- Email: [email protected]
Suspended sediment transport on a temperate, microtidal mudflat, the Danish Wadden Sea
Marine Geology 173 (2001) 69±85
www.elsevier.nl/locate/margeo
Suspended sediment transport on a temperate, microtidal mud¯at, the Danish Wadden Sea T.J. Andersen*, M. Pejrup Institute of Geography, University of Copenhagen, éster Voldgade 10, DK-1350 Kùbenhavn K, Denmark Received 24 February 2000; accepted 13 December 2000
Abstract Time series measurements of suspended sediment transport and bed level change on a microtidal mud¯at of the Danish Wadden Sea were carried out between February 1997 and September 2000. The data show that the mud¯at is generally accreting except for periods with onshore winds when wave-induced erosion takes place during periods of low water levels at the site. In general, a continuous net input of mud was recorded during periods of weak or offshore winds, i.e. the import is not event controlled, and this was interrupted by episodic loss of mud during a few tidal periods with onshore winds. The net landward transport of suspended sediment during calm weather or periods of offshore wind is caused by settling and scour lag and tidal asymmetry. A very large input of suspended material was observed in the period following a strong storm, the landward directed net transport during the ®ve tidal periods following the storm being equivalent to approximately 40% of the net annual accumulation in the area. The typical depth of sediment reworking at the intertidal study site is 2±5 cm. Some seasonal variation can be deduced from the data set with a tendency for deposition during spring and summer and erosion in the winter period. The seasonality was especially clear at the most landward station where accretion following periods of ice-formation in the area is due to grounding of sediment transported by ice-¯oes but otherwise deposition is mainly taking place in the warmer seasons. Net accretion over the three-year measuring period along the transect varies between 0.6 and 1.9 cm a 21. Deposition is largest at the most landward station and the accretion found on the basis of repeated bed level measurements compares well with 210Pb and 137Cs dating. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Mud¯at; Deposition; Cohesive; Sediment; Transport; Reworking
1. Introduction A quanti®cation of ®ne-grained sediment accumulation is important both for environmental and economic reasons. Heavy metals, nutrients, pesticides and herbicides tend to stick to ®ne-grained material and knowledge of the transport pathways and the * Corresponding author. Tel.: 145-35-32-25-03; fax: 145-35-3225-01. E-mail address: [email protected] (T.J. Andersen).
potential for erosion and deposition of the material will consequently give information on the fate of these toxic or otherwise environmentally harmful substances. Deposition of ®ne-grained material in ports or navigation channels is also an issue of great economic importance, which calls for knowledge of the potential for erosion and deposition in estuarine environments. Since mud¯ats are both quantitatively and qualitatively very important sinks for ®ne-grained material, a thorough understanding of the processes governing
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(00)00164-X
70
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 1. Sedimentological map of the study area showing the locations of station A and B off the east coast of Rùmù. The subtidal parts of the tidal basin are mainly sandy.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
deposition and erosion in these environments is of foremost importance. In addition, mud¯ats are important biotopes for a number of macrofaunal species and act as feeding places for both birds and nursery for ®sh fry. Detailed investigations of suspended sediment transport on intertidal mud¯ats have been undertaken, e.g. Frostick and McCave (1979), Pejrup (1988), Kirby et al. (1992), Amos (1995), O'Brien (1998), Christie et al. (1999), and O'Brien et al. (2000). The area occupied by mud¯ats in the European Wadden Sea has been reduced over past centuries by land reclamation (e.g. Flemming and Nyandwi (1994) and Reise (1998)). With the prospect of an accelerating sea-level rise, concern has arisen as to the fate of the remaining mud¯ats and salt marshes. As part of the EU-funded MAST III project INTRMUD concerned with mud¯at sedimentation and classi®cation, studies of suspended sediment transport have been undertaken on an intertidal mud¯at of the microtidal Danish Wadden Sea. In order to obtain a broader picture of the suspended sediment transport of ®ne-grained material in the study area, measurements on both an intertidal and a subtidal site were carried out during a number of tidal periods and additional information on sedimentation was gathered from repeated bed level measurements along a mud¯at transect. This paper presents the results of these investigations. 2. Study area The investigated mud¯at is situated in the
71
microtidal Lister Dyb tidal basin (Fig. 1). The Lister Dyb tidal basin is situated between the islands of Sylt and Rùmù, on the one hand, and the mainland of Jutland, on the other. Due to the presence of dams connecting the islands with the mainland, the only pathway of exchange of water with the North Sea is through the Lister Dyb tidal inlet. The mean tidal range is approximately 1.8 m and the water column is normally not strati®ed, the estuary being classi®ed as a well-mixed coastal plain estuary. The tidal ¯ats of the basin are mainly sandy but extensive mud¯ats have formed in the areas close to the dams. The study site is situated 2 km south of the Rùmù Dam along the east coast of Rùmù island close to the village Kongsmark. Due to the location of the mud¯at off the east coast of Rùmù, the wind is offshore at the mud¯at during periods of the prevailing westerly winds in which the barrier coastline in general is experiencing onshore winds. This should be kept in mind when looking at the data, which will be presented. The pro®le of the mud¯at along the transect at Kongsmark is shown in Fig. 2 along with the position of station A. The innermost 150 m of the pro®le is concave whereas the rest of the pro®le is almost ¯at with a slope of 0.0011. A 0.5 m high salt marsh cliff is situated 0.25 m above mean high water spring tide (MHWS) followed by 20 m of Spartina sp. marsh to MHWS. The rest of the pro®le has no vegetation and is generally smooth with local disturbances caused by ice deformations (e.g. 150 m from the salt mash cliff). The present average yearly
Fig. 2. Elevation pro®le of the mud¯at transect at Kongsmark. Letter A marks the site at which time series measurements of suspended sediment transport were carried out.
72
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 3. Scatter diagram showing data used for the calibration of the OBS-sensor at station A and B. Suspended sediment concentrations with fecal pellet contents above and below 15% are distinguished. The dotted line regression was used at station A for turbidities higher than 117 NTU. The solid line regression was used for all other data. Solid line: Y 4 1 1:28X 1 0:011X 2 , n 27; r2 0:95: Dotted line: Y 2561 1 7:30X, n 20; r2 0:64:
accumulation of ®ne-grained material in the bay, calculated on the basis of 210Pb-datings by Pejrup et al. (1997), amounts to 57 000 t of which 64% is imported from the North Sea, 14% stems from ¯uvial supply and 15% is derived from primary production, 5% comes from local erosion of salt marsh cliffs, and only 2% is contributed from atmospheric deposition. According to 210Pb and 137 Cs dating (Andersen et al., 2000) the present accumulation rate at the study site is in the order of 8 mm a 21. The bed material on the mud¯at is very ®ne-grained with a sand content of less than 5%. The material is generally highly aggregated, up to 60% comprising fecal pellets produced by the gastropod Hydrobia ulvae. This aggregation is in¯uencing both the erodibility of the bed (Austen et al., 1999; Andersen, 2001) and the settling velocities of the suspended material (Andersen, 2001). Station B is located in the shallow tidal channel 2.6 km south of station A (Fig. 1). The channel drains the mud¯at at Kongsmark and is approximately 500 m wide with a mean water depth of 2.5 m. The bed is muddy with 60% mud and 40% ®ne-grained sand. The fecal pellet content of the bed material is only about 10% at this site (measured December 1999, contents in the summer could be higher).
