Journal
Journalof
ofCoastal
CoastalResearch
Research
SI 64
pg
45 -- pg
49
ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Variation in longshore sediment transport under low to moderate
conditions on barred macrotidal beaches
A. Cartier†‡ and A. Héquette†‡
† Laboratoire d’Océanologie et de Géosciences,
‡ Univ Lille Nord de France, F-59000
UMR CNRS 8187 LOG, Université du Littoral Côte Lille, France.
d’Opale, 32 Ave Foch, 62930 Wimereux, France
[email protected]
[email protected]
ABSTRACT
Cartier, A. and Héquette, A., 2011. Variation in longshore sediment transport under low to moderate conditions on
barred macrotidal beaches. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal
Symposium), – . Szczecin, Poland, ISSN 0749-0208.
The aim of this study was to estimate longshore sediment transport on three sandy barred macrotidal beaches of
Northern France using sediment traps. Measurements of longshore sediment transport were carried out at several
locations across the intertidal zone during rising and falling tides in order to obtain estimates of longshore
sediment flux from the lower to the upper beach and coupled with high-frequency (2 Hz) hydrodynamic data using
a series of hydrodynamic instruments. 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. Although, limited variation of longshore sediment
transport was observed, cross-shore variation showed that sediment fluxes were higher on the middle to lower
beach, which can be explained by a decrease in tidal flow velocity towards the upper beach as well as wave-energy
dissipation over the beach and intertidal bars. Longshore sediment transport direction appeared to be controlled by
both incoming wave and tidal current direction, depending on the hydrodynamic zones, sediment transport being
tidally-dominated in the shoaling zone and mostly wave-induced in the inner surf zone.
ADITIONAL INDEX WORDS: Sediment traps, Hydrodynamics, Northern France
INTRODUCTION
Characterization of longshore sediment transport (LST) and its
alongshore and cross shore distribution across the beach are
fundamental to the understanding of coastal morphodynamics and
to the effective design of coastal engineering. Although longshore
sediment transport processes have been extensively studied, most
studies were conducted in micro- to meso-tidal environments, and
comparatively only a few papers are specifically concerned with
longshore sediment transport on macrotidal beaches (e.g.,
Davidson et al., 1993; Levoy et al., 1994; Masselink and
Pattiaratchi, 2000). It has long been recognized from a large
number of field and laboratory experiments that variation of
longshore sediment transport is strongly controlled by longshore
currents generated by waves breaking at an angle with the
shoreline (e.,g., Komar and Inman, 1970; Kamphuis, 1991). In
macrotidal coastal environments, additional factors affect
sediment transport, which also depends on the interactions of tidal
currents with wave oscillatory flows and longshore currents
generated by obliquely incident breaking waves (Davidson et al.,
1993; Cartier and Héquette, 2011).
For a number of years, sediment transport studies were mostly
focused on meso- to macroscale processes and estimated from
beach volume change near coastal engineering structures such as
jetties or groins that block littoral drift, for example, or from
fluorescent tracer experiments (Voulgaris et al., 1998). During the
last decades, with the development of high frequency sensors
technology, sediment transport has also 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. Accurate calibration of acoustic or optical
data is often problematic in natural environments, however, due to
the presence of organic matter in the water column or to grain size
variability (Battisto et al., 1999)
In this paper, we present the results of field measurements
carried out on three macrotidal beaches of northern France for
evaluating alongshore sediment transport in the intertidal zone
under the action of wave and tidally-induced currents. In order to
avoid the problems mentioned above in the turbid and organic-rich
coastal waters of northern France (Vantrepotte et al., 2007), this
study was based on sediment trap experiments. Wave and current
measurements were also obtained during these experiments, using
a series of hydrodynamic instruments deployed across the
intertidal zone.
STUDY AREA
This study has been conducted on three sandy barred beaches of
northern France from November 2008 to March 2010. The first
field experiment site (Zuydcoote, ZY) is located near the Belgian
border, facing the North Sea; the second site (Wissant Bay, WI) is
on the shore of the Dover Strait, while the third study site
(Hardelot, HA) is located on the coast of the eastern English
Channel (Figure 1). Mean sediment size is 0.20 mm at Zuydcoote,
0.22 mm at Wissant and 0.23 mm at Hardelot. The field sites were
selected based of their tidal range, which increases from
approximately 5m at Zuydcoote to nearly 10m at Hardelot. The
Journal of Coastal Research, Special Issue 57, 2011
45
1
Longshore sediment transport on barred macrotidal beaches
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.
