RELATIONSHIPS BETWEEN WAVE-INDUCED CURRENTS AND SEDIMENT GRAIN SIZE ON A SANDY TIDAL-FLAT G.C. MALVAREZ, J.A.G. COOPER, AND D.W.T JACKSON Coastal Studies Research Group, School of Environmental Studies, University of Ulster, Coleraine BT52 1SA Northern Ireland e-mail:
[email protected]
ABSTRACT: Studies of sedimentation on sandy tidal-flats have long acknowledged that tidal flows and waves are the most influential hydrodynamic forcing factors operating on surficial sediments. Tidal flows influence the long-term evolution of tidal-flats because of the asymmetry of tidal regimes (flood and ebb). Tidal current activity is mainly confined to channels. Outside the channels, mainly on the upper tidalflats, tidal current velocities decrease and sediment entrainment is frequently ascribed to wave action. The role of wave-induced processes on intertidal-flat sediments is frequently stated but has seldom been investigated, apparently because of methodological constraints. In this paper we report on investigations into the role of wave processes in determining surficial sediment distribution patterns on a sandy tidal-flat at Newtownards, at the northern end of Strangford Lough, Northern Ireland (spring tidal range 3.5 m). The main aim of this paper is to investigate the potential relationship between waveinduced processes and surficial sediments without attempting to achieve a comprehensive analysis of other sedimentation. The methodological approach involves numerically modeling wave propagation and comparing this with sediment distribution on a controlled field site. The method enables the role of waves on tidal-flats to be more fully assessed. Waves were modeled at a variety of tidal levels and wave data were extracted from the model grid at positions co-incident with sediment sampling locations. Multiple correlation of sediment mean grain size with simulated wave-induced orbital velocity showed a distinctive variation in correlation coefficients according to water level. The best (and statistically significant) correlations occur when water levels occupy a vertical range between 20.15 and 1.0 m local Ordnance Level (OD). In the absence of strong tidal currents, sediment distribution appears to be best explained by waves acting at these water levels. It is inferred that at lower-than-optimum water levels, wave energy is less influential on sediment texture since (1) part of the tidal-flat is still exposed and (2) the fetch over which the waves are generated is reduced. At higher-than-optimum water levels, wave energy may be sufficient to transport sediment, but the wave penetration in the column of water is not sufficient to permit wave energy dissipation at the sediment surface. Tidal currents are dominant at low tidal elevations so outside channels wave-induced currents exert most influence on the tidal-flats.
INTRODUCTION
The study of tidal-flat sedimentation has received much attention, because flats are readily accessible, dynamic and tend to occur in relatively sheltered locations. Methodological approaches to investigation of tidal-flat sedimentation are widely varied and focus commonly on the examination of forcing factors (tides, waves, wind) and the interplay with the morphology (intertidal zone, channels, salt marshes). Tidal flows have been at the center of extensive investigations on tidal-flat sedimentation, and their role is commonly recognized to be of major importance in the long term evolution of marine estuaries and tidal-flats (Ridderinkhof 1997; Wood et al. 1998). Much research has also been directed at the interplay of biological and physical aspects of sediment transport on tidal-flats (e.g., Bell et al. 1997; Widdows et al. 1998; Ruddy et al. 1998). While it is readily JOURNAL OF SEDIMENTARY RESEARCH, VOL. 71, NO. 5, SEPTEMBER, 2001, P. 705–712 Copyright q 2001, SEPM (Society for Sedimentary Geology) 1527-1404/01/071-705/$03.00
acknowledged that physical factors influence tidal-flat sedimentation (Paterson et al. 1990), predicted trends of erosion and deposition that may be influenced strongly by biological controls including surface vegetation (Frostick and McCave 1979), binding diatoms (Paterson 1989), attached molluscs (Meadows et al. 1998) and burrowing infauna (Meadows et al. 1990). The role of waves in tidal-flat sedimentation is acknowledged to be particularly important in the intertidal area (Reineck 1967; Amos 1995). The threshold of sediment motion under oscillatory waves has been investigated in laboratory studies (Komar and Miller 1973) and it was found that for medium-grained sands the threshold of motion is exceeded while the boundary-layer flow is still laminar. Thus, on sandy tidal-flats affected by frequent wave action, waves may be expected to exert a control on sedimentation patterns. High-frequency wind waves dominate estuaries when sheltered from open seas. Wind waves exhibit a broad spectra of frequency and direction (Holthuijsen et al. 1989) and therefore may be subject to sharp directional changes induced by refraction and/or diffraction. The effects on local morphodynamics are that wave-induced potential sediment transport may occur in multiple