3. Methods 3.1. Bed level measurements The temporal variation of the bed level along the transect was measured at 5 points using a combination of measurements relative to a reference rod and to buried metal plates. At each point two 2 m long iron rods were pushed vertically into the bed spaced about 1.8 m apart. Bed level were measured by placing a 2 m long aluminium bar on top of the two rods and measuring the vertical distance between the rod and the bed surface at 20 cm intervals. The accuracy of the measurements is estimated at ^2 mm. The measurements were conducted in the period February 1997± September 2000 at intervals of 2±4 weeks, but longer intervals had sometimes to be chosen in the winter periods because of formation of ice. Because the iron rods are normally bent or otherwise disturbed by ice, metal reference plates (20 £ 20 cm 2) were buried horizontally in the bed at each station, thereby allowing resetting the iron rods. 3.2. Water column measurements At station A the current velocity, water depth, salinity, water temperature, and suspended sediment
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
concentration (by means of an optical backscattering sensor, OBS) were measured 20 cm above the bed every 10 min for a total of 430 tidal periods with an Aanderaa RCM9 sensor package. Additional measurements of current velocity, water depth, suspended sediment concentration and in situ settling velocities of the suspended sediment were obtained at a number of occasions using an in¯atable dinghy. Current velocities were measured with an OTT 31 current meter, and water samples being collected with an integrating sampler (Nilsson, 1969). The water samples were taken 20 cm above the bed, i.e. at the height of the OBS sensor, in order to calibrate the OBS readings. In situ settling velocities of the suspended material were determined using Braystoke SK110 settling tubes. The same equipment con®guration was used at station B in the tidal channel, the sensor height being 50 cm above the bed in this case. The OBS sensor was calibrated during all measuring seasons at site A and in October and December at site B. In general, the NTU readings (Nephelometer Turbidity Units) and the suspended sediment concentrations (SSC) were well correlated, a larger scatter being observed at high turbidities at site A due to suspension of fecal pellets (Fig. 3). These pellets are more compact than the material normally found in suspension, thus producing lower turbidity readings compared to mass concentrations. It is not possible to accurately compensate for this effect, but since high fecal pellet contents are only found in suspension over short periods this will not signi®cantly alter the calculated transport rates. The calibration data at site A was split into two groups, one with a fecal pellet content below, the other above 15%. At site B the fecal pellet content is much lower than at site A (t10% compared to about 60%) and the calibration curve for low fecal pellet contents was used in this case. The wind data was obtained from a measuring station of the Danish Meteorological Institute situated in the North of Rùmù. Only for the last month of the measuring period wind data was obtained from a station 50 km north of the study area because of an instrumental malfunction of the local station. 3.3. Transport calculations Suspended sediment transport per unit width, Qs
73
21
(kg m ), was determined on the basis of integrated ¯ux calculations. This was achieved by step-wise integration of the product between velocity, depth and concentration Qs
n X i1
Ui Di SSCi Di;
where Di is the time interval between successive measurements (600 s), U the velocity measured 20 cm above the bed, D the water depth and SSC is the suspended sediment concentration measured 20 cm above the bed. It was assumed that U and C measured 20 cm above the bed was representative for average water column values. This is obviously an approximation but measurements of both the vertical distribution of velocity and SSC at the site showed only minor variations with depth (Andersen, 1999). A similar argument justi®es the use of the measurements 50 cm above the bed at the tidal channel site, although a larger vertical variation of the velocity is observed here. However, this is partly counterbalanced by a larger vertical variation in suspended sediment concentrations. The computed net sediment transport is obviously biased by the residual water transport. Consequently, residual net sediment transport is only used in the discussion for tidal periods with a net water transport close to zero. 4. Results 4.1. Bed level measurements The results of the bed level measurements from three stations covering more than three years are illustrated in Fig. 4. Some seasonality is observed, especially at the most landward station (station 20) where rapid deposition generally took place from late winter to early summer followed by erosion in the summer, autumn and winter. The systematic seasonal variation is less pronounced at stations 225 and 575 but deposition normally took place in spring, summer and autumn whereas erosion prevailed in the winter periods. The net deposition from March 1997 to September 2000 was 3.3, 2.8 and 1.0 cm a 21 for station 20, 225 and 575, respectively, based on linear regression. The average net deposition at station 575 m calculated on the basis of 210Pb and 137Cs dating amounts
74 T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 4. Temporal changes in relative bed levels measured at three points on the mud¯at transect at Kongsmark.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85 21
0.7±0.8 cm a (Andersen et al., 2000) which is less than the deposition calculated on the basis of the bed level measurements. However, the dry bulk density for the top 40 cm of the bed on which the 210Pb dating was based is 0.59 g cm 23 compared to only 0.34 g cm 23 for the top 5 mm of the bed (average for 1999). Correcting the bed level measurements for the lower density of the surface material, an equivalent net deposition of 0.6 cm a 21 is found which is close to the sedimentation found on the basis of 210Pb dating. Similar corrections for station 20 and 225 gives net accretion of 1.9 and 1.6 cm yr 21, respectively. 4.2. Time series of suspended sediment transport Time series data on suspended sediment transport were collected both on the mud¯at (station A) and in a nearby tidal channel (station B). At station A, the data covered more than 400 tidal cycles in the period July± October 1997 and April±November 1998. At station B the measurements were carried out for almost 160 tidal cycles in the periods 21 May±18 June and 19 October±13 December 1999. 4.3. Suspended sediment transport at station A: onshore winds An example of the suspended sediment transport on the mud¯at during a period of onshore winds is illustrated in Fig. 5. The E±SE winds had typical speeds of 5±8 m s 21. In this situation the residual ¯ow is seawards (about 1000 m 3 m 21 tide 21) and the suspended sediment ¯ux is strongly seawards, on average reaching about 500 kg m 21 per tidal cycle for the seven tidal periods. The transport weighted mean SSC during the ¯ood cycle was 347 mg l 21, whereas it was 449 mg l 21 for the ebb cycle. Consequently, the SSC of the water draining the mud¯at during the ebb period had a mean concentration that was about 100 mg l 21 higher than that of the water entering the mud¯at during the ¯ood cycle. 4.4. Suspended sediment transport at station A: offshore winds At times of offshore winds at the study site (westerly winds) the average SSC is low even for wind speeds up to 10±15 m s 21, when the SSC
75
increases because of a generally higher SSC in the tidal basin as a whole. An example from a period with northwesterly winds (offshore at the study site) with speeds of 5±10 m s 21 is shown in Fig. 6. The SSC varies between 20 and 250 mg l 21 with clear asymmetries: the SSC along the leading edge of the water ¯ooding the mud¯at is comparatively high (50±250 mg l 21) but the SSC quickly decreases and is consistently lower during the ebb cycle. The result is a net import of suspended material to the mud¯at. For the six tidal periods shown in Fig. 6 the residual transport of water was 2 3500 m 3 m 21, whereas the landward suspended sediment transport was about 40 kg m 21 for the period. The systematic, temporal variation of SSC over a tidal cycle was similar during periods of stronger offshore winds. 4.5. Suspended sediment transport in the tidal channel (station B) Typical maximum current velocities at station B are 40±50 cm s 21, the period of low current velocities around high-water slack being much longer than at low-water slack. The period with velocities below 10 cm s 21 is about 20 min at low-water slack compared to 100 min at high-water slack. Because of this tidal current asymmetry, and the advection of the turbidity maximum, the SSC generally shows a clear minimum around high water slack and much higher SSC around low water slack. Typical SSC during calm periods is about 20±100 mg l 21 but this may increase to more than 1000 mg l 21 during and after periods of very strong winds. 4.6. Suspended sediment transport during periods of mixed winds A time series data set of water depth, current velocity, SSC and accumulated suspended sediment ¯ux of ®ve representative tidal periods during summer is illustrated in Fig. 7. Typical wind speeds are generally below 10 m s 21 and wind directions are variable. Southwesterly winds of 3±10 m s 21 were interrupted by a short period of easterly winds (3±8 m s 21). The SSC was typically in the range 40±250 mg l 21 with no marked asymmetry between the ¯ood and ebb cycles and low SSC at high water slack. The residual ¯ux of water was slightly landward with a net landward transport of 6000 m 3 m 21 in the ®ve tidal periods.
76
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 5. Time series of wind speed (m s 21), Wind direction (d N), water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station A on the mud¯at during a period of onshore winds (4±8 m s 21) in November 1998.
When calculating the net transport of suspended material, a landward directed transport of 1400 kg m 21 was found, part of which was due to the residual ¯ux of water.
4.7. Suspended sediment ¯ux during and after a storm surge On 3 December 1999 Denmark experienced the
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
77
Fig. 6. Time series of wind speed (m s 21), Wind direction (d N), water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station A on the mud¯at during a period of offshore winds in September 1998. Note the different scaling of the axis for SSC and Qs.
strongest storm since measurements began in the 19th century. Mean wind speeds reached up to 44 m s 21 at the study site. This resulted in a major storm surge in the area with water levels rising to 4.35 m above mean sea level along the mainland coast. A self-recording
current meter (Aanderaa RCM9) was deployed at station B during this period but unfortunately the current meter sensor had a malfunction. In spite of this problem the other recorded data from the instrument gave a very clear picture of the suspended
78
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 7. Time series of water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station B in the tidal channel during a period of mixed winds of mainly low velocities in June 1999.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
79
Fig. 8. Time series of wind speed (m s 21), Wind direction (d N), water depth (cm), and suspended sediment concentration SSC (mg l 21) at station B during a storm surge in December 1999.
sediment transport during and especially after the storm surge. The time series data of the storm surge are shown in Fig. 8. The maximum water depth at site B was 700 cm and typical maximum SSC were 800±1000 mg l 21, which is about an order of magnitude higher than observed during calm periods. The SSC was high during most of the storm surge with a tendency for decreasing values during the falling stage, although it remained high over the whole period following the storm surge. There was considerable, but systematic variation. Thus, during the ¯ood cycles typical SSCs were 500±1000 mg l 21, whereas during the ebb cycles they generally decreased to values of 100±500 mg l 21. Although no current data is available, it seems reasonable to conclude that at the study site the net transport of suspended material was slightly landwards during the storm surge and strongly landwards in the ®rst few days following the surge.
In order to make an approximate computation of the net ¯ux of suspended material in the period right after the storm surge, current measurements from the same site during a period with similar water level ¯uctuations were used (hereafter called ªreference periodº). The average water level was approximately one meter higher during the period after the storm surge. Nevertheless, both the current direction and approximate current magnitude were probably fairly similar due to the very similar water level ¯uctuations. 2D hydrodynamical modelling from the area carried out recently support these approximations (U. Lumborg and A. Windelin, personal communication). Water depth, current velocity, SSC and cumulative suspended sediment transport are shown in Fig. 9 for the ®rst ®ve tidal cycles after the storm surge together with data from the reference period. Whereas the net suspended sediment transport during the reference period was 11.5 t m 21, the net transport was 123 t m 21 for the period after the storm surge and
80
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 9. Time series of water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station B in the tidal channel after the storm surge of December 1999. Also shown is the reference period with current velocities used for the calculation of the suspended sediment transport in the period after the storm surge (dotted lines).
