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Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach
Marine Geology, 110 (1993) 333-353 Elsevier Science Publishers B.V., Amsterdam
333
Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach Mark
A . D a v i d s o n a, P a u l E . R u s s e l l a ' l , D a v i d A . H u n t l e y b a n d J a c k H a r d i s t y c
aMarine Geosciences Group, Department of Geology, University of Wales, P.O. Box 914, Cardiff CF1 3 YE, UK blnstitute of Marine Studies, Polytechnic South-West, Drake Circus, Plymouth PL4 8AA, UK CSchool of Geography, The University, Cottingham Road, Hull HU6 7RX, UK (Received February 11, 1992; revision accepted September 29, 1992)
ABSTRACT Davidson, M.A., Russell, P.E., Huntley, D.A. and Hardisty, J., 1993. Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach. Mar. Geol., l l0: 333-353. Time-series of nearbed horizontal flow velocities and suspended sediment concentrations obtained from a colocated electromagnetic current meter (EMCM) and optical backscatter sensor (OBS), respectively, are used to examine the relative importance of steady and fluctuating components to the total sediment transport over a full tidal cycle on a macrotidal, intermediate beach (Spurn Head, UK). Fluctuating sediment fluxes are decomposed into gravity and infragravity contributions using cospectral techniques. The relative importance of the oscillatory (gravity and infragravity) and steady (mean) transport components to the total sediment transport is analysed throughout the tidal cycle. A continuum of 34 discrete suspended sediment-cross-shore velocity co-spectra are computed over a full tidal cycle for the OBS and EMCM measurements 0.10 m above the bed. These net transport spectra vary greatly both with cross-shore location and tidal state. In particular, a marked asymmetry in transport processes is evident between the flood and ebb tides, with high levels of sediment resuspension and transport occurring on the ebbing tide approximately two hours after high water (just seaward of the breakpoint). At this time the dominant transport was directed offshore (co-spectral peak, 0.04 kg/m2/s) at incident wave frequency. Typical patterns are observed in transport spectra outside the surf zone and within the inner surf zone. Outside the narrow surf zone cross-shore transport spectra show weak offshore transport (co-spectral peak = 0.002 kg/m2/s) associated with bound long waves and stronger onshore transport (co-spectral peak=0.006 kg/m2/s) at incident wave frequencies. Conversely, cospectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m2/s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m2/s) occurring at incident wave frequencies.
Introduction T i d a l effects o n b e a c h p r o c e s s e s h a v e r e c e i v e d c o m p a r a t i v e l y little a t t e n t i o n in the l i t e r a t u r e d e s p i t e the fact t h a t a s i g n i f i c a n t p r o p o r t i o n o f the w o r l d ' s b e a c h e s are s i t u a t e d in m e s o t i d a l o r m a c r o tidal e n v i r o n m e n t s ( W r i g h t et al., 1982a). T h i s paper considers how the importance of the three
Correspondence to: M.A. Davidson, School of Civil and Structural Engineering, University of Plymouth, Palace Court, Palace Street, Plymouth, Devon PL1 2DE, UK. 1Present address: Institute of Marine Studies, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Elsevier Science Publishers B.V.
main transport components
(mean, gravity band
a n d i n f r a g r a v i t y b a n d ) c h a n g e s o v e r a full tidal cycle o n a m a c r o t i d a l b e a c h . M e a s u r e m e n t s are o b t a i n e d f r o m the i n n e r s u r f z o n e to well o f f s h o r e of the breakers. The total shore-normal (cross-shore) sediment t r a n s p o r t r a t e at a n y g i v e n h e i g h t a b o v e the b e d is m a d e u p o f the s u m o f t h e m e a n a n d o s c i l l a t i n g t r a n s p o r t c o m p o n e n t s . T h i s was e x p r e s s e d by Jaffe et al. (1984) as:
c u ~ cuq- c'u ' Total = Mean + Oscillatory
334
where c and u are the instantaneous concentration and cross-shore velocity measurements; the overbars represent time averages, and the primes denote fluctuations about the means. The oscillatory transport component may be further decomposed, using co-spectral techniques, to give the transport contributions as a function of frequency. Since the original work of Huntley and Hanes (1987), several workers (e.g. Russell, 1990; Beach and Sternberg, 1991; Osborne and Greenwood, 1992) have used the co-spectrum between velocity and suspended sand concentration to examine the net sediment transport contribution at different frequencies. In the longshore direction, the oscillatory transport contribution approximates to zero as the fluctuating components of longshore velocity and concentration are generally uncorrelated (e.g. Sternberg et al., 1984: Russell, 1990). However, in the cross-shore direction, the velocity fluctuations associated with both incident (gravity band) waves and infragravity waves are often closely correlated with sediment concentration fluctuations, so that the oscillatory transport component may dominate the total cross-shore sediment transport. For example, Jaffe et al. (1984) instrumented a bar crest within the surf zone and found the net sediment transport to be directed onshore, due to the dominance of the oscillatory component (directed onshore in this case) over the mean offshore transport associated with the steady nearbed offshore flow (undertow). Huntley and Hanes (1987) presented data from outside a low energy (breaker height = 0.25 m) surf zone. Their results showed a strong onshore transport under the incident waves and a weaker offshore transport under the infragravity waves. Russell (1990) and Beach and Sternberg (1991) have presented high energy data (breaker height = 3 m, Russell, 1990; offshore significant swell height = 3 to 5 m, Beach and Sternberg, 1991) from inside the surf zone which show that offshore transport at low frequency is often the dominant transport component in the inner surf and swash zones. The mechanism put forward by Huntley and Hanes (1987) to explain the offshore transport at low frequency is that, outside the surf zone, high wave groups are phase locked to the troughs of long period oscillations (Longuet-Higgins and
M A DAVIDSON ET At.
Stewart, 1964), so that the high suspended sand concentrations occurring under the high wave groups (Larsen, 1982) are transported offshore by the seaward directed nearbed current associated with the troughs of the long waves. Inside the surf zone, the situation is rather different. After being released at the breakpoint (Longuet-Higgins and Stewart, 1964), the long waves may become resonantly trapped in the nearshore by processes of reflection and refraction forming edge waves (Huntley, 1981). Eckart (1951) presented a model which showed edge wave energy to increase exponentially towards the shoreline, so it may be expected that the infragravity transport component will be large in the inner surf zone. Many field experiments (Downing, 1983: Sternberg et al., 1984: Beach and Sternberg, 1988) have confirmed this. Most measurements have shown these high transport rates at low frequency in the inner surf zone to be directed offshore (Beach and Sternberg, 1988, 1991; Russell, 1990). Nevertheless, it is the phase lag between concentration and velocity that determines the transport direction (and magnitude), and this phase will vary with both crossshore position and height above the bed.
Experimental set-up, study area and methods The data analysed in this paper forms part of an extensive data base collected during the British Beach and Nearshore Dynamics (B-BAND) field experiment (see Russell et al., 1991). Data presented in this contribution was collected from the 16th to 25th April 1991, at Spurn Head, U.K. (Fig. 1). This site is a 5 km long spit extending southwards at its northern end, and veering southwest to its tip in the mouth of the Humber Estuary, on the East Yorkshire coast. The field site is positioned on the eastern side of the spit facing into the North Sea. The beach profile (Fig. 2) consists of a steep high tide beach (gradient = 0.0975) comprised of fine to medium gravels (and occasionally larger boulders) and a shallow sloping (gradient=0.023) low tide terrace consisting of a lens of well sorted medium sands (Dso = 0.35 mm) overlying boulder clays. An extensive shingle ridge (called the Stoney Binks) runs parallel to Spurn Head, 3 km offshore from
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the study area• The Stoney Binks "dries out" during spring low tides consequently reducing incident wave energy on the beach during the low tide period. This macrotidal environment (tidal range = 3-6 m) experiences strong rectilinear tidal currents which run parallel to the shoreline in a southwesterly direction on the flood and a northeasterly direction on the ebb. The field site is exposed to wind waves from a broad window extending clockwise from the northeast through to the south• Most significant in this area are the strong swells from the north which refract and impinge obliquely on the beach setting up strong southwesterly longshore currents within the surf zone. The field experiment at Spurn Head involved the deployment of 7 pressure transducers (PTs), 10 electromagnetic current meters (EMCMs), 9 optical back scatter (OBSs) sensors and 2 self generated noise (SGN) sensors. This paper reports on some preliminary analysis of the resulting 130 Mbyte data set. Detailed analyses have been focused initially on data from one set of colocated PT, E M C M and OBS sensors (rig A2). These sensors were situated on the low tide terrace of the beach profile (Fig. 2). The E M C M and OBS were posi-
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tioned 0.1 m above the bed, and had a horizontal separation alongshore of 0.5 m. The sensors were secured by burying their mountings below the sand surface, and the 200 m cables were buried back up to the logging equipment in the mobile laboratory at the head of the beach. Care was taken to ensure that the instrument mountings were buried in excess of 10 cm below the sand level so as to prevent self-suspension by the rigs themselves. Depth of disturbance measurements and beach profiles taken before and after the tide indicated maximum disturbance depths of 5 cm and bed level changes of less than 1 cm over the tidal cycle. The OBS's were mounted to a slender (3 mm diameter) stem which protruded out of the sediment-water interface from the subsurface mountings. No significant signs of scour were observed around the stem during inspections at low water times. Prior to the field deployment, the EMCM's were calibrated by towing at a number of speeds in both the positive and negative directions. This technique provides measurements of oscillatory currents accurate to within _+ 10% and mean flow with uncertainties of + 2 to 3 cm/s (see Guza et al., 1988). Zero offsets were also recorded in still water in the field before and after the experiment. The OBS sensors were calibrated in a re-circulating tank using surface sediment taken from the field site in the close vicinity of the sensor array. Laboratory tests indicate that uncertainties in the suspended sediment concentration measurements are less than + 10%. More details on the B-BAND experimental techniques and set-up are given in Russell et al. (1991). Results presented in this paper cover a full tidal cycle, over consecutive low water periods, during the neap tide (tidal range = 3,2 m on the flood, and 4.0 m on the ebb) of the 23rd April 1991. This tide was selected due to the high, regular groupy waves which occurred on this day. An example time-series from the EMCM and OBS recorded just seawards of the breaker zone is shown in Fig. 3. A total of 34, 17 minute, data runs digitised at 2Hz (2048 data points) spanned this recording period (Fig. 4). The tidal progression over the instruments allowed a profile of measurements to be recorded across, and well beyond, the surf zone. At high water the sensors were positioned a distance of 85 m offshore,
M A. D A V I D S O N ET AL.