pathways linked to slight variations in topography of the seabed. Intertidal-flat topography commonly lacks relief (outside channels), and therefore a highly sensitive analysis of wave propagation and decay is essential if energy gradients at different stages of the tide are to be interpreted in terms of their effect on sediment entrainment and/or transport. Wave energy dissipation, related to bottom friction and breaking, can be directly linked to water depth in shallow environments (Collins 1976) and thus the expanding/contracting tidal prism affects wave fetch and water depth. Variations in tidal water level can affect wave sediment interaction. Wave parameters are, however, difficult to assess across a broad spatial zone, and the study of their effect on tidal-flats is further hampered by difficulties of measurement and simulation under regular tidal variation. Thus their direct impact on tidal-flat sedimentation has remained poorly studied. Several studies have demonstrated a broad link between wave energy and tidal-flat morphology (Boyd et al. 1992; Ryan and Cooper 1998). At a local scale, Carling (1982) investigated the role of waves by deploying a wave pole at a single location and using time-averaged results as a measure of wave intensity. Although the results apparently showed a link between wave action and tidal-flat behavior at the scale of the whole tidalflat, these results were based on data from a single point and cannot adequately explain variability within a tidal-flat. In this paper we focus only on the relationship between the distribution of wave energy and sediment grain sizes on a sandy tidal-flat at Newtownards, at the head of Strangford Lough, Northern Ireland. The objectives of the study are (1) to achieve a better understanding of wave propagation over tidal-flat areas; (2) to determine whether there is any direct link between simulated wave parameters and distribution of surficial sediments on the intertidal-flat, using a combined field and numerical modeling approach; (3) to investigate the role of tide-induced water-level variation on the relationship between wave induced orbital velocities and sediment size distribution on a variety of water-depth scenarios; and (4) to demonstrate a combined modeling and fieldwork approach to the problem. Study Area The study area is located at the northern end of Strangford Lough, on the eastern coast of Northern Ireland (Fig. 1). The Lough is an elongate
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FIG. 1.—Location of Strangford Lough in eastern Northern Ireland. Location of profiles and sediment sampling stations. Note that only two of the eleven transect lines are labeled. Samples were taken along all 11 transect lines following the same labeling criteria (A 5 High Water Mark; H 5 Low Water Mark).
embayment with a north–south axis (c. 30 kilometers long). It is bordered by bedrock topped by glacial sediments and contains many drowned drumlins within its margins. Contemporary marine conditions in Strangford Lough are characterized by an average tidal range of 3.0 m (spring tide range 3.5 m) with little amplification of the tidal wave in the Lough. It is connected to the Irish Sea by the Narrows, in which tidal velocities reach 3.5 m s21. These strong currents are confined to central deep channels and are significantly weakened ca. 14 kilometers north of the Narrows. The northern end of the Lough is affected by minor tidal forces confined to the major tidal channels. Tidal modeling (KMM 1993) indicates that currents on the Newtownards tidal-flat reach a maximum at mid-tide levels (c. 0.1 m s21). Waves develop within the Lough and propagate following local wind direction. The wave climate is dominated by these wind waves within the Lough with limited fetch. Irish Sea swell does not penetrate the Lough with sufficient energy to have any significant morphodynamic effects in the northern end. Wind directions affecting the wave climate at Newtownards are typically from the southeast and effective fetch is about 16 kilometers (BBV 1997). Central regions of the Lough reach 60 m in depth whereas the margins of the system are characterized by a variety of intertidal-flats, bedrock, and relict glacial shorelines. The intertidal areas at Newtownards exhibit a flat or gently sloping sandy surface (siliciclastic sediments on the flats are generally fine sands, ranging from 0.10 mm to 0.25 mm) intersected by a major drainage channel (Fig. 1). Examination of tidal-flat cross sections shows that the intertidal region is 1000–1200 m wide along the western shore (including the area fronting an existing sea wall) and only 300–500m along the eastern shore. This means that the flats along the western margin are of much lower gradient (0.003) than those on the eastern shore (0.007). Cross sections of the tidal-flat generally have a marked break in slope that separates a gently inclined upper tidal-flat from a steeper lower section leading to the low-tide mark at the margin of the tidal and storm channel that drains from Newtownards. METHODS
To investigate the relationship between wave-induced currents and the distribution of surficial sediment grain sizes, a combined field data gath-
