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Sediment transport over an intertidal mudflat: field investigations and estimation of fluxes within the “Baie de Marenngres-Oleron” (France)
Continental Shelf Research 20 (2000) 1635}1653
Sediment transport over an intertidal mud#at: "eld investigations and estimation of #uxes within the `Baie de Marennes-Olerona (France) Ph. Bassoullet *, P. Le Hir , D. Gouleau, S. Robert IFREMER, Centre de Brest, B.P. 70, 29280 Plouzane& , France CREMA, B.P. 5, 17137 L+Houmeau, France Received 31 May 1999; received in revised form 25 January 2000; accepted 28 January 2000
Abstract This contribution analyses the various hydrodynamic forcings responsible for sediment dynamics over the intertidal mud#at of Brouage (Baie de Marennes-OleH ron). This mud#at is characterized by a gentle slope and an extensive ridge and runnel network. The sediment dynamics have been thoroughly investigated by continuous measurements of turbidity, waves and tidal currents, core measurements and bed level monitoring. Turbidity measurements highlight the importance of waves to the sediment resuspension over the mud#at. Conversely, higher turbidities during spring tides in the channel, further o!shore, indicate the major contribution of the tide to sediment transport within the embayment. Overall, a large amount of mobile sediment is consistently present within the bay, either as suspensions during spring tides, or as #uid mud deposits in the runnels of the mud#at during neap tides. Residual sediment #uxes, computed from the measurements, proved to be onshore during spring tides, but o!shore during periods of wave domination. Finally, a sediment dynamics overview for the area is proposed on the basis of the #ux estimations. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Intertidal environment; Sediment transport; In situ measurements; Ultrasonic device; Fluxes; Baie de Marennes-OleH ron
1. Introduction and overview of study site Located along the French Atlantic coastline, the Baie de Marennes-OleH ron is macro-tidal, with a range reaching 6 m during spring tides. Between OleH ron Island, to * Corresponding author. Tel.: 33-298-22-43-44; fax: 33-298-22-45-94. E-mail address: [email protected] (Ph. Bassoullet). 0278-4343/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 0 4 1 - 8
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the west, and the shoreline to the east, a central channel separates two large intertidal areas; 60% of the total area of the embayment is occupied by these intertidal zones (Fig. 1). Morphological and sedimentary characteristics of this bay have been studied previously (e.g. Tesson, 1973; Germaneau and Robert, 1995). The economical context of the Baie de Marennes-OleH ron is important as it is the premier production site of oysters in Europe. Whilst the western part of the study area is mainly sandy, the eastern part is composed of cohesive sediments. This latter mud#at covers an expanse of 15 km in a north}south and 4.5 km in a west}east direction. The Brouage mud#at, characterized by a gentle slope (of about 1 : 1000) and an extensive ridge and runnel network, has been investigated thoroughly during the INTRMUD project. As reported by Eisma (1998), the `elongated dendritic channel systema type, represented in this area of the Baie de Marennes-OleH ron, is particular and seldom encountered. The prevailing sedimentary structures, whose initiation has been discussed largely from
Fig. 1. The Brouage mud#at study area (Baie de Marennes-OleH ron). Location of the transect showing the sampling stations (0) } (5) and the location of the points A, B, C and D equipped (for 3, 20, 30 and 15 days, respectively) for the monitoring of the hydrodynamics.
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various environments * e.g. the initiation of longitudinal bedforms obtained from strati"cation e!ects (Dyer, 1982) * impose a very particular suspended sediment transport pattern over the mud#at. The prevailing tidal current pattern runs subparallel to the alignment of the bedforms. A simulation obtained from the depth-averaged SAM-2DH numerical model (Le Hir et al., 2000) shows the distribution of maximum current speeds and directions during a mean spring #ood tide (with a range of 5 m) and the bathymetry of the area. (Fig. 2). In a synthesis paper published on tidal #ow and sediment transport over the mud#ats, Ridderinkhof (1998) emphasized the importance of tidal asymmetries in controlling sediment transport processes and in particular the asymmetry of the tidal
Fig. 2. Bathymetry, maximum current speeds and directions over the mud#at from SAM 2DH model (Le Hir et al., this volume).
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slack duration. The Brouage mud#at is an example of this. In addition, an inversion of the #ow asymmetry has been noticed even for similar spring tidal ranges (Le Hir et al., 2000). The present contribution aims to improve knowledge about sediment budgets over the Brouage mud#at, taking into account the prevailing hydrodynamics and meteorological conditions. In practice, the mud#at is subjected to signi"cant variations in bed level; these are in response to large advective transports between the lower and upper mud#ats. Moreover, the intense shell"sh culture undertaken over the lower part of the intertidal system (oyster reefs in Fig. 1) may induce an input of biodeposits; these may be transported to the upper #at under the combined action of waves and tidal currents. It is well known that the "rst centimetres of intertidal mud#at sediments are a!ected by processes such as consolidation, drying e!ects during air-exposure, temperature changes over the #at during a tidal cycle (e.g. Anderson, 1973; Anderson and Howell, 1984; Paterson et al., 1990; Bassoullet and Jestin, 1995), but also liquefaction by waves. As part of the INTRMUD Project, both "eld investigations and mathematical modelling were used to provide a better understanding of the various transport processes and to start a model of the response of the mud#at to physical forcings, in relation to sediment behaviour and biota. This contribution presents sedimentological and physical characteristics of the sur"cial muds, continuous bed level monitoring and hydrodynamic measurements. Subsequently, a discussion of the residual sediment #uxes, computed from the measurements, enables the relative e!ects of tide and waves on the sediment transport, to be distinguished.
