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Tidal banks are common in the southern North Sea where they form linear shore-parallel or slightly oblique sand bodies. A 13 day field experiment was conducted in February 2007 on a macrotidal barred sandy beach of northern France, on the southwestern shore of the North Sea, in order to assess the effects of a shallow nearshore sand bank on beach/nearshore hydrodynamics. Two Acoustic Doppler Current Profilers (ADCP) were moored in the surf zone and two electromagnetic current meters were deployed on the middle beach. The instruments were deployed along two shore-perpendicular transects, the first being located onshore of a sand bank extending along an area characterized by shoreline retreat, while the other was positioned in a prograding area of the coastline, about 2 km eastward of the bank edge. Results obtained in the intertidal zone during moderate wind events, showed that significant wave height was similar or even slightly higher behind the bank compared to the other experimental site, indicating that the bank did not significantly enhance wave energy dissipation. Strong flood tidal currents, up to 0.7 m/s, were recorded at both sites during the experiment, but current speed was generally higher behind the bank, presumably because tidal flows are constrained in the channel located between the sand bank and the beach. These results suggest that sand banks do not necessarily protect the coast from the action of incoming waves and may locally favor downcurrent sediment deposition at the coast due to increased sediment transport in nearshore channel.
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Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 59 - 63 ICS2009 (Proceedings) Portugal ISSN
Effects of nearshore sand bank and associated channel on beach
hydrodynamics : implications for beach and shoreline evolution
A. Héquette, M.H. Ruz, A. Maspataud and V. Sipka
Laboratoire d'Océanologie et de Géosciences (UMR CNRS 8187),
Université du Littoral Côte d’Opale, Wimereux, 62930 France
email :
A. and S
V., 2009. Effects of nearshore sand bank and associated
channel on beach hydordynamics: implications for beach and shoreline evolution. Journal of Coastal Research,
SI 56 (Proceedings of the 10th International Coastal Symposium), 59 – 63. Lisbon, Portugal, ISBN
Tidal banks are common in the southern North Sea where they form linear shore-parallel or slightly oblique sand
bodies. A 13 day field experiment was conducted in February 2007 on a macrotidal barred sandy beach of
northern France, on the southwestern shore of the North Sea, in order to assess the effects of a shallow nearshore
sand bank on beach/nearshore hydrodynamics. Two Acoustic Doppler Current Profilers (ADCP) were moored in
the surf zone and two electromagnetic current meters were deployed on the middle beach. The instruments were
deployed along two shore-perpendicular transects, the first being located onshore of a sand bank extending along
an area characterized by shoreline retreat, while the other was positioned in a prograding area of the coastline,
about 2 km eastward of the bank edge. Results obtained in the intertidal zone during moderate wind events,
showed that significant wave height was similar or even slightly higher behind the bank compared to the other
experimental site, indicating that the bank did not significantly enhance wave energy dissipation. Strong flood
tidal currents, up to 0.7 m/s, were recorded at both sites during the experiment, but current speed was generally
higher behind the bank, presumably because tidal flows are constrained in the channel located between the sand
bank and the beach. These results suggest that sand banks do not necessarily protect the coast from the action of
incoming waves and may locally favor downcurrent sediment deposition at the coast due to increased sediment
transport in nearshore channel.
Tidal sand bank, macrotidal coast, North Sea
The shoreface and inner shelf of the southern North Sea is
characterized by the presence of numerous tidal banks forming
linear shore-parallel or slightly oblique sand bodies (L
et al.,
et al., 1999) Although a large body of
literature has been dedicated to the dynamics of tidal sand banks,
notably in the North Sea (e.g., D
and H
, 1999; B
et al., 1994; D
et al., 2004; H
, 2008), only a few studies have been conducted on the
influence of nearshore banks on shoreline behaviour (e.g., S
et al., 2008) even if it is generally considered that these nearshore
sand bodies may have important effects on coastal hydrodynamics
and shoreline evolution (M
and O’C
, 1996:
et al., 1999). In this paper, we present the results of a
field experiment conducted on the coast of northern France, on the
southwestern shore of the North Sea, in order to assess the effects
of a shallow nearshore sand bank on beach and nearshore
hydrodynamics. The study was carried out on a barred sandy
beach (ridge-and-runnel) east of Dunkirk, such intertidal bar-
trough morphology being typical of the macrotidal coast of
northern France (R
and A
, 2002). The beach is
backed by coastal dunes that are eroding in places (Fig. 1B) in
response to high water levels associated with stormy conditions,
although aeolian deposition and dune development also occurs at
some locations (Fig. 1C). Seaward, a shallow sand bank (Hills
Bank) extends along the coast over a distance of about 9 km. The
crest of the bank may be exposed at spring low tides, forming a
shoal at a distance of about 1400 m from the beach in the study
area (Fig. 1). The bank is separated from the beach by a 10 to 15
m deep channel (Fig. 1C), sub-parallel to the coastline, that is
sometimes dredged for navigation.
Due to macrotidal conditions (mean spring tidal range at
Dunkirk: 5.6 m), tidal currents are strong with peak near-surface
velocities reaching 1.5 m.s
during spring tides in narrow
interbank channels. Tidal currents are alternating in the coastal
zone, flowing almost parallel to the coastline. Flood currents are
oriented towards the east-northeast and ebb currents towards the
west-southwest. Measurements in various sectors of the coastal
zone show that the speeds of flood currents exceed those of the
ebb, resulting in a flood-dominated asymmetry responsible for a
net regional sediment transport to the east-northeast (H
al., 2008). The speed of tidal currents decreases onshore, however,
whilst the action of wave oscillatory flows become dominant in
the shallower water depths of the upper shoreface and intertidal
zone (A
et al., 1990). Winds mainly come from the
southwest and northeast, but the strongest winds mostly originate
from west to southwest. Associated with this wind regime is a
fetch-limited environment dominated by short period waves from
southwest to west originating from the English Channel, followed
by waves from the northeast to north generated in the North Sea.
Offshore modal significant wave heights are less than 1.5 m, but
Journal of Coastal Research, Special Issue 56, 2009
Effects of nearshore sand bank on beach hydrodynamics
may episodically exceed heights of 4 m during storms (D
, 2004). Wave heights are much lower at the coast due
to significant refraction and shoaling over the shallow banks and
low gradient shoreface of the southern North Sea.
