Anthony J. Bowen

Sediment waves on the monterey fan levee: A preliminary physical interpretation

Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.

Acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness

Acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface, seawards of the breaking zone, during a mild storm event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases: it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break-off bed shear stress is exceeded. No direct measurements of bedforms were available and the combined wave-current interaction model of Grant and Madsen (J. Geophys. Res., Vols. 84 (1979) and 87 (1982)) with extensions to movable bed roughness (ripple dimensions from Carstens et als. 1969 laboratory data (U.S. Army Corps Eng. CERC Tech. Mem. 28) and to sediment-induced self-stratification (Glenn and Grant, J. Geophys. Res., Vol. 92, 1983) was used to predict the current and suspended sand concentration profile. Matching the measured and modeled concentrations at 2 cm above the bed, the values of the resuspension coefficient γ0 were found to decrease as excess skin friction increased, in a manner similar to that suggested by Drake and Cacchione (Cont. Shelf Res., Vol. 9, 1989) but were an order of magnitude larger. Using the much smaller ripple dimensions of Nielsen (J. Geophys. Res., Vol. 86, 1981) changed the model results very little. Significant differences were found between the time-averaged suspended sand profiles and those predicted by the model. Sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents (the current being the sum of steady and wave-induced components). The wave-induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5–10 cm above the bed and offshore transport in the few centimetres closest to the bed: little net transport occurred above 15 cm. These fluxes show a weak dependence on the wave energy, becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased, again consistent with the influence of the ripple steepness.

Parametrization of orbital velocity asymmetries of shoaling and breaking waves using bispectral analysis

A bispectral analysis of electromagnetic flow meter data collected from four nearshore environments was undertaken to investigate the spatial variation of velocity skewness and asymmetry for shoaling and breaking surface gravity waves. The data base provides a wide range of wave conditions and beach characteristics (including planar and barred beaches). A simple parametrization for bispectral evolution (amplitude and phase) due to wave shoaling, which is based on the Ursell parameter, is proposed and tested. The results show that the parametrization provides both a good qualitative and reasonably quantitative prediction of velocity skewness and asymmetry arising from phase-coupling between wind-wave frequencies. Infragravity wave velocity skewness and asymmetry are also examined.