2. Methodology A detailed view of the study area is presented in Fig. 1. Points A}D represent the locations for the monitoring of the hydrodynamics; the transect includes six stations (numbered 0}5 from the shore to the sea) sampled at monthly intervals from March 1997 to May 1998. 2.1. Sediment characteristics At each site along the transect, three cores (15 cm long, 8 cm in diameter) were performed during spring low tides. The measured parameters (every centimetre) were: water content in the sediments, dry density and grain-size distribution. The resistance of a mud to erosion is usually parameterised according to the critical shear stress for erosion. This shear stress can be correlated to the shear strength given by a shear vane tester. Vane shear strength measurements have been obtained within the upper centimetre of the mud by means of an H60 Geonor hand vane tester; this consists of a 150 mm diameter vane "tted speci"cally for soft muds (range 0}10 N m\) and integrating over 10 mm thickness in the vertical. Replicates (n"3 minimum) are performed for every measurement. This arrangement permits the
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evaluation of local di!erences on the six stations transects, together with seasonal variations in the properties of the sur"cial sediment. Radioisotopes pro"les (Be, Pb) were also analyzed for the station 0, within the #uid mud and for three stations: 1, between 2 and 3 (for ridge and runnel) and station 4, up to 70 cm depth. A full description of the methods used is provided in Gouleau et al. (2000). The aim was to obtain a description of the sedimentary structures and an estimation of the sedimentation rates in the upper, middle and lower mud#ats. 2.2. Bed elevation monitoring Only continuous bed level monitoring may distinguish which process is involved in a depletion or accretion; namely variation of tidal current, wind-generated waves and currents, dewatering, or changes in response to biological activity. The biology is considered as a very important factor within such a context and "eld studies have identi"ed relationships between biota and sediment stability (e.g. Widdows et al., 2000). Biota can act as sediment stabiliser (e.g. benthic algal "lms), or destabiliser (e.g. bioturbation) (RiethmuK ller et al., 2000); therefore, the balance between the two processes can vary both spatially and temporally. Bed level monitoring was carried out on the Brouage mud#at using poles, in pairs, "xed within the mud (the speci"c position of these poles allowed three measurements between them which eliminated the scouring e!ects). However, this monitoring method integrates the short-term variation of the hydrodynamics (in particular, calm and wave-dominated conditions) occurring between the surveys. In order to continuously record (at a programmed sampling rate) the bed level changes * erosion or deposition * in response to any particular event (storm, calm), an autonomous bed elevation monitor (ALTUS) has been developed and deployed. This submersible system acts as an echosounder. The transducer is excited at a 2 MHz frequency and acts both as a transmitter and a receiver. This transducer is located on a light frame at a given distance (0.2}2 m) from the bed (Fig. 3) and this arrangement prevents any sediment scouring under the transducer. A separate container includes the electronics: the datalogger, a pressure sensor and the power supply. Detailed speci"cations of the system are presented in Jestin et al. (1998). This equipment permits data recordings either for some days, with a high sampling rate (once every 2 min), or for a few months at a sampling rate of about once per hour. Its high frequency enables to quantify #uid mud deposits upon a more consolidated bed. The other speci"cations of the device concern (i) the accuracy: $2 or $5 mm (according to the altitude ranges: 0.2}0.7 m and 0.2}2.0 m, respectively) and a resolution of 0.6 mm for both cases; (ii) the recording of a &maximum echo' in order to validate the data and the recording of the absolute pressure to obtain the water height. 2.3. Hydrodynamics Continuous measurements (at a sampling rate of 4 Hz for 1 min, 6 times/h) of horizontal components of the #ow (electromagnetic current meter) at 17 cm above the
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Fig. 3. ALTUS: sketch of the bed elevation system.
bed; of turbidity (optical backscatter sensors OBS) at 12 and 40 cm above the bottom; and of the total pressure near the bed were recorded. Measurements were collected over 20 days (20 October to 10 November 1994), from a SAMPLE station moored in the channel (point C, Fig. 1). This station consists of an autonomous system of programmable measurements (detailed speci"cations can be found in Jestin et al., 1994). During this same period a second SAMPLE station was located on the mud#at, at point B (see Fig. 1) recording (at 5 Hz for 1 min, 8 times/h): horizontal velocities (32 cm), total pressure (water height and waves) and turbidities at 12 and 32 cm. A problem with the pressure sensor of the station C has prevented correlations between the wave data at both locations. In addition, the SAMPLE station was deployed within the `ridge and runnela area of the mud#at (point D), for two weeks (17}29 November 1997) in order to identify the contribution of these sedimentary structures to sediment dynamic processes; these results are presented as a separate paper in this volume (Whitehouse et al., 2000). Measurements were recorded at 4 Hz * with OBS turbidimeters located at 3 levels (2, 21 and 43 cm) over a runnel; with a fourth at 10 cm above the neighbouring ridge, at the same altitude as the upper one located above the runnel * to provide an estimation of the concentrations of suspended particulate matter (SPM). Pressure (water heights and waves), conductivity, temperature and horizontal #ow (tidal current and orbital velocities) were also recorded simultaneously.
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In order to enable reliable estimation of sediment #uxes, all the OBS sensors were accurately calibrated with SPM from the waters (or from bottom sediments) of the environments to be monitored.
3. Results and discussion 3.1. Sediment characteristics and spatial variability In terms of spatial variations over the mud#at: the sur"cial sediments of the mud#at do not exhibit signi"cant changes in grain-size distribution. The sediments are mainly represented by clay/silt sizes, with a "ne sand fraction decreasing from the lower #at (15}20%) to the upper mud#at ((5% at the station 0). This "ne sand fraction is composed mainly of quartz (75}80%) over the lower and middle parts of the #at and of carbonates (up to 55%) over the upper mud#at (from the station 2, landward); the latter is related to the presence of foraminifera and often shell debris. In the bottoms of the runnels and creeks, associated with the intertidal #at, #uid mud is often observed. Despite its high mobility in response to the tidal currents, this #uid mud shows a "ner grain-size and an increase of carbonate content landward. The bulk density distribution (Fig. 4) of the upper two centimetres of the intertidal #at (averaged upon 15 months at the 6 stations) indicates that the highest values (1.3}1.4 kg m\) occur over the middle #at stations in comparison with the upper and lower mud#ats (1.25}1.30 kg m\) without appreciable seasonal changes. Below 2 cm,
Fig. 4. Correlations between bulk density, shear strength (averaged values/n"45) and position on the mud#at. Inset: correlation between shear strength and bulk density, for measurements obtained at all stations.
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bulk density is quite homogenous (of about 1.40}1.45 kg m\) for all the stations. This pattern can be expressed by the fact that #uid mud generally crosses the middle #at without settling. Deposition of low-density sediment usually occurs on the upper #at, sometimes on the lower #at, but only in the runnels over the remainder of the intertidal area. The Be (half-life 53.3 days) measurements corroborate these conclusions (Gouleau et al., 2000). The Be technique can measure a `temporarya sedimentation rate, on a very short-time scale, and can identify the movement of #uid mud along the transect. The highest activities were measured at stations 0 and 4, in #uid mud (sedimentation rates of about 21 cm yr\ in runnels compared to 7 cm yr\ on ridges). Actually, this #uid mud settles rapidly in the runnel area of the middle #at (station 3) and in the upper #at but can be easily resuspended by wave action as it will be seen further. The Pb (half-life 22.3 yr) measurements inform about the long-term sedimentation rates: very low in runnels (0.07 cm yr\) compared to those on ridges (0.7 cm yr\). Averaged values * over the entire study period (n"45) * of vane shear strength measurements, obtained in the upper centimetre for each station of the transect, con"rm the spatial di!erentiation on the mud#at transect; correlation with the measured bulk densities is reasonable (Fig. 4). 3.2. Bed level variations Bed level variations in Fig. 5 (measured using a system of poles), at each station of the transect, exhibit seasonal changes; however, the correlation of these with the
Fig. 5. Examples of relative bed level change measurements (by means of poles "xed in the mud), compared with the initial situation from March 1997. (a) July 1997 situation; (b) October 1997 situation.