seawards of a narrow ( ~ 20 m width) surf zone. During the low water period the instrument array was positioned within the inner surf zone. In spite of a reduction in incident wave height at low tide caused by the sheltering effect of the Stoney Binks, the surf zone was somewhat broader ( ~ 50 m) at this time due to the dissipative profile of the low tide terrace. As a consequence of the large tidal range, fairly short record lengths had to be selected in order to obtain stationary spectral estimates. In this case 17 minutes was selected as the maximum record length. Each of the 2048 point records were divided into 8 sequential non-overlapping segments, giving 15 segments with 50% overlaps. These segments were tapered with a Welch window (Press et al., 1986) and Fourier transformed. Raw spectra were ensemble averaged to give smoothed spectral estimates with 27 degrees of freedom (Nuttal, 1971) and a bandwidth of 0.0078 Hz. Upper and lower confidence limits for these spectra are 1.8 and 0.62 times the spectral estimates, respectively (at the 95% confidence level). Results
1. Waves and currents The incident wave climate on this day was comprised of very groupy swell waves approaching from the north east with significant breaker height and period of 1.5 m and 10 s, respectively. Each of the 17 minute flow velocity time-series was separated into steady and fluctuating components. Using spectral techniques the oscillatory components were further decomposed into gravity and infragravity contributions. Variations in the total, gravity (0.05
337
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338
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Fig. 4. Variation of water depth with run number at the sensor location. the height of the EMCM (0.1 m) the steady crossshore flow velocities are directed offshore (the undertow) within the surf zone, and weakly onshore outside the surf zone. Using the variance and mean flow plots in Figs. 5 and 6 the times when the breaker line passed over the sensors on the flood and ebb tide can be estimated with some confidence. From the shoreline the gravity wave variance is expected to increase seaward to the breaker line and then decrease seawards beyond the breaker line. In Fig. 5a this would suggest that the breaker line was at the sensor rig during runs 9-10 and 28-30. Outside the surf zone the (longshore) tidal currents, in quadrature with the elevation, reach magnitudes of 0.35 m/s even on this neap tide. Within the surf zone longshore flows are dominated by strong southwesterly wave-driven currents forced by swell approaching obliquely from the northeast. The observed longshore current pattern is entirely consistent with the breakpoint being at runs 9-10 and 28-29. Work is in progress to quantify, and model, the interaction between tidal and wave-driven longshore currents at this site.
2. Sediment resuspension and transport This section presents data for sediment suspension over the tidal cycle followed by an analysis of
the sediment transport processes. The total sediment transport is broken down into its steady and oscillatory components and the relative importance of these components to the total transport in both the cross-shore and longshore directions is subsequently assessed. Detailed data analyses have been focused on one pair of colocated EMCM and OBS sensors positioned 10 cm above the bed, with minor reference to sensors at other vertical heights and spatial locations. Depth-integrated transport rates have not been calculated due to the large uncertainties involved with fitting instantaneous velocity and suspended sediment profiles to the point measurements.
Sediment resuspension As well as the OBS at 10 cm above the bed, two other OBS's were deployed at 19 and 27 cm above the bed, on rig A2. At all three heights, the steady (Fig. 7a) and oscillatory (Fig. 7b) SSC components showed a marked asymmetry over the tidal cycle, with almost an order of magnitude increase in the measured SSC's on the ebb tide. This suspension asymmetry was observed at the other rigs during the same tide. The bottom OBS variance at rig A3 (located shore-parallel to rig A2, 20 m to the northeast) is included on Fig. 7b, and shows a similar
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TIDAL ASYMMETRY IN SUSPENDED SAND TRANSPORT
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pattern to that from the bottom OBS on rig A2. The suspension asymmetry was also observed on other tidal cycles e.g. 18th April at rig A2 (Fig. 7c, steady components; Fig. 7d, oscillatory components) and has been noted in data from other macrotidal beach sites e.g. Llangennith, South Wales (Russell, 1990). The strong suspension asymmetry observed in Fig. 7a and b is not so apparent in the total velocity variance (Fig. 5a). Therefore, factors other than the strength of the current velocity must be responsible for producing the observed tidal asymmetry
in the sediment suspension. The possible influence of changes in bedform morphology, dewatering of the beach and localised bed level changes are considered in the discussion section.
Oscillatory transport As stated previously, the oscillatory longshore transport is negligible, and therefore only the oscillatory cross-shore transport is considered here. The oscillatory cross-shore transport was exam-
340
M.A. DAVIDSON
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TIDAL ASYMMETRYIN SUSPENDEDSANDTRANSPORT
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ined by computing cross-spectra between the crossshore current velocity (u) and the suspended sediment concentration (c) time-series for the EMCM and OBS at 10 cm above the bed. Sediment transport patterns occurring on the flooding and ebbing tides are examined separately.
Flooding tide Three dimensional plots showing how the u spectra and c spectra change with water depth over
the instruments on the flooding tide are shown in Fig. 8a and b, respectively. The typical pattern of incident wave energy ( f = 0.11 Hz) decaying shorewards of the breakpoint (depth = 1.8 m), accompanied by an increase in infragravity energy, is depicted in the u spectra (Fig. 8a). Although the c spectra (Fig. 8b) have peaks at low frequency, incident wave frequency and the first harmonic of the incident wave frequency (corresponding to the three main peaks in Fig. 8a), the basic pattern of
342
M.A. DAVIDSON ET AL 2.0
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the u spectra (Fig. 8a) is not repeated in the c spectra (Fig. 8b), again indicating that factors other than the current velocity are influencing the amount of sediment in suspension. Detailed cross-spectral analysis results are presented for two runs on the incoming tide; run 15,
outside the surf zone (depth=2.8 m; Fig. 9a-f), and run 1, in the inner surf zone (depth = 0.45 m; Fig. 10a-f). Outside the surf zone the c spectrum (Fig. 9a) has three main peaks at low frequency, incident wave frequency and at the first harmonic of the incident waves, whereas although the u
343
T I D A L A S Y M M E T R Y IN S U S P E N D E D S A N D T R A N S P O R T
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Fig. 10. Cross-spectral analyses of suspended sediment and cross-shore current velocity for run 1 (inner surf zone, flooding tide). Plots include, (a) SSC auto-spectrum, (b) u-velocity auto-spectrum, (c) c - u co-spectrum with 95% confidence limits. Phase points corresponding to the co-spectral (net transport) peaks are distinguished by circles, (d) c - u phase spectrum, (e) c - u cross-spectrum, and (f) c - u coherence spectrum.
spectrum (Fig. 9b) shows these peaks, it is strongly dominated by the incident wave peak at f = 0.11 Hz. The co-spectrum (Fig. 9c) is similar to that presented by Huntley and Hanes (1987), with a dominant onshore transport occurring under the
incident waves, and a smaller offshore transport occurring under the long period waves. There is also significant onshore transport associated with the first harmonic of the incident waves. The phase lag (Fig. 9d) between ¢ and u is critical in determin-
344
ing the net sediment transport direction and magnitude. At infragravity frequencies, phase differences are greater than ~/2 radians or less than - 7r/2 radians giving offshore transport, whereas at gravity band frequencies phase differences are between g/2 and - ~ / 2 radians, giving onshore transport. 95% confidence limits for the phase spectrum have been calculated from the coherence estimates (Jenkins and Watts, 1968), and are plotted on Fig. 9d. These results illustrate that the co-spectral (net transport) peaks are significant both in their magnitude and direction. The gross transport rate is represented by the c-u cross-spectrum (Fig. 9e), and again shows the three main transport peaks. The dominant transport process is associated with the highly coherent peak (Fig. 9f) at incident wave frequency, which, in this case, produces onshore transport. The spectra from the inner surf zone (Fig. 10) have two main peaks; at low frequency and at the incident wave frequency. Harmonic oscillations have been totally dissipated in this region of the surf zone. The low frequency peak dominates the c spectrum (Fig. 10a) and u spectrum (Fig. 10b), and gives a strong offshore transport at low frequency (Fig. 10c and d). Sediment transport is directed onshore under the incident waves, but this process is secondary to the offshore transport occurring at low frequency. Note that the peak transport rates within the inner surf zone are an order of magnitude higher than those measured outside the surf zone. Russell (1990) and Beach and Sternberg (1991) have presented c u co-spectra from the inner surf zone which also show the offshore transport at low frequency to be the dominant transport mechanism in this zone.
M A . D A V I D S O N ET AL.
The infragravity wave energy increases towards the shoreline but, in contrast to the flood, does not dominate, even closest to the shoreline. The c spectra (Fig. 1 lb) show high suspension to be associated with the incident wave velocity maximum, although the peak spectral suspension values occur at infragravity frequencies. At both gravity and infragravity frequencies, peak concentrations are ten to twenty times higher in magnitude than on the flood tide. The resulting 3-D transport spectra (Fig. 1 lc) are dominated by a very strong offshore transport at incident wave frequencies at the seawards edge of the surf zone (depth = 1.5 3 m). Detailed cross-spectral analysis results are presented for run 24 (depth= 2.7 m: Fig. 12a f) which represents a cross-section through this area of maximum offshore transport shown in Fig. l lc. The three main peaks in the suspension (once again at low frequency, incident wave frequency and the first harmonic of the incident wave frequency) are clearly shown in the c spectrum (Fig. 12a), whilst the u spectrum (Fig. 12b) shows the strong incident wave dominance also observed outside the surf zone on the flooding tide (Fig. 9b). However, the c u phase relationship (Fig. 12d) is markedly different from that on the flooding tide (Fig. 9d), and results in offshore directed transport at all frequencies less than 0.27 Hz, so that the net transport at low frequency, incident wave frequency and the first harmonic of the incident wave frequency is directed offshore in all cases (Fig. 12c). This strong offshore transport on the ebbing tide is the dominant feature of the oscillatory sediment transport results obtained, with peak values in excess of four times larger than those observed over the rest of the tide, even in the inner surf zone.