ering and modeling study was undertaken. Sediment was sampled at 77 locations across the tidal-flat. Wave parameters were simulated and data extracted from the same stations as sediment samples to obtain geographically matching data sets. Positioning of sampling and hydrodynamic data was carefully conducted to minimize errors. Hydrodynamic parameters were not recorded using empirical approaches, as simultaneous measuring of wave height and wave orbital velocity was unfeasible on 77 stations across the flats over a tidal cycle. Numerical modeling was conducted using a calibrated and a tested wave propagation model specifically designed for shallow-water wave propagation (HISWA; HIndcasting of Shallow Water wAves. Holthuijsen et al. 1989; Booij and Holthuijsen 1995). In the present study, wind speeds selected for simulation of wave conditions were chosen for a high-energy or storm condition (initial significant wave height 5 1.5 m; significant wave period 5 4.5 seconds; southeasterly approach) coupled with a 10 m s21 wind from the southeast. Ryan and Cooper, (1998) demonstrated that waves in Strangford Lough are fetch limited and the largest significant wave height likely to occur at Newtownards is in the region of 1.5 m. A comprehensive set of wave energy simulations derived from a wind data set recorded at Newtownards (Malvarez et al. 2001) showed no apparent correlation between waves and sediment distribution under lower (including modal) wave conditions because wave-induced orbital velocities were below the predicted threshold for sediment motion at most sites. Data on sediments and waves were combined and simple statistical methods were used to examine relationships between the two sets of variables. Input data for the numerical wave propagation simulator were sea-bed topography of the tidal-flats and deep water regions, initial wave geometry (deep-water waves), wind speed and direction, and water levels. Topography and Bathymetry (D-GPS) Topographic surveys were carried out using a Trimble 4400 Ssi Differential Global Positioning System (D-GPS) at sub-centimeter accuracy. Profile lines were set up to measure cross sections of the topography of the tidal-flats (Fig. 1). Profile lines 1 to 3 run from mean low water mark
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FIG. 2.—Bathymetry and procedure used in modeling wave propagation. Boxes represent computational area and output. The computational area includes deep-water boundary to provide off shore wave conditions. The output area is the geographical extent used in Figures 4, 5, and 6.
distance between successive samples varied from 50 m for short cross sections near the sea wall (northern end) to 150 m along longer transect lines in the southern part of the survey area. An average of seven samples were taken per profile line. The samples were then wet sieved to establish mud content and later dried at 958C overnight. The sample grain size was analyzed using an automated settling column linked to a computer that provided moment measures and settling velocities for the sand fraction. The analyses provide the median, mean, sorting, and skewness of the distribution as well as settling velocities. Results from the sediment particle analysis are directly geo-referenced because sample location was recorded in the field with D-GPS during collection. This facilitates mapping of the spatial distribution of textural information and guarantees reliability of subsequent analysis of spatial variability in sediment particle size and correlations.
(MLWM) to mean high water mark (MHWM) at the eastern side of the upper flats. Profiles 4, 5, 6, 7 and 8 run perpendicular to the sea defenses towards the MLWM. Finally, profiles 9, 10, and 11 are designed to delineate cross sections from the western MHWM to the MLWM of the main tidal channel. The arrangement of the profile lines enabled a topographic characterization of the upper flats based on benchmarks linked to the Irish Grid coordinate system and Ordnance Datum (OD) at Belfast. Real time kinematic (RTK) surveying was conducted on an all terrain vehicle (ATV), equipped with extra-wide tires to reduce incision and disturbance on the tidal-flats. Waypoints were extracted from a map and loaded into the database of the data collector. Data recorded during fieldwork was downloaded onto a desktop computer using TRIMMAP software, which enabled post-processing of survey data and normalization of coordinate systems. Maps based on the field survey were produced using the Surfer 6 package. Topographic data from the high-resolution topographic D-GPS surveys were appended to the digital bathymetric files, which described seabed topography of the deep-water areas. Deep water soundings were digitized and interpolated from Admiralty Chart 2156 (surveyed 1967 and published 1968). All coordinates were then normalized to Irish Grid Reference and elevations were reduced to meters (OD Belfast). The resulting combined synthetic topography of the intertidal-flats and seabed were organized in a 10 3 10 m lattice and used in the wave propagation model (Fig. 2).