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prevailing hydrodynamics is not always obvious. It is commonly stated that winter and spring months are periods of greatest bed elevation change, with erosion during winter months followed by rapid deposition during spring months (e.g. Anderson and Black, 1981). However, for the Brouage mud#at, the results obtained have highlighted that the slight variation is of the same range as the uncertainty of the levelling measurement (about$5 mm). Nevertheless, the method allows some generalizations to be made. In autumn, deposition dominates across the transect (Fig. 5) whilst erosion dominates over the upper #at (stations 0 and 1) and to seaward (from station 3) in July; which requires some explanation. Stations on the upper #at are characterized by a rapid succession of erosional/depositional events due to #uid mud inputs during spring tides (discussed later) and this appears in some cases to erase the classic seasonal cycle. It remains uncertain if the variations in July represent an e!ective erosion or are the result of a dewatering e!ect. Observations obtained over a 3 yr study period indicate that the mud#at appears to be in apparent equilibrium; the reason for which will be explained further. Continuous bed level monitoring (ALTUS device) was carried out at point D (see Fig. 1) over two periods: 17}24 November 1997 in a runnel (Fig. 6) and 03}28 December 1997 on a ridge coupled to the SAMPLE benthic multiparameter (hydrodynamics) station. A typical seven-day recording is shown in Fig. 7, showing results
Fig. 6. View of the instrumented site, on the ridge and runnel zone of the mud#at, showing the SAMPLE and ALTUS devices (November 1997- point D).
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Fig. 7. Bed elevation change recording (from ALTUS) and hydrodynamic measurements * Brouage mud#at (on a runnel at the point D * 17}25 November 1997). In graph (c): >-axes represent: the distance transducer * bed, the bed level changes (from an initial zero). Small grey dots symbolize the yuid mud and black dots, the denser mud.
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from the bed elevation monitor (ALTUS), an optical backscatter sensor (OBS) * located at 21 cm above bed -, and a high-resolution pressure transducer, to measure mean water depth and waves. The signi"cant wave height is computed as four times the standard deviation of the pressure #uctuations near the bed, neglecting the wave pressure decay over the water column. Until the 20 November (see Fig. 7(a) and (b)), in the afternoon, spring tidal currents caused high SPM concentrations (of about 3}4 kg m\) in the water column and the initial level (28.2 cm, symbolized by black dots) is constant until the 20th in the morning. Subsequently, higher wave (see Fig. 7(a)) actually produces a slight erosion; this is visible in Fig. 7(c), in the course of the 20 November. With increasing wave height, more than 1 cm of erosion is recorded. From 21 November, the wave action decreases, combined with the tidal current decreasing. SPM concentrations are much lower (0.2 kg m\) at this time (see Fig. 7(b)) and deposition becomes e!ective from 23 November during neap tide: about 1 cm of a fresh #uid mud layer appears (symbolized by grey dots in Fig. 7(c)). Beneath this deposit, the `hard bottoma (in black dots) is located at 29.8 cm, that is to say 16 mm more in comparison with the initial reference level and due to both erosion and slight compaction of the mud. The end of the recording (24 November) represents partly resuspended #uid mud in response to higher wave activity. 3.3. Hydrodynamics and suspended sediment monitoring Field measurements of waves, currents and SPM concentrations were carried out on the Brouage mud#at and in the channel seaward of the tidal in order to understand the suspended sediment dynamics over the mud#at and in the embayment; likewise to appreciate the relative e!ects of wave and tidal activity on sedimentation processes. Fig. 8 exhibits the 20 days of SAMPLE station recordings for the two locations (points B and C). For the channel location, the turbidities appear quite well correlated with tidal forcing. SPM concentrations reach 2 kg m\ near the bottom under spring tide conditions. Over the mud#at, although a rather good correlation between #ow intensity and tidal range can be observed, these mechanisms do not appear to have any e!ect on the SPM concentrations. However, these concentrations are correlated with wave activity at point B, especially just above the bed (i.e. with the sensor at 12 cm). The dominance of wave e!ects on the lower turbidimeter, when the tidal #ow is weak, con"rms this nature of the wave boundary layer, close to the bed. Concentrations always appear high: 0.2}1.0 kg m\ (under wave-dominated conditions) and can reach up to 2.5 kg m\, at the beginning of the #ood and at the end of the ebb phase of the tidal cycle. Details obtained on wave-induced resuspension are presented in Fig. 9, where data from four successive tides of similar amplitudes are presented. Waves became signi"cant for the last two tides only, and exhibited maxima heights around high water. This feature is in agreement with an observed linear relationship established previously between maximum wave height and water depth, in response to wave dissipation on a uniform slope (Le Hir et al., 2000). Waves around high water produce a net increase
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Fig. 8. SAMPLE station recordings: Brouage mud#at (point B) and channel (point C), October}November 1994. Waves, current speed and suspended particulate matter (for point locations, see Fig. 1).
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Fig. 9. Details of the recordings: points B and C, October}November 1994. Waves, current speed and suspended particulate matter.
in SPM concentrations over the mud#at (Fig. 9(b) and (c), point B), whereas, at both ends of the tidal immersion period, the concentrations are often lower. These are controlled by the tidal currents. Within the channel (Fig. 9(d), point C), a signi"cant increase in SPM is observed at low water slack during a period of wave activity; this indicates that wave-induced resuspensions can be transported down into the channel, contributing to a general increase in turbidity within the basin. Data recorded during the experiment undertaken in November 1997, over a runnel (point D: 750 m to seaward from point B) provide evidence to arrive at the same conclusion (see Fig. 7). Mud resuspension by wave activity was clearly observed on the intertidal mud#at, mainly during high tide.
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3.4. Estimation of suspended sediment yuxes From the time series of water height, SPM concentrations at two levels, together with current measurements, a total suspended sediment load and local #ux can be derived. First, #ow velocities are computed at the turbidimeter levels, assuming a logarithmic velocity pro"le and a roughness length of z "1 cm. Then the depth integrated suspended load is computed by distributing uniformly the measured suspended sediment concentrations over sections of the water height that surround the turbidimeters. In other words, the water column is split into two sections where the concentration is assumed to be uniform. At the "rst section, located from the bottom up to 30 cm, the SPM concentration measured at the 21 cm level is a!ected. Above, up to the surface, the concentration is assumed to be the one measured at 43 cm. Fluxes are computed as the product of the local #ow velocity by the suspended load for each section, and then are summed over the total water height. Tests have been run for checking that #ux results are not too sensitive to the hypotheses made for such computations. Results are presented for measurements at point B (20 October to 10 November 1994) (Fig. 10), and for measurements at point D (17}29 November 1997) (Fig. 11). In Fig. 10, the correlation between the suspended load and the signi"cant wave height is apparent; it is as clear as the previously described correlation between local turbidity and wave activity. However, the #ux does not show such a correlation, as it is also related to the tidal #ow. Over the intertidal zone, suspended sediment #uxes are dependent mainly upon the tides, although the concentration is correlated essentially with the waves. Nevertheless, the residual #ux, represented by the time-integration of the #ux (plotted in Fig. 10 as a dotted line), proves to be dependent both upon tide and wave forcing. For instance, during days 2 and 20, when the waves are high and the tidal amplitude is at a mean level, the integrated #ux decreases; this denotes o!shore residual sediment transport. In contrast, between days 11 and 18, on spring tide and in the absence of very large waves, the time-integrated #ux increases and reveals a net onshore transport. The above observations agree with the general concepts of net deposition of sediment on the upper intertidal #at, in response to tidal forcing, together with erosion of the #at under wave activity. Unfortunately data are not numerous enough to give an idea of the critical wave height (wind speed) capable of reversing the sediment transport. Measurements obtained at point D (Fig. 11), in the middle of the #at, are qualitatively similar to those described previously, with higher depth-integrated suspended loads and #uxes and still a residual o!shore #ux under wave forcing. The tidally induced onshore #ux is less clear than at point B, but the tidal amplitude here is also lower. It should be noted that on neap tide and even when the waves are signi"cant (e.g. the 8th day in Fig. 11), the total load and the sediment #ux remain negligible; this is due to the low mixing and weak advection, in response to small currents. In other words, waves constitute a dominant forcing mechanism for resuspension over intertidal #ats. However, the sediment transport and redistribution induced by waves are controlled mainly by the tidal conditions.