Streamer Traps
LST rates were estimated using streamer traps similar to the
original design of 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 1m.
These measurements were used to compute estimates of
suspended sediment transport at discrete elevations above the bed
as well as depth-integrated sediment flux. Calculations of the
sediment flux from sand traps were carried out according to the
procedure of Rosati and Kraus (1989) (Eq.1):
N
N −1
i =1
i =1
F = h∑ F (i ) + ∑ a(i ) * FE (i )
Figure 1. Location of experimental sites
northern coast of France is affected by semi diurnal tides
responsible for relatively strong tidal currents that flow almost
parallel to the shoreline in the coastal zone. The dominant wave
directions in the region are 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. Because of the flood-dominated asymmetry of the tidal
currents, which may be reinforced by the prevailing southwesterly
winds, the net regional sediment transport in the coastal zone is
directed northward along the eastern Channel coast and toward the
east-northeast along the southern North Sea coast (Sedrati and
Anthony, 2007; Héquette et al., 2008).
(1)
Where F is the depth integrated flux in kg.s-1.m-1, h is the height
of the streamer opening in meters, F(i) is the sediment flux at a
streamer I, a(i) is the distance between neighboring streamers,
FE(i) is the sediment flux between neighboring streamers and N is
the total number of streamers.
The sediment traps were deployed along two shoreperpendicular transects with a spacing of about 100 m to
investigate the alongshore variability in longshore transport rate.
Measurements of longshore sediment transport were carried out at
several locations across the intertidal zone (stoss side of intertidal
bars and in the runnels) during rising and falling tides in order to
obtain estimates of longshore sediment flux from the lower to the
upper beach (Figure 2). Although the sediment traps were usually
deployed in similar water depths, ranging from 0.8 to 1.4 m,
METHODOLOGY
Hydrodynamic instrumentation
Three different hydrographic instruments were used during the
field experiments. One Acoustic Doppler Current Profiler
(ADCP), and two electromagnetic wave and current meters
(Midas Valeport©, and InterOcean ADW S4) were deployed
across the intertidal zone from the lower to the upper beach. The
instruments were either deployed on the stoss side of intertidal
bars (ridge) or in troughs (runnel) between the bars (Figure 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 current velocity (Vl),
and mean current velocity (Vm) and direction. Current velocity was
measured at different elevations above the bed depending on each
instrument. The ADW 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
Figure 2. Field methodology. Numbers on the left side refer to
the successive positions of sediment trap during rising (1-5) and
falling (6-10) tide. Codes on the right side refer to beach
morphology where UB and LB are Upper and Lower Beach
respectively; R and T correspond to ridges and troughs.
sediment transport measurements took place in various
hydrodynamic zones including shoaling, breaker and surf zones,
depending on wave activity during sampling. For safety reasons,
sand transport measurements were conducted only under low to
moderate wave energy conditions (Hs max ≅ 0.70m).
Journal of Coastal Research, Special Issue 57, 2011
46
2
2
Cartier and Héquette
Cartier and Héquette
RESULTS
RESULTS
Comparison of LST with hydrodynamic data
Comparison
of LST with
Two field experiments
were hydrodynamic
conducted at each data
site between
Twoand
field
experiments
at each site
between
2008
2010,
involving were
more conducted
than 700 sediment
samples
that
2008
involving186
more
than 700 sediment
samples
that
were and
used2010,
to compute
depth-integrated
sediment
fluxes.
were
used
to
compute
186
depth-integrated
sediment
fluxes.