Hydrodynamic measurements were carried out in the coastal
zone east of Dunkirk (Fig. 1) during a two-week field experiment
in February 2007. Two Acoustic Doppler Current Profilers
(ADCP) were moored in the nearshore zone in approximately 1.45
and 1.9 m below Hydrographic Datum (HD) and two
electromagnetic wave and current meters (Valeport Midas current
meters) were deployed on the middle beach at an elevation of
about 2.2 m above HD (Fig. 1). All instruments were programmed
to measure wave parameters at a frequency of 2 Hz for 512
consecutive seconds (8 minutes 32 seconds burst record duration),
every 15 minutes. Spectral analyses of the raw data yielded values
of significant wave height (H
), period and direction. Mean current
speed and direction were also recorded by all instruments. The
electromagnetic current meters recorded velocity components at
15 cm above the bed during 1 minute every 15 minutes, providing
values of mean near-bottom flow speed and direction. The ADCP
data collected at a frequency of 2 Hz were used for computing
time-averaged residual current velocity and direction over 5
minutes at 1 m above the bed.
The instruments were deployed along two shore-perpendicular
transects, the first being located onshore of the Hills Bank in an
area characterized by shoreline retreat (Site 1, Fig. 1B), while the
other was positioned in a prograding area of the coastline (Site 2,
Fig. 1C), about 2 km eastward of the bank edge. Because the
ADCPs were not deployed at the same water depth, the data
obtained from these instruments can not be easily compared as
wave height and tidal current velocity decrease with water depth
et al., 1990). The similar elevations of the instruments
located on the beach, however, allow comparisons of
hydrodynamic data obtained on a coastal stretch sheltered by the
bank (Site 1) with those collected in an area that is not directly
protected by the bank (Site 2). Unfortunately, one of the current
meter deployed on the beach failed during several days and the
period of common measurements with the other instrument was
limited to three days. Beach and nearshore hydrodynamic
measurements were complemented by wave data collected at an
offshore wave buoy (Westhinder) in 27 m water depth,
approximately 36 km seaward of the Belgian coast (Fig. 1),
providing significant wave height every 15 minutes. Hourly wind
data (speed and direction) were obtained from the Meteo-France
meteorological station of Dunkirk.
Several wind events occurred during the experiment. Winds
essentially came from southwest to northwest with speeds
generally ranging from 9 to 10 m s
. Associated with these wind
events, offshore wave heights in excess of 1.5 m were measured
on several occasions, reaching about 2.2 m on 26 February (Fig. 2)
in response to northwesterly winds that exceeded 11 m s
. Wave
measurements at the shallow nearshore ADCP stations showed a
considerable decrease in wave height from the offshore to
nearshore stations (Fig. 2), due to refraction and wave energy
dissipation across the shelf and shoreface. It is noteworthy that
wave heights were quite similar at both nearshore sites, showing
that the Hills Bank was not responsible for significant wave
energy dissipation during these events. Wave height was
nevertheless slightly lower at site 1 (behind the bank) on some
occasions (on 21, 24 and 25 February, for example), but because
the instrument at this site was in shallower water depth (-1.45 m
HD) than the one located in the nearshore zone at site 2 (-1.9 m
HD), such decrease in wave height is not necessarily due to the
presence of the bank as wave heights are expected to be lower in
shallower water depths.
Figure 1. Location of the study area and instruments deployed during the field experiment (14 to 27 February 2007). A) Nearshore
bathymetry profile at sites 1 and 2; B) Photograph of eroding coastal dune at site 1; C) Photograph of incipient dune development on
the upper beach at site 2.
Journal of Coastal Research, Special Issue 56, 2009
Héquette et al.
Measurements of wave heights on the beach also showed an
important reduction in wave height from the nearshore stations to
the middle beach (Fig. 2), revealing very significant energy
dissipation across the intertidal zone. Comparison of significant
wave heights recorded on the beach at both sites during conditions
of moderate wave agitation (offshore H
ranging from about 0.9 to
1.8 m, Fig. 2) indicates, however, that wave height was generally
higher at site 1 (Fig. 3A), showing again that the proximity of the
bank did not result in more energy dissipation of the incoming
waves. Lower wave heights at site 2 may probably be explained
by the low gradient, highly dissipative, slopes (tan β) of the
nearshore/shoreface (0.0045) and beach (0.012), whereas the
somewhat steeper coastal profile at site 1 (nearshore slope: 0.01;
beach slope: 0.017) (Fig. 1A) would result in less wave energy
Another difference in hydrodynamics between the two middle
beach stations concerns the speeds of the mean currents. As Figure
3B shows, the middle beach is strongly dominated by eastward-
flowing flood currents that can be temporarily reinforced by
shore-parallel winds, like on the afternoon of 24 February when
winds with speeds of 8 to 9 m s
were blowing from the
southwest. Our measurements reveal that mean current speeds
were higher at site 1, compared to site 2, whether the mean flow is
essentially tidally-induced (i.e., during moderate wind conditions)
or intensified by shore-parallel winds. These data suggest that a
longshore gradient in mean current velocity develops on the beach
during flood, which may be related to the canalization of the tidal
flows between the bank and the coast. The action of ebb currents
is limited to very short time intervals on this part of the beach
(Fig. 3B), which therefore appears to be mainly affected by flood-
currents that can potentially induce eastward-directed sediment
transport. The two instruments in the nearshore zone were always
submerged and consequently recorded complete tidal cycles of
flood and ebb currents. As one would expect, the speeds of the
tidal current currents were higher than those measured in the
intertidal zone and flood currents were still dominant, with
maximum flood current velocities reaching about 0.7 m s
several tides whilst ebb current velocities never exceeded 0.4 m s
Several authors have suggested that nearshore tidal banks may
play a significant role on shoreline evolution, notably along the
coast of northern France. According to A
et al. (2006) and
et al. (2008), the formation of an extensive sand flat
near Calais, about 30 km west of Dunkirk, would be related to the
onshore migration of a prominent nearshore sand bank. Shoreline
progradation of more than 150 m during the last decades
et al., 2008) is probably related to onshore sediment
transfer from this bank that is almost completely welded to the
beach nowadays. The exact mechanisms that could be responsible
for this shoreward movement of sand are not known however, as
modeling of shoreface sediment transport in that area suggests
longshore rather than onshore transport (H
et al., 2008).
In their study of a massive headland-associated nearshore sand
bank off the coast of prince Edward Island, Canada, S
et al
(2008) demonstrate how this bank may have been responsible for
sediment deposition and infilling of embayments on the nearby
coast. Using detailed multibeam bathymetry data and three-
dimensional current modeling, they show that sediment is not
necessarily supplied from the bank to the coast, but that the bank
causes significant disturbance of the coastal hydrodynamics that
control sediment transport. In contrast, very high coastal erosion
rates have been observed onshore of an elongated headland-
associated sand bank that extends across Wissant Bay, on the
French coast of the Dover Straight, which may be partly due to
variations in incident wave propagation across the bay caused by
changes in bank morphology during the 20
century (A
and H
, 2006).