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Fig. 10. Time-series from SAMPLE station recordings, point B, October}November 1994, including suspended sediment #uxes, time- and depth-integrated suspended sediment load (for point locations, see Fig. 1).
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Fig. 11. Time-series from SAMPLE station recordings, point D, November 1997, including suspended sediment #uxes, time- and depth-integrated suspended sediment load (for point D location, see Fig. 1).
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Although the computation of #uxes described here is somewhat approximate, quanti"cation of the residual budgets can be attempted. During the spring tidal phase (Fig. 10), the residual onshore #ux is approximately 20 000 kg m\ (i.e. over 8 days), at point B located 1 km seaward of the shore. If this net input to the upper part (km) of the #at is deposited evenly as mud, with a dry density of 500 kg m\ (after consolidation; which is proved correct), a net accretion of 20 000 kg m\/1000 m/ 500 kg m\"0.04 m is obtained. Such a tidally induced deposition over a fortnightly tidal cycle (remembering that neap tides do not contribute to the sediment transport) is in agreement with the soft mud deposits of about 10 cm which have been observed commonly on this upper intertidal #at. With respect to the wave erosion, the observed o!shore #ux of around 2000 kg m\ over a single tide (e.g. on the 2nd day, Fig. 10) can be related to an overall erosion depth of 0.4 cm spread over the upper 1 km of the #at. Such an erosion is consistent with the measurements obtained from the ALTUS probe (see Fig. 7). The computations of residual #uxes take no account of the contribution of runnels to the overall pattern during the low water slack. Field observations show that since the #at begins to be uncovered, a #ow establishes in the runnels, carrying high SPM concentrations. The concentration of this #uid mud #ow decreases progressively together with the current intensity. Typically, this transient #ux can be estimated (taking into account the "eld data) as the transport of a 50 kg m\ concentration, over a height of 0.1 m and a width of 0.20 m/m alongshore (each runnel is approximately 30 cm wide and the wavelength of these bedforms is about 1.5 m), at a speed of 0.10 m s\ during 10 s (&3 h); then the corresponding o!shore #ux is 50 kg m\;0.10 m s\;0.1 m;0.20;10 s"1000 kg m\ per tide. The order of magnitude of this resulting o!shore #ux has to be compared to the onshore tidal residual #ux of 20 000 kg m\ over an 8-day period. The estimation is too crude to attribute any signi"cance to the di!erence, but it shows that longitudinal bedforms, organized either as a ridge and runnel network or as a dendritic channel system (both are present on the Brouage mud#at) are probably important factors in the residual sediment transport. 4. Conclusion For intertidal mud#ats where bedforms (i.e. cross-shore aligned ridge and runnel networks) are well developed, sediment transport over the #at is especially sensitive to the relative hydrodynamic forcing mechanisms (tidal currents and wave action). On the basis of the various measurements discussed in this paper, various conclusions can be drawn, as: (1) Under wave-dominated conditions: mud resuspension due to waves may be signi"cant (e.g. SPM'2 kg m\); at least on the elevated part of the intertidal #at, it occurs preferentially around the high water slack period. Within the channel, o!shore from the mud#at, the higher SPM concentrations during the low water slack on spring tide conditions, suggest an input of resuspended material from the mud#at,
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(2) Tidal forcing: controls the transport and redistribution of the suspended material and is particularly important during calm spring tides, producing a pronounced onshore residual #ux. Generally, there is an o!shore #ux under wavy conditions and an onshore #ux due to tide highlighting the importance of these conditions upon the overall development of the mud#at. Over annual timescales, the antagonist e!ects of tides and waves appear to lead to a quasi-equilibrium for the intertidal mud#at.
Acknowledgements This work was funded partly by the Commission of the European Communities, Directorate General for Science, Research and Development, as part of the INTRMUD collaborative research programme (Contract Number MAS3-CT95-0022). The authors would like to thank HerveH Jestin for the assistance he provided in the "eld and for discussions. Special thanks to K.R. Dyer, M.B. Collins and M.C. Christie who reviewed the initial manuscript and provided constructive and helpful criticisms.
References Anderson, F.E., 1973. Observations of some sedimentary processes acting on a tidal #at. Marine Geology 14, 101}116. Anderson, F.E., Black, L., 1981. A temporal and spatial study of mud#at erosion and deposition. Journal of Sedimentary Petrology 51 (3), 729}736. Anderson, F.E., Howell, B.A., 1984. Dewatering of an unvegetated muddy tidal #at during exposure. Dessication or drainage ? Estuaries 7 (3), 225}232. Bassoullet, Ph., Jestin, H., 1995. Characterization of intertidal mud#at environments by means of in situ experiments. MAST II - G8 Morphodynamics, Final Overall Meeting, Gdansk' (Poland), 4}8 September Vol. 6. pp. 47}50. Dyer, K.R., 1982. The initiation of sedimentary furrows by standing internal waves. Sedimentology 29, 885}889. Eisma, D., 1998. Macrotidal deposits. In: Kennish, M.J., Lutz, P.L. (Eds.), Intertidal Deposits: River Mouths, Tidal Flats and Coastal Lagoons. CRC Press LLC, Boca Raton, pp. 19}130. Germaneau, J., Robert, S., 1995. Etude morpho-seH dimentaire de la vasie`re intertidale de MontportailBrouage: Atlas des facie`s. Rapport IFREMER DRV/95.25 RA-DEL/95.14, 111 p. (in French). Gouleau, D., Jouanneau, J.M., Weber, O., Sauriau, P.G., 2000. Short and long term sedimentation on Montportail-Brouage intertidal mud#at, Marennes-OleH ron Bay (France). Continental Shelf Research 20 (12/13), 1513}1530. Jestin, H., Le Hir, P., Bassoullet, Ph., 1994. The `SAMPLE Systema, a new concept of benthic station. Oceans'94-Osates, Brest (France), 13}16 September, Vol. 3. pp. 278}283. Jestin, H., Bassoullet, Ph., Le Hir, P., L'Yavanc, J., Degres, Y., 1998. Development of ALTUS, a high frequency acoustic submersible recording altimeter to accurately monitor bed elevation and quantify deposition or erosion of sediments. Oceans'98-IEEC/OES Conference, Nice (France), Conference Proceedings, Vol. 1/3. pp. 189}194. Le Hir, P., Roberts, W., Cazaillet, O., Christie, M., Bassoullet, P., Bacher, C., 2000. Characterization of intertidal #at hydrodynamics. Continental Shelf Research 20 (12/13), 1433}1459.
