Journal of Coastal Research
SI 56
59 - 63
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
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 : [email protected]
ABSTRACT
HÉQUETTE, A., RUZ, M.H., MASPATAUD, A. and SIPKA, 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, ISSN 0749-0258.
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.
ADITIONAL INDEX WORDS: Tidal sand bank, macrotidal coast, North Sea
INTRODUCTION
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 (LANCKNEUS
et al., 1994; TESSIER 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., DYER and HUNTLEY, 1999; BERNÉ
et al., 1994; DELEU et al., 2004; HORRILLO-CARABALLO and
REEVE, 2008), only a few studies have been conducted on the
influence of nearshore banks on shoreline behaviour (e.g., SHAW
et al., 2008) even if it is generally considered that these nearshore
sand bodies may have important effects on coastal hydrodynamics
and shoreline evolution (MCDONALD and O’CONNOR, 1996:
CORBAU 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 bartrough morphology being typical of the macrotidal coast of
northern France (REICHMÜTH and ANTHONY, 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-1 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 (HÉQUETTE et
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 (AUGRIS 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
59
Effects of nearshore sand bank on beach hydrodynamics
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.
may episodically exceed heights of 4 m during storms (DELFT
HYDRAULIC, 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.
METHODS
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 (Hs), 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
(AUGRIS 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.
RESULTS
Several wind events occurred during the experiment. Winds
essentially came from southwest to northwest with speeds
generally ranging from 9 to 10 m s-1. 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-1. 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.
Journal of Coastal Research, Special Issue 56, 2009
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Héquette et al.
Figure 2 Time-series of significant wave height (Hs) 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.
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 Hs 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 ) 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
loss.
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 eastwardflowing 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-1 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 floodcurrents 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-1 during
several tides whilst ebb current velocities never exceeded 0.4 m s-1.
DISCUSSION
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 ANTHONY et al. (2006) and
CHAVEROT 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
(CHAVEROT 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 (HÉQUETTE et al., 2008).
In their study of a massive headland-associated nearshore sand
bank off the coast of prince Edward Island, Canada, SHAW 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 threedimensional 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 headlandassociated 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 20th century (AERNOUTS
and HÉQUETTE, 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, CORBAU 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
Journal of Coastal Research, Special Issue 56, 2009
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Effects of nearshore sand bank on beach hydrodynamics
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 (CORBAU et
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 onshoredirected transport and are commonly associated with foredune
accretion (AAGAARD et al., 2004; COOPER and NAVAS, 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).
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.
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, REICHMÜTH and ANTHONY (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
REICHMÜTH and ANTHONY (2002), but this would have no effects
CONCLUSION
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.
Journal of Coastal Research, Special Issue 56, 2009
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Héquette et al.
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ACKNOWLEDGEMENTS
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
figures.
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