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85 21
of this 17 t m was supplied during the ®rst two tidal cycles. With an approximate channel width of 500 m and a uniform bed level this corresponds to a total net suspended sediment transport in the channel of 12,000 t for the full period and 8000 t for the ®rst two cycles following the storm surge.
5. Discussion 5.1. Bed level measurements The net deposition found on the basis of the full period corresponds to deposition rates of 1.9, 1.6 and 0.6 cm yr 21 for the stations 20, 225 and 575, respectively, when correcting for the lower density of the surface material. Only a small portion of the sediment deposited at the mud¯at contributes to net accumulation. This indicates that the mud¯at is close to a dynamic equilibrium in which deposition during some periods is almost counter-balanced by erosion during other periods. There is good agreement between the net-sedimentation found on the basis of 210 Pb and 137Cs dating (giving the net-deposition during the last 50±100 years) and the sedimentation found on the basis of the three years of measurements of bed-level changes. This suggests that the net deposition for periods of only a few years could compare reasonably well with long term accretion rates, although high reworking of the surface material of the mud¯at obviously takes place (large annual variation in bed levels, Fig. 4). However, this similarity may be purely co-incidental. The bed level data from station 20 show repeated deposition during spring seasons followed by erosion during summer and autumn. The deposition is believed to be caused both by sedimentation of material from ice-¯oes (Pejrup and Andersen, 2000) and by sedimentation in areas covered by bio®lms, and therefore with restricted erosion (Austen et al., 1999; Andersen, 2001). Sedimentation of ice-rafted sediments will take place in winter/early spring when the ice melts and this type of deposition will obviously have a strong seasonality with erosion during the winter period (sediment being incorporated into the ice ¯oes) and deposition during the spring season (ice melt). Similarly, enhanced deposition and/or restricted erosion due to the presence of bio®lms
81
will also tend to show some seasonality (e.g. Underwood and Paterson, 1993; Andersen, 2001). Bio®lms were found at the most landward station at the mud¯at at Kongsmark in May 1997 (Austen et al., 1999) and during most of 1999 and 2000 and a tendency for stronger development of the bio®lms in spring and summer was observed during 1999 (Andersen, 2001). The temporal variation of the sedimentation induced by the two mechanisms mentioned above is expected to be highest on the upper mud¯at where mat-forming benthic diatoms are generally more abundant due to higher availability of light. This part of the intertidal, together with the salt marsh also receives most of the ice-deposited sediment (Pejrup and Andersen, 2000). The bed level measurements at the most landward station con®rm this interpretation as a clear tendency for deeper and more rapid reworking of the bed surface is observed at this site (see Fig. 4). The tendency for deposition during spring and summer and erosion during autumn and winter at the two seaward stations could be explained by seasonal variations in temperature, solar radiation, and wind climate. Higher temperature and radiation will enhance biological activity, which, in turn, will affect the erodibility of the mud¯at. Studies on seasonal variations in the erodibility of an intertidal mud¯at have been reported by Amos et al. (1988), Underwood and Paterson (1993), O'Brien (1998) and Widdows et al. (2000). A tendency for higher erosion thresholds in the warm seasons was generally found and this was explained as a consequence of increased evaporation and/or increased stabilization induced by benthic diatoms. However, studies in 1999 and 2000 of the seasonal erodibility of the Kongsmark mud¯at have shown increased erodibility in the warmer seasons except for the most landward station where the erosion threshold remained high during the study period (Andersen, 2001b). It is likely that the increase in sediment stability in the winter period is due to lower feeding activity of the dominant macro-faunal species H. ulvae and consequently a lower content of fecal pellets in the bed material. The aggregate structure and hence the settling velocity of the suspended material is also a function of biological activity (Andersen, 2001), with larger aggregates (increasing contents of fecal pellets) favoring deposition during the warmer seasons. Less
82
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
aggregation and lower settling velocities due to lower temperatures during winter periods, by contrast, will hamper deposition of suspended material, while the stronger wind climate in winter will promote resuspension. These processes, together with the incorporation of surface sediments in grounded ice, will also explain the tendency for erosion of the mud¯at during late autumn and winter. Erosion in the winter period was also observed on the macrotidal Skef¯ing mud¯at in the Humber Estuary (Christie et al., 1999) and in the Deben Estuary (Frostick and McCave, 1979). However the system is very complex and tidal basins might be importing ®ne-grained material in the winter during calm periods or in periods following storm surges (see later). From Fig. 4 it is also seen that there is a tendency for phase-reversal between station 20 and the two seaward stations. It seems as if the large amount of material deposited during late winter and spring at station 20 is redistributed over the entire mud¯atpro®le later in the year. The reason is probably that the temporary strong deposition at the landward part of the mud¯at creates a steeper pro®le, which is not in equilibrium with the forces generated by tidal currents and waves. 5.2. Suspended sediment transport at station A Typical maximum current velocities at station A on the mud¯at were 25±30 cm s 21. From vertical pro®les of current velocities, bed shear stresses induced by currents alone were calculated to be in the order of 0.05±0.2 N m 22 (Andersen, 1999; Austen et al., 1999). Clearly, such low bed shear stresses are unable to erode the bed at the site, erosion thresholds between 0.16 and more than 3 N m 22 having been determined by Austen et al. (1999). Tidal ¯ats are very sensitive to wind-induced wave action because of the low water depths, which favor the generation of high bed shear stresses which, in turn, promote sediment resuspension (e.g. Freeman et al., 1994). Wave-induced resuspension is evidently an important process at the microtidal Kongsmark site, especially as the bed material is very ®negrained, the SSC rising to at least 5000 mg l 21 under even small breaking waves (Andersen, 1999). The strong in¯uence of waves means that the net transport of suspended material is strongly dependent on wind
speed and direction. This is consistent with results presented by Pejrup (1986), Christie et al. (1999) and Bassoullet et al. (2000). The calculation of suspended sediment transport at station A revealed that the net transport was highly dependent on the residual water transport, even though clear asymmetries in the SSC can be observed over a tidal period. However, the difference in SSC between ¯ood and ebb will indicate the net transport direction (offshore/onshore), although exact amounts cannot be calculated. The much higher SSC of the ebb compared to the ¯ood during periods of onshore winds clearly shows that the mud¯at is eroded during these inundation periods. The time series show that the suspended sediment transport was directed from the local tidal channel towards the mud¯at during periods of calm weather or offshore winds. This is primarily due to wind-shelter during these events, waves at the site being small and incapable of eroding the mud¯at surface. Some of the material brought to the mud¯at during innundation will settle out during high-water slack but will not be resuspended during the following ebb-period as the current alone is incapable of eroding the bed. The suspended sediment transport rates are low under situations of offshore winds and the net transport found for the six tidal periods in Fig. 6 is small and it would take 300 tidal periods with similar conditions to explain the total annual net sedimentation on the mud¯at. Analysis of the full data set from station A shows that net import of suspended material to the mud¯at primarily takes place in the form of small but repeated imports during each tidal cycle in periods of weak or offshore winds, i.e. the import is not event controlled. The net export on the other hand is generally restricted in time to a few tidal periods with onshore winds at the site. 5.3. Suspended sediment transport in the tidal channel (station B) The data set shows that the net transport of suspended material in the channel during calmer periods are fairly low due to low SSC and only minor asymmetries between ¯ood and ebb SSC. The net transport of ®ne-grained suspended material is directed landwards as a result of settling Ð and scour lag and tidal asymmetry. During windy periods larger
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
