SEDIMENT TRANSPORT on a macrotidal Ridge and RunnelBEACH 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)
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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.
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 OmahaBeach, 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 cross-shore profile in order to quantify wave and current conditions during the experiment.
STUDY AREA
OmahaBeach 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 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 OmahaBeach from northwesterly to northeasterly directions. The tidal range at OmahaBeach 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.
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 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 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.
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).
Figure 4. Main cross-shore profile evolution
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
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.
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 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
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 cross-shore 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 approach obtains good indications about tidally averaged sand transport, but transport quantification should be taken with caution due to method assumptions and field deployment.
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.
Figure 7. Shoaling wave patterns measured with H2, 27/05/99
local high tide at 19:31 GMT