Similar reasoning involving disturbing effects of nearshore tidal
banks on coastal hydrodynamics has been applied by several
authors to the sand banks of the southern North Sea. Based on
wave propagation modeling, C
et al. (1999), suggested that
the nearshore sand banks near Dunkirk induce complex
deformations of wave propagation that generate alternating
patterns of energy concentration and dissipation along the coast.
According to these authors, the Hills Bank, significantly protect
the coast from wave attack by dissipating the energy of incoming
Figure 2 Time-series of significant wave height (H
) measured offshore (Westhinder buoy), in the nearshore zone and on the beach east
of Dunkirk between 14 and 27 February 2007 (see Fig. 1 for location of the instruments). Water depths and elevation are relative to
Hydrographic Datum.
Journal of Coastal Research, Special Issue 56, 2009
Effects of nearshore sand bank on beach hydrodynamics
waves. They attribute coastal erosion in this sector to the
combined action of tidal currents and shore-parallel winds from
WSW, but they had no wave or current measurements for
supporting this hypothesis. In their study of beach morphology
east of Dunkirk, R
and A
(2002) also
considered that the presence of the Hills Bank controls the
exposure of the beach to wave energy, which would represent a
major factor explaining the variability of the observed intertidal
bar morphology.
Although we acknowledge that waves can undergo significant
refraction and shoaling over the shallow sand banks of the
southern North Sea, resulting in energy dissipation, our
measurements show that the Hills Bank does not significantly
reduce wave energy, at least during moderate energy conditions
like the ones encountered during this study. Wave energy
dissipation is probably high at low water levels, especially when
waves break over the crest of the bank. During such conditions,
the bank can certainly protect the beach and may play a role on the
morphodynamics of the intertidal zone, notably on the
morphological behaviour of the intertidal bars as suggested by
and A
(2002), but this would have no effects
on the upper beach and coastal dunes. During high water level
conditions, however, which may be associated with positive
surges and/or high astronomical tides, the dissipation of wave
energy is lower due to greater water depths over the bank,
resulting in less protection for the upper beach and coastal dunes.
This is especially the case for short period waves, which are
representative of the fetch-limited North Sea, that undergo
refraction and shoaling in shallow water depths (compared to long
period swells that characterize open seas). An additional factor
that can explain why the coastal dunes are not efficiently protected
by the bank is the presence of a relatively deep channel between
the bank and the beach, probably allowing waves to reform. Wave
energy dissipation actually appears very significant across the
gently sloping nearshore/shoreface at site 2.
Our data suggest that the proximity to the coast of the Hills
Bank and associated channel constitutes a major barrier limiting
onshore sediment transport, which conversely favors alongshore
transport. The channel between the bank and the beach represents
a relatively deep trough characterized by high-velocity tidal flows
that are likely responsible for longshore sediment transport. The
coarse nature of the sediments on the channel floor (C
al., 1999) attests the strength of tidal currents that winnow the
finer-grained sediments. Most of the sand that is driven landward
by onshore-directed wave oscillatory flows across the bank is
therefore probably transported alongshore in the channel, in
response to flood-dominated tidal currents, and can hardly be
transported onshore to the beach. Eastward of the bank, however,
sand can more likely be transported onshore by waves over the
gently sloping shoreface (Fig. 1A), potentially resulting in a
higher sediment supply to the beach. As shown in several studies,
gently sloping dissipative shoreface profiles increase onshore-
directed transport and are commonly associated with foredune
accretion (A
et al., 2004; C
and N
, 2004).The
observed eastward decrease in mean current velocity on the beach,
which should result in downcurrent deposition, is another factor
that may contribute to the accretion observed in the eastern sector
near the Belgium border (site 2).
Nearshore sand banks may have different effects on coastal
hydrodynamics, including the redistribution of wave energy along
the coast through wave refraction and disturbance of current
patterns, depending on the depth, orientation and distance of the
bank to the coast. Our wave measurements on the beach at two
different sites along the shore show that the presence of a bank in
the nearshore zone does not necessarily result in more wave
energy dissipation at the coast, even with offshore wave heights
exceeding 2 m. This may be explained by the presence of a
relatively deep channel between the bank and the beach in which
waves can reform and by a steeper nearshore slope compared to
the second experimental site located eastward of the bank. Our
current measurements also suggest that the occurrence of a
channel close to the coast results in tidal flow canalization, which
favours longshore sediment dispersal rather than cross-shore
transport. This could limit onshore sediment transport to the beach
by shoreward-directed wave oscillatory flows and may explain the
apparent sediment deficit and coastal dune erosion observed
behind the bank. Conversely, nearshore sediment can probably be
more easily driven onshore by waves on the low gradient
nearshore slope eastward of the bank where a decrease in
longshore current velocity also contributes to sand deposition, thus
favouring shoreline progradation.
Figure 3. A) Comparison between significant wave heights
measured on the middle beach at site 1 and site 2 from 22 to 25
February 2007; B) Time-series of mean current velocity on the
middle beach at sites 1 and 2.
Journal of Coastal Research, Special Issue 56, 2009
Héquette et al.
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This study was partly funded by the French “Agence Nationale
pour la Recherche” (ANR) through the project VULSACO
(VULnerability of SAndy COast systems to climatic and anthropic
changes) and by European funds (FEDER) through the
INTERREG IIIA project Beaches At Risk. Bathymetry data were
provided by the French “Service Hydrographique et
Océanographique de la Marine” (SHOM). The authors would like
to thank Mr. Hans Pope of the Belgian “Agency for Maritimes
Services and Coast – Division COAST” for providing the offshore
wave data. Thanks are also due to Denis Marin for drafting of the
... Shoal bypassing (i.e. sediments that move with the littoral currents around the ebb-tidal delta, forming shoal banks that evolve from the ebb delta, detach from the main lobe and attach to the shoreline downdrift of the tidal inlet) influences the adjacent shoreline through channel compression and temporary erosion of the barrier before attachment of the bypassed sediment mass onto the barrier and consequent shoreline accretion (Gaudiano and Kana, 2001;Héquette et al., 2009;Kana, 1995;McBride et al., 2013) Héquette et al. (2009) analysed the effects of nearshore sand banks and associated channel and discovered that sand banks did not necessarily enhance wave dissipation, but rather constrained tidal flows leading to higher velocities and scouring. ...