Among these, 79 depth-integrated sediment fluxes were computed
Among
these, 79trap
depth-integrated
using
sediment
measurements sediment
obtained fluxes
close towere
the computed
wave and
using sediment trap measurements obtained close to the wave and
sediment flux values obtained during conditions of equivalent
sedimentvelocities
flux values
obtained
of that
equivalent
current
or wave
heightsduring
(Fig. 3),conditions
and the fact
similar
current velocities
or wave
heights (Fig.
and thewave
fact that
similar
transport
rates may
be associated
with3),
different
or current
transport rates
may
associated
withcan
different
or current
conditions,
imply
thatbevariations
of LST
not be wave
explained
by the
conditions,
implyorthat
variations
LST but
canrather
not beby
explained
by the
action of waves
mean
current of
alone,
a combination
action
of waves
current
alone, but
rather by a Least-square
combination
of
these
and/oror mean
by other
forcing
mechanisms.
of these and/or
other forcing
mechanisms.
regression
analysesbynevertheless
revealed
a better Least-square
relationship
regression
analyses sediment
nevertheless
a better
relationship
between longshore
fluxrevealed
and mean
longshore
current
between comparatively
longshore sediment
flux height,
and mean
longshore
current
velocity,
with wave
showing
that longshore
velocity,
with wave
height,
showing
that longshore
transport comparatively
strongly depends
on the
strength
of the
transport which
strongly
on induced
the strength
of the
longshorea
current,
can depends
be tidally
but more
commonly
current,
whichof can
be tidally induced
but more
combination
wave-generated
and tidal
flows.commonly
Results ofa
combination
wave-generated
flows.
of
Vm atResults
0.2 m and
multiple
linearofregression
analyses and
usingtidal
Hs and
Vm at 0.2
m and
multiple
lineartheregression
analyses
usingcurrent
Hs andvelocity
0.4
m above
bed showed
that mean
accounts
0.4
above the bed
showed
mean current
accounts
for m
approximately
60%
of thethat
variance,
while velocity
wave height
can
for approximately
explain
about 30%. 60% of the variance, while wave height can
explain
aboutLST
30%.commonly shows a relationship with the wave
Although,
Although,
LST
commonly
shows a1970;
relationship
with 1991),
the wave
breaking
angle
(Komar
and Inman,
Kamphuis,
no
breaking was
angle
(Komar
and Inman,
1970;
Kamphuis,
no
relation
found
between
LST and
wave
angle (R²1991),
= 0.20),
relation
found
LST andofwave
angle (R²
0.20),
which is was
probably
duebetween
to the influence
tidal currents
that=interact
whichwave-induced
is probably due
to the influence
ofthese
tidal macrotidal
currents that
interact
with
longshore
currents on
beaches.
with wave-induced longshore currents on these macrotidal beaches.
Alongshore variability of LST
Alongshore
of sediment
LST fluxes were sampled on
A total of 60 variability
depth-integrated
Figure 3. Relationship between LST and (A) significant wave
Figure 3. Relationship between LST and (A) significant wave
height (Hs) for each field experiment, and (B) mean current
height
each
field experiment,
(B) above
mean the
current
velocity(H
(Vs)m)for
at 0.2
m (squares)
and 0.4 m and
(circles)
bed.
velocity (Vm) at 0.2 m (squares) and 0.4 m (circles) above the bed.
current meters, allowing a comparison between LST and
current
meters,
allowing a comparison between LST and
hydrodynamic
parameters.
hydrodynamic
parameters.
Our results showed
a classical bottomward increase in sediment
Our results
classical bottomward
increase
sediment
transport,
withshowed
a high avariability
in total sediment
fluxindepending
transport,
with aand
high
variability
total sediment
depending
on wave energy
mean
currentin
strength.
In spite flux
of some
scatter
on wave
and mean current
strength. Insediment
spite of some
in
the energy
data, comparison
of measured
fluxscatter
with
in
the data, comparison
sediment
flux sites
with
hydrodynamic
data obtainedof atmeasured
the sediment
collection
hydrodynamic
data obtained
the sediment
collection
sites
showed
that sediment
transportat increased
with both
significant
showed
that and
sediment
transport
increased
with3Aboth
wave
height
mean current
velocity
(Figure
and significant
3B). High
wave height
mean
current
velocity
(Figure
3Avelocity
and 3B).values
High
height and
values
(>0.4
m) and
strong
current
wave
(>0.4 m)associated
and strongwith
current
were always
highvelocity
transportvalues
rate
(>
0.4 height
m.s-1) values
-3 -1) were
associated
with high
transport
(> 10.4
m.s
kg.s-1.m-1)always
while the
lowest sediment
fluxes
(< 1 x rate
10x 10
-1
4 1 x-110-3
.m-1) while related
the lowest
sediment
fluxes (< 1(<x 0.3
10(>
kg.s .m-1)kg.s
are exclusively
to low
wave amplitude
-1
-1
4
-1
kg.s
.m
)
are
exclusively
related
to
low
wave
amplitude
(<
0.3
m) and weak mean flow (< 0.2 m.s ). The range of sediment
-1
).m under
The range
of-1,sediment
m) and weak
flow (<wide
0.2 for
m.sV
0.2 m.s
but with
transport
rates ismean
particularly
-1
0.2 m.s
, but with
transport rates
is particularly
widerange
for Vmofunder
increasing
current
speed, the
values
is narrowed,
increasing
current
the current
range speed
of values
is narrowed,
may correspond
to a
especially above
0.4 speed,
m.s-1. This
-1
. Thisthe
current
speedismay
correspond
especially
above above
0.4 m.swhich
threshold value
transport
dominated
by to
thea
threshold
mean flow.value above which the transport is dominated by the
mean
flow. LST can reach high values during low wave and
However,
However,
LST can The
reach highest
high values
low wave
current
conditions.