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Paterson, D.M., Crawford, R.M., Little, C., 1990. Subaerial exposure and changes in the stability of intertidal estuarine sediments. Estuarine Coastal Shelf Science 30, 541}556. Ridderinkhof, H., 1998. Sediment transport in intertidal areas. In: Kennish, M.J., Lutz, P.L., (Eds.), Intertidal Deposits: River Mouths, Tidal Flats and Coastal Lagoons. CRC Press LLC, Boca Raton, pp. 363}381. RiethmuK ller, R., Heineke, M., KuK hl, H., Keuker-RuK diger, R., 2000. Chlorophyll a concentration as an index of sediment surface stabilisation by microphytobenthos. Continental Shelf Research 20 (10/11), 1351}1372. Tesson, M., 1973. Aspects dynamiques de la seH dimentation dans la baie de Marennes-OleH ron (France). Thesis, University of Bordeaux I, 128 pp. (in French). Whitehouse, R.J.S., Le Hir, P., Bassoullet, P., Dyer, K.R., Mitchener, H.J., Roberts, W., 2000. The in#uence of bedforms on #ow and sediment transport over intertidal mud#ats. Continental Shelf Research 20 (10/11), 1099}1124. Widdows, J., Brown, S., Brinsley, M.D., Salkeld, P.N., Elliot, M., 2000. Temporal changes in intertidal sediment erodability: in#uence of biological and climatic factors. Continental Shelf Research 20 (10/11), 1275}1289.
Sediment transport over an intertidal mud#at: "eld investigations and estimation of #uxes within the `Baie de Marennes-Olerona (France) Ph. Bassoullet *, P. Le Hir , D. Gouleau, S. Robert IFREMER, Centre de Brest, B.P. 70, 29280 Plouzane& , France CREMA, B.P. 5, 17137 L+Houmeau, France Received 31 May 1999; received in revised form 25 January 2000; accepted 28 January 2000
Abstract This contribution analyses the various hydrodynamic forcings responsible for sediment dynamics over the intertidal mud#at of Brouage (Baie de Marennes-OleH ron). This mud#at is characterized by a gentle slope and an extensive ridge and runnel network. The sediment dynamics have been thoroughly investigated by continuous measurements of turbidity, waves and tidal currents, core measurements and bed level monitoring. Turbidity measurements highlight the importance of waves to the sediment resuspension over the mud#at. Conversely, higher turbidities during spring tides in the channel, further o!shore, indicate the major contribution of the tide to sediment transport within the embayment. Overall, a large amount of mobile sediment is consistently present within the bay, either as suspensions during spring tides, or as #uid mud deposits in the runnels of the mud#at during neap tides. Residual sediment #uxes, computed from the measurements, proved to be onshore during spring tides, but o!shore during periods of wave domination. Finally, a sediment dynamics overview for the area is proposed on the basis of the #ux estimations. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Intertidal environment; Sediment transport; In situ measurements; Ultrasonic device; Fluxes; Baie de Marennes-OleH ron
1. Introduction and overview of study site Located along the French Atlantic coastline, the Baie de Marennes-OleH ron is macro-tidal, with a range reaching 6 m during spring tides. Between OleH ron Island, to * Corresponding author. Tel.: 33-298-22-43-44; fax: 33-298-22-45-94. E-mail address: [email protected] (Ph. Bassoullet). 0278-4343/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 0 4 1 - 8
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the west, and the shoreline to the east, a central channel separates two large intertidal areas; 60% of the total area of the embayment is occupied by these intertidal zones (Fig. 1). Morphological and sedimentary characteristics of this bay have been studied previously (e.g. Tesson, 1973; Germaneau and Robert, 1995). The economical context of the Baie de Marennes-OleH ron is important as it is the premier production site of oysters in Europe. Whilst the western part of the study area is mainly sandy, the eastern part is composed of cohesive sediments. This latter mud#at covers an expanse of 15 km in a north}south and 4.5 km in a west}east direction. The Brouage mud#at, characterized by a gentle slope (of about 1 : 1000) and an extensive ridge and runnel network, has been investigated thoroughly during the INTRMUD project. As reported by Eisma (1998), the `elongated dendritic channel systema type, represented in this area of the Baie de Marennes-OleH ron, is particular and seldom encountered. The prevailing sedimentary structures, whose initiation has been discussed largely from
Fig. 1. The Brouage mud#at study area (Baie de Marennes-OleH ron). Location of the transect showing the sampling stations (0) } (5) and the location of the points A, B, C and D equipped (for 3, 20, 30 and 15 days, respectively) for the monitoring of the hydrodynamics.
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various environments * e.g. the initiation of longitudinal bedforms obtained from strati"cation e!ects (Dyer, 1982) * impose a very particular suspended sediment transport pattern over the mud#at. The prevailing tidal current pattern runs subparallel to the alignment of the bedforms. A simulation obtained from the depth-averaged SAM-2DH numerical model (Le Hir et al., 2000) shows the distribution of maximum current speeds and directions during a mean spring #ood tide (with a range of 5 m) and the bathymetry of the area. (Fig. 2). In a synthesis paper published on tidal #ow and sediment transport over the mud#ats, Ridderinkhof (1998) emphasized the importance of tidal asymmetries in controlling sediment transport processes and in particular the asymmetry of the tidal
Fig. 2. Bathymetry, maximum current speeds and directions over the mud#at from SAM 2DH model (Le Hir et al., this volume).
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slack duration. The Brouage mud#at is an example of this. In addition, an inversion of the #ow asymmetry has been noticed even for similar spring tidal ranges (Le Hir et al., 2000). The present contribution aims to improve knowledge about sediment budgets over the Brouage mud#at, taking into account the prevailing hydrodynamics and meteorological conditions. In practice, the mud#at is subjected to signi"cant variations in bed level; these are in response to large advective transports between the lower and upper mud#ats. Moreover, the intense shell"sh culture undertaken over the lower part of the intertidal system (oyster reefs in Fig. 1) may induce an input of biodeposits; these may be transported to the upper #at under the combined action of waves and tidal currents. It is well known that the "rst centimetres of intertidal mud#at sediments are a!ected by processes such as consolidation, drying e!ects during air-exposure, temperature changes over the #at during a tidal cycle (e.g. Anderson, 1973; Anderson and Howell, 1984; Paterson et al., 1990; Bassoullet and Jestin, 1995), but also liquefaction by waves. As part of the INTRMUD Project, both "eld investigations and mathematical modelling were used to provide a better understanding of the various transport processes and to start a model of the response of the mud#at to physical forcings, in relation to sediment behaviour and biota. This contribution presents sedimentological and physical characteristics of the sur"cial muds, continuous bed level monitoring and hydrodynamic measurements. Subsequently, a discussion of the residual sediment #uxes, computed from the measurements, enables the relative e!ects of tide and waves on the sediment transport, to be distinguished.