... Shoal bypassing (i.e. sediments that move with the littoral currents around the ebb-tidal delta, forming shoal banks that evolve from the ebb delta, detach from the main lobe and attach to the shoreline downdrift of the tidal inlet) influences the adjacent shoreline through channel compression and temporary erosion of the barrier before attachment of the bypassed sediment mass onto the barrier and consequent shoreline accretion (Gaudiano and Kana, 2001;Héquette et al., 2009;Kana, 1995;McBride et al., 2013) Héquette et al. (2009) analysed the effects of nearshore sand banks and associated channel and discovered that sand banks did not necessarily enhance wave dissipation, but rather constrained tidal flows leading to higher velocities and scouring. ...
Shorelines bordering ebb deltas are often subject to deformation by the approach and attachment of ebb-tidal shoals. Although this is a common behaviour, little attention has been paid to the link between shoal migration and the multi-annual development of the adjacent beach-dune system. This study aims to understand beach-dune systems' behaviour during the approach phase of an ebb-tidal shoal attachment event. Topographic and bathymetric data of the barrier island of Texel (NL) was used to calculate shoreline position, beach and dune volume, dunefoot position, and shoal displacement rates. Two classical beach-dune interaction models were used to understand the observed behaviour: the surfzone-beach-dune model and the sediment budget model. The analysis revealed how a larger bank on the land-facing side of the ebb delta split into two smaller shoals with different morphological behaviors. One of the shoals (S1) moved consistently landwards, recently attaching the shore, whereas the second shoal (S2) is still evolving and presents a milder landward movement than S1. In parallel to the landward movement of S1, the adjacent coastline was subject to erosion, partially counterbalanced by nourishments. However, despite the erosion, dunes are growing in volume, suggesting a net positive transfer of sand from the beach to the dunes. In the surfzone-beach-dune model, regular nourishments positively affect beach state by maintaining sufficient beach size for aeolian sediment transport and reducing shoreline erosion. In the budget model, dune volume growth may occur despite a slightly negative beach budget. Framing Texel in the budget model implies that the beach budget before the start of the approach phase is crucial in defining the behaviour of the dune system during this phase. If the shoal approach significantly reduces the beach budget, the system may cross the budget threshold that dune building is possible. Thus, the dune system may evolve to an erosional state, including dune ridge dissection and blowout development. Hence, nourishments have been essential in keeping the current morphological state during the attachment of the shoal, as they restrict the system from crossing the negative morphological threshold. Thus, without the nourishments, beach erosion driven by the shoal might have led to morphological changes in the dune system that could lead to years of a weakened level of protection until foredune recovery after attachment.
... The region is dominated by waves from southwest to west, originating from the English Channel, followed by waves from the northeast to north, generated in the North Sea. The presence of several sand banks on the shoreface and the inner shelf and the gentle beach slopes that characterize the coast of Northern France are responsible for strong wave energy dissipation, resulting in modal significant wave heights lower than 0.6 m in the intertidal zone (Héquette et al., 2009). Wave heights can nevertheless exceed 1.5 m on the foreshore during extreme events. ...
... Wave heights were considerably lower in the intertidal and nearshore zones, however, due to significant refraction and energy dissipation of the incident waves originating from the NNE to NE that propagated over the offshore sand banks and gently sloping inner shelf of the southern North Sea. Our measurements also show that significant wave height is strongly modulated by water depth in the coastal zone, the higher wave heights being always recorded at high tide while much lower wave heights were observed during the rising or falling tide ( Fig. 2A & 2B), this dependence of wave height on water depth having been commonly observed in previous studies conducted on beach hydrodynamics in the region (Anthony et al., 2004;Héquette et al., 2009). ...
Although a large number of studies were dedicated to the observation and prediction of wave height at breaking, field investigations were essentially carried out in micro- to mesotidal environments. Here we report the results of a study conducted on a barred macrotidal beach of northern France during which video observations of wave breaking were carried out simultaneously with measurements of wave data using a series of hydrodynamic instruments deployed across the intertidal and the nearshore zones and an offshore wave buoy. Our results show that waves were breaking when the ratio of wave height to depth (Hb/hb) was comprised between approximately 0.2 to 0.45, the lower value being slightly lower than what is commonly reported in the literature for macrotidal beaches. Comparison of measured and calculated wave height at breaking using a series of predictive equations revealed that the Rattanapitikon et al. (2003) formula gave the best results for our data set. Predicted breaker heights were nevertheless overestimated when using offshore wave measurements and better results were obtained with wave data recorded in 5 m water depth instead. This can be explained by the presence of offshore tidal sand banks that are responsible for significant wave refraction and wave energy dissipation. These results contribute to determine more precisely the range of appropriate breaker index values for barred macrotidal beaches and highlight that nearshore wave parameters should be used rather than offshore wave statistics for predicting breaking wave height and breaking depth where sand banks strongly alter wave characteristics
... The ridge-and-runnel intertidal beach appears to be an area of intense sand exchange with the subtidal zone: export by the tidal channels when the tide is out and import by the incoming waves when they break in plunging. Previous studies along the northern French coast have highlighted the primary role of subtidal nearshore banks in supplying sand to coastal systems (Aernouts, 2005;Aubry, 2010;Corbau, 1995;Garlan, 1990;Héquette et al., 2009;Latapy et al., 2020;Tessier et al., 1999). They form a huge sedimentary reservoir wherein onshore and longshore migration has been demonstrated. ...
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Battiau-Queney, Y.; Ventalon, S.; Abraham, R.; Sipka, V.; Cohen, O., and Marin, D., 0000. Bedforms and sedimentary features related to water-depth variations in a sandy tidal-flat environment. Journal of Coastal Research, 00(00), 000-000. Charlotte (North Carolina), ISSN 0749-0208. This study highlighted the variability of a wide sandy macrotidal coastal system on short timescales. Changes in water depth and exposure length were the main drivers of this variability. In the study area, the coastal system consists of three units: a lower ridge-and-runnel beach, a 1000-m-wide tidal flat, and a sandy backshore. This research is based on sedimentary features that can record physical forces, especially the bedforms at the beach surface, and their relationship with beach gradient, exposure length, and changing water depth, according to tidal, weather, and marine conditions. It also demonstrated the ability of scanning electron microscope analysis of quartz grain microtextures to indicate the level of marine and eolian energy in the coastal system. The widespread wave and current sand ripples on the tidal flat showed great variability in time and space. A minimum water depth of 0.30 to 0.50 m was required for their development. The role of wind-induced waves and the frequent interference of tidal, wave, and wind forcing mechanisms are emphasized. The development of some complex sand ripples extended over a complete lunar tidal cycle. Wind-generated sand ripples were observed only on the backshore. Their absence on the tidal flat, despite the high level of eolian energy attested by quartz microtextures, and the poor development of dunes on the backshore are explained by factors impeding eolian sand transport at the beach surface such as extreme fetch segmentation and possible shell armoring of the beach surface during dry periods. This study demonstrated the short-term variability of the beach morphology as opposed to the long-term stability of the coastal system as a whole, in which the ultradissipative tidal flat, characterized by limited sand supply, low wave energy, and high but inefficient wind energy, plays a key role. ADDITIONAL INDEX WORDS: Marine energy, wind energy, SEM quartz microtextures, sand ripples, wind waves, sediment supply, French North Sea coast.
