David A. Huntley

inssev: An instrument to measure the size and settling velocity of flocs in situ

Abstract An instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments. inssev —IN Situ SEttling Velocity instrument—is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column. The settling flocs are viewed using a miniature video system. Subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 μm. From this information floc density can be estimated. The main feature of the instrument is its ability to video flocs in situ, irrespective of the concentration in the estuary, with as little disturbance to their hydrodynamic environment as possible. Preliminary field testing in the Tamar Estuary has given useful results in flow velocities up to 0.4 m s −1 and in concentrations up to 200 mg 1 −1 . The video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres.

A mechanism for the generation of wave-driven rhythmic patterns in the surf zone

The coupling between topographic irregularities and wave-driven mean water motion in the surf zone is examined. This coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation. This circulation, in turn, creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large-scale bed forms. To investigate this coupling mechanism, the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases. First, the breaking line is considered to be fixed, and second, the perturbations in water depth that produce a displacement of the breaker line are accounted for. The first case shows that the basic topography can be unstable with respect to two different modes: a giant cusp pattern with shore-attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect. In the second case, the varying breaker line may have a strong influence on the circulation. This is clear for the giant cusp topography whose growth is totally inhibited. In contrast, the morphology and the growth of the crescentic pattern remains almost unchanged.

Continuous measurements of suspended sand concentration in a wave dominated nearshore environment

Abstract A miniature optical backscatter sensor (MOBS) and an electro-magnetic flowmeter were deployed seaward of the surf zone at Pte Sapin, New Brunswick during the first Canadian Coastal Sediment Study (C 2 S 2 ), autumn 1983. The MOBS had sensing elements at five vertical locations above the sea bed, and was sampled at 10 Hz. The suspension of sand is well correlated with the passage of individual waves and also with wave groups, with the influence of wave groups progressively more dominant at higher elevations above the bed. Suspension appears to be stronger during the onshore directed phase of the wave motion than during the offshore motion and there is evidence that initiation of suspension may be determined by fluid acceleration more than velocity. Time lags between suspension events at different heights suggest that vertical gradients in sediment flux are more important than horizontal gradients.

Self-organization mechanisms for the formation on nearshore crescentic and transverse sand bars

The formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow-water equations and on a simplified sediment transport parameterization. By assuming normally approaching waves and a saturated surf zone, rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed. Two types of bedforms appear: one is a crescentic bar pattern centred around the breakpoint and the other, herein modelled for the first time, is a transverse bar pattern. The feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations. Basically, the waves stir up the sediment and keep it in suspension with a certain cross-shore distribution of depth-averaged concentration. Then, a current flowing with (against) the gradient of sediment concentration produces erosion (deposition). It is shown that inside the surf zone, these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms. Numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro-morphodynamic input conditions to which of the patterns develops. It is suggested that crescentic bar growth would be favoured by high-energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment. In intermediate conditions mixed patterns may occur.

Long-wave forcing by the breaking of random gravity waves on a beach

This paper presents new laboratory data on long–wave (surf–beat) forcing by the random breaking of shorter gravity water waves on a plane beach. The data include incident and outgoing wave amplitudes, together with shoreline oscillation amplitudes at long–wave frequencies, from which the correlation between forced long waves and short–wave groups is examined. A detailed analysis of the cross–shore structure of the long–wave motion is presented, and the observations are critically compared with existing theories for two–dimensional surf–beat generation. The surf beat shows a strong dependency on normalized surf–zone width, consistent with long–wave forcing by a time–varying breakpoint, with little evidence of the release and reflection of incident bound long waves for the random–wave simulations considered. The seaward–propagating long waves show a positive correlation with incident short–wave groups and are linearly dependent on short–wave amplitude. The phase relationship between the incident bound long waves and radiated free long waves is also consistent with breakpoint forcing. In combination with previous work, the present data suggest that the breakpoint variability may be the dominant forcing mechanism during conditions with steep incident short waves.

Breakpoint generated surf beat induced by bichromatic wave groups

Abstract This paper presents new experimental data on 2-D surf beat generation by a time-varying breakpoint induced by bichromatic wave groups. The experimental investigation covers a broad range of wave amplitudes, short wave frequencies, group frequencies and modulation rates. The data include measurements of incident and outgoing wave amplitudes, breakpoint position, shoreline run-up and the cross-shore structure of both the short and long wave motion. Surf beat generation is shown to be in good agreement with theory [Symonds, G., Huntley, D.A., Bowen, A.J., 1982. Two dimensional surf beat: long wave generation by a time-varying breakpoint. J. Geophys. Res. 87, 492–498]. In particular, surf beat generation is dependent on the normalised surf zone width, which is a measure of the phase relationship between the seaward and shoreward breakpoint forced long waves, and linearly dependent on the short wave amplitude. The cross-shore structure of the long wave motion is also consistent with theory; at maximum and minimum surf beat generation, the mean breakpoint coincides with the nodal and anti-nodal points, respectively, for a free long wave standing at the shoreline. A numerical solution, using measured data as input, additionally shows that the phase relationship between the incident bound long wave and the outgoing breakpoint forced wave is consistent with the time-varying breakpoint mechanism.

Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach

Abstract 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 co-spectral 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/m 2 /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/m 2 /s) associated with bound long waves and stronger onshore transport (co-spectral peak = 0.006 kg/m 2 /s) at incident wave frequencies. Conversely, co-spectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m 2 /s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m 2 /s) occurring at incident wave frequencies.

Ripple generation under the combined influences of waves and currents on the Canadian continental shelf

Abstract A multi-parameter instrument package (Ralph) was deployed for 15 days in 22 m of water on Sable Island Bank, Scotian Shelf. The instrument successfully recorded mean current velocity, statistics on near-bed wave motion, wave height, period and propagation direction, and time-lapse photographs of the seabed. The seabed at the deployment site was composed of well-sorted, fine sand (0.23 mm mean diameter) which moved during peaks in tidal flow and during periods of high wave activity. Eight distinct bed types were seen in time-lapse imagery: (1) wave ripples; (2) straight-crested current ripples; (3) linguoid current ripples; (4) wave and current ripples; (5) transitional wave ripples; (6) transitional current ripples; (7) poorly developed (biodegraded) ripples; and (8) flat bed. Each bed type occurred at well-defined combinations of near-bed wave motion, mean current speed and biodegradation. Wave-current ripples intermediate in form and orientation, were not seen. Bedform types were found to be well separated in plots of wave Reynolds number (Umb·d0/ν) and dimensionless mean flow (U100/Ws). A better separation was found if the wave Shields parameter (after Grant and Madsen , 1979 , Journal of Geophysical Research, 84, 1797–1808) and the current Shields parameter (after Sternberg , 1972 , Shelf sediment transport, process and pattern, pp. 61–83) were used. Bedform stability plots showed that wave ripple threshold is influenced by currents, and current ripple threshold is influenced by waves. Despite this, ripple type is defined by the partitioned, wave and current components of stress and not the total stress as no intermediate (wave-current) ripple type was observed. Thresholds for sand transport and the generation of well-developed bedforms were influenced by the combined, near-bed motions of waves and currents even when the bed exhibited wave ripples or current ripples only. The threshold for sand transport under combined flows is vague, but is adequately represented by a line drawn between the threshold for transport under unidirectional flow based on the modified Shields curve of Miller et al. (1977 , Sedimentology, 24, 507–527) (θc= 0.04) and the threshold specified for pure wave motion of Komar and Miller (1973 , Journal of Sedimentary etrology, 43, 1101–1110) (θw = 0.04). A first approximation to a threshold criterion for fine sand is proposed as [θw + θc]crit = 0.04.

Waves, long waves and nearshore morphology

Abstract Recent field measurements on beaches of different slopes have established that wave motion at periods substantially longer than the incident waves dominates the velocity field close to the shore. Analysis of a number of extensive data sets shows that much of this long wave motion is in the form of progessive edge waves, though forced wave motion, standing edge waves and free waves propagating away from the shore may also contribute to the energy. Theoretically, the drift velocities in bottom boundary layers due to edge waves show spatial patterns of convergence and divergence which may move sediment to form either regular crescentic or cuspate features when only one edge wave mode dominates, or a bewildering array of bars, bumps and holes when several phase-locked modes exist together. Convincing field demonstration of the link between nearshore topography and edge waves only exists for the special case of small-scale beach cusps on steep beaches, formed by edge waves at the subharmonic (twice the period) of the incident waves. At longer periods the link is proving more difficult to establish, due to the longer time-scales of topographic changes, the interaction between pre-existing topography and the water motion, and the observation of broad-banded edge wave motion which is not readily linked to topography with a well-defined scale. These ideas are, however, central to the study of nearshore processes, as most of the plausible alternate hypotheses do not seem to lead to quantitative predictions. Clearly, further theoretical and observational work is essential.

Investigation of a self-organization model for beach cusp formation and development

A recent numerical investigation of “self-organization” [Werner and Fink, 1993] suggests that the feedback process between currents and sediment response can result in “self-organized” patterns and can be used to predict beach cusp formation and spacing. A similar model based on self-organization is tested here in order to understand the processes occurring during beach cusp formation and development, to evaluate the sensitivity toward the parameters used, and to examine how the model might relate to field observations. Results obtained confirm the validity of the self-organization approach and its capacity to predict beach cusp spacing, with values in fair agreement with the available field measurements, with most of the input parameters primarily affecting the rate of the process rather than the final spacing. However, changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior. Cusp spacing tends to change with time, and cusp regularity shows large long-term variations. Cusps are found to be accretionary in the swash zone, and in agreement with most observations, mean flows are horn divergent over developed topography. Simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed. Results indicate that preexisting large-amplitude cusps are destroyed if their spacing is substantially different from that expected under self-organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach. These findings support the hypothesis that self-organization is a robust mechanism for beach cusp formation.