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ESTIMATION OF LONGSHORE AND CROSS-SHORE
SEDIMENT TRANSPORT ON SANDY MACROTIDAL BEACHES
OF NORTHERN FRANCE
ADRIEN CARTIER1,2, ARNAUD HEQUETTE1,2
1.
2.
Laboratoire d’Océanologie et de Géosciences, UMR CNRS 8187 LOG, Université du
Littoral Côte d’Opale, 32 Ave Foch, 62930 Wimereux, France.
Univ Lille Nord de France, F-59000 Lille, France.
[email protected], [email protected]
Abstract:
In this paper, data on longshore and cross-shore sediment transport
were obtained during six field experiments, on three sandy macrotidal barred beaches
of northern France. This study is based on sediment trap experiments, following the
method of Kraus (1987), and completed by wave and current measurements using a
series of hydrodynamic instruments deployed across the intertidal zone. Results showed
that longshore sediment transport increased with both wave height and mean flow, but
no relation was found with wave angle which is probably due to the influence of tidal
currents that interact with wave-induced longshore currents. Cross-shore sediment flux
was generally higher than longshore flux, suggesting that shore-perpendicular sediment
transport associated with wave oscillatory currents probably represents a major factor
controlling the cross-shore migration of intertidal bars.
Introduction
Estimation of sand transport is needed for most coastal engineering designs and
constitutes a major issue for a better understanding of the dynamics of sandy
beaches. Longshore and cross-shore sediment transport are one of the most widely
studied processes in the coastal literature because movement of sand in the
nearshore zone by waves and currents is of fundamental importance to the longterm sediment budget of a stretch of coast and plays a key role in shaping coastline.
Although, a large number of studies have been carried out on this topic, most of
them were focused on measuring sand transport along micro- and mesotidal beaches
(Bayram et al., 2001; Kumar et al., 2003). In comparison, only a few studies have
been conducted on macrotidal beaches (e.g., Davidson et al., 1993; Levoy et al.,
1994; Voulgaris et al., 1998; Corbau et al, 2002) where sediment transport results
from the complex interactions of tidal currents with longshore currents generated by
obliquely incident breaking waves, this complexity being further increased by the
large variations in water level that induce significant horizontal translations of the
surf zone. For a number of years, sediment transport studies were mostly focused on
meso- to macroscale processes and estimated from beach morphology change, for
example, or from fluorescent tracer experiments (Voulgaris et al., 1998). With the
fast evolution of high frequency sensors technology, sediment transport has also
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been determined using optical backscatter sensors (Davidson et al., 1993; Tonk and
Masselink, 2005) or more recently, acoustic backscatter profiler instruments that
can provide high resolution suspended sediment concentration estimates. Proper
calibration of acoustic or optical data is often problematic, however, due to the
presence of organic matter in the water column or to grain size variability (Battisto
et al., 1999). The use of optical or acoustic backscatter sensors for estimating
sediment flux is further complicated in the highly-turbulent surf zone where
breaking waves generate air bubbles that can induce erroneous sediment
concentration estimates (Puleo et al., 2006).
In this paper, we present the results of field measurements carried out on three
macrotidal beaches of northern France for evaluating alongshore and cross-shore
sediment transport in the surf zone under the action of wave and tidally-induced
currents. In order to avoid the problems mentioned above in the turbid and organicrich coastal waters of northern France (Vantrepotte et al., 2007), this study was
based on sediment trap experiments completed by wave and current measurements
using a series of hydrodynamic instruments deployed across the intertidal zone.
Study area
Fig. 1. Location map showing the three study sites, northern France.
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This study has been conducted on three sandy macrotidal barred beaches of northern
France (Figure 1). The first field experiment site (Zuydcoote) is located near the
Belgian border, facing the North Sea; the second site (Wissant Bay) is on the shore of
the Dover Strait, while the third study site (Hardelot) is located on the coast of the
eastern English Channel. Mean sediment size is 0.20 mm at Zuydcoote, 0.22 m at
Wissant and 0.23 mm at Hardelot. Tidal range increases from the Belgium border to
the English Channel, spring tide amplitude ranging from approximately 5 m at
Zuydcoote to nearly 10 m at Hardelot. Due to the large tidal range, tidal currents are
strong along the coast of northern France, and flow almost parallel to the coastline.
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. Wave energy is strongly reduced at the coast due to significant wave
refraction and shoaling over the shallow sand banks of the eastern Channel and
southern North Sea.
