Morphodynamics of intertidal bars on
wave-tide-dominated beaches: the role of longshore currents
Coastal Geomorphology and Shoreline
Management Unit - EA 3599
189 A, Avenue Maurice Schumann
59140 Dunkerque
Tél. : 03-28-23-76-21 Fax : 03-28-23-76-23
Equipe
d'Accueil
3599
Edward J. ANTHONY and Mouncef SEDRATI
[email protected] ; [email protected]
Beaches with multiple intertidal bar-trough systems have received increasing attention since the 1950s when the term "ridge and runnel" was first applied by King and Williams (1949) to describe their
particular morphology. A number of recent studies have explored the roles of wave shoaling, surf and swash, and of vertical and horizontal tidal excursion rates on the mobility of intertidal bars
(Masselink and Anthony 2001 ; Kroon and Masselink 2002 ; Anthony et al. 2004, 2005 ; Masselink 2004 ; Masselink et al., 2006). It may be suspected, however, from the spatial variability of the intertidal
morphology of these beaches, that some of the temporal cross-shore profile variability is induced by important longshore processes (Anthony et al. 2004, 2005 ; Sedrati and Anthony 2005) that have
recieved little attention in the morphodynamic studies devoted to these beaches. The present study contributes to a better understanding of the interactivity between these processes by comparing the
morphology and hydrodynamics of two macrotidal beaches in northern France characterised by spring tidal ranges exceeding 7 m and by equilibrium sediment budgets.
a
The results discussed in this paper concern two beaches, respectively the Dune
d'Amont (Wissany Bay) facing the Dover Strait, and Merlimont facing the eastern
English Channel (Fig. 1). These sites are typical of mixed wave-tide dominated environments (Anthony and Orford, 2002; Anthony et al. 2004).They are affected by semidiurnal tides with spring tide ranges that decrease northwards from 8.2 m in Merlimont to 7.2 m in Wissant Bay. Both sites are subject to strong offshore tidal currents
with mean velocities ranging from 0.5 to 2 m. During conditions of significant wind
stress (sustained wind speeds > 8 m.s-1 ), the peak spring tide velocities are two to
five times larger than 'normal' spring tide current velocities. The waves in Merlimont
are essentially from west to southwest, in response to the synoptic winds. In Wissant
Bay they are from west to west-southwest, with fetch and coastal orientation conditions restricting the incidence of southwesterly waves. Recorded offshore waves
heights for both sites range from 0.25 to 1.5 m during modal conditions, and may attain up to 3 m during storms. South to southwesterly winds are largely dominant on
these beaches.Wind forcing significantly enhances the mean current velocities recorded on these beaches (Anthony et al. 2004, 2005 ; Sedrati and Anthony 2005). These
sites thus display a relatively more energetic wave and tidal current regime than
most other intertidal barred beaches described in the literature.
NORTH
SEA
ra
it
ENGLAND
D
Isle of Wight
ov
s
r
e
t
Wissant
Bay
Merlimont
NNE
A
H
C
H
ENGLIS
L
FRANCE
0
50
100
150 km
Fig.1: Location map of Merlimont and Dune d'Amont (Wissant Bay).
b
c
5m
Fig. 2: Aerial photographs (a, b) showing respectively Merlimont and
Dune d'Amont beaches, and ground photograph (c) depicting longshore
migrating crescentic dunes attached to bar faces.
The present study contributes to a beter understanding of the interactivity between tide and wave processes by comparing the
morphology and hydrodynamics of these two macrotidal beaches (Fig. 2). The data from on which this study is based comprise hydrodynamic and topographic measurements
carried out during two field experiments conducted respectively from April 24 to May 2, 1997 (Merlimont) and from January 16 to 28, 2004 (Dune d'Amont). The full results
are reported in Anthony et al. (2004, 2005) and Sedrati and Anthony 2005.