flux during
obtained
near and
the
-1
current
conditions.
The(1,6highest
flux-1.mobtained
near was
the
), for example,
hydrodynamic
insruments
x 10-1 kg.s
-1
-1
-1
kg.s
), for example,
was
hydrodynamic
(1,6 of
x 100.5
the strongest
measured
for insruments
a mean flow
m.s-1.m while
-1
measured current
for a mean
flow(1.29
of 0.5
) is while
relatedthe
to strongest
a lower
recorded
velocity
m.s-1m.s
-1
-3
-1
-1
) is3B).
related
to a lower
recorded
current
.m )m.s
(Figure
The variability
in
sediment flux
(7.2 velocity
x 10 kg.s(1.29
sediment flux (7.2 x 10-3 kg.s-1.m-1) (Figure 3B). The variability in
A perpendicular
total of 60 depth-integrated
fluxes
were100
sampled
on
two
beach profilessediment
(P1 and P2)
spaced
m apart.
two perpendicular
beach profiles
(P1 variation
and P2) spaced
100 m
Our
results show limited
alongshore
as revealed
byapart.
high
Our
show limited
alongshore
as revealed
by high
linearresults
regression
coefficients,
exceptvariation
during one
field experiment
linear regression
coefficients,
one (Figure
field experiment
during
which more
variabilityexcept
was during
observed
4). The
during
which
morefrom
variability
was toobserved
4). The
consistency
in LST
one transect
the other (Figure
can be explained
consistency
LST from
one transectuniformity
to the otherofcanhydrodynamic
be explained
by
a high indegree
of alongshore
by a highas degree
of hydrodynamic
processes
revealedofby alongshore
simultaneousuniformity
measurements
of waves and
processes
revealed
by simultaneous measurements of waves and
currents onasboth
transects.
currents
on both
transects.sediment transport flux was generally
Although
longshore
Althoughalongshore,
longshore significant
sediment transport
generally
equivalent
differencesflux
werewasobserved
in
equivalent
alongshore,
significantin differences
were observed
in
some
occasions
due to variations
beach morphology,
which can
some occasions
to variations
in beach
morphology,
which
can
result
in a localdue
increase
in current
velocity.
This was
notably
result
in aduring
local ebb
increase
velocity.
Thislocated
was notably
observed
whenina current
sediment
trap was
in the
observedofduring
ebb when
a sediment
wasflow
located
in the
vicinity
a migrating
intertidal
channel intrap
which
canalization
vicinity of a migrating intertidal channel in which flow canalization
Figure 4. Comparison of LST between two shore-perpendicular
Figure
4. Comparison
of LST between two shore-perpendicular
perpendicular
beach profiles.
perpendicular beach profiles.
associated with runnel drainage locally increased
associated
runnel
increasedin
transport. Inwith
one of
these drainage
occasions, locally
the differences
transport. rates
In onereached
of these2.4occasions,
the -1differences
in
.m-1 between
transport
x 10-2 kg.s
-2
-1 -1
transport
transects. rates reached 2.4 x 10 kg.s .m between
transects.
