SEDIMENT TRANSPORT ON A MACROTIDAL RIDGE AND RUNNEL BEACH
DURING ACCRETION CONDITIONS
Alexis Stépanian, Brigitte Vlaswinkel*, Franck Levoy and Claude Larsonneur
Abstract: In May 1999 a field campaign has been carried out at Omaha beach,
a macrotidal beach situated along the coastline of Normandy in North France.
This beach is characterised as a typical ridge and runnel beach and four
intertidal bars can be distinguished. During a period of two weeks extensive
morphological and hydrodynamic data have been collected. Key objective
during the field period was to understand the three-dimensional development of
the ridges and runnels and to recognise the processes responsible for origin and
mobility of the ridges. The paper presented here attempts to combine integrated
sand transport rates, measured by fluorescent tracers injected at the seaward
flanks of the two highest ridges, with the residual topographic evolution of the
intertidal beach. Fair weather conditions prevailed and as a result a slight
onshore migration of the ridges has been monitored (max. 12 m). The
dispersion of the tracer clouds reveals a dominance of cross-shore over
longshore transport. However, this can not be comfirmed by the hydrodynamic
data measured seaward of the breaker line. It illustrates the limited influence
shoaling wave processes have on sand transport at the injection points during
conditions of onshore bar migration. A predominance of swash action is
assumed to be the main acting cross-shore sediment transportation process.
INTRODUCTION
Ridge and runnel beaches are a common feature in intertidal areas along the coastlines of
northwestern Europe. This type of longshore bars (Greenwood and Davidson-Arnott, 1979)
__________________
Continental and Coastal Morphodynamics – UMR CNRS 6143 - University of CAEN - 24, rue
des Tilleuls - 14002 CAEN - FRANCE
*) Present address: Department of Marine Geology and Geophysics – Rosenstiel School of
Marine and Atmospheric Science / University of MIAMI - 4600 Rickenbacker Causeway MIAMI - FL 33149 U.S.A.
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Stépanian et al.
usually develops in environments with the following characteristics (Orford and Wright,
1978): (1) abundant fine to medium sand, (2) low beach gradient, (3) fetch limited wave
climate and (4) high tidal range. Several field studies have been performed in order to
investigate to which extent hydrodynamic processes are responsible for the origin of ridge
construction and mobility (e.g. King and Williams, 1949; Mulrennan, 1992). Classical
sequences of ridge and runnel beach evolution are well described by Mulrennan (1992) with
ridge accretion and/or onshore migration during low energy conditions and destruction and/or
offshore movement during high energy conditions. Erosion can affect the entire beach profile
and this can result in a smooth featureless profile, or ridges can be partially eroded and
described as “semi-permanent” features. Even though the main mobility factor is recognised
to be incident waves, the ridge formation process is still unknown (Short, 1999). According to
King (1972), field experiments highlighted swash action as a possible origin of ridge
construction by “an attempt of the waves to produce an equilibrium swash zone gradient, on
a beach of which the overall gradient is flatter than the equilibrium gradient” (cited in Short,
1999). However the swash processes need a phase of stationary water level to build up ridge
morphology during a tidal cycle. This is the case only for high and low water slacks and
hence ridge formation in the middle foreshore can hardly be explained by this 'swash bar'
theory.
This paper deals with part of the results of a field campaign carried out during May 1999 at
Omaha Beach, a macrotidal ridge and runnel beach in the north of France. The work
presented here tries to combine integrated sand transport, measured by fluorescent tracers,
with the residual topographic evolution of a ridge and runnel system on a weekly time scale
during fair weather conditions. Additionally hydrodynamic data are acquired along a crossshore profile in order to quantify wave and current conditions during the experiment.
STUDY AREA
Omaha Beach is a 6 km long sandy beach between rocky headlands, along the coastline of
Calvados in Normandy (Figure 1). The inshore profile is relatively steep with a –10 m contour
located at 1400 m from the shoreline.
Figure 1. Location of the study area
The intertidal zone is characterised by a wide stretch of sandy beach containing several
parallel bars incised by drainage channels. The width of the intertidal zone is approximately
350 meters at low water spring tide. The mean intertidal gradient (tan) is 0.015. The beach
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Stépanian et al.
consists of fine to medium grained sand. Median grain sizes (D50) are found to be between
185 m and 230 m with a slight decrease from the high-tidal zone to the low-tidal zone. In
the subtidal zone, two breaker bars are recognised by bathymetric survey.