An automated ‘‘Dobie’’ water-level and wave recording system (Green 1998) was deployed near Killinchy (Fig. 1). The interrogation period was set to 30 minutes (Fig. 3). Data from the water-level record were calibrated using a barometer before deployment, and output records were normalized using D-GPS for real elevation control. The tidal record was used to input sequential water levels up to 50 times into a wave propagation model.
Sediment Sampling and Analysis
Wind-Data Recording
Sediment samples consisted of about 30 grams of surficial (2–4 cm depth) sediments taken at variable intervals along the profile lines. The
Wind data was recorded using a Weather Wizard III meteorological station installed at Ards Airfield (Ulster Flying Club, fig. 1) at an elevation
Water-Level Recording
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G.C. MALVAREZ ET AL. Model output (significant wave height, wave energy dissipation, waveinduced stress and near-bottom orbital velocity) was returned at fixed geographic points coinciding exactly with the co-ordinates of the sediment samples. Statistical Analysis
FIG. 3.—Measured water levels (m) during semidiurnal tide cycle at Killinchy (Strangford Lough). These water levels were used in the computer simulation of waves. Height units in meters.
of approximately 5 m above the ground to reduce turbulence. Wind velocity and temperature were logged at 30 minute intervals for the period 1997– 1999. Data were retrieved using PC-LINK software via modem. Data sets were analyzed and gust wind speeds associated with storms were used as input to the wave propagation model. Wind and wave directions inside Strangford Lough coincide because of the shape of the estuary and the limited effective fetch, which controls wave generation and propagation in the northern end of the Lough. Wave Propagation Modeling The numerical computer model HISWA was used to generate wave energy scenarios and to analyze spatial distribution of wave-related hydrodynamic parameters. Gridded output of the model was plotted and tabulated for interpretation. Simulation of wave propagation was used initially to assess Irish Sea wave penetration through the Narrows (which proved to be negligible) and then to carry out simulations inside Strangford Lough (Fig. 2) Wave energy dissipation, orbital velocity, and friction coefficients were set to default values in the program (see Holthuijsen et al. 1989 for complete details on computation and background physics). High-resolution (grid cells , 10 square meters) computations of wave propagation were then undertaken over the inshore bathymetry at different water levels. Deep-water wave parameters for Newtownards tidal-flats were established from wind speed and effective fetch from the most common storm direction. Single wave geometry was generated from fetch-limited wind generation, resulting in significant wave heights of 1.5 m, zero crossing periods of 4.5 seconds, and approach from southeast. This wave climate was then coupled with a storm-related wind field (10 m s21) that was assumed homogeneous over the entire study area. Wave propagation, as performed by HISWA, is stationary (i.e., waves propagate simultaneously over the area but there is no time evolution in the model). The results can be interpreted as a ‘‘snapshot’’ of the distribution of wave energy in an area. The water level is therefore also fixed at a single stage of the tidal cycle during each simulation. This was modified using measured tidal elevations, and the model was run in a loop, reading sequential water levels from data recorded for this experiment. Spatial distributions of different wave energy scenarios across the tidalflats were generated at 39 stages of the tide (i.e., more than a full semidiurnal tidal cycle at 30 minute interval readings).