¯uxes of suspended material is observed, the situation during and after the storm surge being an extreme example of this. Little net transport was observed during the storm surge but in the following period a large input of suspended material to this part of the tidal basin was observed. For the ®ve tidal cycles shown in Fig. 9 a suspended sediment transport of 12,000 t was calculated, of which 8000 t were supplied during the ®rst two tidal cycles following the storm surge. The yearly net input to the entire Lister Dyb tidal basin is 57,000 t (Pejrup et al., 1997) of which 28,000 t accumulates in Rùmù Bay. The net import during the ®rst ®ve tidal cycles after the storm surge thus corresponded to approximately 40% of the total yearly accumulation in the entire Rùmù Bay, the import during the ®rst two tidal cycles accounting to about 30%. There is no reason to believe that a strong outward transport of suspended sediment took place during the storm surge itself as the recorded OBS-data during the storm surge showed a weak tendency for higher SSC during ¯ood than ebb. The conclusion is therefore that the storm surge caused a strong inward transport of suspended sediment to this part of the tidal basin equivalent to approximately 40% of the total yearly accumulation. The reason for the strong inward transport of suspended material in this part of the tidal basin is probably that large amounts of ®ne-grained bed material are mobilized in both the tidal basin and the adjacent part of the North Sea during periods of strong westerly winds. The settling velocity of the suspended material will increase with increasing SSC when the turbulence decreases which will have the effect that a net transport due to settling lag (Van Straaten and Kuenen, 1958) will be effective. The seaward transport of suspended material which has been shown to be induced by horizontal diffusion (e.g. Amos and Tee, 1989) is apparently more than balanced by settling Ð and scour lag and tidal asymmetry in this shallow type of estuary. It must be noted that the import of suspended matter after the storm surge only was measured in this restricted part of the tidal basin. No information is available on the suspended sediment concentrations or transport in the rest of the tidal basin. However, it is likely that erosion prevailed in the eastern part of the basin due to strong wave action and it is therefore also likely that at least part of the material imported to
83
Rùmù Bay is sediments which were eroded locally within the tidal basin. Consequently, the result of the storm surge was probably a strong internal redistribution of material within the bay and it is not possible on the basis of the data set to estimate the amount contributed from the North Sea, i.e. the primary import. It was shown by Pejrup et al. (1997) that the net sedimentation on salt marshes within the Lister Dyb tidal basin accounts for about 17% of the total accumulation of ®ne-grained material within the basin. The sedimentation on the salt marshes only takes place during periods of increased water levels, i.e. during periods of strong westerly winds. This implies that the salt marshes are responsible for a substantial part of the import found during these periods and therefore also the storm surge period shown in Fig. 9. It is likely that a major part of this material too is eroded on intertidal ¯ats in the basin (Bartholdy and Anthony, 1998) 6. Conclusions The objective of the present study have been to gain qualitative and quantitative information on the deposition, erosion and transport of ®ne-grained suspended material on an intertidal mud¯at situated in the Lister Dyb tidal basin, the Wadden Sea. Bed level measurements have been carried out for more than three years and the suspended sediment transport during almost 600 tidal cycles have been measured at a station at the mud¯at or in a nearby tidal channel. Reworking of the sediment surface is much larger than the net accumulation, the reworking depth being 3±8 cm as compared to a yearly net accumulation of approximately 0.8 cm determined on the basis of 210 Pb and 137Cs dating. This indicates that the mud¯at is close to dynamic equilibrium and deposition during periods of calm weather or local offshore winds in the study area is balanced by erosion during periods of onshore winds. When correcting for bulk density, the calculated net accumulation on the basis of bed level measurement amounts to 0.6 cm a 21 for the same site as the 210Pb dating. Some degree of seasonality was observed for erosion and deposition on the mud¯at, with a
84
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
tendency for deposition in spring and summer and erosion in the winter. The seasonal signal, however, was sometimes disturbed or even reversed, being mainly controlled by both differences in biological stabilization and aggregation of the bed material and freezing of the sediment surface in cold winters. In addition, temperature-induced viscosity changes of the water and associated changes in settling velocity will produce a seasonal effect as will seasonal changes in wind climate. Reworking is largest close to the salt marsh. This is explained by deposition of soft, ice-rafted sediment in the spring and enhanced net deposition in areas characterized by well developed bio®lms in spring, summer and autumn. With the exception of periods of local onshore winds, the study area received a supply of ®ne-grained suspended material. A very large input of suspended material was found in the period following a strong storm, in particular during the ®ve tidal periods following the storm during which net transport equivalent to approximately 40% of the yearly net accumulation took place. Acknowledgements The ®eldwork was made possible through ®nancial support by the MAST III project INTRMUD (MAS3CT95-0022) and by the Danish Natural Science Research Council, grant no. 9701836. Great thanks goes to Ulrik Lumborg, Anders Windelin, Ole Aarup Mikkelsen and Annette LuÈtzen Mùller for help with ®eldwork and data and to Paul Christiansen, Heini Larsen, Kirsten Simonsen and Ulf Thomas for logistic support. Finally, we would like to thank B.W. Flemming and an anonymous reviewer for their fruitful comments on an earlier version of this manuscript. References Amos, C.L., 1995. Siliclastic tidal ¯ats. In: Perillio, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries. Elsevier, Amsterdam, pp. 273±306. Amos, C.L., Tee, K.T., 1989. Suspended sediment transport processes in Cumberland basin, Bay of Fundy. Journal of Geophysical Research 94 (c10), 14 407±14 417. Amos, C.L., Van Wagoner, N.A., Daborn, G.R., 1988. The in¯u-
ence of subarial exposure on the bulk properties of ®ne-grained intertidal sediment from Minas Basin, Bay of Fundy. Estuarine, Coastal and Shelf Science 27, 1±13. Andersen, T.J., 1999. Suspended sediment transport and sediment reworking at an intertidal mud ¯at, the Danish Wadden Sea. Meddelelser fra Skalling-laboratoriet, Vol. 37 72pp. Andersen, T.J., 2000. The role of fecal pellets in suspended sediment settling velocities at an intertidal mud¯at, the Danish Wadden Sea. In: McAnally, W.H., Mehta, A.J. (Eds.), Coastal and Estuarine Fine Sediment Processes. Elsevier, Amsterdam, pp. 387±401. Andersen, T.J., 2001. Seasonal variation in erodibility of two temperate, microtidal mud¯ats. Estuarine, Coastal and Shelf Science (in press). Andersen, T.J., Mikkelsen, O.A., Mùller, A.L., Pejrup, M., 2000. Deposition and mixing depths on some European intertidal mud¯ats based on 210Pb and 137Cs activities. Continental Shelf Research 20 (12/13), 1569±1591. Austen, I., Andersen, T.J., Edelvang, K., 1999. The in¯uence of benthic diatoms and invertebrates on the erodibility of an intertidal mud¯at, the Danish Wadden Sea. Estuarine, Coastal and Shelf Science 49, 99±111. Bartholdy, J., Anthony, D., 1998. Tidal dynamics and seasonal dependent import and export of ®ne-grained sediment through a back-barrier tidal channel of the Danish Wadden Sea. In: Tidalites: Processes and Products, SEPM Special Publication, Vol. 61, pp. 43±52. Bassoullet, P.H., Le Hir, P., Gouleau, D., Robert, S., 2000. Sediment transport over an intertidal mud¯at: ®eld investigations and estimation of ¯uxes within the Baie de Marennes-Oleron (France). Continental Shelf Research 20 (12/13), 1635±1653. Christie, M.C., Dyer, K.R., Turner, P., 1999. Sediment ¯ux and bed level measurements from a macro tidal mud¯at. Estuarine, Coastal and Shelf Science 49, 667±688. Flemming, B.W., Nyandwi, N., 1994. Land reclamation as a cause of ®ne-grained sediment depletion in backbarrier tidal ¯ats (Southern North Sea). Netherlands Journal of Aquatic ecology 28 (3-4), 299±308. Freeman, D.P., Coates, L.E., Ockenden, M.C., Roberts, W., West, J.R., 1994. Cohesive sediment transport on an inter-tidal zone under combined wave-tidal ¯ow. Netherlands Journal of Aquatic Ecology 28 (3±4), 283±288. Frostick, L.E., McCave, I.N., 1979. Seasonal shifts of sediment within an estuary mediated by algal growth. Estuarine and Coastal Marine Science 9, 569±576. Kirby, R., Bleakley, S.T., Weatherup, C., Raven, P.J., Donaldson, D., 1992. Effect of episodic events on tidal mud ¯at stability, Ardmillan Bay, Strangford Lough, Northern Ireland. In: Mehta, A.J. (Ed.), Nearshore and Estuarine Cohesive Sediment Transport, 42. Paci®c Rim Congress, pp. 378±392. Nilsson, B., 1969. Development of a depth-integrating water sampler. UNGI rapport No. 2, Uppsala Universitet. O'Brien, D.J., 1998. The sediment dynamics of a macrotidal mud¯at on varying timescales. Ph.D. Thesis, University of Wales, 135 pp. O'Brien, D.J., Whitehouse, R.J.S, Cramp, A., 2000. The cyclic development of a macrotidal mud¯at on varying timescales. Continental Shelf Research 20 (12/13), 1593±1619.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85 Pejrup, M., 1986. Parameters affecting ®ne-grained suspended sediment concentrations in a shallow micro-tidal estuary, Ho Bugt. Estuarine Coastal and Shelf Science 22, 241±254. Pejrup, M., 1988. Suspended sediment transport across a tidal ¯at. Marine Geology 82, 187±198. Pejrup, M., Andersen, T.J., 2000. The in¯uence of ice ¯ow on sediment transport, deposition and reworking in a temperate intertidal mud¯at area, the Danish Wadden Sea. Continental Shelf Research 20, 1621±1634. Pejrup, M., Larsen, M., Edelvang, K., 1997. A ®ne-grained sediment budget for the Sylt-Rùmù tidal basin. HelgolaÈnder Meeresuntersuchungen 51, 1±15. Reise, K., 1998. Coastal change in a tidal backbarrier basin of the
85
Northern Wadden Sea: Are tidal ¯ats fading away?. Senckenbergiana Maritima 29 (1/6), 121±127. Underwood, G.J.C., Paterson, D.M., 1993. Seasonal changes in diatom biomass, sediment stability and biogenic stabilization in the Severn Estuary. Journal of Marine Biological Association 73, 871±887. Van Straaten, L.M.J.U., Kuenen, Ph.H., 1958. Tidal action as a cause of clay accumulation. Journal of Sedimentary Petrology 28 (4), 406±413. Widdows, J., Brown, S., Brinsley, M.D., Salkeld, P.N., Elliott, M., 2000. Temporal changes in intertidal sediment erodability: in¯uence of biological and climatic factors. Continental Shelf Research 29, 1275±1289.