... L'étude souligne le rôle essentiel des niveaux d'eau durant l'épisode de tempête, la durée de l'épisode et l'état de la dune et de la plage avant la tempête. La différence de comportement entre les deux sites est aussi à mettre en relation avec la bathymétrie des petits fonds (Héquette et al, 2009). Maspataud et al., 2011). ...
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Il est courant de lire et d'entendre que la hausse du niveau de la mer, l'une des conséquences les plus manifestes du réchauffement climatique, va accélérer l'érosion de nos côtes, menacer de submersion marine de vastes zones littorales urbanisées et faire disparaître nombre d'îles basses habitées. Ces craintes sont-elles justifiées ? Comme un consensus ne vaut pas vérité scientifique, on va essayer de démêler le vrai du faux en partant de l'état des connaissances scientifiques sur la hausse du niveau de la mer et analyser ses effets possibles sur les processus d'érosion des côtes.
... Being able to accurately and consistently monitor beach and nearshore processes provides the foundation for understanding beach dynamics . The control on waves by changing nearshore bathymetry has been the subject of increased research interest, primarily to understand and predict shoreline changes (Hequette & Aernouts, 2010;Hequette et al., 2009;Lazarus & Murray, 2011;Ruessink et al., 2004;Stokes et al., 2015). Nearshore sediment accretion provides protection to the coast during the first high energy events that follow periods of low energy (Dissanayake et al., 2015). ...
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Coastal management and engineering applications require data that quantify the nature and magnitude of changes in nearshore bathymetry. However, bathymetric surveys are usually infrequent due to high costs and complex logistics. This study demonstrates that ground‐based X‐band radar offers a cost‐effective means to monitor nearshore changes at relatively high frequency and over large areas. A new data quality and processing framework was developed to reduce uncertainties in the estimates of radar‐derived bathymetry and tested using data from an 18‐months installation at Thorpeness (UK). In addition to data calibration and validation, two new elements are integrated to reduce the influence of data scatter and outliers: (a) an automated selection of periods of “good data” and (b) the application of a depth‐memory stabilization. For conditions when the wave height is >1 m, the accuracy of the radar‐derived depths is shown to be ±0.5 m (95% confidence interval) at 40 × 40‐m spatial resolution. At Thorpeness, radar‐derived bathymetry changes exceeding this error were observed at time scales ranging from 3 weeks to 6 months. These data enabled quantification of changes in nearshore sediment volume at frequencies and spatial cover that would be difficult and/or expensive to obtain by other methods. It is shown that the volume of nearshore sediment movement occurring at time scale as short as few weeks are comparable with the annual longshore transport rates reported in this area. The use of radar can provide an early warning of changes in offshore bathymetry likely to impact vulnerable coastal locations.
... Numerical modelling was also carried out to predict the medium to long-term morphological evolution of the tidal sand banks of the North Sea region [10,[30][31][32][33]. Numerous studies on sand bank dynamics have analyzed the interactions between sand bank morphology, hydrodynamic processes and sediment transport. Relatively few studies though have been conducted nearshore to evaluate the effect of shallow sand banks on coastal hydrodynamics and associated sediment transport [34][35][36]. Long-term morphodynamic behavior have been investigated in the UK coastal zone using historical charts over a 150 year-long period [16,37,38], whereas in the coastal zone of northern France, the available dataset is more restricted (less than 150 km 2 ) and/or limited to the last few decades. ...
Dans les environnements côtiers, les interactions entre les processus morphodynamiques, océanographiques et anthropiques (agissant sur différentes échelles de temps) contrôlent l'évolution des systèmes littoraux. Cette thèse est axée sur une échelle de temps séculaire afin de déterminer l'influence non seulement des activités humaines, mais aussi l'impact du changement climatique. Le long de la côte nord de la France, des mesures de niveau d'eau ont été effectuées depuis le début du 19ème siècle et conservées dans les archives du Service Hydrographique et Océanographique de la Marine (Shom). Parrallèlement à ces mesures tidales, des levés hydrographiques ont été effectués pour cartographier les fonds marins de cette zone côtière, caractérisée par la présence de nombreux bancs sableux, formant des corps sédimentaires massifs parallèles à subparallèles au rivage. Pour déterminer les tendances à long terme, la numérisation et l'analyse des documents historiques ont été réalisées. Ceci a permis de reconstituer les variations du niveau de la mer à Dunkerque et Calais, et d'évaluer les changements de la morphologie et de la position des bancs sableux. A partir de ces séries inédites, l'étude de l'évolution des niveaux marins moyens et des composantes de marée a révélé des changements significatifs. De plus, l'évolution bathymétrique a montré des variations morphologiques importantes depuis le 19ème siècle, qui sont en grande partie dues à la mobilité des bancs sableux. Afin d'étudier l'influence des changements morphologiques sur les processus hydrodynamiques, une modélisation numérique de la propagation de la houle et de la circulation tidale a été réalisée à l'aide de module de la chaîne de calcul TELEMAC. Dans un contexte d'érosion sédimentaire, une accélération des courants et une augmentation de la hauteur des vagues le long du rivage sont détectées. Inversement, dans un contexte d'accumulation sédimentaire, nos résultats mettent en évidence une diminution de la vitesse des courants et une plus grande dissipation de l'énergie des vagues dans les petits-fonds. Cette étude souligne le rôle des rétroactions morphologiques entre l'hydrodynamique côtière et la morphologie du littoral. L'identification de ces mécanismes à une échelle de temps séculaire est essentielle pour évaluer les facteurs potentiels des changements côtiers.