2. Methodology A detailed view of the study area is presented in Fig. 1. Points A}D represent the locations for the monitoring of the hydrodynamics; the transect includes six stations (numbered 0}5 from the shore to the sea) sampled at monthly intervals from March 1997 to May 1998. 2.1. Sediment characteristics At each site along the transect, three cores (15 cm long, 8 cm in diameter) were performed during spring low tides. The measured parameters (every centimetre) were: water content in the sediments, dry density and grain-size distribution. The resistance of a mud to erosion is usually parameterised according to the critical shear stress for erosion. This shear stress can be correlated to the shear strength given by a shear vane tester. Vane shear strength measurements have been obtained within the upper centimetre of the mud by means of an H60 Geonor hand vane tester; this consists of a 150 mm diameter vane "tted speci"cally for soft muds (range 0}10 N m\) and integrating over 10 mm thickness in the vertical. Replicates (n"3 minimum) are performed for every measurement. This arrangement permits the
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evaluation of local di!erences on the six stations transects, together with seasonal variations in the properties of the sur"cial sediment. Radioisotopes pro"les (Be, Pb) were also analyzed for the station 0, within the #uid mud and for three stations: 1, between 2 and 3 (for ridge and runnel) and station 4, up to 70 cm depth. A full description of the methods used is provided in Gouleau et al. (2000). The aim was to obtain a description of the sedimentary structures and an estimation of the sedimentation rates in the upper, middle and lower mud#ats. 2.2. Bed elevation monitoring Only continuous bed level monitoring may distinguish which process is involved in a depletion or accretion; namely variation of tidal current, wind-generated waves and currents, dewatering, or changes in response to biological activity. The biology is considered as a very important factor within such a context and "eld studies have identi"ed relationships between biota and sediment stability (e.g. Widdows et al., 2000). Biota can act as sediment stabiliser (e.g. benthic algal "lms), or destabiliser (e.g. bioturbation) (RiethmuK ller et al., 2000); therefore, the balance between the two processes can vary both spatially and temporally. Bed level monitoring was carried out on the Brouage mud#at using poles, in pairs, "xed within the mud (the speci"c position of these poles allowed three measurements between them which eliminated the scouring e!ects). However, this monitoring method integrates the short-term variation of the hydrodynamics (in particular, calm and wave-dominated conditions) occurring between the surveys. In order to continuously record (at a programmed sampling rate) the bed level changes * erosion or deposition * in response to any particular event (storm, calm), an autonomous bed elevation monitor (ALTUS) has been developed and deployed. This submersible system acts as an echosounder. The transducer is excited at a 2 MHz frequency and acts both as a transmitter and a receiver. This transducer is located on a light frame at a given distance (0.2}2 m) from the bed (Fig. 3) and this arrangement prevents any sediment scouring under the transducer. A separate container includes the electronics: the datalogger, a pressure sensor and the power supply. Detailed speci"cations of the system are presented in Jestin et al. (1998). This equipment permits data recordings either for some days, with a high sampling rate (once every 2 min), or for a few months at a sampling rate of about once per hour. Its high frequency enables to quantify #uid mud deposits upon a more consolidated bed. The other speci"cations of the device concern (i) the accuracy: $2 or $5 mm (according to the altitude ranges: 0.2}0.7 m and 0.2}2.0 m, respectively) and a resolution of 0.6 mm for both cases; (ii) the recording of a &maximum echo' in order to validate the data and the recording of the absolute pressure to obtain the water height. 2.3. Hydrodynamics Continuous measurements (at a sampling rate of 4 Hz for 1 min, 6 times/h) of horizontal components of the #ow (electromagnetic current meter) at 17 cm above the
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Fig. 3. ALTUS: sketch of the bed elevation system.
bed; of turbidity (optical backscatter sensors OBS) at 12 and 40 cm above the bottom; and of the total pressure near the bed were recorded. Measurements were collected over 20 days (20 October to 10 November 1994), from a SAMPLE station moored in the channel (point C, Fig. 1). This station consists of an autonomous system of programmable measurements (detailed speci"cations can be found in Jestin et al., 1994). During this same period a second SAMPLE station was located on the mud#at, at point B (see Fig. 1) recording (at 5 Hz for 1 min, 8 times/h): horizontal velocities (32 cm), total pressure (water height and waves) and turbidities at 12 and 32 cm. A problem with the pressure sensor of the station C has prevented correlations between the wave data at both locations. In addition, the SAMPLE station was deployed within the `ridge and runnela area of the mud#at (point D), for two weeks (17}29 November 1997) in order to identify the contribution of these sedimentary structures to sediment dynamic processes; these results are presented as a separate paper in this volume (Whitehouse et al., 2000). Measurements were recorded at 4 Hz * with OBS turbidimeters located at 3 levels (2, 21 and 43 cm) over a runnel; with a fourth at 10 cm above the neighbouring ridge, at the same altitude as the upper one located above the runnel * to provide an estimation of the concentrations of suspended particulate matter (SPM). Pressure (water heights and waves), conductivity, temperature and horizontal #ow (tidal current and orbital velocities) were also recorded simultaneously.
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In order to enable reliable estimation of sediment #uxes, all the OBS sensors were accurately calibrated with SPM from the waters (or from bottom sediments) of the environments to be monitored.
3. Results and discussion 3.1. Sediment characteristics and spatial variability In terms of spatial variations over the mud#at: the sur"cial sediments of the mud#at do not exhibit signi"cant changes in grain-size distribution. The sediments are mainly represented by clay/silt sizes, with a "ne sand fraction decreasing from the lower #at (15}20%) to the upper mud#at ((5% at the station 0). This "ne sand fraction is composed mainly of quartz (75}80%) over the lower and middle parts of the #at and of carbonates (up to 55%) over the upper mud#at (from the station 2, landward); the latter is related to the presence of foraminifera and often shell debris. In the bottoms of the runnels and creeks, associated with the intertidal #at, #uid mud is often observed. Despite its high mobility in response to the tidal currents, this #uid mud shows a "ner grain-size and an increase of carbonate content landward. The bulk density distribution (Fig. 4) of the upper two centimetres of the intertidal #at (averaged upon 15 months at the 6 stations) indicates that the highest values (1.3}1.4 kg m\) occur over the middle #at stations in comparison with the upper and lower mud#ats (1.25}1.30 kg m\) without appreciable seasonal changes. Below 2 cm,
Fig. 4. Correlations between bulk density, shear strength (averaged values/n"45) and position on the mud#at. Inset: correlation between shear strength and bulk density, for measurements obtained at all stations.