www.elsevier.nl/locate/margeo
Suspended sediment transport on a temperate, microtidal mud¯at, the Danish Wadden Sea T.J. Andersen*, M. Pejrup Institute of Geography, University of Copenhagen, éster Voldgade 10, DK-1350 Kùbenhavn K, Denmark Received 24 February 2000; accepted 13 December 2000
Abstract Time series measurements of suspended sediment transport and bed level change on a microtidal mud¯at of the Danish Wadden Sea were carried out between February 1997 and September 2000. The data show that the mud¯at is generally accreting except for periods with onshore winds when wave-induced erosion takes place during periods of low water levels at the site. In general, a continuous net input of mud was recorded during periods of weak or offshore winds, i.e. the import is not event controlled, and this was interrupted by episodic loss of mud during a few tidal periods with onshore winds. The net landward transport of suspended sediment during calm weather or periods of offshore wind is caused by settling and scour lag and tidal asymmetry. A very large input of suspended material was observed in the period following a strong storm, the landward directed net transport during the ®ve tidal periods following the storm being equivalent to approximately 40% of the net annual accumulation in the area. The typical depth of sediment reworking at the intertidal study site is 2±5 cm. Some seasonal variation can be deduced from the data set with a tendency for deposition during spring and summer and erosion in the winter period. The seasonality was especially clear at the most landward station where accretion following periods of ice-formation in the area is due to grounding of sediment transported by ice-¯oes but otherwise deposition is mainly taking place in the warmer seasons. Net accretion over the three-year measuring period along the transect varies between 0.6 and 1.9 cm a 21. Deposition is largest at the most landward station and the accretion found on the basis of repeated bed level measurements compares well with 210Pb and 137Cs dating. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Mud¯at; Deposition; Cohesive; Sediment; Transport; Reworking
1. Introduction A quanti®cation of ®ne-grained sediment accumulation is important both for environmental and economic reasons. Heavy metals, nutrients, pesticides and herbicides tend to stick to ®ne-grained material and knowledge of the transport pathways and the * Corresponding author. Tel.: 145-35-32-25-03; fax: 145-35-3225-01. E-mail address: [email protected] (T.J. Andersen).
potential for erosion and deposition of the material will consequently give information on the fate of these toxic or otherwise environmentally harmful substances. Deposition of ®ne-grained material in ports or navigation channels is also an issue of great economic importance, which calls for knowledge of the potential for erosion and deposition in estuarine environments. Since mud¯ats are both quantitatively and qualitatively very important sinks for ®ne-grained material, a thorough understanding of the processes governing
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(00)00164-X
70
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 1. Sedimentological map of the study area showing the locations of station A and B off the east coast of Rùmù. The subtidal parts of the tidal basin are mainly sandy.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
deposition and erosion in these environments is of foremost importance. In addition, mud¯ats are important biotopes for a number of macrofaunal species and act as feeding places for both birds and nursery for ®sh fry. Detailed investigations of suspended sediment transport on intertidal mud¯ats have been undertaken, e.g. Frostick and McCave (1979), Pejrup (1988), Kirby et al. (1992), Amos (1995), O'Brien (1998), Christie et al. (1999), and O'Brien et al. (2000). The area occupied by mud¯ats in the European Wadden Sea has been reduced over past centuries by land reclamation (e.g. Flemming and Nyandwi (1994) and Reise (1998)). With the prospect of an accelerating sea-level rise, concern has arisen as to the fate of the remaining mud¯ats and salt marshes. As part of the EU-funded MAST III project INTRMUD concerned with mud¯at sedimentation and classi®cation, studies of suspended sediment transport have been undertaken on an intertidal mud¯at of the microtidal Danish Wadden Sea. In order to obtain a broader picture of the suspended sediment transport of ®ne-grained material in the study area, measurements on both an intertidal and a subtidal site were carried out during a number of tidal periods and additional information on sedimentation was gathered from repeated bed level measurements along a mud¯at transect. This paper presents the results of these investigations. 2. Study area The investigated mud¯at is situated in the
71
microtidal Lister Dyb tidal basin (Fig. 1). The Lister Dyb tidal basin is situated between the islands of Sylt and Rùmù, on the one hand, and the mainland of Jutland, on the other. Due to the presence of dams connecting the islands with the mainland, the only pathway of exchange of water with the North Sea is through the Lister Dyb tidal inlet. The mean tidal range is approximately 1.8 m and the water column is normally not strati®ed, the estuary being classi®ed as a well-mixed coastal plain estuary. The tidal ¯ats of the basin are mainly sandy but extensive mud¯ats have formed in the areas close to the dams. The study site is situated 2 km south of the Rùmù Dam along the east coast of Rùmù island close to the village Kongsmark. Due to the location of the mud¯at off the east coast of Rùmù, the wind is offshore at the mud¯at during periods of the prevailing westerly winds in which the barrier coastline in general is experiencing onshore winds. This should be kept in mind when looking at the data, which will be presented. The pro®le of the mud¯at along the transect at Kongsmark is shown in Fig. 2 along with the position of station A. The innermost 150 m of the pro®le is concave whereas the rest of the pro®le is almost ¯at with a slope of 0.0011. A 0.5 m high salt marsh cliff is situated 0.25 m above mean high water spring tide (MHWS) followed by 20 m of Spartina sp. marsh to MHWS. The rest of the pro®le has no vegetation and is generally smooth with local disturbances caused by ice deformations (e.g. 150 m from the salt mash cliff). The present average yearly
Fig. 2. Elevation pro®le of the mud¯at transect at Kongsmark. Letter A marks the site at which time series measurements of suspended sediment transport were carried out.
72
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 3. Scatter diagram showing data used for the calibration of the OBS-sensor at station A and B. Suspended sediment concentrations with fecal pellet contents above and below 15% are distinguished. The dotted line regression was used at station A for turbidities higher than 117 NTU. The solid line regression was used for all other data. Solid line: Y 4 1 1:28X 1 0:011X 2 , n 27; r2 0:95: Dotted line: Y 2561 1 7:30X, n 20; r2 0:64:
accumulation of ®ne-grained material in the bay, calculated on the basis of 210Pb-datings by Pejrup et al. (1997), amounts to 57 000 t of which 64% is imported from the North Sea, 14% stems from ¯uvial supply and 15% is derived from primary production, 5% comes from local erosion of salt marsh cliffs, and only 2% is contributed from atmospheric deposition. According to 210Pb and 137 Cs dating (Andersen et al., 2000) the present accumulation rate at the study site is in the order of 8 mm a 21. The bed material on the mud¯at is very ®ne-grained with a sand content of less than 5%. The material is generally highly aggregated, up to 60% comprising fecal pellets produced by the gastropod Hydrobia ulvae. This aggregation is in¯uencing both the erodibility of the bed (Austen et al., 1999; Andersen, 2001) and the settling velocities of the suspended material (Andersen, 2001). Station B is located in the shallow tidal channel 2.6 km south of station A (Fig. 1). The channel drains the mud¯at at Kongsmark and is approximately 500 m wide with a mean water depth of 2.5 m. The bed is muddy with 60% mud and 40% ®ne-grained sand. The fecal pellet content of the bed material is only about 10% at this site (measured December 1999, contents in the summer could be higher).