Geographical information was compiled in a matrix including latitude, longitude, and elevation of the position of the sampling stations and simulated wave parameters. The information on the sediment properties comprises mean grain size (phi and SI units), median, sorting, and skewness of the distribution, and sediment fall velocity. Wave parameters input in the matrix included significant wave height, zero-crossing period, waveinduced bottom orbital velocity, wave energy dissipation rate and wave steepness at the same locations from which sediment was collected. Data sets extracted from the results of the sediment and wave analyses were combined on a 156 column by 77 row matrix for correlation. The matrix containing the combined geographical, hydrodynamic, and sedimentological information were analyzed using the SPSS (statistical software system). Mean grain size and maximum wave orbital velocity were selected from the sediment and hydrodynamic, data sets, respectively. Bivariate correlations were performed between those variables, and the Pearson’s coefficient was taken as an indicator of the relationship of those variables. RESULTS
Simulations of Wave Propagation Wind generated waves affecting the Newtownards tidal-flats have a wide frequency spectrum and the directional sector of propagation is considerably wider that that of oceanic swell. High-frequency waves dominate the wave climate over the tidal-flats, but the total fetch is limited and levels of wave energy are generally low. Wave Near-Bottom Orbital Velocities.—The results of the sequential simulation of wave orbital velocities at the sea floor (m s21) at different water levels are shown in Figure 4. At water level of 0.88 (Fig. 4A) most of the tidal flat is submerged and low wave orbital velocities occur on most of the upper tidal flat. At high tide (Fig. 4C) the entire tidal flat experiences wave orbital velocities greater than 0.1 m s21 and maximum wave orbital velocities occur in the northwestern sector, where low-gradient area minimizes wave energy dissipation and maintains wave velocities. At intermediate water depths (Fig 4B) the zone of highest orbital velocities extends farther seaward than at high tide and is less extensive along the northwestern shore. Note the high orbital velocities on the western tidal flats. The threshold for sediment motion was calculated following Komar and Miller (1973) to illustrate the distribution of potential effects of wave induced currents for a given water level. At a water level of 1.0 m, the threshold for sediment motion was exceeded at 90% of sample stations, and therefore sediment suspension and motion can be expected. Wave Heights.—Most wave energy is dissipated to the south of the study region (Fig. 5). Here, the rapid decrease in model wave height suggests that the decrease in water depth is the main control on wave energy propagation to northern parts of the tidal-flats. However, some energy is carried by waves propagating northwest from the Comber estuary. Over this very flat region wave energy remains intact because of a lack of dissipation. To the east, gradients in reduction of wave height indicate that steeper slopes of the shore increase wave energy dissipation (shown here as reduction in wave height). A sheltered zone to the northeast exhibits gentle gradients of wave height (Fig. 5). Sediment Analysis The variability of grain size across the tidal-flats was limited and ranged from 0.09 to 0.20 mm (i.e., very fine to fine-grained sand; Fig. 6). Finer
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particles occur in the north and southwest sections of the tidal-flats. A zone of higher mean grain size between those two regions contains sediments with an average size of 0.19 mm. Finer material is found at higher elevations (Fig. 6), and coarser sediment normally occurs in the central areas and the southeastern shore, where the topography is steeper. The western shore is nearly homogeneous, with average grain sizes of about 0.16 mm over large areas of the sand flats. Mud fractions of any significance (,10%) were found only in samples taken from the most northern reaches. Correlations between Wave Parameters and Sediment Size Bivariate correlations between calculated wave-orbital velocity and measured sediment texture at 77 points on the tidal-flat surface were tested for significance at the 0.01 and 0.05 confidence levels for tidal water levels 20.5 and 1 2.38 m OD (low and high tide, respectively). Significant correlations (at the 0.01 level) were found (Table 1, Fig. 7) in a distinct range of tidal water levels in the tidal cycle. The most significant correlation (0.631) was at a water level of 0.88 m OD, and significant correlations were restricted to between tidal levels of 20.15 and 1.0 m OD. At higher and lower water levels, correlation coefficients were not significant. DISCUSSION
FIG. 4.—Distribution of predicted wave orbital velocities. A) water level 5 0.88 m; B) water level 5 1.85 m (mid tide) and C) water level 5 2.38 (high tide) when it is proposed that closure depth is exceeded. The experiment included a total of 49 water levels. For clarity, only three tide stages are shown here. Velocities and geographical scales apply to all three diagrams.