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bulk density is quite homogenous (of about 1.40}1.45 kg m\) for all the stations. This pattern can be expressed by the fact that #uid mud generally crosses the middle #at without settling. Deposition of low-density sediment usually occurs on the upper #at, sometimes on the lower #at, but only in the runnels over the remainder of the intertidal area. The Be (half-life 53.3 days) measurements corroborate these conclusions (Gouleau et al., 2000). The Be technique can measure a `temporarya sedimentation rate, on a very short-time scale, and can identify the movement of #uid mud along the transect. The highest activities were measured at stations 0 and 4, in #uid mud (sedimentation rates of about 21 cm yr\ in runnels compared to 7 cm yr\ on ridges). Actually, this #uid mud settles rapidly in the runnel area of the middle #at (station 3) and in the upper #at but can be easily resuspended by wave action as it will be seen further. The Pb (half-life 22.3 yr) measurements inform about the long-term sedimentation rates: very low in runnels (0.07 cm yr\) compared to those on ridges (0.7 cm yr\). Averaged values * over the entire study period (n"45) * of vane shear strength measurements, obtained in the upper centimetre for each station of the transect, con"rm the spatial di!erentiation on the mud#at transect; correlation with the measured bulk densities is reasonable (Fig. 4). 3.2. Bed level variations Bed level variations in Fig. 5 (measured using a system of poles), at each station of the transect, exhibit seasonal changes; however, the correlation of these with the
Fig. 5. Examples of relative bed level change measurements (by means of poles "xed in the mud), compared with the initial situation from March 1997. (a) July 1997 situation; (b) October 1997 situation.
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prevailing hydrodynamics is not always obvious. It is commonly stated that winter and spring months are periods of greatest bed elevation change, with erosion during winter months followed by rapid deposition during spring months (e.g. Anderson and Black, 1981). However, for the Brouage mud#at, the results obtained have highlighted that the slight variation is of the same range as the uncertainty of the levelling measurement (about$5 mm). Nevertheless, the method allows some generalizations to be made. In autumn, deposition dominates across the transect (Fig. 5) whilst erosion dominates over the upper #at (stations 0 and 1) and to seaward (from station 3) in July; which requires some explanation. Stations on the upper #at are characterized by a rapid succession of erosional/depositional events due to #uid mud inputs during spring tides (discussed later) and this appears in some cases to erase the classic seasonal cycle. It remains uncertain if the variations in July represent an e!ective erosion or are the result of a dewatering e!ect. Observations obtained over a 3 yr study period indicate that the mud#at appears to be in apparent equilibrium; the reason for which will be explained further. Continuous bed level monitoring (ALTUS device) was carried out at point D (see Fig. 1) over two periods: 17}24 November 1997 in a runnel (Fig. 6) and 03}28 December 1997 on a ridge coupled to the SAMPLE benthic multiparameter (hydrodynamics) station. A typical seven-day recording is shown in Fig. 7, showing results
Fig. 6. View of the instrumented site, on the ridge and runnel zone of the mud#at, showing the SAMPLE and ALTUS devices (November 1997- point D).
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Fig. 7. Bed elevation change recording (from ALTUS) and hydrodynamic measurements * Brouage mud#at (on a runnel at the point D * 17}25 November 1997). In graph (c): >-axes represent: the distance transducer * bed, the bed level changes (from an initial zero). Small grey dots symbolize the yuid mud and black dots, the denser mud.
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from the bed elevation monitor (ALTUS), an optical backscatter sensor (OBS) * located at 21 cm above bed -, and a high-resolution pressure transducer, to measure mean water depth and waves. The signi"cant wave height is computed as four times the standard deviation of the pressure #uctuations near the bed, neglecting the wave pressure decay over the water column. Until the 20 November (see Fig. 7(a) and (b)), in the afternoon, spring tidal currents caused high SPM concentrations (of about 3}4 kg m\) in the water column and the initial level (28.2 cm, symbolized by black dots) is constant until the 20th in the morning. Subsequently, higher wave (see Fig. 7(a)) actually produces a slight erosion; this is visible in Fig. 7(c), in the course of the 20 November. With increasing wave height, more than 1 cm of erosion is recorded. From 21 November, the wave action decreases, combined with the tidal current decreasing. SPM concentrations are much lower (0.2 kg m\) at this time (see Fig. 7(b)) and deposition becomes e!ective from 23 November during neap tide: about 1 cm of a fresh #uid mud layer appears (symbolized by grey dots in Fig. 7(c)). Beneath this deposit, the `hard bottoma (in black dots) is located at 29.8 cm, that is to say 16 mm more in comparison with the initial reference level and due to both erosion and slight compaction of the mud. The end of the recording (24 November) represents partly resuspended #uid mud in response to higher wave activity. 3.3. Hydrodynamics and suspended sediment monitoring Field measurements of waves, currents and SPM concentrations were carried out on the Brouage mud#at and in the channel seaward of the tidal in order to understand the suspended sediment dynamics over the mud#at and in the embayment; likewise to appreciate the relative e!ects of wave and tidal activity on sedimentation processes. Fig. 8 exhibits the 20 days of SAMPLE station recordings for the two locations (points B and C). For the channel location, the turbidities appear quite well correlated with tidal forcing. SPM concentrations reach 2 kg m\ near the bottom under spring tide conditions. Over the mud#at, although a rather good correlation between #ow intensity and tidal range can be observed, these mechanisms do not appear to have any e!ect on the SPM concentrations. However, these concentrations are correlated with wave activity at point B, especially just above the bed (i.e. with the sensor at 12 cm). The dominance of wave e!ects on the lower turbidimeter, when the tidal #ow is weak, con"rms this nature of the wave boundary layer, close to the bed. Concentrations always appear high: 0.2}1.0 kg m\ (under wave-dominated conditions) and can reach up to 2.5 kg m\, at the beginning of the #ood and at the end of the ebb phase of the tidal cycle. Details obtained on wave-induced resuspension are presented in Fig. 9, where data from four successive tides of similar amplitudes are presented. Waves became signi"cant for the last two tides only, and exhibited maxima heights around high water. This feature is in agreement with an observed linear relationship established previously between maximum wave height and water depth, in response to wave dissipation on a uniform slope (Le Hir et al., 2000). Waves around high water produce a net increase
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Fig. 8. SAMPLE station recordings: Brouage mud#at (point B) and channel (point C), October}November 1994. Waves, current speed and suspended particulate matter (for point locations, see Fig. 1).
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Fig. 9. Details of the recordings: points B and C, October}November 1994. Waves, current speed and suspended particulate matter.
in SPM concentrations over the mud#at (Fig. 9(b) and (c), point B), whereas, at both ends of the tidal immersion period, the concentrations are often lower. These are controlled by the tidal currents. Within the channel (Fig. 9(d), point C), a signi"cant increase in SPM is observed at low water slack during a period of wave activity; this indicates that wave-induced resuspensions can be transported down into the channel, contributing to a general increase in turbidity within the basin. Data recorded during the experiment undertaken in November 1997, over a runnel (point D: 750 m to seaward from point B) provide evidence to arrive at the same conclusion (see Fig. 7). Mud resuspension by wave activity was clearly observed on the intertidal mud#at, mainly during high tide.