3. Methods 3.1. Bed level measurements The temporal variation of the bed level along the transect was measured at 5 points using a combination of measurements relative to a reference rod and to buried metal plates. At each point two 2 m long iron rods were pushed vertically into the bed spaced about 1.8 m apart. Bed level were measured by placing a 2 m long aluminium bar on top of the two rods and measuring the vertical distance between the rod and the bed surface at 20 cm intervals. The accuracy of the measurements is estimated at ^2 mm. The measurements were conducted in the period February 1997± September 2000 at intervals of 2±4 weeks, but longer intervals had sometimes to be chosen in the winter periods because of formation of ice. Because the iron rods are normally bent or otherwise disturbed by ice, metal reference plates (20 £ 20 cm 2) were buried horizontally in the bed at each station, thereby allowing resetting the iron rods. 3.2. Water column measurements At station A the current velocity, water depth, salinity, water temperature, and suspended sediment
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
concentration (by means of an optical backscattering sensor, OBS) were measured 20 cm above the bed every 10 min for a total of 430 tidal periods with an Aanderaa RCM9 sensor package. Additional measurements of current velocity, water depth, suspended sediment concentration and in situ settling velocities of the suspended sediment were obtained at a number of occasions using an in¯atable dinghy. Current velocities were measured with an OTT 31 current meter, and water samples being collected with an integrating sampler (Nilsson, 1969). The water samples were taken 20 cm above the bed, i.e. at the height of the OBS sensor, in order to calibrate the OBS readings. In situ settling velocities of the suspended material were determined using Braystoke SK110 settling tubes. The same equipment con®guration was used at station B in the tidal channel, the sensor height being 50 cm above the bed in this case. The OBS sensor was calibrated during all measuring seasons at site A and in October and December at site B. In general, the NTU readings (Nephelometer Turbidity Units) and the suspended sediment concentrations (SSC) were well correlated, a larger scatter being observed at high turbidities at site A due to suspension of fecal pellets (Fig. 3). These pellets are more compact than the material normally found in suspension, thus producing lower turbidity readings compared to mass concentrations. It is not possible to accurately compensate for this effect, but since high fecal pellet contents are only found in suspension over short periods this will not signi®cantly alter the calculated transport rates. The calibration data at site A was split into two groups, one with a fecal pellet content below, the other above 15%. At site B the fecal pellet content is much lower than at site A (t10% compared to about 60%) and the calibration curve for low fecal pellet contents was used in this case. The wind data was obtained from a measuring station of the Danish Meteorological Institute situated in the North of Rùmù. Only for the last month of the measuring period wind data was obtained from a station 50 km north of the study area because of an instrumental malfunction of the local station. 3.3. Transport calculations Suspended sediment transport per unit width, Qs
73
21
(kg m ), was determined on the basis of integrated ¯ux calculations. This was achieved by step-wise integration of the product between velocity, depth and concentration Qs
n X i1
Ui Di SSCi Di;
where Di is the time interval between successive measurements (600 s), U the velocity measured 20 cm above the bed, D the water depth and SSC is the suspended sediment concentration measured 20 cm above the bed. It was assumed that U and C measured 20 cm above the bed was representative for average water column values. This is obviously an approximation but measurements of both the vertical distribution of velocity and SSC at the site showed only minor variations with depth (Andersen, 1999). A similar argument justi®es the use of the measurements 50 cm above the bed at the tidal channel site, although a larger vertical variation of the velocity is observed here. However, this is partly counterbalanced by a larger vertical variation in suspended sediment concentrations. The computed net sediment transport is obviously biased by the residual water transport. Consequently, residual net sediment transport is only used in the discussion for tidal periods with a net water transport close to zero. 4. Results 4.1. Bed level measurements The results of the bed level measurements from three stations covering more than three years are illustrated in Fig. 4. Some seasonality is observed, especially at the most landward station (station 20) where rapid deposition generally took place from late winter to early summer followed by erosion in the summer, autumn and winter. The systematic seasonal variation is less pronounced at stations 225 and 575 but deposition normally took place in spring, summer and autumn whereas erosion prevailed in the winter periods. The net deposition from March 1997 to September 2000 was 3.3, 2.8 and 1.0 cm a 21 for station 20, 225 and 575, respectively, based on linear regression. The average net deposition at station 575 m calculated on the basis of 210Pb and 137Cs dating amounts
74 T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 4. Temporal changes in relative bed levels measured at three points on the mud¯at transect at Kongsmark.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85 21
0.7±0.8 cm a (Andersen et al., 2000) which is less than the deposition calculated on the basis of the bed level measurements. However, the dry bulk density for the top 40 cm of the bed on which the 210Pb dating was based is 0.59 g cm 23 compared to only 0.34 g cm 23 for the top 5 mm of the bed (average for 1999). Correcting the bed level measurements for the lower density of the surface material, an equivalent net deposition of 0.6 cm a 21 is found which is close to the sedimentation found on the basis of 210Pb dating. Similar corrections for station 20 and 225 gives net accretion of 1.9 and 1.6 cm yr 21, respectively. 4.2. Time series of suspended sediment transport Time series data on suspended sediment transport were collected both on the mud¯at (station A) and in a nearby tidal channel (station B). At station A, the data covered more than 400 tidal cycles in the period July± October 1997 and April±November 1998. At station B the measurements were carried out for almost 160 tidal cycles in the periods 21 May±18 June and 19 October±13 December 1999. 4.3. Suspended sediment transport at station A: onshore winds An example of the suspended sediment transport on the mud¯at during a period of onshore winds is illustrated in Fig. 5. The E±SE winds had typical speeds of 5±8 m s 21. In this situation the residual ¯ow is seawards (about 1000 m 3 m 21 tide 21) and the suspended sediment ¯ux is strongly seawards, on average reaching about 500 kg m 21 per tidal cycle for the seven tidal periods. The transport weighted mean SSC during the ¯ood cycle was 347 mg l 21, whereas it was 449 mg l 21 for the ebb cycle. Consequently, the SSC of the water draining the mud¯at during the ebb period had a mean concentration that was about 100 mg l 21 higher than that of the water entering the mud¯at during the ¯ood cycle. 4.4. Suspended sediment transport at station A: offshore winds At times of offshore winds at the study site (westerly winds) the average SSC is low even for wind speeds up to 10±15 m s 21, when the SSC
75
increases because of a generally higher SSC in the tidal basin as a whole. An example from a period with northwesterly winds (offshore at the study site) with speeds of 5±10 m s 21 is shown in Fig. 6. The SSC varies between 20 and 250 mg l 21 with clear asymmetries: the SSC along the leading edge of the water ¯ooding the mud¯at is comparatively high (50±250 mg l 21) but the SSC quickly decreases and is consistently lower during the ebb cycle. The result is a net import of suspended material to the mud¯at. For the six tidal periods shown in Fig. 6 the residual transport of water was 2 3500 m 3 m 21, whereas the landward suspended sediment transport was about 40 kg m 21 for the period. The systematic, temporal variation of SSC over a tidal cycle was similar during periods of stronger offshore winds. 4.5. Suspended sediment transport in the tidal channel (station B) Typical maximum current velocities at station B are 40±50 cm s 21, the period of low current velocities around high-water slack being much longer than at low-water slack. The period with velocities below 10 cm s 21 is about 20 min at low-water slack compared to 100 min at high-water slack. Because of this tidal current asymmetry, and the advection of the turbidity maximum, the SSC generally shows a clear minimum around high water slack and much higher SSC around low water slack. Typical SSC during calm periods is about 20±100 mg l 21 but this may increase to more than 1000 mg l 21 during and after periods of very strong winds. 4.6. Suspended sediment transport during periods of mixed winds A time series data set of water depth, current velocity, SSC and accumulated suspended sediment ¯ux of ®ve representative tidal periods during summer is illustrated in Fig. 7. Typical wind speeds are generally below 10 m s 21 and wind directions are variable. Southwesterly winds of 3±10 m s 21 were interrupted by a short period of easterly winds (3±8 m s 21). The SSC was typically in the range 40±250 mg l 21 with no marked asymmetry between the ¯ood and ebb cycles and low SSC at high water slack. The residual ¯ux of water was slightly landward with a net landward transport of 6000 m 3 m 21 in the ®ve tidal periods.
76
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 5. Time series of wind speed (m s 21), Wind direction (d N), water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station A on the mud¯at during a period of onshore winds (4±8 m s 21) in November 1998.
When calculating the net transport of suspended material, a landward directed transport of 1400 kg m 21 was found, part of which was due to the residual ¯ux of water.
4.7. Suspended sediment ¯ux during and after a storm surge On 3 December 1999 Denmark experienced the
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
77
Fig. 6. Time series of wind speed (m s 21), Wind direction (d N), water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station A on the mud¯at during a period of offshore winds in September 1998. Note the different scaling of the axis for SSC and Qs.
strongest storm since measurements began in the 19th century. Mean wind speeds reached up to 44 m s 21 at the study site. This resulted in a major storm surge in the area with water levels rising to 4.35 m above mean sea level along the mainland coast. A self-recording
current meter (Aanderaa RCM9) was deployed at station B during this period but unfortunately the current meter sensor had a malfunction. In spite of this problem the other recorded data from the instrument gave a very clear picture of the suspended
78
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 7. Time series of water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station B in the tidal channel during a period of mixed winds of mainly low velocities in June 1999.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
79
Fig. 8. Time series of wind speed (m s 21), Wind direction (d N), water depth (cm), and suspended sediment concentration SSC (mg l 21) at station B during a storm surge in December 1999.
sediment transport during and especially after the storm surge. The time series data of the storm surge are shown in Fig. 8. The maximum water depth at site B was 700 cm and typical maximum SSC were 800±1000 mg l 21, which is about an order of magnitude higher than observed during calm periods. The SSC was high during most of the storm surge with a tendency for decreasing values during the falling stage, although it remained high over the whole period following the storm surge. There was considerable, but systematic variation. Thus, during the ¯ood cycles typical SSCs were 500±1000 mg l 21, whereas during the ebb cycles they generally decreased to values of 100±500 mg l 21. Although no current data is available, it seems reasonable to conclude that at the study site the net transport of suspended material was slightly landwards during the storm surge and strongly landwards in the ®rst few days following the surge.
In order to make an approximate computation of the net ¯ux of suspended material in the period right after the storm surge, current measurements from the same site during a period with similar water level ¯uctuations were used (hereafter called ªreference periodº). The average water level was approximately one meter higher during the period after the storm surge. Nevertheless, both the current direction and approximate current magnitude were probably fairly similar due to the very similar water level ¯uctuations. 2D hydrodynamical modelling from the area carried out recently support these approximations (U. Lumborg and A. Windelin, personal communication). Water depth, current velocity, SSC and cumulative suspended sediment transport are shown in Fig. 9 for the ®rst ®ve tidal cycles after the storm surge together with data from the reference period. Whereas the net suspended sediment transport during the reference period was 11.5 t m 21, the net transport was 123 t m 21 for the period after the storm surge and
80
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
Fig. 9. Time series of water depth (cm), current speed (cm s 21), suspended sediment concentration SSC (mg l 21), water transport per unit width (m 2), and sediment transport per unit width (kg m 21) at station B in the tidal channel after the storm surge of December 1999. Also shown is the reference period with current velocities used for the calculation of the suspended sediment transport in the period after the storm surge (dotted lines).
