spatial resolution of the current fields in the figures is 80 m ×
80 m for half the measurement to preserve the clarity of the images. The spatial
time series of the current field demonstrates the interaction of the local bathymetry
with the impact of the sea bottom morphology on the circulation. In the shallow
areas (northwest and near the shore), the current speed over the shoals is increased
(as expected from the continuity). In addition to that, the direction of the current
field for both flooding and ebbing is steered by the direction of the local geomorphological features. For instance, in the channel, the current direction is the same
with the direction of the channel, or circular current features around the shoal are
formed.
At the beginning of the observations, the ebb was hindered, as extensively
described in Section 3.5.1; the flooding lasted for more than 13 h (Figures 3.10 and
3.12b), from 0 to 13 h, as a response to the trespassing of the low-pressure front
(Figure 3.12c). During the period of the missing ebbing, the mean velocity is relatively low, approximately 0.5 m/s, which does not explain the slight increase in water
level. The explanation for this is that, in the northern part of the tidal inlet, which is
not imaged with the radar but is the main outflow channel, even under these conditions, still there is outgoing flow. Hence, the increase of only 0.5 m of the water level
Storm Impact on the Coastal Geomorphology and Current Field
from south to north was boosted by the storm. Near the shore, on the northeastern
side, the underwater spit embayed by the isoline at 7 m has been propagated during
the storm and, similarly, the geomorphological feature on the northwestern side. In
the west, during the first period, there is a shoal (approximately 3 m deep) that was
eroded approximately 2 m during the storm. The mean difference of the sediment
volume during the two periods is approximately –220,000 m 3 . This result lies within
the error bounds of the method or the mean error of the offset method with a value of
0.24 m, which corresponds to a volume of 295,000 m 3 . Despite this, there is a clear
change in the patterns of the geomorphological structures, which proves the motion
of the sediment.
3.5.3 cuRRent field MonitoRing duRing the StoRM
During period C, the wave field was monitored by a radar for 45 h at 30-min intervals; the current field has been extracted using DiSC with a 40-m spatial resolution,
and it covered an area of 3 km 2 . The time series of the current field shows that, in
shallow areas (over the shoal at the northwest side of the area and close to the shore),
there are often missing values, mainly during the ebb phase and the low-wave conditions. This is due to the limitations of the method. Due to the breaking waves, it is
impossible to be inverted for the determination neither by bathymetry nor current.
In the central shipping channel, there are continuous measurements. The extracted
current velocities are integrated over the wave height, because they have been calculated by the Doppler shift of the current on the waves. As the wave height varies
between 1 and 3 m and the area of investigation is relatively shallow (mean depth is
approximately 8 m), the DiSC current field could be considered as a depth-integrated
current field measurement.
Figures 3.10 and 3.11 illustrate the current field measurements during the trespassing of the low-pressure front and during the stabilization of the air pressure,
respectively. The spatial resolution of the current fields in the figures is 80 m ×
80 m for half the measurement to preserve the clarity of the images. The spatial
time series of the current field demonstrates the interaction of the local bathymetry
with the impact of the sea bottom morphology on the circulation. In the shallow
areas (northwest and near the shore), the current speed over the shoals is increased
(as expected from the continuity). In addition to that, the direction of the current
field for both flooding and ebbing is steered by the direction of the local geomorphological features. For instance, in the channel, the current direction is the same
with the direction of the channel, or circular current features around the shoal are
formed.
At the beginning of the observations, the ebb was hindered, as extensively
described in Section 3.5.1; the flooding lasted for more than 13 h (Figures 3.10 and
3.12b), from 0 to 13 h, as a response to the trespassing of the low-pressure front
(Figure 3.12c). During the period of the missing ebbing, the mean velocity is relatively low, approximately 0.5 m/s, which does not explain the slight increase in water
level. The explanation for this is that, in the northern part of the tidal inlet, which is
not imaged with the radar but is the main outflow channel, even under these conditions, still there is outgoing flow. Hence, the increase of only 0.5 m of the water level