sediment
sediment
sediment
the two
the two
Cross shore variation of LST
Cross
shore
variation
LST was
measured
duringof28LST
days during which 103 sediment
LST
was measured
28 out
daysduring
duringfalling
whichtide,
103 while
sediment
trap
deployments
wereduring
carried
83
trap deployments were carried out during falling tide, while 83
Journal of Coastal Research, Special Issue 57, 2011
Journal of Coastal Research, Special Issue 57, 2011
47
3
Longshore sediment transport on barred macrotidal beaches
Figure.5: Example of depth-integrated longshore sediment flux and wave and current measurements during the June 2009 Hardelot
field experiment (HA09). Circles with letters and numbers correspond to a code for beach morphology (see Fig. 2). Arrows, on the left
graph, show the direction of sediment transport; time of sampling is indicated on the left of y-axis.
deployments took place during rising tide. During each falling or
rising tide, the higher sediment transport rates were generally
obtained on the middle to the lower beach (R2 to R3), while the
lower sediment transport rates were measured either on the upper
beach (UB to R1) or on the lower beach (LB) (Figure 5), mostly
during slack water conditions. Our cross-shore measurements also
showed that more than 80% of the highest sediment fluxes were
sampled on the intertidal bars and less than 20% were measured in
inter-bar troughs.
Similarly to what was observed with sediment transport, current
velocity generally increased from the upper to the lower beach. As
shown on Figure 5, mean current velocity recorded during flood
and part of the ebb was significantly higher on a bar located on the
middle to lower part of the beach (R2) compared to velocities
measured on a bar on the upper beach (R1) (Figure 5). This can be
explained by a decrease in tidal flow velocity towards the upper
beach. Simultaneous hydrodynamic measurements by different
instruments deployed across the intertidal zone also show that
significant wave height tends to decrease toward the upper beach
(Figure 5) due to wave-energy dissipation over the beach and
intertidal bars. Analyses of the influence of beach slope on
sediment transport also suggest that the relatively steeper slopes
associated with the stoss side of intertidal bars may cause a rapid
increase in shoaling wave height and locally induce higher waves,
which could also explain the high transport rates on the intertidal
bars (Cartier and Héquette, 2011).
Factors controlling the direction of LST
In approximately 80% of cases, sediment transport was directed
eastward on the beaches facing the North Sea (Zuydcoote) or the
Dover Strait (Wissant) or northward on the beach located on the
eastern Channel coast (Hardelot). This is to some degree due to
the flood-dominated asymmetry of the tidal currents that is
responsible for higher flood current velocities that are directed
eastward on the northern coast and northward along the eastern
Channel coast (e.g., Fig. 5). In addition, waves mostly originated
from the NW during the field experiments conducted at Zuydcoote
and Wissant and from the SW during the experiments carried out
at Hardelot, which would also favor an eastward-directed transport
at the northern sites and a northward transport at the east Channel
site.
As shown on Figure 5, sediment transport directions are
consistent with the direction of the mean flow that is either
flowing northward or southward depending on the phase of the
tide. Because wave agitation was limited during the HA09
experiment sediment transport was essentially tidally-induced,
sediment being transported in the direction of the tidal current. In
other circumstances, however, our data show that sediment can be
transported in a different direction. Almost 60% of the sediment
transport directions were consistent with expected tidal current
directions, but about 40% appeared to be in the opposite direction.
This can be partly explained by the fact that tidal current reversal
does not occur at high or low tide, but typically after a delay of 2
to 3 hours due to the existence of stationary and progressive tidal
waves in this region (Héquette et al., 2008), but also by variable
wave influence on sediment transport processes depending on the
wave energy level in the intertidal zone. During low wave energy
conditions (<0.20m), streamer traps were not located in the surf
zone, but rather in the shoaling zone where currents are only
slightly generated by breaking waves. In this zone, sediment
transport typically shows regularly reversing, tidally-induced,
shore-parallel directions. Conversely, during higher wave energy
conditions (>0.40m), sand trapping took place in or close to the
breaker zone or in the surf zone, where the direction of residual
sediment transport is strongly controlled by the direction of
incoming waves. During such conditions, longshore sediment
transport can be attenuated or strengthened by tidal currents,
depending on the direction of the tidal flow.
DISCUSSION AND CONCLUSION
Evaluation of LST in the intertidal zone of macrotidal beaches
appeared to be difficult to determine due to the complex
interactions between several forcing parameters. In addition,
several physical constraints (high organic content and fine-grained
suspended sediments) did not allow us to measure sediment fluxes
with high-resolution acoustic or optical sensors, but using
Journal of Coastal Research, Special Issue 57, 2011
48
2
4
Cartier and Héquette
Cartier and Héquette
RESULTS
streamer traps that can only provide time-averaged transport rates
instead of high-frequency suspended sediment transport estimates.