Ebbing tide
On the ebbing tide, 3-D plots illustrating how the u spectra, c spectra and c-u co-spectra change with depth are presented in Fig. l la, b and c, respectively. The u spectra (Fig. 1 l a) show a strong incident wave dominance which has a maximum magnitude outside the surf zone (depth 2--3 m).
Mean transport Mean cross-shore transport
Variations in the steady cross-shore transport contribution over the tidal cycle are shown in Fig. 13. The mean cross-shore transport rate is
Fig. 11. Three-dimensionalplots illustrating the variation with depth, on the ebbing tide, of, (a) the u-velocityauto-spectrum, (b) the SSC auto-spectrum, and (c) the transport spectrum (c-u co-spectrum).
TIDAL ASYMMETRY IN SUSPENDED SAND TRANSPORT
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346
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Fig. 12. Cross-spectral analyses of suspended sediment and cross-shore current velocity for run 25 (outside the surf zone, ebbing tide). Plots include, (a) SSC auto-spectrum, (b) u-velocity auto-spectrum, (c) c - u co-spectrum, (d) c-u phase spectrum with 95% confidence limits. Phase points corresponding to the co-spectral (net transport) peaks are distinguished by circles, (e) c - u crossspectrum, and (f) c - u coherence spectrum.
largest within the surf zone (up to 0.20 kg/m2/s) and is directed offshore (at the height of the sensors) in association with the undertow. Conversely, outside the surf zone, the mean cross-shore transport component is directed onshore reaching maximum values (0.15 kg/mZ/s) during the ebbing tide,
coincident with the time of high sediment suspension. This result, which gives a convergence of the mean nearbed transports in the breaker zone, has been observed by other workers (e.g. Bailard, 1987) and may be related to the formation of a "breakpoint bar".
347
T I D A L A S Y M M E T R Y IN S U S P E N D E D S A N D T R A N S P O R T
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Fig. 13. Variationsin the mean cross-shoreand longshoretransport rates with tidal state.
Mean longshore transport The steady longshore transport component (~.~) is particularly important in this macrotidal environment with a highly oblique incident wave approach. On this day it reached rates of up to 0.6 kg/m2/s (Fig. 13). Tidal currents dictate the magnitude and direction of the steady longshore transport, component outside the surf zone whilst the wave driven longshore currents (directed to the southwest), control these factors inside the surf zone. This is the subject of studies to be reported elsewhere• Total transport Total cross-shore transport In order to examine the relative importance of the cross-shore oscillatory and steady components to the total cross-shore sediment transport rate, the tidal history of these parameters is plotted in Fig. 14a, b and c, for the bottom, middle and top OBS heights, respectively. The results for the bottom OBS height (Fig. 14a) are described first. During the flooding tide, instantaneous suspension and flow velocity time-series are commonly close to quadrature at peak frequencies, resulting in low net oscillatory transport rates. It is the mean component, therefore, which dominates the total net transport rate on the flood tide. However, one hour
after the turn of the tide (run 24) SSC and u timeseries are highly coherent and in anti-phase at incident wave frequencies resulting in high net oscillatory transport rates which are directed offshore. Consequently, it is the oscillatory component which is dominant on the ebbing tide, coincident with the time of high sediment suspension, leading to the highest net transport rates (c'.u'= 0.37 kg/m2/s). During this period of high suspension and high rates of oscillatory transport, the mean transport component is secondary in importance and opposite in sign to the unsteady components. Higher in the water column (Fig. 14b and c) the oscillatory transport is much lower in magnitude and is directed onshore. Therefore, at these higher elevations, the mean component tends to dominate the total transport. A similar result was obtained by Osborne and Greenwood (1992).
Total longshore transport The total longshore transport rate is completely dominated by the mean longshore transport component throughout the tidal cycle. Fluctuations in suspension and longshore current velocity are generally uncorrelated resulting in very low oscillatory longshore transport rates. The total longshore transport rate, therefore, follows the pattern of the mean longshore transport described above.
348
MA DAVIDSONETAL 0.3
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TIDAL ASYMMETRYIN SUSPENDEDSANDTRANSPORT
Discussion
Many of the present results agree with recent observations made by other workers on natural beaches. For example, the cross-shore variations in hydrodynamic processes support the findings of other writers (Holman, 1981; Wright et al., 1982a,b). Although complicated by the influence of the Stoney Binks, the classical pattern for shorewards growth of long wave energy accompanied by decaying incident wave energy in the surf zone is clearly evident in this data set. However, typical of other observations on macrotidal intermediate beaches (e.g. Wright et al., 1982a), long wave oscillations are subordinate to incident wave components throughout the tidal cycle (at the location of the sensors). Similarly, previously observed patterns in sediment transport processes both in the longshore (Sternberg et al., 1984) and cross-shore (Huntley and Hanes, 1987; Russell, 1990; Beach and Sternberg, 1991; Osborne and Greenwood, 1992) directions are also consistent with this data set. The total longshore transport can be adequately represented by the mean longshore transport component alone (i.e. the oscillatory longshore transport is small and can be neglected), whereas the oscillatory cross-shore transport contribution (both at incident wave frequencies and long wave frequencies) is very important in determining the total crossshore transport. At 10 cm above the bed, the patterns of net cross-shore sediment transport (as illustrated by the c u co-spectra) on the flooding tide, both outside the surf zone (e.g. run 15, Fig. 9c) and in the inner surf zone (e.g. run 1, Fig. 10c) show the same dominant transport mechanisms that were noted by Huntley and Hanes (1987; predominantly onshore transport at incident wave frequencies), and Russell (1990) and Beach and Sternberg (1991; predominantly offshore transport at long wave frequencies), respectively. However, by plotting the c - u co-spectra as a continuum across and beyond the surf zone it is clear that these net transport spectra vary greatly
349
with both cross-shore location and tidal state. The asymmetry in the magnitude of the observed suspended sediment concentration and associated transport rates between flood and ebb tides is striking. For this data set, when the complete tidal cycle is studied, the offshore transport at incident wave frequencies seaward of the breaker zone on the ebbing tide is the dominant cross-shore transport feature. This period of high transport occurred primarily as a result of high sediment suspension at gravity frequencies which interacted coherently and in anti-phase with the incident wave flow field. Three possible causes of the observed increased suspension after the tide turns are: (1) dewatering of the beach; (2) localised bed-level changes relative to the OBS; (3) erosion of a ripple field which formed in the slack waters over high tide. Brenninkmeyer (1976) noted that the level of the beach water table tends to lag the tide by 1 to 3 hours. The dry beach (above the level of the water table during the flooding tide) enhances deposition, whereas the saturated beach (below the level of the water table on the ebbing tide) augments erosion (Grant, 1948). Lanyon et al. (1982) state that, on the saturated beach, groundwater effluence occurs during the latter stage of the backwash flow. This process was shown by Grant (1948) to increase turbulence levels in the backwash, and also to dilate the sand grains, both of which would encourage entrainment. Another possible solution for the observed tidal asymmetry is localised accretion of sediment in the region of the OBS sensor, so lowering the relative height of the sensor above the bed, causing it to measure higher SSC's. This seems unlikely as the measured height of the OBS above the bed was 9 cm before the tidal cycle, and 10 cm after the tidal cycle. Extensive profiling and depth-of-disturbance rod data collected during the fieldwork period showed bed level changes on the low tide terrace to be restricted to a few centimetres. The third hypothesis of bedform alteration is
Fig. 14. Variations in the mean, oscillatory and total, cross-shore transport rates with tidal state, for (a) the bottom OBS height (I0 cm), (b) the middle OBS height (19 cm), and (c) the top OBS height (27 cm).
350
M A DAVIDSON E~[ .A.I+.
favoured by the authors. During the period of low tidal transition at high water, both oscillatory and mean currents (including tidal and wave driven components) are at a minimum (Figs, 5 and 6). It is well documented (cf. Dingler and Inman, 1976) that in such weak oscillatory dominated flow fields it is common to observe well developed symmetrical ripples seaward of the surf zone. The only proviso is that the orbital velocities of the waves are above the threshold for the initiation of sediment transport (in this case > 0.23 m/s, see Komar and Miller, 1975). After the turn of the tide there is a sharp increase in both the nearbed oscillatory currents (due to a reduction in the water depth) and mean flows (now flowing in the opposite direction). These larger velocities would be arriving on an over-steep rippled bed, and would cause the bed to efficiently eject sand-laden vortices high into the flow. Such a mechanism has also been described by Vincent et al. (1991). Hence, the high SSC's observed on the ebb tide are likely to be a result of the destruction of the ripple field which occurs during the transition from rippled to plane bed with increasing velocity as the water depth shallows. Once the plane bed condition is re-established
SHEET FLOW
in the surf zone, the observed suspension at height would be expected to decrease. Values quoted in the literature for the critical velocity at the point of transition between rippled bed conditions and sheet flow show a considerable spread. Dingler and Inman (1976) suggest values of the maximum orbital velocity (g/m crit) at transition of between 0.46 and 1.14 m/s. The precise value is a function of the wave orbital velocity and the grain size of the sediment. For example, extrapolation of the data from Horikawa et al. (1982) taking the median grain size (Ds0 = 0.35 ram) and peak wave period ( T = 11 s) for this data set, gives Urn c r i t = 0 . 7 m/s. However, most of the values quoted for um i~iu are derived from laboratory experiments (cf. Manohar, 1955; Carstens, 1966: Dingier and Inman, 1976; Horikawa et al., 1982; Shibayama and Horikawa, 1982) which consider only oscillatory (usually monochromatic) flows. Therefore, some caution should be taken when applying laboratory results to a 3-D field situation where there exists a complex spectrum of sea waves combined with steady flows. Figure 15 shows a plot of wave orbital velocity predicted by cnoidal wave theory (which is appro-
I I I FORMATION I I OF DESTRUCTION OF RIPPLE FIELD SYMMETRICAL I AND RETURN TO SHEET FLOW I RIPPLES I
0.9 ~11.--Region 1 - ~ R e g i o n J
Transition velocity between rippled
bed and sheet flow (Horikawa, 1982)
"
'
I
| ~. 0.8 1 Br
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"
'
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I I
d o,
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"
'
"
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_ _ _
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_~ I'~a~"~th~-o~'- -- -- ~t' ~ -- -- T --J' ~-i F'l~-"Bre~d~wn'"~' u'/'[ - \ A /~ iil ~ ~ of Cnoidal Velocity predicted using V ~ / " ] ~i ! wave t h e orv "~ ICnoidalwavetheory ~ / / ~ 'l 0"6 ! (reference P°S'~°n" " ' " " " ~ V / ~ i Ca 10cm above the bed) + t •o
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o 0.4
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,
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:
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,
. I
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6 t~ .... ~ 4 ~O ¢,
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,
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Fig 15. Variation of the total suspended sediment variance and cross-shore current velocity (predicted using cnoidal wave theory) with tidal state.