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85 21
of this 17 t m was supplied during the ®rst two tidal cycles. With an approximate channel width of 500 m and a uniform bed level this corresponds to a total net suspended sediment transport in the channel of 12,000 t for the full period and 8000 t for the ®rst two cycles following the storm surge.
5. Discussion 5.1. Bed level measurements The net deposition found on the basis of the full period corresponds to deposition rates of 1.9, 1.6 and 0.6 cm yr 21 for the stations 20, 225 and 575, respectively, when correcting for the lower density of the surface material. Only a small portion of the sediment deposited at the mud¯at contributes to net accumulation. This indicates that the mud¯at is close to a dynamic equilibrium in which deposition during some periods is almost counter-balanced by erosion during other periods. There is good agreement between the net-sedimentation found on the basis of 210 Pb and 137Cs dating (giving the net-deposition during the last 50±100 years) and the sedimentation found on the basis of the three years of measurements of bed-level changes. This suggests that the net deposition for periods of only a few years could compare reasonably well with long term accretion rates, although high reworking of the surface material of the mud¯at obviously takes place (large annual variation in bed levels, Fig. 4). However, this similarity may be purely co-incidental. The bed level data from station 20 show repeated deposition during spring seasons followed by erosion during summer and autumn. The deposition is believed to be caused both by sedimentation of material from ice-¯oes (Pejrup and Andersen, 2000) and by sedimentation in areas covered by bio®lms, and therefore with restricted erosion (Austen et al., 1999; Andersen, 2001). Sedimentation of ice-rafted sediments will take place in winter/early spring when the ice melts and this type of deposition will obviously have a strong seasonality with erosion during the winter period (sediment being incorporated into the ice ¯oes) and deposition during the spring season (ice melt). Similarly, enhanced deposition and/or restricted erosion due to the presence of bio®lms
81
will also tend to show some seasonality (e.g. Underwood and Paterson, 1993; Andersen, 2001). Bio®lms were found at the most landward station at the mud¯at at Kongsmark in May 1997 (Austen et al., 1999) and during most of 1999 and 2000 and a tendency for stronger development of the bio®lms in spring and summer was observed during 1999 (Andersen, 2001). The temporal variation of the sedimentation induced by the two mechanisms mentioned above is expected to be highest on the upper mud¯at where mat-forming benthic diatoms are generally more abundant due to higher availability of light. This part of the intertidal, together with the salt marsh also receives most of the ice-deposited sediment (Pejrup and Andersen, 2000). The bed level measurements at the most landward station con®rm this interpretation as a clear tendency for deeper and more rapid reworking of the bed surface is observed at this site (see Fig. 4). The tendency for deposition during spring and summer and erosion during autumn and winter at the two seaward stations could be explained by seasonal variations in temperature, solar radiation, and wind climate. Higher temperature and radiation will enhance biological activity, which, in turn, will affect the erodibility of the mud¯at. Studies on seasonal variations in the erodibility of an intertidal mud¯at have been reported by Amos et al. (1988), Underwood and Paterson (1993), O'Brien (1998) and Widdows et al. (2000). A tendency for higher erosion thresholds in the warm seasons was generally found and this was explained as a consequence of increased evaporation and/or increased stabilization induced by benthic diatoms. However, studies in 1999 and 2000 of the seasonal erodibility of the Kongsmark mud¯at have shown increased erodibility in the warmer seasons except for the most landward station where the erosion threshold remained high during the study period (Andersen, 2001b). It is likely that the increase in sediment stability in the winter period is due to lower feeding activity of the dominant macro-faunal species H. ulvae and consequently a lower content of fecal pellets in the bed material. The aggregate structure and hence the settling velocity of the suspended material is also a function of biological activity (Andersen, 2001), with larger aggregates (increasing contents of fecal pellets) favoring deposition during the warmer seasons. Less
82
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
aggregation and lower settling velocities due to lower temperatures during winter periods, by contrast, will hamper deposition of suspended material, while the stronger wind climate in winter will promote resuspension. These processes, together with the incorporation of surface sediments in grounded ice, will also explain the tendency for erosion of the mud¯at during late autumn and winter. Erosion in the winter period was also observed on the macrotidal Skef¯ing mud¯at in the Humber Estuary (Christie et al., 1999) and in the Deben Estuary (Frostick and McCave, 1979). However the system is very complex and tidal basins might be importing ®ne-grained material in the winter during calm periods or in periods following storm surges (see later). From Fig. 4 it is also seen that there is a tendency for phase-reversal between station 20 and the two seaward stations. It seems as if the large amount of material deposited during late winter and spring at station 20 is redistributed over the entire mud¯atpro®le later in the year. The reason is probably that the temporary strong deposition at the landward part of the mud¯at creates a steeper pro®le, which is not in equilibrium with the forces generated by tidal currents and waves. 5.2. Suspended sediment transport at station A Typical maximum current velocities at station A on the mud¯at were 25±30 cm s 21. From vertical pro®les of current velocities, bed shear stresses induced by currents alone were calculated to be in the order of 0.05±0.2 N m 22 (Andersen, 1999; Austen et al., 1999). Clearly, such low bed shear stresses are unable to erode the bed at the site, erosion thresholds between 0.16 and more than 3 N m 22 having been determined by Austen et al. (1999). Tidal ¯ats are very sensitive to wind-induced wave action because of the low water depths, which favor the generation of high bed shear stresses which, in turn, promote sediment resuspension (e.g. Freeman et al., 1994). Wave-induced resuspension is evidently an important process at the microtidal Kongsmark site, especially as the bed material is very ®negrained, the SSC rising to at least 5000 mg l 21 under even small breaking waves (Andersen, 1999). The strong in¯uence of waves means that the net transport of suspended material is strongly dependent on wind
speed and direction. This is consistent with results presented by Pejrup (1986), Christie et al. (1999) and Bassoullet et al. (2000). The calculation of suspended sediment transport at station A revealed that the net transport was highly dependent on the residual water transport, even though clear asymmetries in the SSC can be observed over a tidal period. However, the difference in SSC between ¯ood and ebb will indicate the net transport direction (offshore/onshore), although exact amounts cannot be calculated. The much higher SSC of the ebb compared to the ¯ood during periods of onshore winds clearly shows that the mud¯at is eroded during these inundation periods. The time series show that the suspended sediment transport was directed from the local tidal channel towards the mud¯at during periods of calm weather or offshore winds. This is primarily due to wind-shelter during these events, waves at the site being small and incapable of eroding the mud¯at surface. Some of the material brought to the mud¯at during innundation will settle out during high-water slack but will not be resuspended during the following ebb-period as the current alone is incapable of eroding the bed. The suspended sediment transport rates are low under situations of offshore winds and the net transport found for the six tidal periods in Fig. 6 is small and it would take 300 tidal periods with similar conditions to explain the total annual net sedimentation on the mud¯at. Analysis of the full data set from station A shows that net import of suspended material to the mud¯at primarily takes place in the form of small but repeated imports during each tidal cycle in periods of weak or offshore winds, i.e. the import is not event controlled. The net export on the other hand is generally restricted in time to a few tidal periods with onshore winds at the site. 5.3. Suspended sediment transport in the tidal channel (station B) The data set shows that the net transport of suspended material in the channel during calmer periods are fairly low due to low SSC and only minor asymmetries between ¯ood and ebb SSC. The net transport of ®ne-grained suspended material is directed landwards as a result of settling Ð and scour lag and tidal asymmetry. During windy periods larger
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