Comparison
of LST
with hydrodynamic
data that
Under the low
to moderate
hydrodynamic conditions
Two field our
experiments
were conducted
at each
sitecontrolled
between
characterize
field measurements,
LST proved
to be
2008both
and significant
2010, involving
thanand
700mean
sediment
samples
that
by
wavemore
height
current
velocity
were
used
to
compute
186
depth-integrated
sediment
fluxes.
(Figures 3). Our data showed that sediment transport was mainly
Among these,
79 mean
depth-integrated
sediment
fluxes
were computed
dependent
on the
flow, especially
above
a velocity
threshold
using
sediment trap0.4measurements
obtained
close toalso
the show
wave that
and
our results
of
approximately
m.s-1. Although
wave action increases sediment remobilization, the absence of
relationship between wave angle and sediment flux shows that the
evaluation of LST rates based on longshore wave energy flux
(e.g., Komar and Inman, 1970; Smith et al., 2009) is not suitable
for estimating longshore sand transport on macrotidal beaches
where tidal currents can modulate the magnitude of longshore
sediment transport
Although sediment transport rates showed a low alongshore
variability, owing to a high degree of longshore hydrodynamic
uniformity, longshore sediment flux were much more variable
across the intertidal zone due to cross-shore variations in
hydrodynamic processes and beach morphology. The higher sand
transport rates were principally measured on the stoss side of
intertidal bars where the relatively steeper slopes induce a rapid
increase of shoaling wave heights, leading to more resuspension of
sand that is carried away by the mean alongshore current.
Longshore sediment flux is also characterized by a net increase in
the middle to lower beach, which can be explained by a decrease
in tidal flow velocity towards the upper beach and by wave energy
dissipation across the intertidal zone. Wave dissipation is more
significant during higher wave conditions (Hs > 0.4 m) and at high
tide when waves propagate over the entire intertidal zone. During
low wave energy conditions, the width of the surf zone is reduced
and beyond the breaker zone, LST appears to be essentially driven
by the tidal flows that are generally responsible for low transport
rates
less efficient
sediment
resuspension.
Inside thewave
surf
Figuredue
3. to
Relationship
between
LST
and (A) significant
zone, however, sand transport is wave-dominated and transport
height (Hlargely
each field
experiment,
(B) mean
current
s) for depends
direction
on the
direction and
of incoming
waves
and
velocity
(V
)
at
0.2
m
(squares)
and
0.4
m
(circles)
above
the
bed.
m
on the angle between the wave crest and the shoreline, which
determines the longshore current direction.
Severalmeters,
authors allowing
described significant
sand transport
current
a comparison
between asymmetry
LST and
on
macrotidal parameters.
beaches (Davidson et al., 1993; Masselink and
hydrodynamic
Pattiaratchi,
Corbau
et al., bottomward
2002). This transport
asymmetry
Our results2000;
showed
a classical
increase in
sediment
was
attributed
the combined
a larger flux
bed depending
roughness
transport,
with to
a high
variability effect
in totalofsediment
and
stronger
nearshore
during
theof falling
tide,
on wave
energymean
and mean
current flows
strength.
In spite
some scatter
inducing
highercomparison
sediment resuspension
ebb than
in the data,
of measuredduring
sediment
fluxduring
with
flood
(Masselinkdata
and obtained
Pattiaratchi,
Although,
tidal currents
hydrodynamic
at 2000).
the sediment
collection
sites
in
the coastal
zone of northern
France
are characterized
by a clear
showed
that sediment
transport
increased
with both significant
flood-dominated
asymmetry,
a tidal (Figure
asymmetry
in 3B).
sediment
wave height and mean
current velocity
3A and
High
transport
could
not(>0.4
be observed
in our
measurements.
The
wave height
values
m) and strong
current
velocity values
variability
longshore
transport
on transport
the surveyed
) were
alwayssediment
associated
with high
rate
(> 0.4 m.s-1in
-3
beaches
to-1)bewhile
essentially
related
to variations
kg.s-1.m
the lowest
sediment
fluxes (<in1 wave
x 10(> 1 x 10appears
-1 mean flow velocity, independently of rising or falling
4
energy
and
kg.s-1.m
) are exclusively related to low wave amplitude (< 0.3
tide
conditions.
m) and
weak mean flow (< 0.2 m.s-1). The range of sediment
transport rates is particularly wide for Vm under 0.2 m.s-1, but with
increasing currentLITERATURE
speed, the rangeCITED
of values is narrowed,
current
speed may
especially
above
0.4 Friedrichs,
m.s-1. This C.