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3.4. Estimation of suspended sediment yuxes From the time series of water height, SPM concentrations at two levels, together with current measurements, a total suspended sediment load and local #ux can be derived. First, #ow velocities are computed at the turbidimeter levels, assuming a logarithmic velocity pro"le and a roughness length of z "1 cm. Then the depth integrated suspended load is computed by distributing uniformly the measured suspended sediment concentrations over sections of the water height that surround the turbidimeters. In other words, the water column is split into two sections where the concentration is assumed to be uniform. At the "rst section, located from the bottom up to 30 cm, the SPM concentration measured at the 21 cm level is a!ected. Above, up to the surface, the concentration is assumed to be the one measured at 43 cm. Fluxes are computed as the product of the local #ow velocity by the suspended load for each section, and then are summed over the total water height. Tests have been run for checking that #ux results are not too sensitive to the hypotheses made for such computations. Results are presented for measurements at point B (20 October to 10 November 1994) (Fig. 10), and for measurements at point D (17}29 November 1997) (Fig. 11). In Fig. 10, the correlation between the suspended load and the signi"cant wave height is apparent; it is as clear as the previously described correlation between local turbidity and wave activity. However, the #ux does not show such a correlation, as it is also related to the tidal #ow. Over the intertidal zone, suspended sediment #uxes are dependent mainly upon the tides, although the concentration is correlated essentially with the waves. Nevertheless, the residual #ux, represented by the time-integration of the #ux (plotted in Fig. 10 as a dotted line), proves to be dependent both upon tide and wave forcing. For instance, during days 2 and 20, when the waves are high and the tidal amplitude is at a mean level, the integrated #ux decreases; this denotes o!shore residual sediment transport. In contrast, between days 11 and 18, on spring tide and in the absence of very large waves, the time-integrated #ux increases and reveals a net onshore transport. The above observations agree with the general concepts of net deposition of sediment on the upper intertidal #at, in response to tidal forcing, together with erosion of the #at under wave activity. Unfortunately data are not numerous enough to give an idea of the critical wave height (wind speed) capable of reversing the sediment transport. Measurements obtained at point D (Fig. 11), in the middle of the #at, are qualitatively similar to those described previously, with higher depth-integrated suspended loads and #uxes and still a residual o!shore #ux under wave forcing. The tidally induced onshore #ux is less clear than at point B, but the tidal amplitude here is also lower. It should be noted that on neap tide and even when the waves are signi"cant (e.g. the 8th day in Fig. 11), the total load and the sediment #ux remain negligible; this is due to the low mixing and weak advection, in response to small currents. In other words, waves constitute a dominant forcing mechanism for resuspension over intertidal #ats. However, the sediment transport and redistribution induced by waves are controlled mainly by the tidal conditions.
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Fig. 10. Time-series from SAMPLE station recordings, point B, October}November 1994, including suspended sediment #uxes, time- and depth-integrated suspended sediment load (for point locations, see Fig. 1).
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Fig. 11. Time-series from SAMPLE station recordings, point D, November 1997, including suspended sediment #uxes, time- and depth-integrated suspended sediment load (for point D location, see Fig. 1).
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Although the computation of #uxes described here is somewhat approximate, quanti"cation of the residual budgets can be attempted. During the spring tidal phase (Fig. 10), the residual onshore #ux is approximately 20 000 kg m\ (i.e. over 8 days), at point B located 1 km seaward of the shore. If this net input to the upper part (km) of the #at is deposited evenly as mud, with a dry density of 500 kg m\ (after consolidation; which is proved correct), a net accretion of 20 000 kg m\/1000 m/ 500 kg m\"0.04 m is obtained. Such a tidally induced deposition over a fortnightly tidal cycle (remembering that neap tides do not contribute to the sediment transport) is in agreement with the soft mud deposits of about 10 cm which have been observed commonly on this upper intertidal #at. With respect to the wave erosion, the observed o!shore #ux of around 2000 kg m\ over a single tide (e.g. on the 2nd day, Fig. 10) can be related to an overall erosion depth of 0.4 cm spread over the upper 1 km of the #at. Such an erosion is consistent with the measurements obtained from the ALTUS probe (see Fig. 7). The computations of residual #uxes take no account of the contribution of runnels to the overall pattern during the low water slack. Field observations show that since the #at begins to be uncovered, a #ow establishes in the runnels, carrying high SPM concentrations. The concentration of this #uid mud #ow decreases progressively together with the current intensity. Typically, this transient #ux can be estimated (taking into account the "eld data) as the transport of a 50 kg m\ concentration, over a height of 0.1 m and a width of 0.20 m/m alongshore (each runnel is approximately 30 cm wide and the wavelength of these bedforms is about 1.5 m), at a speed of 0.10 m s\ during 10 s (&3 h); then the corresponding o!shore #ux is 50 kg m\;0.10 m s\;0.1 m;0.20;10 s"1000 kg m\ per tide. The order of magnitude of this resulting o!shore #ux has to be compared to the onshore tidal residual #ux of 20 000 kg m\ over an 8-day period. The estimation is too crude to attribute any signi"cance to the di!erence, but it shows that longitudinal bedforms, organized either as a ridge and runnel network or as a dendritic channel system (both are present on the Brouage mud#at) are probably important factors in the residual sediment transport. 4. Conclusion For intertidal mud#ats where bedforms (i.e. cross-shore aligned ridge and runnel networks) are well developed, sediment transport over the #at is especially sensitive to the relative hydrodynamic forcing mechanisms (tidal currents and wave action). On the basis of the various measurements discussed in this paper, various conclusions can be drawn, as: (1) Under wave-dominated conditions: mud resuspension due to waves may be signi"cant (e.g. SPM'2 kg m\); at least on the elevated part of the intertidal #at, it occurs preferentially around the high water slack period. Within the channel, o!shore from the mud#at, the higher SPM concentrations during the low water slack on spring tide conditions, suggest an input of resuspended material from the mud#at,
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(2) Tidal forcing: controls the transport and redistribution of the suspended material and is particularly important during calm spring tides, producing a pronounced onshore residual #ux. Generally, there is an o!shore #ux under wavy conditions and an onshore #ux due to tide highlighting the importance of these conditions upon the overall development of the mud#at. Over annual timescales, the antagonist e!ects of tides and waves appear to lead to a quasi-equilibrium for the intertidal mud#at.
Acknowledgements This work was funded partly by the Commission of the European Communities, Directorate General for Science, Research and Development, as part of the INTRMUD collaborative research programme (Contract Number MAS3-CT95-0022). The authors would like to thank HerveH Jestin for the assistance he provided in the "eld and for discussions. Special thanks to K.R. Dyer, M.B. Collins and M.C. Christie who reviewed the initial manuscript and provided constructive and helpful criticisms.
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