¯uxes of suspended material is observed, the situation during and after the storm surge being an extreme example of this. Little net transport was observed during the storm surge but in the following period a large input of suspended material to this part of the tidal basin was observed. For the ®ve tidal cycles shown in Fig. 9 a suspended sediment transport of 12,000 t was calculated, of which 8000 t were supplied during the ®rst two tidal cycles following the storm surge. The yearly net input to the entire Lister Dyb tidal basin is 57,000 t (Pejrup et al., 1997) of which 28,000 t accumulates in Rùmù Bay. The net import during the ®rst ®ve tidal cycles after the storm surge thus corresponded to approximately 40% of the total yearly accumulation in the entire Rùmù Bay, the import during the ®rst two tidal cycles accounting to about 30%. There is no reason to believe that a strong outward transport of suspended sediment took place during the storm surge itself as the recorded OBS-data during the storm surge showed a weak tendency for higher SSC during ¯ood than ebb. The conclusion is therefore that the storm surge caused a strong inward transport of suspended sediment to this part of the tidal basin equivalent to approximately 40% of the total yearly accumulation. The reason for the strong inward transport of suspended material in this part of the tidal basin is probably that large amounts of ®ne-grained bed material are mobilized in both the tidal basin and the adjacent part of the North Sea during periods of strong westerly winds. The settling velocity of the suspended material will increase with increasing SSC when the turbulence decreases which will have the effect that a net transport due to settling lag (Van Straaten and Kuenen, 1958) will be effective. The seaward transport of suspended material which has been shown to be induced by horizontal diffusion (e.g. Amos and Tee, 1989) is apparently more than balanced by settling Ð and scour lag and tidal asymmetry in this shallow type of estuary. It must be noted that the import of suspended matter after the storm surge only was measured in this restricted part of the tidal basin. No information is available on the suspended sediment concentrations or transport in the rest of the tidal basin. However, it is likely that erosion prevailed in the eastern part of the basin due to strong wave action and it is therefore also likely that at least part of the material imported to
83
Rùmù Bay is sediments which were eroded locally within the tidal basin. Consequently, the result of the storm surge was probably a strong internal redistribution of material within the bay and it is not possible on the basis of the data set to estimate the amount contributed from the North Sea, i.e. the primary import. It was shown by Pejrup et al. (1997) that the net sedimentation on salt marshes within the Lister Dyb tidal basin accounts for about 17% of the total accumulation of ®ne-grained material within the basin. The sedimentation on the salt marshes only takes place during periods of increased water levels, i.e. during periods of strong westerly winds. This implies that the salt marshes are responsible for a substantial part of the import found during these periods and therefore also the storm surge period shown in Fig. 9. It is likely that a major part of this material too is eroded on intertidal ¯ats in the basin (Bartholdy and Anthony, 1998) 6. Conclusions The objective of the present study have been to gain qualitative and quantitative information on the deposition, erosion and transport of ®ne-grained suspended material on an intertidal mud¯at situated in the Lister Dyb tidal basin, the Wadden Sea. Bed level measurements have been carried out for more than three years and the suspended sediment transport during almost 600 tidal cycles have been measured at a station at the mud¯at or in a nearby tidal channel. Reworking of the sediment surface is much larger than the net accumulation, the reworking depth being 3±8 cm as compared to a yearly net accumulation of approximately 0.8 cm determined on the basis of 210 Pb and 137Cs dating. This indicates that the mud¯at is close to dynamic equilibrium and deposition during periods of calm weather or local offshore winds in the study area is balanced by erosion during periods of onshore winds. When correcting for bulk density, the calculated net accumulation on the basis of bed level measurement amounts to 0.6 cm a 21 for the same site as the 210Pb dating. Some degree of seasonality was observed for erosion and deposition on the mud¯at, with a
84
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85
tendency for deposition in spring and summer and erosion in the winter. The seasonal signal, however, was sometimes disturbed or even reversed, being mainly controlled by both differences in biological stabilization and aggregation of the bed material and freezing of the sediment surface in cold winters. In addition, temperature-induced viscosity changes of the water and associated changes in settling velocity will produce a seasonal effect as will seasonal changes in wind climate. Reworking is largest close to the salt marsh. This is explained by deposition of soft, ice-rafted sediment in the spring and enhanced net deposition in areas characterized by well developed bio®lms in spring, summer and autumn. With the exception of periods of local onshore winds, the study area received a supply of ®ne-grained suspended material. A very large input of suspended material was found in the period following a strong storm, in particular during the ®ve tidal periods following the storm during which net transport equivalent to approximately 40% of the yearly net accumulation took place. Acknowledgements The ®eldwork was made possible through ®nancial support by the MAST III project INTRMUD (MAS3CT95-0022) and by the Danish Natural Science Research Council, grant no. 9701836. Great thanks goes to Ulrik Lumborg, Anders Windelin, Ole Aarup Mikkelsen and Annette LuÈtzen Mùller for help with ®eldwork and data and to Paul Christiansen, Heini Larsen, Kirsten Simonsen and Ulf Thomas for logistic support. Finally, we would like to thank B.W. Flemming and an anonymous reviewer for their fruitful comments on an earlier version of this manuscript. References Amos, C.L., 1995. Siliclastic tidal ¯ats. In: Perillio, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries. Elsevier, Amsterdam, pp. 273±306. Amos, C.L., Tee, K.T., 1989. Suspended sediment transport processes in Cumberland basin, Bay of Fundy. Journal of Geophysical Research 94 (c10), 14 407±14 417. Amos, C.L., Van Wagoner, N.A., Daborn, G.R., 1988. The in¯u-
ence of subarial exposure on the bulk properties of ®ne-grained intertidal sediment from Minas Basin, Bay of Fundy. Estuarine, Coastal and Shelf Science 27, 1±13. Andersen, T.J., 1999. Suspended sediment transport and sediment reworking at an intertidal mud ¯at, the Danish Wadden Sea. Meddelelser fra Skalling-laboratoriet, Vol. 37 72pp. Andersen, T.J., 2000. The role of fecal pellets in suspended sediment settling velocities at an intertidal mud¯at, the Danish Wadden Sea. In: McAnally, W.H., Mehta, A.J. (Eds.), Coastal and Estuarine Fine Sediment Processes. Elsevier, Amsterdam, pp. 387±401. Andersen, T.J., 2001. Seasonal variation in erodibility of two temperate, microtidal mud¯ats. Estuarine, Coastal and Shelf Science (in press). Andersen, T.J., Mikkelsen, O.A., Mùller, A.L., Pejrup, M., 2000. Deposition and mixing depths on some European intertidal mud¯ats based on 210Pb and 137Cs activities. Continental Shelf Research 20 (12/13), 1569±1591. Austen, I., Andersen, T.J., Edelvang, K., 1999. The in¯uence of benthic diatoms and invertebrates on the erodibility of an intertidal mud¯at, the Danish Wadden Sea. Estuarine, Coastal and Shelf Science 49, 99±111. Bartholdy, J., Anthony, D., 1998. Tidal dynamics and seasonal dependent import and export of ®ne-grained sediment through a back-barrier tidal channel of the Danish Wadden Sea. In: Tidalites: Processes and Products, SEPM Special Publication, Vol. 61, pp. 43±52. Bassoullet, P.H., Le Hir, P., Gouleau, D., Robert, S., 2000. Sediment transport over an intertidal mud¯at: ®eld investigations and estimation of ¯uxes within the Baie de Marennes-Oleron (France). Continental Shelf Research 20 (12/13), 1635±1653. Christie, M.C., Dyer, K.R., Turner, P., 1999. Sediment ¯ux and bed level measurements from a macro tidal mud¯at. Estuarine, Coastal and Shelf Science 49, 667±688. Flemming, B.W., Nyandwi, N., 1994. Land reclamation as a cause of ®ne-grained sediment depletion in backbarrier tidal ¯ats (Southern North Sea). Netherlands Journal of Aquatic ecology 28 (3-4), 299±308. Freeman, D.P., Coates, L.E., Ockenden, M.C., Roberts, W., West, J.R., 1994. Cohesive sediment transport on an inter-tidal zone under combined wave-tidal ¯ow. Netherlands Journal of Aquatic Ecology 28 (3±4), 283±288. Frostick, L.E., McCave, I.N., 1979. Seasonal shifts of sediment within an estuary mediated by algal growth. Estuarine and Coastal Marine Science 9, 569±576. Kirby, R., Bleakley, S.T., Weatherup, C., Raven, P.J., Donaldson, D., 1992. Effect of episodic events on tidal mud ¯at stability, Ardmillan Bay, Strangford Lough, Northern Ireland. In: Mehta, A.J. (Ed.), Nearshore and Estuarine Cohesive Sediment Transport, 42. Paci®c Rim Congress, pp. 378±392. Nilsson, B., 1969. Development of a depth-integrating water sampler. UNGI rapport No. 2, Uppsala Universitet. O'Brien, D.J., 1998. The sediment dynamics of a macrotidal mud¯at on varying timescales. Ph.D. Thesis, University of Wales, 135 pp. O'Brien, D.J., Whitehouse, R.J.S, Cramp, A., 2000. The cyclic development of a macrotidal mud¯at on varying timescales. Continental Shelf Research 20 (12/13), 1593±1619.
T.J. Andersen, M. Pejrup / Marine Geology 173 (2001) 69±85 Pejrup, M., 1986. Parameters affecting ®ne-grained suspended sediment concentrations in a shallow micro-tidal estuary, Ho Bugt. Estuarine Coastal and Shelf Science 22, 241±254. Pejrup, M., 1988. Suspended sediment transport across a tidal ¯at. Marine Geology 82, 187±198. Pejrup, M., Andersen, T.J., 2000. The in¯uence of ice ¯ow on sediment transport, deposition and reworking in a temperate intertidal mud¯at area, the Danish Wadden Sea. Continental Shelf Research 20, 1621±1634. Pejrup, M., Larsen, M., Edelvang, K., 1997. A ®ne-grained sediment budget for the Sylt-Rùmù tidal basin. HelgolaÈnder Meeresuntersuchungen 51, 1±15. Reise, K., 1998. Coastal change in a tidal backbarrier basin of the
85
Northern Wadden Sea: Are tidal ¯ats fading away?. Senckenbergiana Maritima 29 (1/6), 121±127. Underwood, G.J.C., Paterson, D.M., 1993. Seasonal changes in diatom biomass, sediment stability and biogenic stabilization in the Severn Estuary. Journal of Marine Biological Association 73, 871±887. Van Straaten, L.M.J.U., Kuenen, Ph.H., 1958. Tidal action as a cause of clay accumulation. Journal of Sedimentary Petrology 28 (4), 406±413. Widdows, J., Brown, S., Brinsley, M.D., Salkeld, P.N., Elliott, M., 2000. Temporal changes in intertidal sediment erodability: in¯uence of biological and climatic factors. Continental Shelf Research 29, 1275±1289.