The results presented here illustrate that the distribution of wave energy across the Newtownards tidal-flat varies according to the state of the tide and, in particular, that most effective energy dissipation occurs across the tidal-flat at about mid-tide. At lower tidal levels (, 0 m OD) high orbital velocities occur in narrowly defined zones that represent a narrow surfzone towards the seaward margins of the intertidal flat. At levels between 0 and 1 m OD, wave orbital velocities are less intense but are above sediment transport thresholds capable of work across a much greater area of the tidal-flat. At elevations greater than 1 m OD, wave bottom orbital velocities decrease across most of the tidal flat and a narrow zone of high orbital velocity develops at high-tide margin of the tidal-flat. Under these conditions penetration of wave energy through the water column is impeded by the increased water depth, and wave action on the sea bed does not produce sediment movement. Examination of Figure 7 indicates best (and statistically significant) correlations between sediment mean grain size and wave orbital velocity at water levels between 20.15 and 1.0 m OD. At higher and lower water levels the correlation between grain size and wave orbital velocity is not significant. The significant correlations between grain size and orbital velocity exist in the presence of other (not assessed) potential variations in sediment-transport-related factors (including spatially variable algal and diatom concentrations, mucus cementation of grains, disturbances by grazing birds, etc.). These factors were beyond the scope of this study, which aimed only to examine the potential role of wave action as a control on sediment transport. Nonetheless, on the basis of the wave-energy simulations alone, these results suggest that variations in wave energy can explain much of the observed variation in sediment grain size. In addition a number of interesting findings are indicated that relate to the relationship between wave processes and sedimentation. For example, rather than a rapidly migrating zone of intense activity of breaking waves (such as exists at the margin of the advancing or retreating tide) being the main control on sediment texture on such a sandy tidal-flat, prolonged activity at a slightly lower energy level may have a more significant role on the distribution of sediment texture. Thus, when water levels are between 20.15 and 1.0 m OD and wave shoaling processes are active across a broad area of the tidalflat, the strongest relationship is found between wave velocities and sediment texture. Our results indicate that whereas high wave orbital velocities occur at lower stages of the tide as a surf-zone develops, tidal translation causes this zone to migrate rapidly across the low-gradient tidal-flat thus diminishing its role in sediment sorting on the tidal-flat as a whole. The
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FIG. 5.—Distribution of significant wave height (m) on the tidal-flats under storm conditions (initial significant wave height 5 1.5 m; wave period 5 4.5 s; southeast approach). Coordinate system is Irish Grid (m). Horizontal scale bar is in meters. High waves occur in deep water south of the tidal-flat, whereas the tidalflat itself is a zone of dissipation of wave energy (shoaling waves) with low wave heights.
FIG. 6.—Sediment size distribution on Newtownards tidal-flats. The area is grouped into north, middle, and south sections and data relates to all 11 profiles. To simplify interpretation, results are grouped into the three areas.
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TABLE 1.—Pearson’s correlation coefficients of wave orbital velocity (ms21) and mean sediment grain sizes (mm) of 77 stations across the tidal-flats at 36 water levels during one tidal cycle. Time
Tidal Level (OD)
9:15 9:30 9:45 10:00 10:15 10:30 10:45 11:00 11:15 11:30 11:45 12:00 12:15 12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30 15:45 16:00 16:15 16:30 16:45 17:00 17:15 17:30 17:45 18:00 18:15
20.433 20.248 20.066 0.102 0.287 0.462 0.648 0.847 1.043 1.252 1.459 1.671 1.851 2.017 2.177 2.264 2.353 2.386 2.374 2.279 2.173 2.036 1.888 1.732 1.562 1.402 1.227 1.057 0.884 0.713 0.535 0.36 0.183 20.005 20.188 20.377 20.57
Pearson’s Correlation Coefficient 0.151 0.182 0.390** 0.398** 0.436** 0.446** 0.569** 0.617** 0.531** 0.19 0.076 20.074 20.182 20.238* 20.249* 20.253* 20.256* 20.257* 20.257* 20.253* 20.251* 20.239* 20.193 20.11 0.018 0.12 0.114 0.486** 0.631** 0.604** 0.499** 0.438** 0.404** 0.389** 0.182 0.151 0.151
** Correlation is significant at 0.01 level * Correlation is significant at 0.05 level Tide data is from a gauge located at Killinchy, western Strangford Lough (sampling at 15 minute intervals).