TIDAL ASYMMETRYIN SUSPENDEDSANDTRANSPORT
35 |
priate to the asymmetric flow field in this region of the surf zone) against sediment suspension. Values of um are not given for runs 1-4 and 31-34 as cnoidal wave theory is not valid for the shallowest data runs. Also marked on the graph is a theoretical field value for Umcrit equivalent to the extrapolated laboratory results of Horikawa et al. (1982). The intersection of the position of the transition zone indicated by the field data (the vertical dotted line) and the computed Um values (the horizontal dotted line) divides the graph into 3 distinct regions. During the flooding tide (region 1; Fig. 15) values of Um exceed Um crit leading to sheet flow conditions close to the bed. Therefore, at the height of the OBS (0.1 m above the bed), low levels of sediment suspension were recorded. During the high tide period (region 2; Fig. 15) declining nearbed flows lead to the formation of symmetrical ripples in the deeper water outside the surf zone. A sharp increase in Umone hour after the turn of the tide from 0.67 m/s (run 22) to 0.83 m/s (run 24) corresponds exactly with the increase in sediment suspension observed at the height of the OBS (start
of region 3; Fig. 15). It is hypothesised that the observed increase in sediment suspension relates to the destruction of the ripple field which was established over high water, and the advection of sediment to the height of the OBS by vortices generated by strong nearbed flows over the still partially rippled bed. Eventually a plane bed condition is reached (at approximately run 28; Fig. 15) and the sheet flow is once again confined to a narrow region close to the bed similar to the situation on the flooding tide (region 1; Fig. 15). Inman and Bagnold (1963) first described the mechanism by which sand could travel in an offshore direction over a rippled bed with shoreward propagating incident waves (Fig. 16). Under the shoreward stroke, the sand is carried a small distance onshore close to the bed and becomes trapped in a vortex in the lee of the ripple crest. Upon flow reversal the vortex is released upwards and the suspended sand is carried a greater distance offshore by the seaward stroke, so giving a net offshore transport. Thus, the onset of sheet flow over the ripple field during the ebb tide leads to
ONSHORE FLOW y DENSE SEDIMENT FLOW NEAR EBED " ,
"
"~l
~
.
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SEDIMENT SUSPENSION TRANSPORTED OFFSHORE
Fig. 16. Simplistic model for offshore transport at incident wave frequency over a rippled bed (after Inman and Tunstall, 1972).
352 enhanced advection and offshore transport of sediment. Schepers (1978) and Sato (1986) reported similar findings in that their data, collected over sharp crested ripples, showed the resulting sediment transport to be in the opposite direction to that of the largest instantaneous velocities. Although both the dewatering and bedform alteration mechanisms are likely to have an important influence on sediment transport processes, the bedform mechanism is appealing as it adequately explains the high correlation between c" and u' and their antiphase relationship, leading to strong offshore transport at incident wave frequencies. Conclusions
Observations on a macrotidal beach (Spurn Head, Yorkshire, UK) have confirmed that there are some similarities in the hydrodynamic processes and sediment dynamics between highly tidal (this contribution) and meso/micro tidal beaches (other writers). For example: (1) Within the surf zone hydrodynamic infragravity variance grows in a shorewards direction accompanied by a dissipation of incident and harmonic variance (Holman, 1981: Wright et al., 1982b). (2) Infragravity energy is subordinate to gravity wave energy on intermediate beaches (Wright et al., 1982a). (3) Sediment transport occurs primarily at three specific frequencies. TheSe are at gravity, infragravity and incident-harmonic frequencies (cf. Huntley and Hanes, 1987). (4) Longshore sediment transport processes are dominated by mean flows with negligible contributions from oscillatory components (cf. Sternberg et al., 1984). (5) The net cross-shore sediment flux is given by the sum of the steady and oscillatory components (Jaffe et al., 1984). The relative importance of these two components is dependant on several factors. These factors include: (a) the position relative to the surf zone. This affects the strength of the undertow, the magnitude of the various oscillatory components mentioned in (3) above, and the presence/absence of bedforms.
MA DAVIDSONI'.FAk. (b) the height above the bed. This affects the phase relationship between sediment suspension and velocity time-series and thus the magnitude and direction of sediment transport (cf. Hanes and Huntley, 1986; Osborne and Greenwood, 1992). (6) Seaward of the surf zone, sediment transport processes are dominated by the oscillatory incident wave components (cf. Huntley and Hanes, 1987). Strong onshore transport occurs at the incident wave frequencies with weaker offshore transport at infragravity wave frequencies. (7) In the inner surf zone sediment transport processes are dominated by oscillatory, infragravity components (cf. Russell, 1990; Beach and Sternberg, 1991). The resulting nearbed sediment transport is directed predominantly offshore. This contribution has also emphasised important new phenomena which are expected to be unique to macrotidal beaches. Specifically: (l) There is a marked asymmetry in sediment suspension, with suspension values up to an order of magnitude higher on the ebb tide. This pattern is reproduced vertically and spatially, and has been observed at other macrotidal beach sites (cf. Russell, 1990). It is hypothesised that the large sediment suspension and transport observed on the ebb tide, just seaward of the surf zone, correlates with the destruction of a ripple field during the transition period between rippled bed conditions (low current velocities above threshold) and sheet flow (high current velocities > 0.74 m/s). This transition occurs due to a localised shallowing of the water level leading to increased incident wave orbital velocities at the bed. Such high suspension is absent on the flooding tide since bedforms are initially absent, and do not form until high water when wave orbital velocities outside the surf zone are low enough to permit the growth of ripples. (2) High levels of longshore transport (up to 0.6 kg/mZ/s for H1/3 = 1 m) were observed to be associated with steady shore-parallel velocity components (tidal flows and wave driven longshore currents). Wave driven longshore currents dominated longshore transport processes inside the surf zone whilst tidal currents were the major driving force behind sediment transport seaward of the surf zone.
TIDAL ASYMMETRY IN SUSPENDED SAND TRANSPORT
Acknowledgements The authors would like to acknowledge the assistance of Mr. Gareth Lloyd (electronics) and Dr. Adrian Cramp (logistics and review of manuscript). The British Beach And Nearshore Dynamics (B-BAND) programme is supported by the Natural Environment Research Council (NERC) U.K. The Post-Doctoral Research Associates, Dr. Mark Davidson and Dr. Paul Russell, wish to thank the Welsh Office and the NERC, respectively, for funding their work on this project.
References Bailard, J.A., 1987. Surf zone wave velocity moments. Proc. Coastal Hydrodynamics. ASCE, pp. 328-342. Beach, R.A. and Sternberg, R.W., 1988. Suspended sediment transport in the surf zone: Response to cross-shore infragravity motion. Mar. Geol., 80: 61-79. Beach, R.A. and Sternberg, R.W., 1991. Infragravity driven suspended sediment transport in the swash, inner and outer surf zone. Pro~. Coastal Sediments '91. ASCE, pp. I 14-128. Brenninkmeyer, B.M., 1976. Sand fountains in the surf zone. In: R.A. Davis, Jr. and R. Ethington (Editors), Beach and Nearshore Sedimentation. SEPM Spec. Publ., 24: 69-91. Carstens, M.R., 1966. Similarity laws for localised scour. Proc. ASCE J. Hydraul. Div., 92(HY3): 13-36. Dingier, J.R. and Inman, D.L., 1976. Wave formed rippled in nearshore sands. Proc. 15th Conf. Coastal Eng. ASCE, pp. 2109-2126. Downing, J.P., 1983. Field studies of suspended sand transport, Twin Harbours Beach, Washington. Ph.D. Dissert., Univ. Washington, School Oceanogr., WB- 10, 121 pp. Eckart, C., 1951. Surface waves on water with variable depth. Wave Rep. ,I00, Scripps Inst. Oceanogr., Univ. California, San Diego, La Jolla, 99 pp. Grant, U.S., 1948. Influence of watertable on beach aggradation and degradation. J. Mar. Res., 7: 655-660. Guza, R.T, Clifton, M.C. and Rezvani, F., 1988. Field intercomparisons of electromagnetic current meters. J. Geophys. Res., 93:9302-9314. Holman, R.A., 1981. Infragravity energy in the surf zone. J. Geophys. Res., 86(C7): 6442-6450. Horikawa, K., Watanabe, A. and Katori, S., 1982. Sediment transport under sheet flow condition. Proc. 19th Coastal Eng. Conf. ASCE, pp. 1335-1352. Huntley, D.A., Guza, R.T. and Thornton, E.B., 1981. Field observations of surf beat, 1. Progressive edge waves. J. Geophys. Res., 86(C7): 6451-6466. Huntley, D.A. and Hanes, D.M., 1987. Direct measurements of suspended sediment transport. Proc. Coastal Sediments '87. ASCE, pp. 723-737. Inman, D.L. and Bowen, A.J., 1963. Flume experiments on
353 sand transport by waves and currents. Proc. 8th Coastal Eng. Conf. pp. 137-150. Inman, D.L. and Tunstall, E.B., 1972. Phase-dependent roughness control of sand movement. Proc. 13th Coastal Eng. Conf. ASCE, pp. 1155-1171. Jaffe, B.E., Sternberg, R.W. and Sallenger, A.H., 1984. The role of suspended sediment in shore-normal beach profile change. Proc. 19th Coastal Eng. Conf. ASCE, pp. 1983-1996. Komar, P.D. and Miller, M.C., 1975. Sediment threshold under oscillatory waves. Proc. 14th Coastal Eng. Conf. ASCE, pp. 756-775. Lanyon, J.A., Eliot, I.G. and Clarke, D.J., 1982. Observations of shelf waves and bay seiches from tidal and beach groundwater-level records. Mar. Geol., 49: 23-42. Larsen, L.H., 1982. A new mechanism for seaward dispersion of midshelf sediments. Sedimentology, 29: 279-284. Longuet-Higgins, M.S. and Stewart, R.W., 1964. Radiation stresses in water waves: A physical discussion with applications. Deep-Sea Res., 11: 529-562. Manohar, M., 1955. Mechanics of bottom sediment movement due to wave action. Beach Erosion Board Tech. Memo, 75. Nuttal, A.H., 1971. Spectral Estimation by Means of Overlapped Fast Fourier Transforms of Windowed Data. NUSC Rep., No. 4169, Dep. Navy, U.S.A. Osborne, P.D. and Greenwood, B., 1992. Frequency dependent cross-shore sediment transport. 1: A non-barred shoreface. Mar. Geol., 106: 1-24. Press, W.H., Flannery, B.P, Teukolsky, S.A. and Vetterling, W.T., 1986. Numerical Recipies. Cambridge Univ. Press, 818 pp. Russell, P.E., 1990. Field studies of suspended sand transport on a high energy dissipative beach. Ph.D. Thesis, Univ. Wales, 318 pp. (Unpubl.) Russell, P.E., Davidson, M.A., Huntley, D.A., Cramp, A., Hardisty, J. and Lloyd, G. 1991. The British Beach and Nearshore Dynamics (B-BAND) programme. Proc. Coastal Sediments '9 I. ASCE, pp. 371-384. Sato, S., 1986. Oscillatory boundary layer flow and sand movement over ripples. Ph.D. Thesis, Dep. Civ. Eng., Univ. Tokyo. (Unpubl.) Schepers, J.D., 1978. Zandtransport onder invloed van golven en een eenparige stroom bij varierende korreldiameter. MS. Thesis, Delft Tech. Univ. Shibayama, T. and Horikawa, K., 1982. Sediment tarnsport and beach transformation. Proc. 19th Coastal Eng. Conf. ASCE, pp. 1439-1458. Sternberg, R.W., Shi, N.C. and Downing, J.P., 1984. Field investigations of suspended sediment transport in the nearshore zone. Proc. 19th Coastal Eng. Conf. ASCE, pp. 1782-1798. Vincent, C.E., Hanes, D.M. and Bowen, A.J., 1991. Acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness. Mar. Geol., 96: 1-18. Wright, L.D., Nielsen, P., Short, A.D. and Green, M.O., 1982a. Morphodynamics of a macrotidal beach. Mar. Geol., 32: 105-140. Wright, L.D., Guza, R.T. and Short, A.D., 1982b. Dynamics of a high energy dissipative surf zone. Mar. Geol., 45: 41-62.