Methods
Hydrodynamics measurements were carried out using electromagnetic wave and
current meters (InterOcean S4 ADW and Midas Valeport current meters) and Acoustic
Doppler Current Profilers (ADCP) deployed across the intertidal zone from the lower
to the upper beach (Fig. 2). All instruments operated during 9 minutes intervals every
15 min, providing almost continuous records of significant wave height (Hs), wave
period and direction, longshore (Vl) and cross-shore (Vt) current velocity components,
and mean current velocity (Vm) and direction. Current velocity was measured at
different elevations above the bed depending on each instrument. The S4 and Valeport
current meters recorded current velocity at 0.4 m and 0.2 m above seabed respectively,
while the ADCP measured current velocity at intervals of 0.2 m through the water
column from 0.4 m above the bed to the surface. Current velocity at 0.2 m above the
bed was estimated using the ADCP data by applying a logarithmic regression curve
to the measured velocities obtained at different elevations in the water column.
During one field experiment (Zuydcoote, 2009), one of the ADCP only measured
current velocity.
Longshore and cross-shore sediment fluxes were estimated using streamer traps,
following the method proposed by Kraus (1987). The sediment traps consisted of a
vertical array of five individual streamer traps with 63 µm mesh size sieve cloth that
collected sand-size particles at different elevations above the bed, up to a height of
approximately 1 m. These measurements were used to compute estimates of
suspended sediment transport at discrete elevations above the bed as well as depthintegrated sediment flux. Calculations of the sediment flux from sand traps are clearly
detailed in the paper of Rosati and Kraus (1989). Measurements of longshore sediment
transport with the sediment traps were undertaken during 10 minutes at several
locations across the intertidal zone during rising and falling tides in order to obtain
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estimates of longshore sediment flux from the lower to the upper beach during flood
and ebb (Fig. 2). During three of the six field experiments, the sediment traps were
deployed along two shore-perpendicular transects with a spacing of about 100 m to
investigate the alongshore variability in longshore sediment transport (Figure 2).
During five field experiments, a number of cross-shore sediment transport
measurements were carried out simultaneously with longshore sediment sampling.
During four of these experiments, the sediment traps were oriented for measuring
onshore-directed transport, and during one experiment both onshore- and offshoredirected sediment flux were measured (Table 1). A total of 270 streamer trap
deployments were carried out during the study. As shown in Figure 2, sediment traps
were deployed in the vicinity of the wave and current meters, but also at other
locations across the intertidal zone. Bivariate as well as multivariate statistical analyses
were performed between measured sediment flux and hydrodynamic parameters (e.g.,
significant wave height, mean current velocity…) recorded simultaneously, but only
for sediment transport measurements obtained close to hydrographic instruments.
During each field experiment, beach morphology was surveyed using a very high
resolution Differential Global Positioning System (DGPS) with horizontal and vertical
error margins of ± 2 cm and ± 4 cm respectively.
Two field measurement campaigns were conducted on each study site during
conditions of low to moderate wave activity (Hs ranging from approximately 0.1 to 0.7
m), as well as during spring and neap tides, to cover a range of different hydrodynamic
conditions. Field experiments were conducted in November 2008 (ZY08) and in
November-December 2009 (ZY09) at Zuydcoote, in March 2009 (WI09) and in
March-April 2010 (WI10) at Wissant, and in May-June 2009 (HA09) and JanuaryFebruary 2010 (HA10) at Hardelot.
Fig. 2. Representation of deployment of sediment traps and hydrodynamic instruments during the field
experiments. Arrows represent the direction of measured sediment flux. Numbers refer to the successive
positions of sediment traps during rising (1-5) and falling (6-10) tide.
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Results
Range of longshore and cross-shore transport rates
All the sediment trap measurements showed a classical bottomward increase in
sediment transport, with a high variability in total sediment flux depending on wave
energy and mean current strength. Longshore sediment flux reached values up to 2.1 x
10-1 kg.s-1.m-1 during the ZY09 experiment (Table 1), which was the most energetic
event during which sediment sampling took place. Lower rates of sediment transport
were measured in the vicinity of wave and current meters where sediment flux
nevertheless exceeded 1.6 x 10-1 kg.s-1.m-1 for a mean flow velocity of 0.5 m.s-1. The
maximum onshore sediment flux was observed during the same field experiment,
reaching 1.8 x 10-1 kg.s-1.m-1 (Table 1). Close to the hydrodynamic instruments,
lower onshore transport values were obtained with a maximum rate of about
3.2 x 10-2 kg.s-1.m-1. Offshore-directed sediment transport was also measured, but only
during one experiment (WI10), reaching a maximum of 1.1 x 10-2 kg.s-1.m-1 when a
significant wave height of 0.4 m and a mean flow velocity of only 0.15 m.s-1 were
recorded.