Elevation (m IGN 69)
6
0.071
MHWS
0.027
4
0.012
MHWN
2
Bar 1
0.048
0
0.019
0.061
Trough 1
MSL
Bar 2
0.027
0.012
0.019
MLWN
Trough 2
-2
0.009
Overall gradient: 0.012
MLWS
-4
0
100
A
200
300
400
500
600
700 m
Fig. 3: The central beach profile on Merlimont beach, showing slope
segments and loction of currentemeter (A).
The morphology of Merlimont beach during the experiment was characterised
by an overall gradient of 0.012 but by marked variations associeted with two
pronounced intertidal bar-trough systems around the mid-tide zone (Fig. 3).
Dune d'Amont beach exhibited three pronounced bar-trough systems situted in
the mid-tide zone too with an overall gradient of 0.024 (Fig. 4). Both beaches
showed significant meso-scale topographic variations caused by channel systems
entrenched in the troughs, and by pronounced meso-scale bedforms (Fig. 2, c) in
these troughs. Northeastward (Dune d'Amont beach) and northward
(Merlimont) longshore migrating 3D crescentic dunes having wavelengths of
several metres were observed flanking or becoming attached to the landward
and the seaward bar faces (Fig. 2, c)
8
6
Elevation (m IGN 69)
8
0.090
MHWS
4
0.038
Bar 1
0.017
2
Bar 2
0.035
0.065
Figure 5 shows typical hydrodynamic records during calm and high wave energy conditions. During calm conditions,
the peak cross-shore currents, which show strong coherence with wave signatures and are thus wave-generated,
were two to five times weaker than the longshore current velocity peaks (see tides 1, 2 and 3 in Fig. 5, A and tide 8
in Fig. 5, B). However, throughout high wave energy conditions, which correspond with significant wind stress
(sustained wind speeds > 8 m.s-1), the peak longshore current velocities attained up to 1.5 m.s-1 on Dune d'Amont
beach and 0.8 m.s-1 in Merlimont.
The wave-generated cross-shore currents measured on the two beaches during calm and high wave-energy
conditions were thus rather weak, and were subordinate to strong longshore currents generated by tides and by
wind forcing. These strong longshore flows mitigated the intensity of the cross-shore flows and induced the
longshore migration of medium-sized bedforms that largely explained short-term profile change (Fig. 6). The
prominent bar-trough systems in the mid-tide zone of both beaches remained stable in position although they
underwent significant morphological change (Fig. 6).
0
A
0
100
200
28/04/97 AM
27/04/97 AM
m
+0.3
0
-0.3
100
200
300
400
500
600
17/01/04 AM
16/01/04 AM
700
0
Distance (m)
b
200
300
MHWS
4
MHWN
2
MSL
0
MLWN
-2
0
100
200
300
400
Distance (m)
500
600
700
2
3
4
5
6
7
8
6
6
4
4
2
2
26/04/97
28/04/97
0
30/04/97
b
m.s-1
-0.5
offshore
-0.3
-0.3
-0.1
-0.1
0.1
0.1
2
0
26/04/97
28/04/97
m.s-1
0.9
0.7
30/04/97
c
storm
North
-2
-4
0
100
200
300
400
-0.3
South
26/04/97
28/04/97
m.s-1
1.6
1.2
0.8
0.4
0
-0.4
-0.8
-1.2
-1.6
30/04/97
d
m
3.0
1.0
1.0
MHWN
0.5
0.5
MSL
0.0
0.0
500
Distance (m)
Limited cross-shore bar migration on these beaches probably reflects a morphodynamic adjustment
involving pronounced bars resistant to change, and thus liable to promote morphological lag, and wellentrenched troughs maintained by channelled high-energy intertidal flows that evacuate groundwater,
swash overflow and ebb tidal discharge. The study highlights the importance of longshore flows and
their morphological effects. This significant longshore component needs to be fully considered in order
to gain a better understanding of the morphodynamics of beaches with multiple intertidal bars.
This is part of INTERREG III - Project BAR (Beaches at risk).