RESONANT INTERACTIONS FOR WAVES BREAKING.ON A BEACH

A significant portion of the damage by hurricanes is the storm surges. The National Weather Service has developed a dynamical-numerical model to forecast hurricane storm surges. The model is used operationally for prediction, warning, and planning purposes. The model requires fixed oceanographic and real time meteorological input data. The oceanographic data were prepared for the Gulf and East coasts of the U.S. and are stored as an essential part of the program. Meteorological data for any tropical storm are supplied by the forecasters or planners using the model. The model was applied to hurricane Camille 1969. Comparison between the observed and computed surges for Camille was satisfactory for prediction purposes.The main consideration in harbor master planning is to maximize the amount of time that the harbor can be used. The potential level of harbor utilization can be evaluated by analyzing vessel performance during harbor operations in terms of the range of imposed environmental conditions. The harbor utilization level is expressed statistically as the probable amount of time that the harbor can be used as planned.The design and construction of a major ocean outfall and diffuser system for disposal of wastewater effluents is a complex process involving an interplay of requirements originating from various disciplines. These include, among others, considerations of physical oceanography, mixing and dispersion, treatment processes, regulatory requirements, marine geology, economics and construction. The recently completed Sand Island Outfall and the newly designed Barbers Point Outfall are both on the southern coast of the island of Oahu, Hawaii, and are designed for treated sewage effluents from the densely populated portion of the City and County of Honolulu. In this paper, some design considerations of these outfalls will be examined. The emphasis in this paper is on the hydrodynamics, although other design aspects are also discussed briefly.The height of dikes and other coastal structures can only be calculated after determination of the wave run-up. Several formulas for the calculation of wave run-up are developed after model tests as a rule. But the influences of scale effects and natural wind conditions are practically unknown. To clear these questions further investigations and especially field measurements should be carried out. By measuring the markerline of floating trash on the slope of the seadikes the maximum wave run-up could be found out after four storm surges in 1967 and 1973In two graphs it will be shown that on the tidal flats the run-up depends on the waterdepth. The run-up was higher than it could be expected after model tests of 1954. With a newly developed special echo sounder the run-up could be measured in January 1976. The waves and the run-up could be registrated synchronously during two severe storm surges. As shown in Fig. 9 it was found a logarithmic distribution of the wave height, wave period and the higher part of the wave run-up. The found wave run-up is considerably higher than estimated before. The measured 98 % run-up is found about twice the computed value. That is an interesting and important result of the first synchronous recording of wave run-up on sea dikes.In March 1972 the authors firm in association with two Portuguese firms of consulting engineers, Consulmar and Lusotecna, were appointed by the Portuguese Government agency Gabinete da Area de Sines to prepare designs for the construction of a new harbour at Sines on the west coast of Portugal. The location is shown in Figure 1. The main breakwater, which is the subject of this paper, is probably the largest breakwater yet built, being 2 km long and in depths of water of up to 50 m. It is exposed to the North Atlantic and has been designed for a significant wave height of 11 m. Dolos units invented by Merrifield (ref. 1) form the main armour. The project programme required that studies be first made of a wide range of alternative layouts for the harbour. After the client had decided on the layout to be adopted, documents were to be prepared to enable tenders for construction to be invited in January 1973. This allowed little time for the design to be developed and only one series of flume tests, using regular waves, was completed during this period. Further tests in the regular flume were completed during the tender period and a thorough programme of testing with irregular waves was commenced later in the year, continuing until August 1974 when the root of the breakwater was complete and the construction of the main cross-section was about to start. The model tests, which were carried out at the Laboratorio Nacional de Engenharia Civil in Lisbon, were reported by Morals in a paper presented to the 14th International Coastal Engineering Conference in 1974. (ref. 2)Estuaries may be sequentially classified into highly stratified, moderately mixed and vertically homogeneous. An important difference between moderately stratified or vertically homogeneous estuaries, and highly stratified estuaries (salt wedges) is that, in the former, tidal currents are sufficient to cause turbulent mixing of fresh water and sea water over the full depth of the estuary. In the latter, a distinct interface or interfacial layer exists which separates the two nearly homogeneous layers. The vertical advectlon of salt in this two-layer flow is the dominant process in maintaining the salt balance. This paper presents an analytical model describing this process. Experiments have been conducted in the laboratory to compare with the developed theory.

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.

Coherence scales of wave-induced suspended sand concentration fluctuations

Multifreqiiency acoustic backscatter measurements made in the nearshore zone are analyzed to determine the correlation structure of the instantaneous suspended sediment concentration field at horizontal separations of 20 cm. Large-amplitude backscatter fluctuations, representing suspended sediment concentrations more than a factor of 10 greater than the mean and with vertical coherence scales many times greater than the logarithmic wave boundary layer thickness, are shown to be spatially correlated at horizontal separations less than the wave orbital amplitude over successive wave cycles. The time lag between the appearance of fluctuations in different acoustic beams is inversely proportional to the fluid velocity, showing that the fluctuations in backscatter amplitude contain real information on fluctuations in suspended sediment concentration, advected with the fluid. It is found that the instantaneous suspended sediment field in shore-normal cross section is punctuated by spatially discrete clouds extending well above the (logarithmic) wave boundary layer. The fluctuations appear to be present even in the absence of small-scale ripples. It is suggested that the sediment clouds may be due to vortex shedding from megaripples, other bed forms of intermediate scale, or coherent structures in combined wave-current flows.