Maritime coast has potential risk of being significantly prone towards sinusoidal magnitudes of Sea Level Rise (SLR) phenomenon, especially the low-lying coastal areas and river estuaries. Numerical model tools used to analyze the effect of sea level rise with scenario Representative Concentration Pathway (RCP8.5) chosen as the assessment parameters on Kelantan shoreline and delta areas. Primary physical oceanography data consist of tidal elevation, wave parameters, current properties and bathymetry were collected to support the model configuration. Numerical model result illustrated that Kelantan Delta, rivers, and low-lying areas adjacent to riverine will severely affected by higher sea level and current speed magnitudes. Average differences of tidal elevation and current speed ranging from 101 to 104% and 97 to 99% respectively. Projection on the inundation potential in the inland vicinities due to sea level rise are essential to delineate the affected territories and identify flood prone area. Based on the numerical findings, mitigation, adaptation, and awareness program could be outline in order to minimize the socio- economics impacts towards the local communities.KeywordsSea level riseNumerical modelTidal elevationCurrent speedInundation
15 This study concerns an open-coast sandy tidal flat environment. It is located on the French southern 16 coast of the North Sea, where a narrow sand-ridge separates a low-lying hinterland from a macrotidal 17 beach with a wide tidal flat. The purpose of the research is to analyze this coastal system, whose 18 characteristics appear infrequent compared to other tidal flat environments, and explain its historical 19 stability and efficiency to resist against marine erosion and flooding of the rearward lowlands. The 20 coastal system includes 3 parts: backshore, flat intertidal platform (tidal flat, strictly speaking) and 21 intertidal ridge-and-runnel lower beach. All of them are sandy but they differ with their average slope 22 (respectively 9.6 %, 1 ‰ and 1 %) and main dynamic processes. 23 Our research is based on morphological and sedimentological field observations completed with 24 laboratory analyses. Some methods are new, such as bunker-archaeology or the study of quartz grains 25 with Scanning Electron Microscope (SEM). The bunker-archaeology method uses well-preserved 26 This preprint research paper has not been peer reviewed. Electronic copy available at: P r e p r i n t n o t p e e r r e v i e w e d 2 pieces of the German Atlantic Wall to get precise information on the shoreline mobility and beach 27 level changes from 1944 to 2022. A 1400 m-long cross-shore transect was raised in 2021 and 28 compared with other profiles produced between 2008 and 2022. A particle size analysis of 33 samples 29 shows that the sediment is dominated by fine and medium quartz sands, whose sorting varies along the 30 profile according to the percentage of silts. A selection of representative quartz grains was analyzed 31 with SEM to relate their surface microtextures to their sedimentary history. Mechanical and chemical 32 features are identified to evaluate the part of aeolian and marine mechanisms and the level of energy in 33 the transportation and deposition processes in each part of the coastal system. The results of SEM 34 analyses are compared with the sedimentary structures observed in the field to better understand the 35 flow pattern in the coastal system and sediment moving in relationship with the tide cycle, the duration 36 of submersion and aerial exposition and the water depth. The results show that in this ultra-dissipative 37 coastal system, aeolian and marine processes are damped by the natural characteristics of the 38 environment leading to a balanced sediment budget in a zone of limited sand supply and equally 39 limited erosive potential. 40 Keywords: macrotidal beach, bunker-archaeology, SEM quartz microtextures, sediment budget, 41 aeolian dynamics, French northern coast 42 43
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Ridge and runnel features were originally described by King and Williams (1949) from observations at Blackpool beach (U.K.) and laboratory experiments. They were characterised as intertidal, shore-parallel sandbars (ridges), commonly 2 to 6 bars in total, and disconnected from each other by troughs (runnels). The nomenclature ‘ridge and runnel’ was, however, also used by Hayes (1967) to describe multiple-barred beaches but referring to subtidal bars. The more specific term ‘Multiple Inter-Tidal Bars’ (MITB) subsequently adopted for intertidal beaches exhibiting successive shore-parallel sandbars. To date, a detailed understanding of the formation of MITB has remained elusive. It has been suggested that MITB features are the result of both swash and surf zone processes acting on the intertidal beach profile. Those processes are involved in the formation and the long-term stabilisation of MITBs. Despite the long-term persistence of MITB systems they are dynamic at short timescales. Ridge crest positions are regularly modified over each tidal cycle by successive surf and swash processes. At seasonal scales, ridges may undergo erosion and cross-shore migration under high energy conditions (winter) while ridges are well developed during summers. On the basis of 93 separate works on 67 sites we define MITBs, characterise their morphodynamics and assess their global distribution. The distribution of MITB is a function of thresholds in beach slope (<0.02), tidal range (3 to 10m), and wave period (3 to 8 s). They are developed at sites with adequate sediment supply, limited wind, and wave fetch, meso- to macrotidal (>3 m) and on low gradient (wide) intertidal beach slopes.
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The large-scale internal structure of shoreface banks located off Dunkerque (N France) in the Southern Bight of the North Sea is imaged in a very-high resolution seismic reflection study. An interpretation is proposed of the processes controlling the construction and migration of these banks located in a macrotidal coastal environment. The Dunkerque coastal system is characterized by strong, coast-parallel, tidal currents, and northerly moderate storms. The banks are coast-parallel and almost emergent during low-water spring tides. Their length, width and maximum height reach 8 km, 1.5 km and 12 m respectively. They have an asymmetrical profile with a steeper flank dipping either landward or seaward. Bathymetric investigations show that some parts of the banks migrate actively landward, while others tend to elongate or to migrate seaward. The steeper flank indicates the direction of the dominant migration. Seismic data reveal one main feature: the reflectors observed beneath the landward flank dip landward, and the reflectors observed beneath the seaward flank dip seaward. This large-scale internal structure conforms with the bank morphology and reflects the present-day migration mechanisms. According to bathymetric, hydrodynamic and seismic data, the landward migration component is induced by northern storm wave action and the longshore/seaward component is tide-controlled. Seismic data show that this migration pattern was occasionally reversed, some parts of the banks that presently migrate longshore having migrated landward at an earlier time (and vice versa). The predominance of storm-induced landward migration over tide-induced longshore migration (and the reverse) is related to water depth and surrounding seabed morphology evolution. The banks are thought to be recent sedimentary features of a few centuries. They represent very active sand bodies the migration of which induces significant morphodynamic modifications of the coastal system, especially of the beach domain to which they tend to attach.