333
Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach Mark
A . D a v i d s o n a, P a u l E . R u s s e l l a ' l , D a v i d A . H u n t l e y b a n d J a c k H a r d i s t y c
aMarine Geosciences Group, Department of Geology, University of Wales, P.O. Box 914, Cardiff CF1 3 YE, UK blnstitute of Marine Studies, Polytechnic South-West, Drake Circus, Plymouth PL4 8AA, UK CSchool of Geography, The University, Cottingham Road, Hull HU6 7RX, UK (Received February 11, 1992; revision accepted September 29, 1992)
ABSTRACT Davidson, M.A., Russell, P.E., Huntley, D.A. and Hardisty, J., 1993. Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach. Mar. Geol., l l0: 333-353. Time-series of nearbed horizontal flow velocities and suspended sediment concentrations obtained from a colocated electromagnetic current meter (EMCM) and optical backscatter sensor (OBS), respectively, are used to examine the relative importance of steady and fluctuating components to the total sediment transport over a full tidal cycle on a macrotidal, intermediate beach (Spurn Head, UK). Fluctuating sediment fluxes are decomposed into gravity and infragravity contributions using cospectral techniques. The relative importance of the oscillatory (gravity and infragravity) and steady (mean) transport components to the total sediment transport is analysed throughout the tidal cycle. A continuum of 34 discrete suspended sediment-cross-shore velocity co-spectra are computed over a full tidal cycle for the OBS and EMCM measurements 0.10 m above the bed. These net transport spectra vary greatly both with cross-shore location and tidal state. In particular, a marked asymmetry in transport processes is evident between the flood and ebb tides, with high levels of sediment resuspension and transport occurring on the ebbing tide approximately two hours after high water (just seaward of the breakpoint). At this time the dominant transport was directed offshore (co-spectral peak, 0.04 kg/m2/s) at incident wave frequency. Typical patterns are observed in transport spectra outside the surf zone and within the inner surf zone. Outside the narrow surf zone cross-shore transport spectra show weak offshore transport (co-spectral peak = 0.002 kg/m2/s) associated with bound long waves and stronger onshore transport (co-spectral peak=0.006 kg/m2/s) at incident wave frequencies. Conversely, cospectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m2/s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m2/s) occurring at incident wave frequencies.
Introduction T i d a l effects o n b e a c h p r o c e s s e s h a v e r e c e i v e d c o m p a r a t i v e l y little a t t e n t i o n in the l i t e r a t u r e d e s p i t e the fact t h a t a s i g n i f i c a n t p r o p o r t i o n o f the w o r l d ' s b e a c h e s are s i t u a t e d in m e s o t i d a l o r m a c r o tidal e n v i r o n m e n t s ( W r i g h t et al., 1982a). T h i s paper considers how the importance of the three
Correspondence to: M.A. Davidson, School of Civil and Structural Engineering, University of Plymouth, Palace Court, Palace Street, Plymouth, Devon PL1 2DE, UK. 1Present address: Institute of Marine Studies, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Elsevier Science Publishers B.V.
main transport components
(mean, gravity band
a n d i n f r a g r a v i t y b a n d ) c h a n g e s o v e r a full tidal cycle o n a m a c r o t i d a l b e a c h . M e a s u r e m e n t s are o b t a i n e d f r o m the i n n e r s u r f z o n e to well o f f s h o r e of the breakers. The total shore-normal (cross-shore) sediment t r a n s p o r t r a t e at a n y g i v e n h e i g h t a b o v e the b e d is m a d e u p o f the s u m o f t h e m e a n a n d o s c i l l a t i n g t r a n s p o r t c o m p o n e n t s . T h i s was e x p r e s s e d by Jaffe et al. (1984) as:
c u ~ cuq- c'u ' Total = Mean + Oscillatory
334
where c and u are the instantaneous concentration and cross-shore velocity measurements; the overbars represent time averages, and the primes denote fluctuations about the means. The oscillatory transport component may be further decomposed, using co-spectral techniques, to give the transport contributions as a function of frequency. Since the original work of Huntley and Hanes (1987), several workers (e.g. Russell, 1990; Beach and Sternberg, 1991; Osborne and Greenwood, 1992) have used the co-spectrum between velocity and suspended sand concentration to examine the net sediment transport contribution at different frequencies. In the longshore direction, the oscillatory transport contribution approximates to zero as the fluctuating components of longshore velocity and concentration are generally uncorrelated (e.g. Sternberg et al., 1984: Russell, 1990). However, in the cross-shore direction, the velocity fluctuations associated with both incident (gravity band) waves and infragravity waves are often closely correlated with sediment concentration fluctuations, so that the oscillatory transport component may dominate the total cross-shore sediment transport. For example, Jaffe et al. (1984) instrumented a bar crest within the surf zone and found the net sediment transport to be directed onshore, due to the dominance of the oscillatory component (directed onshore in this case) over the mean offshore transport associated with the steady nearbed offshore flow (undertow). Huntley and Hanes (1987) presented data from outside a low energy (breaker height = 0.25 m) surf zone. Their results showed a strong onshore transport under the incident waves and a weaker offshore transport under the infragravity waves. Russell (1990) and Beach and Sternberg (1991) have presented high energy data (breaker height = 3 m, Russell, 1990; offshore significant swell height = 3 to 5 m, Beach and Sternberg, 1991) from inside the surf zone which show that offshore transport at low frequency is often the dominant transport component in the inner surf and swash zones. The mechanism put forward by Huntley and Hanes (1987) to explain the offshore transport at low frequency is that, outside the surf zone, high wave groups are phase locked to the troughs of long period oscillations (Longuet-Higgins and
M A DAVIDSON ET At.
Stewart, 1964), so that the high suspended sand concentrations occurring under the high wave groups (Larsen, 1982) are transported offshore by the seaward directed nearbed current associated with the troughs of the long waves. Inside the surf zone, the situation is rather different. After being released at the breakpoint (Longuet-Higgins and Stewart, 1964), the long waves may become resonantly trapped in the nearshore by processes of reflection and refraction forming edge waves (Huntley, 1981). Eckart (1951) presented a model which showed edge wave energy to increase exponentially towards the shoreline, so it may be expected that the infragravity transport component will be large in the inner surf zone. Many field experiments (Downing, 1983: Sternberg et al., 1984: Beach and Sternberg, 1988) have confirmed this. Most measurements have shown these high transport rates at low frequency in the inner surf zone to be directed offshore (Beach and Sternberg, 1988, 1991; Russell, 1990). Nevertheless, it is the phase lag between concentration and velocity that determines the transport direction (and magnitude), and this phase will vary with both crossshore position and height above the bed.