The annual modal value of the offshore significant wave height is measured around 0.35 m
with a peak period of 6.5 s (MNATET, 1995). This wave climate is due to the short fetch in
the Channel between France and The United Kingdom. Waves mainly approach Omaha
Beach from northwesterly to northeasterly directions. The tidal range at Omaha Beach varies
from 3.5 at neap tide to 6.5 m at spring tide. Mean spring tidal range is around 5.5 m. The
tidal curve is asymmetric with a rapid flood period and a longer ebb phase. The highest tidal
currents are measured at mid tide during flood.
EXPERIMENT SET-UP AND DATA ANALYSIS
Daily monitoring of beach topography was done using a kinematic DGPS mounted on a
customised unit and towed by a quad vehicle. This will give a good insight of the 3D
morphological evolution of the beach and ridge mobility. 2D cross-shore profiles have been
computed from each Digital Elevation Model (DEM). This topographic method allows
surveying a greater number of topographic points than optical classical ways. The method
accuracy ( 3 cm) is evaluated using control points at the beach.
During the campaign hydrodynamic data concerning incident waves and currents have been
recorded along a cross-shore profile (Figure 2), in order to quantify nearshore processes
during contrasting weather conditions. However calm weather conditions dominated during
the entire field period. Mini-frames containing an electromagnetic current meter and pressure
sensor (S4DW) were deployed on the seaward flank of the bars in order to measure orbital
velocities and instantaneous water level fluctuations (sampling frequency: 2 Hz) as well as
mean currents. In the runnels, S4 current meters were deployed to measure mean currents as
well. Mean longshore Vm and cross-shore Um currents are computed by average on 9
minutes.
4
Electromagnetic current meter + pressure sensor
elevation above datum (m)
3
Electromagnetic current meter
high tide level
tracer injection
2
1
C1
A
H2
0
B
C2
H3
-1
C3
H4
low tide level
-2
-3
0
50
100
150
200
250
300
350
400
offshore distance from dune foot (m)
Figure 2. Main cross-shore instrumented profile
Significant waves Hs are computed in incident frequency band (0.05-0.33 Hz). Located in an
offshore position but still in the intertidal beach, mini-frame H4 is used as tide gauge and
measures boundary conditions concerning incident waves. Mini-frame H2 and mini-frame H3
3
Stépanian et al.
give indications of local wave height, mean current and orbital velocities on the seaward flank
of the bars. For mini-frame H2, times series of orbital cross-shore velocities are processed to
obtain an asymmetry ratio for incident waves. All processing steps are discussed in Kroon
(1994). Instrumentation is limited by the height above bottom of sensors and 80 cm of water
depth is required above the sensors (Pierowicz and Boswood, 1995). As a consequence swash
and surf zone dynamics have not been monitored.
During the tidal cycle of the 27th-28th May, a fluorescent tracer experiment was performed.
Two tracer injections were carried out on the seaward flanks of the two highest ridges during
the first low tide on May 27th. A shallow circular pit was dug with a diameter of 150
centimetres and a depth of 2.5 centimetres, and filled with the tagged sand. Tracer injection at
site A contained 10 kg of red dyed native sand with D50 of 259 µm and site B contained 25 kg
of green dyed native sand with D50 of 289 µm. Mean grain sizes have become coarser due to
the painting process, but comparison of cumulative and absolute frequency curves shows that
the modal class is more or less similar throughout the three sediment samples (original sand,
tracer sand and field collected sand). In order to obtain an assessment of the thickness of the
mobile sand layer, several holes were dug around the two injection points and filled with
black tracer sand up to the surface level, according to the methods of King (1951). During the
subsequent low tide the small holes were excavated and the average depth to which the
coloured sand has been removed was taken as a measure of the thickness of sand moving
layer. Tracer sands were recovered during the next low tide in the night of 27th to 28th May.
Tracer principle
Fluorescent tracer experiments have been widely used in coastal studies (among others:
Duane and James, 1980; Yasso, 1966), in particular for evaluation of longshore transport rate.
The method employed here is based on White and Inman (1989) and is explained in detail in
Levoy et al. (1997). In order to quantify sediment transport, several assumptions have to be
made (Voulgaris et al., 1998): (1) fluorescent particles have identical geometric
characteristics as natural sand, (2) tracers are dispersed by the same hydrodynamic agents as
the original sand, (3) their displacement can be used to evaluate sediment transport rates and
(4) thickness of mobile sand is constant on each point of tracer cloud. Visual counting of
fluorescent grains at the surface was done at night during the next low tide, following a
circular grid around the injection points. This Lagrangian spatial integration method is used to
survey the behaviour in space and time of the tracer cloud (Madsen, 1989). The transport
velocity of an average sand grain can be deduced by measuring the movement of the tracer
centroid in a known period of time. The depth of the mobile sand layer needs to be measured
in order to evaluate the sediment transport rate. The average duration of tidal inundation of
each tracer is determined by means of the height of the injection point and by using tidal
measurements.
