35
Storm Impact on the Coastal Geomorphology and Current Field
when employing a multidisciplinary approach for diagnosing the dynamics of beach
and littoral zones. Typically, the monitoring data of physical processes are acquired
with in situ probes, which provide long-term time series data with high temporal
resolution. Their main disadvantages are the high cost of deployment and maintenance, as well as the limited spatial information. On the other hand, satellite remote
sensing products have good spatial coverage, but their temporal and spatial resolutions in littoral zones are insufficient for resolving the scales of dominant phenomena. Similarly, bathymetric surveys are rare and expensive due to the long shipping
time. Consequently, the temporal evolution of the morpho-dynamically active areas
is almost always undersampled by in situ measurements, or in other words, huge
sand transports during storm events cannot be monitored. The bathymetry in most
of the studies is considered static, even though it represents the response of the system to the forcing. By combining the advantages of the in situ observations with the
remote sensing data, the ideal instrument is a ground-based synoptic sensor, which
provides two-dimensional (2-D) time series data from the sea surface, independent
of external factors such as daylight or meteorological conditions. X-band radars are
devices that fulfill those observational requirements.
This chapter is focused on the determination of the bathymetry and current field
based on the analysis of inhomogeneous radar image sequences of a dynamic and
dispersive surface. The core of the method is the analysis of the wave field properties
in intermediate depths and their inversion by a known wave theory. The principle
of the method is the wave shoaling as the wave field approaches the shore or shallow areas as well as the depths where it interacts with the sea bottom and where the
waves are shortened. By having series of sea surface images, the determination of
the local phase velocity is possible by geolocating the distance between successive
wave crests based on the inversion for the estimation of the local bathymetry and
current field. The method is independent of the imaging device but requires image
sequences. This general principle has been used in several investigations (Seemann
et al. 2000b; Hasan and Takewaka 2007). In this study, a relatively new algorithm is
applied, which is the Dispersive Surface Classificator (DiSC; Senet et al. 2008). The
objective of this investigation is the measurement of the sea surface current field and
the estimation of the bathymetric change in the coastal zone of Sylt during a 10-day
severe storm.
3.1.1 aRea of inveStigation
The island of Sylt is the northernmost sandy barrier island of the Frisian island chain
on the German North Sea coast; it is located about 30 km off the mainland, close
to the Danish border. The shape of the island is oblong due to the hydrodynamic
impact. This study focuses on the large sandy spit system at the northern end of the
island, List West, which was formed during the Holocene (Dietz and Heck 1952).
The contemporary surface geological formation is based on the periodic growth and
migration of sand dunes (Lindhorst et al. 2008), which propagate toward the tidal
channel system to the north and the leeward side of the island. Nowadays, since
1978, the shoreline has stabilized by regular beach nourishment, approximately
every second year (Doddy et al. 2004).
Storm Impact on the Coastal Geomorphology and Current Field
when employing a multidisciplinary approach for diagnosing the dynamics of beach
and littoral zones. Typically, the monitoring data of physical processes are acquired
with in situ probes, which provide long-term time series data with high temporal
resolution. Their main disadvantages are the high cost of deployment and maintenance, as well as the limited spatial information. On the other hand, satellite remote
sensing products have good spatial coverage, but their temporal and spatial resolutions in littoral zones are insufficient for resolving the scales of dominant phenomena. Similarly, bathymetric surveys are rare and expensive due to the long shipping
time. Consequently, the temporal evolution of the morpho-dynamically active areas
is almost always undersampled by in situ measurements, or in other words, huge
sand transports during storm events cannot be monitored. The bathymetry in most
of the studies is considered static, even though it represents the response of the system to the forcing. By combining the advantages of the in situ observations with the
remote sensing data, the ideal instrument is a ground-based synoptic sensor, which
provides two-dimensional (2-D) time series data from the sea surface, independent
of external factors such as daylight or meteorological conditions. X-band radars are
devices that fulfill those observational requirements.
This chapter is focused on the determination of the bathymetry and current field
based on the analysis of inhomogeneous radar image sequences of a dynamic and
dispersive surface. The core of the method is the analysis of the wave field properties
in intermediate depths and their inversion by a known wave theory. The principle
of the method is the wave shoaling as the wave field approaches the shore or shallow areas as well as the depths where it interacts with the sea bottom and where the
waves are shortened. By having series of sea surface images, the determination of
the local phase velocity is possible by geolocating the distance between successive
wave crests based on the inversion for the estimation of the local bathymetry and
current field. The method is independent of the imaging device but requires image
sequences. This general principle has been used in several investigations (Seemann
et al. 2000b; Hasan and Takewaka 2007). In this study, a relatively new algorithm is
applied, which is the Dispersive Surface Classificator (DiSC; Senet et al. 2008). The
objective of this investigation is the measurement of the sea surface current field and
the estimation of the bathymetric change in the coastal zone of Sylt during a 10-day
severe storm.
3.1.1 aRea of inveStigation
The island of Sylt is the northernmost sandy barrier island of the Frisian island chain
on the German North Sea coast; it is located about 30 km off the mainland, close
to the Danish border. The shape of the island is oblong due to the hydrodynamic
impact. This study focuses on the large sandy spit system at the northern end of the
island, List West, which was formed during the Holocene (Dietz and Heck 1952).
The contemporary surface geological formation is based on the periodic growth and
migration of sand dunes (Lindhorst et al. 2008), which propagate toward the tidal
channel system to the north and the leeward side of the island. Nowadays, since
1978, the shoreline has stabilized by regular beach nourishment, approximately
every second year (Doddy et al. 2004).