most effective and widespread sediment transport under wave action appears to occur while shoaling waves affect the tidal-flat surface for a prolonged period (i.e., when water level is rising over large areas after flooding tidal channels). The results further suggest that to examine the role of wave energy on sedimentation on the tidal-flats requires analysis at different water levels. Interestingly, the periods of maximum wave influence at mid-tide are also likely to be coincident with maximum tidal current velocities, although in the case of Strangford Lough model studies (KMM 1993) have shown that tidal current velocities are relatively low on the tidal-flats. It is thus possible that previous workers who have ascribed sediment transport and sorting on tidal-flats solely to tidal currents, may have disregarded the contribution of wave-induced forces, which may also exert their maximum influence at mid-tide. Although the results presented here are based on a single storm wave condition (lower incident waves produced sub-threshold orbital velocities), they illustrate the combined role of wave and tidal variation in determining sediment distribution on tidal-flats that has long been acknowledged but seldom demonstrated. Furthermore, our methodological approach, using a combination of field measurement and numerical modeling at a variety of tidal elevations demonstrates the potential for further investigation of the role of waves on tidal-flats at a relatively large spatial scale, which is not readily achieved by deployment of field instrumentation. The type of analysis we propose here can readily be incorporated into spatially extensive studies of other influential factors in sediment transport on intertidal-flats as an aid to interpretation of sedimentation controls. The results in this paper enable a preliminary insight into the influence
FIG. 7.—Graph showing the correlation coefficients between wave orbital velocity and sediment grain size with a varying water level.
of spatial variations in wave energy as a control on sediment transport and distribution across a sandy intertidal-flat. The results presented are for storm wave conditions because analyses of low wave conditions did not produce orbital velocities above the threshold for sediment motion (Malvarez et al. 2001). Similarly, modeled tidal current velocities (KMM 1993) are low across the tidal-flat considered, and even during spring mid tide, conditions do not exceed threshold velocities for the grain sizes encountered. In the absence of alternative sediment-transporting dynamic factors our results, based on storm-related wave conditions, offer a potential mechanism for controlling sediment textural variation on the tidal-flats. Although the relationships reported from this investigation cannot be regarded as conclusive evidence of a cause-and-effect relationship between wave-induced currents and sediment texture, in the absence of alternative sediment sorting mechanisms it is probable that wave energy exerts a control on sediment texture. Furthermore, the methodological approach reported here offers the potential for future advances in understanding the role of waves on tidal-flat sedimentation. ACKNOWLEDGMENTS
This research utilized data gathered under the Strangford Lough Sea Defenses Monitoring Programme funded by Rivers Agency, Department of Agriculture, and Rural Development Northern Ireland (DARDNI), to whom we are grateful. Thanks are extended to the technical staff of the School of Environmental Studies (especially Robert Stewart) for assistance with fieldwork and sediment analyses. REFERENCES
AMOS, C.L., 1995, Siliciclastic tidal-flats, in Perillo, G.M.E., ed., Geomorphology and Sedimentology of Estuaries: Amsterdam, Elsevier Developments in Sedimentology 53. BELL, R.G., HUME, T.M., DOLPHIN, T.J., GREEN, M.O., AND WALTERS, R.A., 1997, Characterisation of physical environmental factors on an intertidal sandflat, Manukau Harbour, New Zealand: Journal of Experimental Marine Biology and Ecology, v. 216, No. 1–2, p. 11–31. BBV (BINNIE, BLACK AND VEATCH, CONSULTING ENGINEERS) 1997, Strangford Lough Sea Defenses at Newtownards: Environmental Assessment Report, Internal Report, March 1997. BOYD, R., DALRYMPLE, R., AND ZAITLIN, B.A., 1992, Classification of clastic coastal depositional environments: Sedimentary Geology, v. 80, p. 139–150.