We also thank the Nord Pas-de-Calais Regional Council for research support in various ways.
1
2
3
4
17/01/04
5
6
19/01/04
7
8
21/01/04
offshore
onshore
26/04/97
28/04/97
30/04/97
17/01/04
19/01/04
storm
21/01/04
storm
NE
storm
SW
17/01/04
19/01/04
21/01/04
17/01/04
19/01/04
21/01/04
m
3.0
1.5
MLWN
700 m
0.5
1.5
MHWS
600
0.3
onshore
2.0
6
500
m.s-1
-0.5
2.0
Distance (m)
4
m
8
2.5
500
Fig. 6: (a) Daily net profile change of Merlimont (A) and Dune d'Amont beaches (B) corresponding to the hydrodynamic records shown in Fig. 5 ; (b)
reference profile for both sites
FEDER
1
-0.1
d
400
2.5
400
8
Elevation (m IGN 69)
Elevation (m IGN 69)
8
-4
100
d
6
a
0.1
18/01/04 AM
26/04/97 AM
0
300
Fig. 4: The central profile on Dune d'Amont beach, showing slope
segments and loction of currentemeter (A).
0.7
19/01/04 AM
MLWN
Overall gradient: 0.024
0.3
29/04/97 AM
m
+0.3
0
-0.3
m
8
0.7
c
0.025
0.005
-4
0.5
a
A
Trough 2
0.5
B
Bar 3
-2
0.3
c
0.022
0.019
Trough 1
B
0
b
MSL
0.040
A
a
MHWN
Fig. 5: Typical records of water levels, mean currents and significant wave heights measured
on Merlimont (A) and Dune d'Amont beaches (B); (a) water levels, (b) long-shore currents,
(c) cross-shore currents, and (d) the significant wave heights.
REFERENCES
Anthony, E.J., Levoy, F., and Monfort, O. (2004) "Morphodynamics of intertidal bars on a megatidal beach, Merlimont, Northern France," Mar. Geol
208, 73-100.
Anthony, E.J., Levoy, F., Monfort, O., and Degryse-Kulkarni, C. (2005) "Short-term intertidal bar mobility on a ridge-and-runnel beach, Merlimont,
Northern France," Earth Surf. Process. Landforms 30, 81-93.
King, C.A.M., Williams, W.W., 1949. Formation and movement of sand bars by wave action. Geogr. J. 113, 70- 85.
Kroon, A., Masselink, G., (2002) "Morphodynamics of intertidal bar morphology on a macrotidal beach under low-energy wave conditions, North
Lincolnshire, England," Mar. Geol. 190, 591- 608.
Masselink, G. (2004) "Formation and evolution of multiple intertidal bars on macrotidal beaches: application of a morphodynamic model," Coast. Eng
51 (8-9), 713-730.
Masselink, G., and Anthony, E.J., (2001) "Location and height of intertidal bars on macrotidal ridge and runnel beaches," Earth Surf. Process. Landf. 26,
759-774.
Masselink, G., Kroon, A., & Davidson-Arnott, R.G.D. (2006) "Morphodynamics of intertidal bars in wave-dominated coastal setting - A rewiew,"
Geomorphology. 73, Issuses 1-2, 33-49.
Sedrati, M., (2002) "Analyse des courants sur un estran macrotidal : Baie de Wissant, nord de la France," Mémoire de DEA. Université du Littoral
Côte d'Opale, Dunkerque. 55 pp.
Sedrati, M., Cohen, O., and Anthony, E.J., (2003) "Interaction entre le régime des courants tidaux et la variation de la distribution longitudinal des
sables sur un estran macrotidal ," J. Rech. Océanogr.
Sedrati, M., and Anthony, E.J., (2005) The role of longshore currents in intertidal bar mobility on a macrotidal beach under high-energy wave
conditions, Dune d'Amont, Wissant Bay, Northern France. Proceedings Coastal Dynamics, 2005, ASCE, paper on CD-ROM.