Experimental set-up, study area and methods The data analysed in this paper forms part of an extensive data base collected during the British Beach and Nearshore Dynamics (B-BAND) field experiment (see Russell et al., 1991). Data presented in this contribution was collected from the 16th to 25th April 1991, at Spurn Head, U.K. (Fig. 1). This site is a 5 km long spit extending southwards at its northern end, and veering southwest to its tip in the mouth of the Humber Estuary, on the East Yorkshire coast. The field site is positioned on the eastern side of the spit facing into the North Sea. The beach profile (Fig. 2) consists of a steep high tide beach (gradient = 0.0975) comprised of fine to medium gravels (and occasionally larger boulders) and a shallow sloping (gradient=0.023) low tide terrace consisting of a lens of well sorted medium sands (Dso = 0.35 mm) overlying boulder clays. An extensive shingle ridge (called the Stoney Binks) runs parallel to Spurn Head, 3 km offshore from
335
T I D A L A S Y M M E T R Y IN S U S P E N D E D S A N D T R A N S P O R T
the study area• The Stoney Binks "dries out" during spring low tides consequently reducing incident wave energy on the beach during the low tide period. This macrotidal environment (tidal range = 3-6 m) experiences strong rectilinear tidal currents which run parallel to the shoreline in a southwesterly direction on the flood and a northeasterly direction on the ebb. The field site is exposed to wind waves from a broad window extending clockwise from the northeast through to the south• Most significant in this area are the strong swells from the north which refract and impinge obliquely on the beach setting up strong southwesterly longshore currents within the surf zone. The field experiment at Spurn Head involved the deployment of 7 pressure transducers (PTs), 10 electromagnetic current meters (EMCMs), 9 optical back scatter (OBSs) sensors and 2 self generated noise (SGN) sensors. This paper reports on some preliminary analysis of the resulting 130 Mbyte data set. Detailed analyses have been focused initially on data from one set of colocated PT, E M C M and OBS sensors (rig A2). These sensors were situated on the low tide terrace of the beach profile (Fig. 2). The E M C M and OBS were posi-
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tioned 0.1 m above the bed, and had a horizontal separation alongshore of 0.5 m. The sensors were secured by burying their mountings below the sand surface, and the 200 m cables were buried back up to the logging equipment in the mobile laboratory at the head of the beach. Care was taken to ensure that the instrument mountings were buried in excess of 10 cm below the sand level so as to prevent self-suspension by the rigs themselves. Depth of disturbance measurements and beach profiles taken before and after the tide indicated maximum disturbance depths of 5 cm and bed level changes of less than 1 cm over the tidal cycle. The OBS's were mounted to a slender (3 mm diameter) stem which protruded out of the sediment-water interface from the subsurface mountings. No significant signs of scour were observed around the stem during inspections at low water times. Prior to the field deployment, the EMCM's were calibrated by towing at a number of speeds in both the positive and negative directions. This technique provides measurements of oscillatory currents accurate to within _+ 10% and mean flow with uncertainties of + 2 to 3 cm/s (see Guza et al., 1988). Zero offsets were also recorded in still water in the field before and after the experiment. The OBS sensors were calibrated in a re-circulating tank using surface sediment taken from the field site in the close vicinity of the sensor array. Laboratory tests indicate that uncertainties in the suspended sediment concentration measurements are less than + 10%. More details on the B-BAND experimental techniques and set-up are given in Russell et al. (1991). Results presented in this paper cover a full tidal cycle, over consecutive low water periods, during the neap tide (tidal range = 3,2 m on the flood, and 4.0 m on the ebb) of the 23rd April 1991. This tide was selected due to the high, regular groupy waves which occurred on this day. An example time-series from the EMCM and OBS recorded just seawards of the breaker zone is shown in Fig. 3. A total of 34, 17 minute, data runs digitised at 2Hz (2048 data points) spanned this recording period (Fig. 4). The tidal progression over the instruments allowed a profile of measurements to be recorded across, and well beyond, the surf zone. At high water the sensors were positioned a distance of 85 m offshore,
M A. D A V I D S O N ET AL.
seawards of a narrow ( ~ 20 m width) surf zone. During the low water period the instrument array was positioned within the inner surf zone. In spite of a reduction in incident wave height at low tide caused by the sheltering effect of the Stoney Binks, the surf zone was somewhat broader ( ~ 50 m) at this time due to the dissipative profile of the low tide terrace. As a consequence of the large tidal range, fairly short record lengths had to be selected in order to obtain stationary spectral estimates. In this case 17 minutes was selected as the maximum record length. Each of the 2048 point records were divided into 8 sequential non-overlapping segments, giving 15 segments with 50% overlaps. These segments were tapered with a Welch window (Press et al., 1986) and Fourier transformed. Raw spectra were ensemble averaged to give smoothed spectral estimates with 27 degrees of freedom (Nuttal, 1971) and a bandwidth of 0.0078 Hz. Upper and lower confidence limits for these spectra are 1.8 and 0.62 times the spectral estimates, respectively (at the 95% confidence level). Results
1. Waves and currents The incident wave climate on this day was comprised of very groupy swell waves approaching from the north east with significant breaker height and period of 1.5 m and 10 s, respectively. Each of the 17 minute flow velocity time-series was separated into steady and fluctuating components. Using spectral techniques the oscillatory components were further decomposed into gravity and infragravity contributions. Variations in the total, gravity (0.05
337
T I D A L A S Y M M E T R Y IN S U S P E N D E D S A N D T R A N S P O R T
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338
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Fig. 4. Variation of water depth with run number at the sensor location. the height of the EMCM (0.1 m) the steady crossshore flow velocities are directed offshore (the undertow) within the surf zone, and weakly onshore outside the surf zone. Using the variance and mean flow plots in Figs. 5 and 6 the times when the breaker line passed over the sensors on the flood and ebb tide can be estimated with some confidence. From the shoreline the gravity wave variance is expected to increase seaward to the breaker line and then decrease seawards beyond the breaker line. In Fig. 5a this would suggest that the breaker line was at the sensor rig during runs 9-10 and 28-30. Outside the surf zone the (longshore) tidal currents, in quadrature with the elevation, reach magnitudes of 0.35 m/s even on this neap tide. Within the surf zone longshore flows are dominated by strong southwesterly wave-driven currents forced by swell approaching obliquely from the northeast. The observed longshore current pattern is entirely consistent with the breakpoint being at runs 9-10 and 28-29. Work is in progress to quantify, and model, the interaction between tidal and wave-driven longshore currents at this site.
2. Sediment resuspension and transport This section presents data for sediment suspension over the tidal cycle followed by an analysis of
the sediment transport processes. The total sediment transport is broken down into its steady and oscillatory components and the relative importance of these components to the total transport in both the cross-shore and longshore directions is subsequently assessed. Detailed data analyses have been focused on one pair of colocated EMCM and OBS sensors positioned 10 cm above the bed, with minor reference to sensors at other vertical heights and spatial locations. Depth-integrated transport rates have not been calculated due to the large uncertainties involved with fitting instantaneous velocity and suspended sediment profiles to the point measurements.
Sediment resuspension As well as the OBS at 10 cm above the bed, two other OBS's were deployed at 19 and 27 cm above the bed, on rig A2. At all three heights, the steady (Fig. 7a) and oscillatory (Fig. 7b) SSC components showed a marked asymmetry over the tidal cycle, with almost an order of magnitude increase in the measured SSC's on the ebb tide. This suspension asymmetry was observed at the other rigs during the same tide. The bottom OBS variance at rig A3 (located shore-parallel to rig A2, 20 m to the northeast) is included on Fig. 7b, and shows a similar
339
TIDAL ASYMMETRY IN SUSPENDED SAND TRANSPORT
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pattern to that from the bottom OBS on rig A2. The suspension asymmetry was also observed on other tidal cycles e.g. 18th April at rig A2 (Fig. 7c, steady components; Fig. 7d, oscillatory components) and has been noted in data from other macrotidal beach sites e.g. Llangennith, South Wales (Russell, 1990). The strong suspension asymmetry observed in Fig. 7a and b is not so apparent in the total velocity variance (Fig. 5a). Therefore, factors other than the strength of the current velocity must be responsible for producing the observed tidal asymmetry
in the sediment suspension. The possible influence of changes in bedform morphology, dewatering of the beach and localised bed level changes are considered in the discussion section.
Oscillatory transport As stated previously, the oscillatory longshore transport is negligible, and therefore only the oscillatory cross-shore transport is considered here. The oscillatory cross-shore transport was exam-
340
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TIDAL ASYMMETRYIN SUSPENDEDSANDTRANSPORT
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ined by computing cross-spectra between the crossshore current velocity (u) and the suspended sediment concentration (c) time-series for the EMCM and OBS at 10 cm above the bed. Sediment transport patterns occurring on the flooding and ebbing tides are examined separately.
Flooding tide Three dimensional plots showing how the u spectra and c spectra change with water depth over
the instruments on the flooding tide are shown in Fig. 8a and b, respectively. The typical pattern of incident wave energy ( f = 0.11 Hz) decaying shorewards of the breakpoint (depth = 1.8 m), accompanied by an increase in infragravity energy, is depicted in the u spectra (Fig. 8a). Although the c spectra (Fig. 8b) have peaks at low frequency, incident wave frequency and the first harmonic of the incident wave frequency (corresponding to the three main peaks in Fig. 8a), the basic pattern of
342
M.A. DAVIDSON ET AL 2.0
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Fig. 9. Cross-spectral analyses o f s u s p e n d e d s e d i m e n t a n d cross-shore current velocity for run 15 (outside the surf zone, f l o o d i n g i n c l u d e , (a) SSC a u t o - s p e c t r u m , (b) u-velocity a u t o - s p e c t r u m , (c) c - u co-spectrum, (d) c - u p h a s e s p e c t r u m with 95% confidence l i m i t s . P h a s e p o i n t s c o r r e s p o n d i n g to the co-spectral (net t r a n s p o r t ) p e a k s a r e d i s t i n g u i s h e d by circles, (e) c - u crossspectrum, a n d (f) c - u coherence spectrum. tide). Plots
the u spectra (Fig. 8a) is not repeated in the c spectra (Fig. 8b), again indicating that factors other than the current velocity are influencing the amount of sediment in suspension. Detailed cross-spectral analysis results are presented for two runs on the incoming tide; run 15,
outside the surf zone (depth=2.8 m; Fig. 9a-f), and run 1, in the inner surf zone (depth = 0.45 m; Fig. 10a-f). Outside the surf zone the c spectrum (Fig. 9a) has three main peaks at low frequency, incident wave frequency and at the first harmonic of the incident waves, whereas although the u
343
T I D A L A S Y M M E T R Y IN S U S P E N D E D S A N D T R A N S P O R T
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Fig. 10. Cross-spectral analyses of suspended sediment and cross-shore current velocity for run 1 (inner surf zone, flooding tide). Plots include, (a) SSC auto-spectrum, (b) u-velocity auto-spectrum, (c) c - u co-spectrum with 95% confidence limits. Phase points corresponding to the co-spectral (net transport) peaks are distinguished by circles, (d) c - u phase spectrum, (e) c - u cross-spectrum, and (f) c - u coherence spectrum.