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BOOIJ, N., AND HOLTHUIJSEN, L.H. 1995, HISWA (HIndcasting of Shallow Water wAves) User Manual, Prediction of Stationary, Short-crested Waves in Shallow Water with Ambient Currents, Version 110.43: Delft University of Technology, Department of Civil Engineering, 87 p. CARLING, P.A., 1982, Temporal and spatial variation in intertidal sedimentation rates: Sedimentology, v. 29, p. 17–23. COLLINS, J.I., 1976, Wave modeling and hydrodynamics, in Davis R., and Ethington R.L., eds., Beach and Nearshore Sedimentation, SEPM, Special paper 24, p. 54–68. FROSTICK, L., AND MCCAVE, I.N., 1979, Seasonal variability of sedimentation on a mudflat mediated by algal growth: Estuarine, Coastal, and Shelf Science, v. 9, p. 569–576. GREEN, M.O., 1998, DOBIE the intelligent wave gauge, Operator Manual, NIWA (National Institute of Water and Atmospheric Research Ltd.), Manufactured by NIWA Instrument Systems, Christchurch, New Zealand. HOLTHUIJSEN, L.H., BOOIJ, N., AND HERBERS, T.H.C., 1989, A prediction model for stationary, short crested waves in shallow water with ambient currents: Coastal Engineering, v. 13, p. 23–54. KMM (KIRK, MCCLURE, MORTON CONSULTING ENGINEERS), 1993, Computational models for management of water quality in Strangford Lough, Report to Department of Environment (NI), Environmental Protection Division, Belfast. KOMAR, P.D., AND MILLER, M.C., 1973, The threshold of sediment movement under oscillatory water waves: Journal of Sedimentary Petrology, v. 43 p. 1101–1110. MALVAREZ, G.C., COOPER, J.A.G., JACKSON, D.W.T., AND NAVAS, F., 2001, The role of wave action on sedimentation of tidal-flats: application of high spatial resolution numerical modeling in Strangford Lough, Northern Ireland: Journal of Coastal Research, Special Issue (ICS 2000 Conference Proceedings), in press. MEADOWS, P.S., MEADOWS, A., WEST, F.J.C., SHAND, P.S., AND SHAIK, M.A., 1998, Mussels and mussel beds (Mytilus edulis) as stabilizers of sedimentary environments in the intertidal zone, in Black, K.S., Paterson, D.M., and Cramp, A., eds., Sedimentary Processes in the Intertidal Zone: Geological Society of London, Special Publication 139, p. 331–347. MEADOWS, P.S., TAIT, J., AND HUSSAIN, S.A., 1990, Effects of estuarine infauna on sediment stability and particle sedimentation: Hydrobiologia, v. 190, p. 263–266.
PATERSON, D.M., 1989, Short term changes in the erodibility of intertidal cohesive sediments related to the migratory behaviour of epipelic diatoms: Limnology and Oceanography, v. 34, p. 223–234. PATERSON, D.M., CRAWFORD, R.M., AND LITTLE, C., 1990, Subaerial exposure and changes in the stability of intertidal estuarine sediments: Estuarine, Coastal and Shelf Science, v. 30, p. 541–556. REINECK, H.E., 1967, Layered sediments of tidal-flats, beaches and shelf bottoms of the North Sea, in Lauff, G.H., ed., Estuaries: American Association for the Advancement of Science, Special Publication, v. 83, p. 191–206. RIDDERINKHOF, H., 1997, Sediment transport in intertidal areas, in Eisma D., ed., Intertidal Deposits: Boca Raton, Florida, CRC Press, p. 363–381. RUDDY, G., TURLEY, C.M., AND JONES, T.E.R., 1998, Ecological interaction and sediment transport on an intertidal mudflat I. Evidence for a biologically mediated sediment–water interface, in Black, K.S., Paterson, D.M., and Cramp, A., eds., Sedimentary Processes in the Intertidal Zone: Geological Society of London, Special Publication 139, p. 135–149. RYAN, N.M., AND COOPER, J.A.G., 1998, Spatial variability of tidal-flats in response to wave exposure: examples from Strangford Lough, Northern Ireland, in Black, K.S., Paterson, D.M., and Cramp, A. eds., Sedimentary Processes in the Intertidal Zone: Geological Society of London, Special Publication 139, p. 221–230. WIDDOWS, J., BRINSLEY, M., AND ELLIOTT, M., 1998, Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna, in Black, K.S., Paterson, D.M., and Cramp, A., eds., Sedimentary Processes in the Intertidal Zone: Geological Society of London, Special Publication 139, p. 85–98. WOOD, R.G., BLACK, K.S., AND JAGO, C.F., 1998, Measurements and preliminary modeling of current velocity over an intertidal mudflat, Humber Estuary, U.K., in Black, K.S., Paterson, D.M., and Cramp, A. eds., Sedimentary Processes in the Intertidal Zone: Geological Society of London, Special Publication 139, p. 167–176. Received 17 November 1999; accepted 22 January 2001.