Generation of edge waves in shallow water

Theoretical growth rates for resonantly driven edge waves in the nearshore are estimated from the forced, shallow water equations of motion for the case of a plane sloping bed. The forcing mechanism arises from spatial and temporal variations in radiation stress gradients induced by a modulating incident wave field. Only the case of exact resonance is considered, where the difference frequencies and wavenumbers satisfy the edge wave dispersion relation (the specific carrier frequencies are not important, only the forced difference values). The forcing is examined in the region seaward of the breakpoint and also within the fluctuating region of surf zone width. In each region, the forcing is dominated by the cross-shore gradient of onshore directed momentum flux, except for large angles of incidence and the lowest edge wave modes. Outside the surf zone, the spatial and temporal variation of the forcing is determined by considering the interaction of two progressive shallow water waves approaching the beach obliquely. In the surf zone, incident wave amplitudes are assumed to be proportional to the water depth. Thus inside the breakpoint, radiation stress gradients are constant and no forcing occurs. However, at the breakpoint, gradients arising from breaking and nonbreaking waves are turned on and off (like a wave maker) with timescales and length scales determined by the modulation of the breaker position. The forcing in this region is stronger, with inviscid growth rates resulting in edge waves growing to the size of the incident waves of the order of about 10 edge wave periods, a factor of 2–10 times larger than in the offshore region. Using a simple parameterization for frictional damping, edge wave equilibrium amplitudes are found to depend linearly on the ratio tan β/Cd, where β is the beach slope and Cd is a bottom drag coefficient. For tan β/Cd about 3–10, equilibrium amplitudes can be as much as 75% of the incident waves over most of the infragravity portion of the spectrum. When the forcing is turned off, these dissipation rates result in a half-life decay timescale of the order of 10–30 edge wave periods.

Spatially correlated depth changes in the nearshore zone during autumn storms

Acoustic reflection measurements of seabed elevation were made at Stanhope Beach, Prince Edward Island, during three successive storms in October-November 1989. The instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2.2 m. Bed elevation measurements were made at 30-min intervals with a range resolution of -+ 1 mm at horizontal spacings between 22 cm and 1.5 m, using an array of four acoustic sounders operating at 1, 2.25, and 5 MHz. It is shown that such an array can be used as (1) an interferometer to determine bedform properties and migration rates, (2) a leveling device to determine changes in local bottom slope, and (3) an erosion/deposition gauge. In the interferometric mode, time domain autocorrelation and cross-correlation analyses of the bottom elevations are used to estimate the wavelengths, migration rates, and migration directions of sand ripples and megaripples. The ripple wavelengths (10-15 cm) and heights (3-5 cm) are comparable to measurements made by divers. Ripple migration rates were 1-6 cm h -1 directed offshore in the direction of the mean shore normal near-bottom current and increased with increasing mean current speed. Evidence of megaripple-like features (2-5 cm height; 1.5-3 m length) is found, also migrating in the direction of the mean shore normal near-bottom drift, at speeds of about 20 cm h -1 . Bedload transport rates in the onshoreoffshore direction are estimated from the bedform migration rates. The ripple transport rate was small (an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift). In contrast, the large-scale bedform transport appears to have been comparable to the mean current transport of suspended sediment. Interestingly, it appears that the total bedform transport was comparable to that predicted by Watanabes (1981) net bedload transport formula, provided a coefficient appropriate to irregular wave conditions is used. During the third and highest-energy storm event, the measurements indicate transition to flat bed, and generalized local erosion at a maximum rate of 8 cm h -1 . This erosion event was associated with a general shoreward migration of the bar.

INFRAGRAVITY WAVES IN STORM CONDITIONS

The equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth. The effect of the turbulent surface oscillations are discussed but neglected in the following analysis, where the equations are applied to the energy balance in a surf zone wave motion. This leads to results for the wave height variation and the velocity of propagation. The results cannot be reconciled completely with measurements and the concluding discussion is aimed at revealing how the model can be improved.A three-dimensional morphodynamic model of sequential beach changes Is presented. The model Is based on variations in breaker wave power generating a predictable sequence of beach conditions. The spectrum of beach conditions from fully eroded-dissipatlve to fully accreted reflective is characterised by ten beach-stages. Using the breaker wave power to beach-stage relationship the model Is applied to explain temporal, spatial and global variations In beach morphodynamlcs.The agents of initial damage to the dunes are water, which undermines them, and animals (including man) which damage the protective vegetation by grazing or trampling. Of these, man has recently assumed predominant local importance because of the popularity of sea-side holidays and of the land-falls of certain marine engineering works such as oil and gas pipelines and sewage outfalls. The need is therefore increasing for active dune management programmes to ensure that under these accentuated pressures, the coast retain an equilibrium comparable with that delicately balanced equilibrium which obtains naturally at a particular location.