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In order to understand regional variations in coastal behavior on Prince Edward Island, Canada, we investigated the role of Milne Bank, a submarine batik at East Point, the eastern tip of the island. The objective was to determine how the batik might facilitate transfer sediment from the eroding north coast to the adjacent sediment-rich South coast. The study utilized grain-size and seismic data collected on Milne Batik in 1989 and multibeam sonar surveys in 1997 and 1999. The disturbing effect of East Point on the hydrodynamic regime controls sediment transport. The northern boundary of the batik is a steep sand wave located where southward tidal and wave-driven currents rounding East Point suddenly decelerate. Sand from the north coast enters Milne Bank and is carried south in a field of migrating sand waves that are shed from the northern bounding sand wave toward the prograding end of the batik. Milne Batik is a major sediment sink, rather than a link between the eroding north coast and the sediment-rich south-facing coast. Longshore transport in nearshore bars is more likely to be responsible for continued sediment accumulation on the south coast. Embayments on the south coast have filled up in a cascading fashion, each one facilitating sediment bypassing when it has reached full capacity.
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The internal structure of the Middelkerke Bank (one of the Flemish Banks located in the southern North Sea off the coast of Oostende, Belgium) has been studied in the framework of the Marine Science and Technology (MAST) program co-funded by the European Community. A dense grid of high and very high resolution seismic profiles has been used, as well as several vibrocorings. Seven major seismic units can be identified in the Quaternary sediments, bounded by major discontinuities correlated across the whole study area. The lower units clearly appear as being deposited during periods of relative low sea level (channel infillings, shoreface, estuarine and/or ebb-tidal delta deposits). The present shape of the bank results partly from recent erosional processes, reworking the underlying deposits. Thus, the lower part of the bank as a morphological feature does not consist of “offshore tidal sands”. The master bedding of the upper part of the bank consists of inclined reflectors, dipping at an angle of about 5° in the same direction as the bank's “steep” face. These reflectors, very similar to those described by Houbolt (1968), are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms.
The morphology of three macrotidal ridge and runnel beaches in northern France was analysed from over 200 profiles in order to identify intertidal spatial and short-term (weeks) profile variability, at both the inter-site and intra-site levels. The beaches, essentially composed of medium to fine sand, are exposed to fetch-limited waves variably dissipated by nearshore sand banks. They have spring tidal ranges of 5.6 to 7.2 m, and sediment budgets ranging from equilibrium to deficient or surplus. The results show that spatial and temporal morphological variability is controlled by: (1) variations in exposure to wave action that depend on the proximity of nearshore sand banks, as well as on protection offered by artificial structures; and (2) by the state of the beach sediment budget. Where equilibrium sediment budget conditions prevail, as in the Dunkerque-Est sector, the beach exhibits a regular alternation of ridges and runnels that represent a cross-shore alternation of fluid-bed interaction domains involving surf/swash activity and channel flow conditions. Energy dissipation at the bed is spent in the construction and destruction of wave and tidal micro- and meso-scale bedforms, leaving little scope for macro-scale ridge migration or change in form, except under exceptionally high wave energy conditions. Chronic sediment losses, as in Wissant Bay, or gains, as in Calais-Hoverport, are recycled respectively alongshore and to embryo dunes and are not necessarily translated in terms of significant meso-scale (years) beach volumetric changes. The short-term beach sediment budget changes however favour active bed readjustments that explain distortion of the regular ridge and runnel form and marked profile mobility, even under low to moderate wave energy conditions.
Differential maps have been digitalized and compared from successive bathymetric and topographic surveys performed under different meteorological conditions from May 1992 to December 1994 in the shoreface-to-beach system of the 30 km long coastal area extending from Gravelines to the French-Belgian border in southernmost North Sea. The data, which concern a low-relief sandy coastal system exposed to a macrotidal regime with flood-driven eastward sediment transport, have been interpreted according to meteorological, hydrodynamical and aerodynamical characteristics. A balanced sediment budget was observed, which contradicts previous data suggesting a progressive long-term erosional trend. This result indirectly underlines the key-role on the sedimentary budget of exceptional events such as severe storms. The two types of erosion identified comprise the action of frontal waves and the combined interaction of tidal currents and wind. Five hydrodynamical cells have been recognized along the shoreface. The corresponding segmentation is attributed to the specific distribution of the wave energy along the coast due to the presence of submarine banks responsible for the deformation of the wave propagation. The morphological changes of the beach depend on the tide and swell action responsible for the construction/migration/destruction of the ridge and runnel system.
Sand banks and elongated sand ridges occur in many coastal and shelf seas where there is abundant sand and where the currents are strong enough to move sediment, but they have a wide variety of forms. Their generation requires a source of mobile sediment, either from the local sea bed, or from coast erosion. Most appear to have been created during the post-glacial rise in sea level, but they have been subsequently modified by changing currents and waves, thus losing their relict characteristics. A descriptive classification scheme is developed to unify the approaches of marine geologists and physical oceanographers, which emphasizes the formation and present hydrodynamic setting in their long-term development. Open shelf linear ridges (Type 1) are up to 80 km long, average 13 km wide and are tens of metres in height. They are oriented at an angle to the flow, are asymmetrical and appear to migrate in the direction of their steep face. They appear to be in near equilibrium with the flow. These contrast with linear ridges formed in mouths of wide estuaries, which are aligned with the flow, and which migrate away from their steeper face (Type2A). In narrow-mouthed estuaries and inlets, tidal currents are strong only close to the mouth and waves are more dominant. The banks then form close to the mouth as ebb and flood deltas (Type 2Bi). When the coast is retreating, the ebb delta forms a primary source of sand to the nearshore region, which can become modified by storm flows into `shore attached ridges’ at angles to the coastline (Type 2Bii). Tidal eddies produced by headlands can create `banner banks’ (Type 3A), but when the headland is retreating alternating or `en-echelon’ ridges can be formed which can become isolated from the coast as it recedes (Type 3B). Coastal retreat and rising sea level can then cause the ridges to become moribund. Thus the majority of ridges rely on sea level rise for their origin. Theoretical and modelling studies of the shorter term response to present hydrodynamic forcing are generally confined to Types 1 ridges and 3A banks. The most promising work considers the coupled system of hydrodynamics, sediment transport and morphology on Type 1 ridges, and predicts features such as the ridge spacing and angle to the flow. Type 3A sand banks are clearly related to the flow patterns produced by the headlands, and the models can reproduce the eddy structures and sand bank extents. Nevertheless, the vital role of shoreline processes has not been fully incorporated into the models, and there is little modelling of ebb tidal deltas or other Type 2 banks. There thus appears to be a wide scope for modelling the generation, evolution and stability of sank banks under the scenario of rising sea level and coastal retreat.