Table 1. Maximum and minimum values of longshore, onshore and offshore sediment transport for each field
experiment (units in kg.s-1.m-1).
Longshore
On shore
Off shore
Max
min
Max
min
Max
Min
ZY08
1.0E-05
3.2E-05
3.9E-03
2.7E-04
-
-
ZY09
2.1E-01*
2.7E-05
1.8E-01
9.1E-05
-
-
WI09
2.8E-02
4.9E-05
2.6E-03
1.9E-04
-
-
WI10
9.4E-04
5.8E-05
8.9E-03
3.7E-04
1.1E-02
2.8E-04
HA09
1.7E-03
1.9E-05*
9.4E-04
7.2E-05
-
-
HA10
8.4E-03
3.9E-04
2.4E-02
3.9E-03
-
-
* Bold characters represent the highest values while italics correspond to the lowest.
Relationship between longshore sediment transport and hydrodynamics
During each experiment, longshore sediment transport increased with both significant
wave height and mean current velocity. This positive relationship appeared extremely
variable, however, from a site to another. Figure 3 represents longshore sediment
transport related to significant wave height for all field experiments. In spite of some
scatter in the data, the trend indicates that sediment transport increases with wave
height. Low sediment transport (< 1.0 x 10-4 kg.s-1.m-1) is mainly associated with small
wave heights (< 0.3 m), but only a small increase in wave height appears to
induce significantly larger sediment transport. Sediment flux generally exceeds
1.0 x 10-3 kg.s-1.m-1 when Hs reaches 0.4 m, representing a ten-fold increase in
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sediment transport for an increase in wave height of approximately 0.2 m. However,
high sediment transport values can also be associated with low wave conditions, such
as during the WI09 experiment when a sediment transport rate of 2.4 x 10-2 kg.s-1.m-1
was measured with a significant wave height of 0.19 m for example (Fig. 3). The
variability in sediment flux values obtained during conditions of equivalent wave
heights, and the fact that similar transport rates may be associated with different wave
heights, even on the same beach and during the same field experiment (e.g., WI09),
suggest that waves do not represent the only factor controlling sediment transport.
Figure 4 shows the relationship between longshore sediment transport and mean
current velocity. Least-square regression analyses show that longshore sediment flux is
better correlated with current velocity than with wave height as revealed by higher
determination coefficients. Low transport rates, which are ranging from 1.0 x 10-4 to
1.0 x 10-5 kg.s-1.m-1, are always associated with low current velocity (< 0.2 m.s-1),
whereas sediment flux is always higher than 1.0 x 10-3 kg.s-1.m-1 when velocity
exceeds 0.4 m.s-1,
Fig. 3. Relationship between longshore sediment transport and significant wave height (Hs) for each field
experiment.
Fig. 4. Relationship between longshore sediment transport and mean current velocity (Vm) at 0.2 m (squares)
and 0.4 m (circles) above the bed.
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Similarly to what was observed with wave heights, high sediment transport rates can
also be observed with lower current velocities. The highest current velocity recorded
during sampling reached 1.29 m.s-1 and was associated with a sediment flux of 7.2 x
10-3 kg.s-1.m-1, but the maximum longshore transport rate measured near a
hydrodynamic instrument peaked at 1.6 x 10-1 kg.s-1.m-1, which is almost two orders of
magnitude higher, for a mean flow of only 0.5 m.s-1(Fig. 4). The range of sediment
transport rates is particularly wide for Vm under 0.2 m.s-1, but with increasing current
speed, the range of values is narrowed, especially above 0.4 m.s-1. This current speed
may correspond to a threshold value above which the transport is dominated by the
mean flow. The higher variability in sediment transport rates observed for current
velocities under 0.4 m.s-1, and especially for velocities less than 0.2 m.s-1, may be
explained by a greater influence at low current velocities of other processes such as
wave oscillatory flows that interact with the mean flow. This variability may also
reflect the lower limit of the sampling method that could barely provide accurate
estimates of sediment flux under low energy conditions.
Longshore sediment flux appeared to be more closely related to the product of
significant wave height and current velocities (Hs*Vm) than to wave height or mean
current velocity alone as shown by the better correlation coefficients obtained (R²0.4m
= 0.46 and R²0.2m = 0.58), suggesting that the observed changes in sediment transport
rates are better explained by the combined action of wave and current rather than by
the action of wave or currents solely acting at the bed. In addition, results of multiple
linear regression analyses using Hs and Vm at 0.2 m and 0.4 m above the bed showed
that mean current velocity accounts for approximately 60% of the variance while wave
height can explain about 30%. Conversely to what was observed in other studies
(Komar and Inman, 1970), our correlation analyses showed no relation between
longshore transport rates and wave angle (R² = 0.20), which is probably due to the
influence of tidal currents that interact with wave-induced longshore currents on these
macrotidal beaches.