RESULTS
Topographic evolution of the beach during the campaign
Figure 3 presents the topographic evolution of the intertidal beach (contour plot) between
18/05/99 and 2/06/99 superimposed on the initial DEM (18/05/99). In this way the main
accretion and erosion zones are distinguished in relation to morphology. The range of changes
between –5 cm to 5 cm is not plotted because these changes are considered to be negligible
compared with the accuracy of the survey method. The topographic evolution shows that the
intertidal area is fairly stable throughout the campaign without significant morphological
4
Stépanian et al.
0.35
0.25
accretion
changes. The maximum mobility is observed in the high-tidal zone and on the higher part of
the mid-tidal zone, whereas the beach exhibits no remarkable evolution close to the low water
line. Main erosion zones are located in the runnels and on the seaward flank of the bars.
Accretion zones are concentrated at the top and onshore flank of the bars, in particular the two
highest ridges, and on the berm.
-0.05
-0.15
-0.25
A
erosion
0.05
(cm)
0.15
-0.35
B
Figure 3. Residual topographic evolution throughout the campaign
It has to be noticed that a swash bar has been growing up on the berm during one tidal cycle at
28/05/99, and thus, this can explain part of the morphological change in the high-tidal zone.
Volumetric difference between the two surveys is evaluated around – 660 m3, which is a low
value compared to the survey surface (70000 m2).
elevation above datum (m)
4
18-may
3
R1
02-june
2
R2
1
0
R3
-1
R4
-2
residual evolution
-3
0.4
accretion
0.2
0.0
erosion
-0.2
0
50
100
150
200
250
300
350
400
offshore distance from dune foot (m)
Figure 4. Main cross-shore profile evolution
5
Stépanian et al.
Such morphological change results in a slight onshore migration of the crest of the ridges R1
and R2 (Figure 4) respectively of 13 and 5 m (Table 1). The ridges R3 and R4 exhibit no
significant morphological changes compared to the accuracy of the survey.
Table 1. Parameters of landward movement of ridge crests
ridge number
X1
Z1
X2
Z2
18-may
69
1.78
146
0.62
2-june
57
1.90
141
0.69
evolution (m)
-12
0.12
-5
0.07
-0.263
0.004
rate change per
tidal cycle m.tide
-1
-0.632 0.006
Xi : cross-shore location of ridge crest from dune foot
Zi : elevation of ridge crest
Hydrodynamic conditions during the campaign
The morphological evolution described above occurred during 18 days of low energetic
conditions (Figure 5), following a short storm event with significant wave height reaching 4
m at the lower beach. The periods of significant wave height associated are around 5 s. Wave
incidence varies with wind conditions. The highest waves recorded are associated with
onshore winds from northern directions, offshore winds corresponding with very low
agitation close to the shoreline.
1.5
(a)
tracer experiment
1
0.5
0
4
(b)
2
0
-2
2/6
1/6
31/5
30/5
29/5
28/5
27/5
26/5
25/5
24/5
23/5
22/5
21/5
20/5
19/5
18/5
-4
Figure 5. Hydrodynamic conditions during the campaign
(a) offshore significant wave height (m)
(b) Tides chart – water elevation referred to datum (m)
Fluorescent tracer experiment of 27/05/99
The fluorescent tracer experiment was executed during a neap tidal cycle when wave
conditions were calm. An offshore significant wave height Hs0 of 0.35 m was measured with a
6
Stépanian et al.
peak period Tp0 of 5 s. The wind was onshore and very modest (3 m/s). Wave incidence was
approximately normal to the shoreline. No significant morphological evolution was observed
during the tidal cycle of the experiment.
Results summarised in Table 2 exhibit a similar behaviour of tide-averaged total transport
between the two sites. Cross-shore transport is dominant over longshore transport and
transport rates remain very low. The average velocity of tracer advection was about 1 cm/min.
This results in a displacement ranging from 3 to 6 metres of the tracer cloud mass centroïd
during the tidal cycle. Accompanying sediment transport rates were small, not exceeding 0.4
kg.min-1.m-1. The average thickness of the moving layer varied from 1.5 cm for the second bar
and 2.5 cm for the first bar. According to King (1951) this small depth of sand disturbance is
in agreement with calm weather conditions and a fine to medium grained sandy beach.