Identification of the relevant forcing factors behind coastal change is a major societal demand in the face of rising sea level. Changes in the patterns of coastal erosion and accretion at a century scale in Dundrum Bay, Northern Ireland, are here attributed to natural changes in the nearshore bathymetry of sufficient magnitude to alter incident wave-energy dispersal. Simulations of wave propagation across the nearshore reveal a marked change in the nearshore patterns of energy dispersal and related sediment- transport pathways between 1861 and 1968. The 1861 bathymetry is associated with a simulated longshore drift divide in the middle of the bay and with sediment dispersal to the southeast and northeast, whereas the 1968 bathymetry is associated with a consistent northeastward drift throughout the bay. These changes are consistent with recorded shoreline changes: sediment accumulation and foredune accretion in the northeast and northeastward spit elongation across a tidal inlet. This study reveals a case in which purely natural changes in the seafloor at a century scale can be related to long-term shoreline changes. Similar changes during relative sea-level rise during the Holocene were potentially capable of significant modification of shoreline morphology and dynamics. Nearshore extraction of sand for beach nourishment purposes may have consequences of similar magnitude for adjacent beaches.
Analyses of wind data collected at Dunkirk (northern coast of France) from 1956 to 2000 revealed two distinct periods of higher storminess: 1956-1962 and especially 1972-1977. These two periods represent 57% of the total observations of winds with velocities ≥ 16 m s−1. Storm frequency was lower during the last two decades of the 20th century, but a long-term trend in storminess could not be confidently established from this 45 year-long wind record characterized by significant decadal variations. Shoreline evolution between 1949 and 2000 was determined using series of geometrically rectified aerial photographs, showing a high temporal and spatial variability and no clear relationship with storminess in most cases. During the 1960s and 1970s, the shoreline significantly advanced seaward at several sites, although 1972-1977 corresponds to the period of maximum storm activity in our data set. Conversely, shoreline retreated at most sites during the late 1980s and 1990s while storm activity considerably decreased. The analysis of tide gauge data recorded in Dunkirk harbour between 1956 and 2000 showed that the relative frequency of high water levels that reached the upper beach zone and coastal dunes during storms increased significantly after 1983, although the annual number of storm events was lower, which may explain the more rapid coastal retreat at several locations during the late 1980s and 1990s. Long-term coastal dune erosion and shoreline retreat are apparently not primarily determined by storm frequency and intensity, because periods of higher storminess did not result in more rapid retreat or more general coastal erosion, but appear to be more likely related to occurrences of high water levels and variations in sediment budget. French L'analyse de données anémométriques enregistrées à Dunkerque de 1956 à 2000 a révélé deux périodes de plus forte tempétuosité: 1956-1962 et particulièrement 1972-1977, ces deux périodes totalisant à elles seules 57% des enregistrements de vents de vitesses ≥ 16 m s−1. Bien que la fréquence des tempêtes a été moins élevée pendant les deux dernières décennies du 20ème siècle, une tendance à long terme dans l'activité des tempêtes n'a pas pu être établie à partir de ces 45 années de données de vent caractérisées par d'importantes variations décennales. L'évolution du trait de côte entre 1949 et 2000, déterminée à l'aide de séries de photographies aériennes, a montré une forte variabilité spatio-temporelle et peu de relation avec l'historique des tempêtes. Pendant les années 1960 et 1970, la ligne de rivage a progradé vers le large sur plusieurs sites, bien que les tempêtes aient été nettement plus fréquentes entre 1972 et 1977 que pendant toute autre période entre 1956 et 2000. A l'inverse, la plupart des sites ont connu un recul du trait de côte à la fin des années 1980 et pendant les années 1990 alors que la fréquence des tempêtes a fortement diminué. L'analyse des données marégraphiques recueillies à Dunkerque entre 1956 et 2000 a montré que la fréquence relative des hauts niveaux d'eau ayant pu atteindre le haut de plage et les dunes littorales pendant les tempêtes a augmenté de façon significative après 1983, bien que le nombre annuel de tempêtes a été moindre après cette date; ceci pourrait expliquer le recul plus rapide du trait de côte sur plusieurs sites pendant la fin des années 1980 et les années 1990. A long terme, l'érosion des dunes littorales et le recul de la ligne de rivage ne semblent pas être principalement déterminés par la fréquence et l'intensité des tempêtes, car les périodes de plus forte tempétuosité ne correspondent pas à des périodes de recul plus rapide du trait de côte ou à une érosion plus généralisée. L'évolution du trait de côte paraît plutôt liée à la fréquence des hauts niveaux d'eau et à des variations du bilan sédimentaire.
This study looks at the complex interactions between sedimentology, morphology and hydrodynamics near a kink in an offshore tidal sandbank (Westhinder) and combines all these data to understand the kink's characteristics. The Westhinder sandbank lies in the northern part of the Belgian continental shelf, where the main hydrodynamical agents are the tidal currents. The data sets include two multibeam surveys, 59 surficial Van Veen grab samples and a hydrodynamical and sediment transport model (set up by MUMM). The different data sets point to a triple division of the kink region. The parts north and south of the kink are covered with larger dunes, culminating at the bank's crest into a symmetrical very-large dune. Here, the bank is actively maintained by a net sediment transport up both bank flanks and by higher residual water transport strengths. The kink part of the bank lies deeper, is characterized by a steeper eastern flank and by a faster and eastward movement of the very-large dunes over both bank flanks. Coarse sediments west of the kink reveal that the peak flood current is hindered by the changing orientation of the bank northward into a more rectilinear N–S position. The finest sediments are found on the lee slope and might be washed out from the kink's stoss slope. A combination of all the results points to a possible breakthrough in the kink, a hypothesis which is worked out into a scheme.
Offshore sandbanks provide a major source of sediment and may have a significant impact on the evolution of the nearby shoreline. The prediction of sandbank evolution is extremely important for coastal engineers and managers. However, it is evident from the existing literature that additional investigations into the evolution of offshore sandbanks are needed. The present paper describes qualitative and quantitative investigations of the evolution of the Great Yarmouth sandbanks. A depth-integrated tidal model has been used to calculate the residual currents arising from the sequence of sandbank configurations observed from historic charts over a 150 year period. Tidal simulations were performed with a nested model covering the Southern North Sea and Great Yarmouth zone (east coast of the UK). Simulations were carried out for a period corresponding to several spring-neap cycles to calculate residual currents and were compared directly with field measurements. Model calibration and validation was performed against published tidal elevation charts based on many observations, tidal predictions and field measurements. These detailed comparisons have been used to determine the errors in the model predictions. Overall, the performance of the model is considered good. The model output has been analysed to investigate qualitative links between the sandbank configuration and the residual flow statistics. The results demonstrate that the long-term changes in sandbank morphology can be explained from the pattern of tidal residual currents.