Alongshore variability in longshore sediment transport
Figure 5 shows the comparison of longshore sediment flux simultaneously measured
along two shore-perpendicular profiles (P1 and P2) spaced 100 m apart. In most cases,
similar sediment transport rates were measured on both transects, except during the
ZY08 field experiment during which more variability was observed. The consistency
in longshore sediment transport from one transect to the other can be explained by a
high degree of alongshore uniformity in hydrodynamic parameters as the comparison
of wave height and mean current velocity measured on both transects revealed.
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Fig. 5. Comparison of longshore sediment fluxes measured on two shore-perpendicular beach profiles.
Although longshore sediment transport flux was generally equivalent alongshore, large
differences were observed in some occasions due to local variations in beach
morphology. During the WI09 experiment, for example, variations in sediment
transport rates reached 2.4 x 10-2 kg.s-1.m-1 between the two transects. These large
differences in longshore sediment flux were observed during ebb when a sediment trap
was located in the vicinity of a migrating intertidal channel in which flow canalization
associated with runnel drainage locally increased sediment transport.
Comparison between longshore and cross-shore sediment transport
85 cross-shore sediment trap deployments were carried out during longshore sediment
sampling, among which 76 sampled onshore sediment flux and 9 were oriented for
measuring offshore-directed transport. All these measurements were used for
comparing the magnitude of cross-shore sediment flux with simultaneously measured
longshore sediment flux, but only 47 cross-shore transport measurements, which were
realized near hydrodynamic instruments, could be compared with hydrodynamic
parameters.
Onshore sediment transport
Figure 6 shows the comparison between longshore and onshore-directed sediment
transport. Although there is a good correspondence between longshore and onshore
transport, our data show that approximately 70% of onshore sediment fluxes are
higher than longshore sediment fluxes. The differences in sediment transport rates are
generally significant as onshore sediment flux is on average 3 to 4 times higher than
longshore flux and up to 30 times higher.
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Fig. 6. Comparison of longshore and onshore depth-integrated flux for each field experiment.
Although one would expect that onshore sediment transport would be directly related
to wave action and orbital velocities, no relation was found between onshore transport
rate and Hs. Onshore transport rates were also poorly correlated with Vt, indicating a
weak relationship with cross-shore currents. A slightly higher, but still poor correlation
(R2 = 0.2), was obtained with mean longshore current velocity (Vl) measured at 0.4 m
above the bed. Moreover, only 40% of mean cross-shore velocities were greater than
longshore velocities during sediment sampling, while 70% of the onshore fluxes
measured near hydrodynamic instruments were higher than longshore sediment fluxes.
Fig. 7. Example of cross-shore and longshore velocity variations recorded by a Valeport electromagnetic
current meter during a one minute interval (HA09).
These apparent contradictions can be explained by the different timescales
corresponding to the sediment sampling interval, to the frequency rates of acquisition
of hydrodynamic data, or to the length of time during which the processes responsible
for sediment transport operate. Sediment transport rates represent 10-minute timeaveraged values while wave heights and current speeds are mean (or spectral) values
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computed from pressure and velocity components recorded at a frequency of 2 Hz
during 9 minute bursts. As can be seen on Figure 7, representing an example of crossshore and longshore velocity variations recorded during a 1-minute interval, crossshore velocities are characterized by relatively large, high-frequency, onshore-offshore
velocity fluctuations resulting from wave orbital motion. In this example, maximum
cross-shore velocity reaches a value of 0.76 m.s-1 whereas maximum longshore
velocity is only 0.18m.s-1. When averaged over this 1-minute interval, however, both
cross-shore and longshore velocities approximately equal to zero, which obscures the
fact that instantaneous cross-shore velocities are extremely variable and much higher
than longshore velocities.
Fig. 8. Example of differences between onshore (Qon) and longshore (Ql) sediment flux as a function of
significant wave height (Hs) and local beach slope (tanβ), HA09 field experiment. P1 and P2 refer to sampling
transect 1 and 2 respectively.