Table 2. Patterns of tracer dispersion during the experiment
site
centroid
migration
X
Y
mean
velocity
m
m
m
cm.min
-1
mean
transport
-1
g.min .m
recovery
rates
volumetric rate
-1
3
-1
m .min .m
-1
3
-1
m .tide .m
-1
%
A
3.04
3.03 -0.21
0.99
393
0.000245
0.077
81
B
5.78
5.71 -0.90
1.39
287
0.000180
0.075
132
X and Y correspond respectively to cross-shore component (onshore positive) and longshore
component (southeast positive) of centroid migration
The hydrodynamic data acquired during the tracer experiment are plotted in Figure 6. The
highest waves are recorded during the flood phase and reach their maximum one-hour and a
half before high tide. A slight decrease of the wave height is recorded during ebb phase. Mean
longshore currents are tidally driven and dominant at the lower part of the beach. Mean crossshore currents are determined by incident wave height and the rate of water elevation changes,
which can reach 2.8 cm.min-1 at mid tide. They are generally positive during flood and
negative during ebb. In the runnels, hydrodynamic measurements (not plotted here) show that
mean longshore currents (averaged on 1 min) are tidally driven and reach their maximum
(respectively 15 and 30 cm.s-1 for C1 and C3) during ebb phase, which relates to the drainage
of the runnel. Mean longshore currents are increasing seaward, according to past observations
done at macrotidal beaches (Wright et al., 1982, Short, 1991).
Figure 7 shows patterns of shoaling wave conditions measured with mini-frame H2. Due to its
high location at the beach, it records mainly wave conditions during the upper part of the tidal
curve. Hs/h parameter is the ratio of significant wave height versus local water depth and
hence characterises the local hydrodynamics and type of wave breaking. According to Kroon
(1994), a ratio Hs/h < 0.4 indicates that waves do not break and recorded processes concern
the shoaling wave zone. In this shoaling wave zone, despite non-negligible onshore orbital
velocities, the asymmetry ratio remains around 0.5. This result indicates equilibrium between
onshore-offshore water movements near the sediment bed and illustrates the small influence
of shoaling wave processes on sand transport at this position.
DISCUSSION
On a tidal time scale, during low energetic conditions, the tracer experiment clearly exhibit
the onshore direction of sand transport on the seaward flanks of the intertidal bars. This tracer
7
Stépanian et al.
approach obtains good indications about tidally averaged sand transport, but transport
quantification should be taken with caution due to method assumptions and field deployment.
5
h (m)
3
1
(a)
-1
-3
0.4
Hs (m)
0.3
H2
H3
H4
0.2
(b)
0.1
0
40
<Vm> (cm/s)
20
H2
H3
0
(c)
-20
-40
10
<Um> (cm/s)
H2
5
H3
0
(d)
-5
2:00
1:00
0:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
-10
Figure 6. Mean characteristics of waves across the beach, 27/05/99
Local high tide at 19:31 GMT
(a) Water elevation at mini-frame H4 (m) – (b) significant wave height (band frequency 0.05 0.33 Hz) (m) – (c) 9 min averaged longshore currents <Vm> (positive southeast) – (d) 9 min
averaged cross-shore currents <Um> (positive onshore)
At site A, the minute-averaged volumetric transport rate is slightly higher. Whereas tidally
averaged sand transport at site B is of the same order, due to different immersion times,
respectively for A and B, 41% and 55% of the tidal duration. Previous studies performed in
other macrotidal ridge an runnel beaches give a similar range of transport rates. Levoy et al.
(1998) measured tidally averaged volumetric transport rates between 0.028 to
0.22 m3.tide-1.m-1. Such results were found in contrasting weather conditions, in the lower
intertidal zone of a beach in North France. This experiment however was performed during
tidal cycles with higher incident wave energy. At Niewpoort beach in Belgium, Voulgaris et
al. (1998) computed mean tidally averaged sand transport rates of 0.027 to 0.13 m3.tide-1.m-1,
with comparable wave agitation and a similar location in the beach profile as present study.
The relation between sand tracer dispersion and local measured hydrodynamic is not clear,
because of the lack of hydrodynamic data in very shallow water depths. Close to site B,
recorded mean currents with mini-frame H2 cannot explain the onshore component of tracer
migration, as tidally driven longshore currents dominate cross-shore currents. The orbital
asymmetry ratio is computed to be close to 0.5, which indicates equilibrium between onshore
and offshore velocities and thus onshore sediment transport cannot occur.