Battisto,
G. M.,
T., Miller,
H. C.correspond
and Resio,toD.a
threshold
aboveofwhich
is dominated
by the
T.,
1999. value
Response
OBS the
to transport
mixed grain-size
suspensions
mean flow.
during
Sandyduck’97.Coastal sediment 99 (Long Island, New
However,
can reach high values during low wave and
York),
1, pp. LST
297-312.
current
conditions.
The highest
flux
obtained
near the
Cartier, A. and Héquette,
A., 2011
(in press).
Estimation
of
kg.s-1.m-1on
), sandy
for example,
was
hydrodynamic
insruments (1,6
x 10-1transport
longshore
and cross-shore
sediment
macrotidal
-1
while Sediments
the strongest
measuredof for
a mean
flowProceedings
of 0.5 m.s Coastal
beaches
northern
France.
'11,
recordedFlorida,
current2-6velocity
(1.29 m.s-1) is related to a lower
Miami,
May
2011.
-3
-1
-1
.m ) (Figure
3B).Ferreira,
The variability
in
sediment
fluxC.,(7.2
x 10 kg.s
Corbau,
Ciavola,
P., Gonzalez,
R. and
O., 2002.
Measurements of Cross-Shore Sand fluxes on a Macrotidal Pocket
sediment flux values obtained during conditions of equivalent
Beach
Beach,
Atlantic
current(Saint-Georges
velocities or wave
heights
(Fig.Coast,
3), andSW
theFrance).
fact thatJournal
similar
of
Coastalrates
Research,
SI36,
182-189.with different wave or current
transport
may be
associated
Davidson,
M.that
A.,variations
Russell, of
P.,LST
Huntley,
Hardisty,
J.,
conditions,
imply
can notD.beand
explained
by the
1993.
in suspended
sand by
transport
on a
action ofTidal
wavesasymmetry
or mean current
alone, but rather
a combination
macrotidal
intermediate
beach.
Marinemechanisms.
Geology. 3-4,
110, 333of these and/or
by other
forcing
Least-square
353.
regression analyses nevertheless revealed a better relationship
Héquette,
A., Hemdane,
Y. and mean
Anthony,
E. J., current
2008.
between
longshore
sediment flux
longshore
Sediment
transport under
wave
andheight,
currentshowing
combined
on a
velocity, comparatively
with
wave
thatflows
longshore
tide-dominated
shoreface,
coast of ofFrance.
Marine
transport strongly
depends northern
on the strength
the longshore
Geology.
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can be tidally induced but more commonly a
Kamphuis,ofJ.W.,
1991. Alongshore
sediment
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combination
wave-generated
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flows.
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above the bed showed that mean current velocity accounts
P. D. 60%
and ofInman
D. L., 1970.
Longshore
for Komar,
approximately
the variance,
while wave
heightsand
can
transport
on beaches.
explain about
30%. Journal of Geophysical Research. 30, 55145527.
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Kraus,angle
N. C.,(Komar
1987. Application
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traps for1991),
obtaining
breaking
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LSTtransport
and wave
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(R²surf
= 0.20),
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ACKNOWLEDGEMENTS
Figure
4. Comparison
of LST
between
shore-perpendicular
This study
was funded
by the
Frenchtwo
Centre
National de la
perpendicular
beach
profiles.
Recherche Scientifique (CNRS) through the PLAMAR Project of
the Programme “Relief de la Terre”. The authors would like to
thank all thewith
students
and personnel
their help
during sediment
the field
associated
runnel
drainage for
locally
increased
experiments.
transport.
In one of these occasions, the differences in sediment
transport rates reached 2.4 x 10-2 kg.s-1.m-1 between the two
transects.
Cross shore variation of LST
LST was measured during 28 days during which 103 sediment
trap deployments were carried out during falling tide, while 83
Journal of Coastal Research, Special Issue 57, 2011
Journal of Coastal Research, Special Issue 57, 2011
49