A number of studies showed that onshore sediment transport in the surf zone is largely
controlled by high-frequency cross-shore velocity variations due to wave-induced
oscillatory currents and by wave breaking processes that contribute to sand
resuspension (e.g., Davidson et al., 1993; Austin et al., 2009), which are all dependent
of beach slope. Beach slope (tanβ) was surveyed at the sediment sampling sites during
each sediment trap deployment and the dominance of onshore or longshore transport
was determined by subtracting longshore sediment flux from onshore sediment flux
(Qon – Ql) where Qon is the onshore sediment flux and Ql is the longshore flux. The
data collected during several experiments suggest that onshore transport tends to
exceed longshore transport with increasing Hs and tanβ (Figure 8). These results show
that shoreward-directed sediment transport increases with wave energy, but also
suggest that the relatively steeper slopes associated with the stoss side of intertidal bars
may cause a rapid increase in shoaling wave height, which can favour asymmetrical,
onshore-directed, wave oscillatory flows and hence onshore sediment transport.
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Offshore sediment transport
Offshore sediment fluxes were measured during only one experiment (WI10),
simultaneously with longshore and onshore sediment transport, representing a total of
9 seaward-directed transport measurements. In all cases but one, cross-shore sediment
flux, which could be either onshore- or offshore-directed, was higher than the
longshore flux (Figure 9a). Offshore sediment transport rates were always higher than
longshore transport rates, offshore sediment transport being on average approximately
7 times greater than longshore fluxes.
Fig. 9. Comparison of longshore sediment flux with (a) onshore (Qon), offshore (Qoff), and (b) net cross-shore
(Qnet) sediment flux during the WI10 experiment.
However, offshore-directed transport was sometimes higher or lower than onshore
transport, which is likely due to differences in orbital velocity asymmetries of
shoaling and breaking waves across the intertidal zone. Non-directional net crossshore sediment flux (Qnet) was estimated as:
Qnet =
(Qon − Qoff ) 2
Qon − Qoff
(1)
The results show that even net cross-shore sediment transport was generally higher
than longshore transport (Figure 9b), revealing that under the hydrodynamic
conditions encountered during this field experiment significant quantities of sand were
transported across the beach.
Conclusion
Assessing the role of controlling hydrodynamic processes on longshore and crossshore sediment transport in the breaker and surf zone of macrotidal beaches appeared
to be difficult to determine due to the complex interactions between several forcing
2141
parameters. In addition, several physical constraints did not allow us to measure
sediment transport with high-frequency sensors, but using streamer traps that provide
time-averaged transport rates. Moreover, for safety reasons, sand trapping was not
conducted during high wave energy conditions (Hs > 1m).
During the low to moderate energy conditions that characterized our sand trap
measurements, longshore sediment transport proved to be dependent on both
significant wave height and mean current velocity (Figures 3 and 4), but appeared to
be mainly controlled by the mean flow, especially beyond a velocity of 0.4 m.s-1.The
lack of relationship between wave angle and sediment flux shows that the wave
energetic approach is not adapted for estimating sediment transport on macrotidal
beaches where tidal currents can modulate the magnitude of horizontal sediment
transport. Although, longshore sediment flux can be very variable across the intertidal
zone, sediment transport rates showed a low variability alongshore due to a high
degree of longshore hydrodynamic uniformity. Higher sediment transport rates can
locally be observed, however, near drainage channels associated with intertidal bartrough beach topography.
It is generally accepted that sediment transport in the southern North sea and Dover
strait is strongly controlled by shore parallel tidal currents, resulting in net longshore
sediment transport on the shoreface (Héquette et al., 2008) and in the intertidal zone
(Sedrati and Anthony, 2007). Our results show that onshore sediment transport in the
surf zone can also be higher than longshore sediment transport. Based on fluorescent
tracer experiments carried out on a macrotidal beach of Belgium, Voulgaris et al.
(1988) also suggested that onshore transport may be higher than longshore transport,
but this was restricted to the swash-dominated upper beach. The fact that macrotidal
beaches appear to be either dominated by longshore- or cross-shore sediment transport
can probably be explained by the differences in spatial and temporal scales associated
with various measurement techniques used in different studies. Fluorescent tracers, for
example, can be used for assessing residual sediment transport across the intertidal
zone during a complete tidal cycle while streamer trap measurements provide timeaveraged estimates of local sediment flux over a time interval of only a few minutes.
Although long-term residual sediment transport may be longshore-dominated on the
macrotidal beaches of northern France, our results showing that important shoreperpendicular sediment transfer takes place across the beach at short timescales,
suggesting that cross-shore transport may have been sometimes overlooked in these
coastal environments. At near instantaneous timescale, cross-shore sediment transport
associated with wave-induced oscillatory currents probably represents a major factor
controlling the cross-shore migration of intertidal bars.
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Acknowledgements
This study was funded by the French Centre National de la Recherche Scientifique
(CNRS) through the PLAMAR Project of the Programme “Relief de la Terre”. The
authors would like to thank all the students and personnel for their help during the
field experiments.
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