spectrum (Fig. 9b) shows these peaks, it is strongly dominated by the incident wave peak at f = 0.11 Hz. The co-spectrum (Fig. 9c) is similar to that presented by Huntley and Hanes (1987), with a dominant onshore transport occurring under the
incident waves, and a smaller offshore transport occurring under the long period waves. There is also significant onshore transport associated with the first harmonic of the incident waves. The phase lag (Fig. 9d) between ¢ and u is critical in determin-
344
ing the net sediment transport direction and magnitude. At infragravity frequencies, phase differences are greater than ~/2 radians or less than - 7r/2 radians giving offshore transport, whereas at gravity band frequencies phase differences are between g/2 and - ~ / 2 radians, giving onshore transport. 95% confidence limits for the phase spectrum have been calculated from the coherence estimates (Jenkins and Watts, 1968), and are plotted on Fig. 9d. These results illustrate that the co-spectral (net transport) peaks are significant both in their magnitude and direction. The gross transport rate is represented by the c-u cross-spectrum (Fig. 9e), and again shows the three main transport peaks. The dominant transport process is associated with the highly coherent peak (Fig. 9f) at incident wave frequency, which, in this case, produces onshore transport. The spectra from the inner surf zone (Fig. 10) have two main peaks; at low frequency and at the incident wave frequency. Harmonic oscillations have been totally dissipated in this region of the surf zone. The low frequency peak dominates the c spectrum (Fig. 10a) and u spectrum (Fig. 10b), and gives a strong offshore transport at low frequency (Fig. 10c and d). Sediment transport is directed onshore under the incident waves, but this process is secondary to the offshore transport occurring at low frequency. Note that the peak transport rates within the inner surf zone are an order of magnitude higher than those measured outside the surf zone. Russell (1990) and Beach and Sternberg (1991) have presented c u co-spectra from the inner surf zone which also show the offshore transport at low frequency to be the dominant transport mechanism in this zone.
M A . D A V I D S O N ET AL.
The infragravity wave energy increases towards the shoreline but, in contrast to the flood, does not dominate, even closest to the shoreline. The c spectra (Fig. 1 lb) show high suspension to be associated with the incident wave velocity maximum, although the peak spectral suspension values occur at infragravity frequencies. At both gravity and infragravity frequencies, peak concentrations are ten to twenty times higher in magnitude than on the flood tide. The resulting 3-D transport spectra (Fig. 1 lc) are dominated by a very strong offshore transport at incident wave frequencies at the seawards edge of the surf zone (depth = 1.5 3 m). Detailed cross-spectral analysis results are presented for run 24 (depth= 2.7 m: Fig. 12a f) which represents a cross-section through this area of maximum offshore transport shown in Fig. l lc. The three main peaks in the suspension (once again at low frequency, incident wave frequency and the first harmonic of the incident wave frequency) are clearly shown in the c spectrum (Fig. 12a), whilst the u spectrum (Fig. 12b) shows the strong incident wave dominance also observed outside the surf zone on the flooding tide (Fig. 9b). However, the c u phase relationship (Fig. 12d) is markedly different from that on the flooding tide (Fig. 9d), and results in offshore directed transport at all frequencies less than 0.27 Hz, so that the net transport at low frequency, incident wave frequency and the first harmonic of the incident wave frequency is directed offshore in all cases (Fig. 12c). This strong offshore transport on the ebbing tide is the dominant feature of the oscillatory sediment transport results obtained, with peak values in excess of four times larger than those observed over the rest of the tide, even in the inner surf zone.
Ebbing tide
On the ebbing tide, 3-D plots illustrating how the u spectra, c spectra and c-u co-spectra change with depth are presented in Fig. l la, b and c, respectively. The u spectra (Fig. 1 l a) show a strong incident wave dominance which has a maximum magnitude outside the surf zone (depth 2--3 m).
Mean transport Mean cross-shore transport
Variations in the steady cross-shore transport contribution over the tidal cycle are shown in Fig. 13. The mean cross-shore transport rate is
Fig. 11. Three-dimensionalplots illustrating the variation with depth, on the ebbing tide, of, (a) the u-velocityauto-spectrum, (b) the SSC auto-spectrum, and (c) the transport spectrum (c-u co-spectrum).
TIDAL ASYMMETRY IN SUSPENDED SAND TRANSPORT
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345
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346
M.A DAVIDSON ET AL. 4.0 3.S
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Fig. 12. Cross-spectral analyses of suspended sediment and cross-shore current velocity for run 25 (outside the surf zone, ebbing tide). Plots include, (a) SSC auto-spectrum, (b) u-velocity auto-spectrum, (c) c - u co-spectrum, (d) c-u phase spectrum with 95% confidence limits. Phase points corresponding to the co-spectral (net transport) peaks are distinguished by circles, (e) c - u crossspectrum, and (f) c - u coherence spectrum.
largest within the surf zone (up to 0.20 kg/m2/s) and is directed offshore (at the height of the sensors) in association with the undertow. Conversely, outside the surf zone, the mean cross-shore transport component is directed onshore reaching maximum values (0.15 kg/mZ/s) during the ebbing tide,
coincident with the time of high sediment suspension. This result, which gives a convergence of the mean nearbed transports in the breaker zone, has been observed by other workers (e.g. Bailard, 1987) and may be related to the formation of a "breakpoint bar".
347
T I D A L A S Y M M E T R Y IN S U S P E N D E D S A N D T R A N S P O R T
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Fig. 13. Variationsin the mean cross-shoreand longshoretransport rates with tidal state.
Mean longshore transport The steady longshore transport component (~.~) is particularly important in this macrotidal environment with a highly oblique incident wave approach. On this day it reached rates of up to 0.6 kg/m2/s (Fig. 13). Tidal currents dictate the magnitude and direction of the steady longshore transport, component outside the surf zone whilst the wave driven longshore currents (directed to the southwest), control these factors inside the surf zone. This is the subject of studies to be reported elsewhere• Total transport Total cross-shore transport In order to examine the relative importance of the cross-shore oscillatory and steady components to the total cross-shore sediment transport rate, the tidal history of these parameters is plotted in Fig. 14a, b and c, for the bottom, middle and top OBS heights, respectively. The results for the bottom OBS height (Fig. 14a) are described first. During the flooding tide, instantaneous suspension and flow velocity time-series are commonly close to quadrature at peak frequencies, resulting in low net oscillatory transport rates. It is the mean component, therefore, which dominates the total net transport rate on the flood tide. However, one hour
after the turn of the tide (run 24) SSC and u timeseries are highly coherent and in anti-phase at incident wave frequencies resulting in high net oscillatory transport rates which are directed offshore. Consequently, it is the oscillatory component which is dominant on the ebbing tide, coincident with the time of high sediment suspension, leading to the highest net transport rates (c'.u'= 0.37 kg/m2/s). During this period of high suspension and high rates of oscillatory transport, the mean transport component is secondary in importance and opposite in sign to the unsteady components. Higher in the water column (Fig. 14b and c) the oscillatory transport is much lower in magnitude and is directed onshore. Therefore, at these higher elevations, the mean component tends to dominate the total transport. A similar result was obtained by Osborne and Greenwood (1992).
Total longshore transport The total longshore transport rate is completely dominated by the mean longshore transport component throughout the tidal cycle. Fluctuations in suspension and longshore current velocity are generally uncorrelated resulting in very low oscillatory longshore transport rates. The total longshore transport rate, therefore, follows the pattern of the mean longshore transport described above.
348
MA DAVIDSONETAL 0.3
• E
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TIDAL ASYMMETRYIN SUSPENDEDSANDTRANSPORT
Discussion
Many of the present results agree with recent observations made by other workers on natural beaches. For example, the cross-shore variations in hydrodynamic processes support the findings of other writers (Holman, 1981; Wright et al., 1982a,b). Although complicated by the influence of the Stoney Binks, the classical pattern for shorewards growth of long wave energy accompanied by decaying incident wave energy in the surf zone is clearly evident in this data set. However, typical of other observations on macrotidal intermediate beaches (e.g. Wright et al., 1982a), long wave oscillations are subordinate to incident wave components throughout the tidal cycle (at the location of the sensors). Similarly, previously observed patterns in sediment transport processes both in the longshore (Sternberg et al., 1984) and cross-shore (Huntley and Hanes, 1987; Russell, 1990; Beach and Sternberg, 1991; Osborne and Greenwood, 1992) directions are also consistent with this data set. The total longshore transport can be adequately represented by the mean longshore transport component alone (i.e. the oscillatory longshore transport is small and can be neglected), whereas the oscillatory cross-shore transport contribution (both at incident wave frequencies and long wave frequencies) is very important in determining the total crossshore transport. At 10 cm above the bed, the patterns of net cross-shore sediment transport (as illustrated by the c u co-spectra) on the flooding tide, both outside the surf zone (e.g. run 15, Fig. 9c) and in the inner surf zone (e.g. run 1, Fig. 10c) show the same dominant transport mechanisms that were noted by Huntley and Hanes (1987; predominantly onshore transport at incident wave frequencies), and Russell (1990) and Beach and Sternberg (1991; predominantly offshore transport at long wave frequencies), respectively. However, by plotting the c - u co-spectra as a continuum across and beyond the surf zone it is clear that these net transport spectra vary greatly
349
with both cross-shore location and tidal state. The asymmetry in the magnitude of the observed suspended sediment concentration and associated transport rates between flood and ebb tides is striking. For this data set, when the complete tidal cycle is studied, the offshore transport at incident wave frequencies seaward of the breaker zone on the ebbing tide is the dominant cross-shore transport feature. This period of high transport occurred primarily as a result of high sediment suspension at gravity frequencies which interacted coherently and in anti-phase with the incident wave flow field. Three possible causes of the observed increased suspension after the tide turns are: (1) dewatering of the beach; (2) localised bed-level changes relative to the OBS; (3) erosion of a ripple field which formed in the slack waters over high tide. Brenninkmeyer (1976) noted that the level of the beach water table tends to lag the tide by 1 to 3 hours. The dry beach (above the level of the water table during the flooding tide) enhances deposition, whereas the saturated beach (below the level of the water table on the ebbing tide) augments erosion (Grant, 1948). Lanyon et al. (1982) state that, on the saturated beach, groundwater effluence occurs during the latter stage of the backwash flow. This process was shown by Grant (1948) to increase turbulence levels in the backwash, and also to dilate the sand grains, both of which would encourage entrainment. Another possible solution for the observed tidal asymmetry is localised accretion of sediment in the region of the OBS sensor, so lowering the relative height of the sensor above the bed, causing it to measure higher SSC's. This seems unlikely as the measured height of the OBS above the bed was 9 cm before the tidal cycle, and 10 cm after the tidal cycle. Extensive profiling and depth-of-disturbance rod data collected during the fieldwork period showed bed level changes on the low tide terrace to be restricted to a few centimetres. The third hypothesis of bedform alteration is
Fig. 14. Variations in the mean, oscillatory and total, cross-shore transport rates with tidal state, for (a) the bottom OBS height (I0 cm), (b) the middle OBS height (19 cm), and (c) the top OBS height (27 cm).