8
Stépanian et al.
h (m)
4
(a)
2
0
Hs/h
0.4
(b)
0.2
U1/3on (cm/s)
0
60
(c)
40
20
asymetry ratio
0
0.7
(d)
0.6
0.5
0.4
0.3
17:00
18:00
19:00
20:00
21:00
22:00
23:00
Figure 7. Shoaling wave patterns measured with H2, 27/05/99
local high tide at 19:31 GMT
(a) water elevation (m) - (b) ratio Hs/h - (c) U1/3on (cm/s): significant onshore velocity of
incident waves (band frequency: 0.05 - 0.33 Hz) - (d) asymmetry ratio of incident waves
As there exists no indication of onshore transport in high water depths, it can be assumed that
transport processes in shallow water depths are responsible for the landward movement of the
tracers. In the surfzone and breaker zone, sediment transport is controlled by re-suspension
due to breaking waves and transport by longshore tidally-driven currents (Wright et al., 1982)
or offshore flows due to low frequency oscillations (Davidson et al. 1993). Since suspended
load mode is closely dependent on mean currents (in this case longshore dominant), the
dominant onshore transport mode should be bedload. Thus the main onshore transport
process, which is responsible for tracer dispersion should be the swash zone processes. The
efficiency of swash action is related to vertical water elevation rates, respectively 2.2 cm.mn -1
and 2.8 cm.m-1 for site A and B during flood and -1.8 cm.mn-1 and -2.2 cm.mn-1 during ebb.
These rapid elevation rates imply that the injection points are not under influence of swash
processes for long periods of time. However this short period of time seems to be sufficient
for the swash action to be the dominant agent for onshore sediment transportation.
A comparison between the results of the tracer experiment and the residual topographic
evolution during the campaign exhibits a similar behaviour between the onshore direction of
tracer dispersion and the landward migration of the ridges. Furthermore erosion zones are
closely related to the seaward flanks of bars where tracer injection points are located.
Throughout the campaign, the average rate of onshore ridge migration is around 0.63 m.tide-1
for the first ridge and 0.26 m.tide-1 for the second ridge. Theses values are low, compared to
the measured onshore component of centroïd migration (respectively 3.04 m and 5.78
m.tide-1). These differences could be explained by (1) uncertainties due to the assumptions
made for the tracer method and (2) only one tracer experiment was performed in specific
wave and tide conditions. However, the integration of tidally averaged sand transport rate
9
Stépanian et al.
over the campaign duration is close to volumetric changes observed on the landward slip face
of the two bars. Furthermore, the highest ridge migration rate is observed at the first bar (site
A), whereas tidally averaged sand transport is comparable with site B. The difference of
inundation time between the two sites implies that bars located higher at the beach are under
influence of more intense wave processes than the bars lower at the beach, due to the
modulation of wave processes by the tide. On a weekly time scale, the frequency of
inundation varies from 15% to 25% (mean to spring tide) for site A and 40% to 55% for site
B. This involves that, during fair weather conditions, the ridge mobility is more dependent on
incident wave energy and related processes than on immersion time. Additionally shallow
water stirring processes control most of the sediment transportation, even at the middle part of
the beach where water elevation rates are the highest (3 cm.min-1).
CONCLUSIONS
This study at Omaha Beach, a typical macrotidal ridge and runnel beach, was performed
during fair weather conditions. Under these low energy conditions, an onshore migration of
ridges has been monitored during two weeks. The different ridges along the cross-shore
profile exhibit differential morphodynamic behaviour and different average rates of onshore
migration. The higher ridges are the most mobile with average rates of landward migration
between 0.2 and 0.6 m.tide-1, whereas lower bars stay relatively stable throughout the survey.
Measured tidally averaged sand transports by fluorescent tracers are comparable with results
of previous studies in similar environments (Levoy et al., 1998, Voulgaris et al., 1998). In
conditions of bar accretion, analysis of shoaling wave processes shows their inefficiency to
drive onshore sediment and hence the predominance of swash action is assumed to be the
main sediment transport process by bedload mode on the seaward flank of bars. Even if swash
processes enhance the ridge mobility on a beach configuration with pre-existing intertidal
bars, the influence of swash action as bar formation process remains to demonstrate with
hydrodynamic data in very shallow water depths.
ACKNOWLEDGEMENTS
This study is part of the PNEC project (National Project of Coastal Environment), funded by
INSU-CNRS. The authors wish to thank all field experiment participants and Olivier Monfort
and Jean-Marc Rousset for processing routines. Meteorological data are provided by
METEOFRANCE (National Meteorological Agency).
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