350
M A DAVIDSON E~[ .A.I+.
favoured by the authors. During the period of low tidal transition at high water, both oscillatory and mean currents (including tidal and wave driven components) are at a minimum (Figs, 5 and 6). It is well documented (cf. Dingler and Inman, 1976) that in such weak oscillatory dominated flow fields it is common to observe well developed symmetrical ripples seaward of the surf zone. The only proviso is that the orbital velocities of the waves are above the threshold for the initiation of sediment transport (in this case > 0.23 m/s, see Komar and Miller, 1975). After the turn of the tide there is a sharp increase in both the nearbed oscillatory currents (due to a reduction in the water depth) and mean flows (now flowing in the opposite direction). These larger velocities would be arriving on an over-steep rippled bed, and would cause the bed to efficiently eject sand-laden vortices high into the flow. Such a mechanism has also been described by Vincent et al. (1991). Hence, the high SSC's observed on the ebb tide are likely to be a result of the destruction of the ripple field which occurs during the transition from rippled to plane bed with increasing velocity as the water depth shallows. Once the plane bed condition is re-established
SHEET FLOW
in the surf zone, the observed suspension at height would be expected to decrease. Values quoted in the literature for the critical velocity at the point of transition between rippled bed conditions and sheet flow show a considerable spread. Dingler and Inman (1976) suggest values of the maximum orbital velocity (g/m crit) at transition of between 0.46 and 1.14 m/s. The precise value is a function of the wave orbital velocity and the grain size of the sediment. For example, extrapolation of the data from Horikawa et al. (1982) taking the median grain size (Ds0 = 0.35 ram) and peak wave period ( T = 11 s) for this data set, gives Urn c r i t = 0 . 7 m/s. However, most of the values quoted for um i~iu are derived from laboratory experiments (cf. Manohar, 1955; Carstens, 1966: Dingier and Inman, 1976; Horikawa et al., 1982; Shibayama and Horikawa, 1982) which consider only oscillatory (usually monochromatic) flows. Therefore, some caution should be taken when applying laboratory results to a 3-D field situation where there exists a complex spectrum of sea waves combined with steady flows. Figure 15 shows a plot of wave orbital velocity predicted by cnoidal wave theory (which is appro-
I I I FORMATION I I OF DESTRUCTION OF RIPPLE FIELD SYMMETRICAL I AND RETURN TO SHEET FLOW I RIPPLES I
0.9 ~11.--Region 1 - ~ R e g i o n J
Transition velocity between rippled
bed and sheet flow (Horikawa, 1982)
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'
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_~ I'~a~"~th~-o~'- -- -- ~t' ~ -- -- T --J' ~-i F'l~-"Bre~d~wn'"~' u'/'[ - \ A /~ iil ~ ~ of Cnoidal Velocity predicted using V ~ / " ] ~i ! wave t h e orv "~ ICnoidalwavetheory ~ / / ~ 'l 0"6 ! (reference P°S'~°n" " ' " " " ~ V / ~ i Ca 10cm above the bed) + t •o
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:
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6 t~ .... ~ 4 ~O ¢,
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Fig 15. Variation of the total suspended sediment variance and cross-shore current velocity (predicted using cnoidal wave theory) with tidal state.
TIDAL ASYMMETRYIN SUSPENDEDSANDTRANSPORT
35 |
priate to the asymmetric flow field in this region of the surf zone) against sediment suspension. Values of um are not given for runs 1-4 and 31-34 as cnoidal wave theory is not valid for the shallowest data runs. Also marked on the graph is a theoretical field value for Umcrit equivalent to the extrapolated laboratory results of Horikawa et al. (1982). The intersection of the position of the transition zone indicated by the field data (the vertical dotted line) and the computed Um values (the horizontal dotted line) divides the graph into 3 distinct regions. During the flooding tide (region 1; Fig. 15) values of Um exceed Um crit leading to sheet flow conditions close to the bed. Therefore, at the height of the OBS (0.1 m above the bed), low levels of sediment suspension were recorded. During the high tide period (region 2; Fig. 15) declining nearbed flows lead to the formation of symmetrical ripples in the deeper water outside the surf zone. A sharp increase in Umone hour after the turn of the tide from 0.67 m/s (run 22) to 0.83 m/s (run 24) corresponds exactly with the increase in sediment suspension observed at the height of the OBS (start
of region 3; Fig. 15). It is hypothesised that the observed increase in sediment suspension relates to the destruction of the ripple field which was established over high water, and the advection of sediment to the height of the OBS by vortices generated by strong nearbed flows over the still partially rippled bed. Eventually a plane bed condition is reached (at approximately run 28; Fig. 15) and the sheet flow is once again confined to a narrow region close to the bed similar to the situation on the flooding tide (region 1; Fig. 15). Inman and Bagnold (1963) first described the mechanism by which sand could travel in an offshore direction over a rippled bed with shoreward propagating incident waves (Fig. 16). Under the shoreward stroke, the sand is carried a small distance onshore close to the bed and becomes trapped in a vortex in the lee of the ripple crest. Upon flow reversal the vortex is released upwards and the suspended sand is carried a greater distance offshore by the seaward stroke, so giving a net offshore transport. Thus, the onset of sheet flow over the ripple field during the ebb tide leads to
ONSHORE FLOW y DENSE SEDIMENT FLOW NEAR EBED " ,
"
"~l
~
.
S._ _ r..u~.r~ i
cLouDs
FLOW
SEDIMENT SUSPENSION TRANSPORTED OFFSHORE
Fig. 16. Simplistic model for offshore transport at incident wave frequency over a rippled bed (after Inman and Tunstall, 1972).
352 enhanced advection and offshore transport of sediment. Schepers (1978) and Sato (1986) reported similar findings in that their data, collected over sharp crested ripples, showed the resulting sediment transport to be in the opposite direction to that of the largest instantaneous velocities. Although both the dewatering and bedform alteration mechanisms are likely to have an important influence on sediment transport processes, the bedform mechanism is appealing as it adequately explains the high correlation between c" and u' and their antiphase relationship, leading to strong offshore transport at incident wave frequencies. Conclusions
Observations on a macrotidal beach (Spurn Head, Yorkshire, UK) have confirmed that there are some similarities in the hydrodynamic processes and sediment dynamics between highly tidal (this contribution) and meso/micro tidal beaches (other writers). For example: (1) Within the surf zone hydrodynamic infragravity variance grows in a shorewards direction accompanied by a dissipation of incident and harmonic variance (Holman, 1981: Wright et al., 1982b). (2) Infragravity energy is subordinate to gravity wave energy on intermediate beaches (Wright et al., 1982a). (3) Sediment transport occurs primarily at three specific frequencies. TheSe are at gravity, infragravity and incident-harmonic frequencies (cf. Huntley and Hanes, 1987). (4) Longshore sediment transport processes are dominated by mean flows with negligible contributions from oscillatory components (cf. Sternberg et al., 1984). (5) The net cross-shore sediment flux is given by the sum of the steady and oscillatory components (Jaffe et al., 1984). The relative importance of these two components is dependant on several factors. These factors include: (a) the position relative to the surf zone. This affects the strength of the undertow, the magnitude of the various oscillatory components mentioned in (3) above, and the presence/absence of bedforms.
MA DAVIDSONI'.FAk. (b) the height above the bed. This affects the phase relationship between sediment suspension and velocity time-series and thus the magnitude and direction of sediment transport (cf. Hanes and Huntley, 1986; Osborne and Greenwood, 1992). (6) Seaward of the surf zone, sediment transport processes are dominated by the oscillatory incident wave components (cf. Huntley and Hanes, 1987). Strong onshore transport occurs at the incident wave frequencies with weaker offshore transport at infragravity wave frequencies. (7) In the inner surf zone sediment transport processes are dominated by oscillatory, infragravity components (cf. Russell, 1990; Beach and Sternberg, 1991). The resulting nearbed sediment transport is directed predominantly offshore. This contribution has also emphasised important new phenomena which are expected to be unique to macrotidal beaches. Specifically: (l) There is a marked asymmetry in sediment suspension, with suspension values up to an order of magnitude higher on the ebb tide. This pattern is reproduced vertically and spatially, and has been observed at other macrotidal beach sites (cf. Russell, 1990). It is hypothesised that the large sediment suspension and transport observed on the ebb tide, just seaward of the surf zone, correlates with the destruction of a ripple field during the transition period between rippled bed conditions (low current velocities above threshold) and sheet flow (high current velocities > 0.74 m/s). This transition occurs due to a localised shallowing of the water level leading to increased incident wave orbital velocities at the bed. Such high suspension is absent on the flooding tide since bedforms are initially absent, and do not form until high water when wave orbital velocities outside the surf zone are low enough to permit the growth of ripples. (2) High levels of longshore transport (up to 0.6 kg/mZ/s for H1/3 = 1 m) were observed to be associated with steady shore-parallel velocity components (tidal flows and wave driven longshore currents). Wave driven longshore currents dominated longshore transport processes inside the surf zone whilst tidal currents were the major driving force behind sediment transport seaward of the surf zone.
TIDAL ASYMMETRY IN SUSPENDED SAND TRANSPORT
Acknowledgements The authors would like to acknowledge the assistance of Mr. Gareth Lloyd (electronics) and Dr. Adrian Cramp (logistics and review of manuscript). The British Beach And Nearshore Dynamics (B-BAND) programme is supported by the Natural Environment Research Council (NERC) U.K. The Post-Doctoral Research Associates, Dr. Mark Davidson and Dr. Paul Russell, wish to thank the Welsh Office and the NERC, respectively, for funding their work on this project.
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