Showing papers on "Waves and shallow water published in 1988"

Motion of Waves in Shallow Water, Interaction between Waves and Sand Bottoms (1946)

Abstract: The loss of energy of a travelling water wave, due to the mechanism of the formation of sand ripples and water vortices on a sandy bed, becomes of practical importance when models are used to predict full-scale foreshore movements. On the assumption that the bottom-water oscillation is nearly simply harmonic, the mechanism was studied by oscillating a section of bed through still water. The pitch, p, of the sand ripple formed was found to vary as the square root of the grain diameter, independently of the speed and of the grain density, for amplitudes, R, of water motion exceeding this pitch. But for smaller amplitudes the pitch shortens with decreasing amplitude of movement. The mean drag coefficient, k, in the case of artificial rigid ripples, was measured directly. For R/p less than unity, k remains constant. For R/p greater than unity, k was found to vary as (R/p)-0.75. These results are compared with the case of steady flow. The critical water speeds and amplitudes for first disturbance of grains on a smoothed surface was also measured, over a wide range of grain diameters and densities. The results conform closely to a simple empirical expression.

Effects of water‐depth mismatch on matched‐field localization in shallow water

Abstract: This article discusses the impact of incorrect estimates of the water column depth on matched‐field source localization in a shallow water environment. Computer calculations were performed for the case of a nominal 100‐m depth water column subject to water‐depth variations of up to ±3.5 m, which would be caused by long‐period ocean swell or by tidal changes. The environment was assumed to be range independent (by proper choice of the geometry); thus the question of rough surface scattering was not an issue. The calculations incorporated source depths of 25, 50, and 75 m, a propagation distance of 4 km, an acoustic frequency of 150 Hz, and a linear vertical receiving array. The array consisted of 21 hydrophones with an interelement spacing of 2.5 m, and it spanned the center one‐half of the water column (25‐ to 75‐m depth). The matched‐field algorithm utilized in this study is the high‐resolution maximum‐likelihood estimator. A primary result of the work is that, as the output of the matched‐field processo...

Island wakes in deep and shallow water

Abstract: Observations of island wakes from the continental shelf have been used by various authors to suggest that the Reynolds number is not an appropriate parameter for wake behaviour in shallow water. It is shown here that wake behaviour both in deep and shallow water is well described by the Reynolds number if it is derived correctly, by taking into account frictional effects in horizontal as well as vertical boundary layers.

Semidiurnal tides in the northeast Atlantic from a finite element numerical model

Abstract: An in-time spectral method is used to solve the complete nonlinear problem of tide propagation simultaneously over oceanic and coastal areas For each constituent of the tidal spectrum, the corresponding elliptic modal problem is written under a variational form and solved by using a finite element technique The model is applied to the computation of the dominant M2 wave over the northeast Atlantic The numerous in situ data over this domain and the large extent of the European continental shelf allow analysis of the gain of accuracy obtained by different components characterizing the model To get a mean accuracy of less than 2 cm and 2° for the amplitudes and phases of the sea surface elevation, it is found that (1) the use of a refined mesh over the shallow areas and of a quasi-linearized version of the quadratic Chezy friction law is essential; (2) loading effects computed by means of a Green integral have to be taken into account; and (3) accurate boundary conditions need to be prescribed at the entrances of dynamically active shallow water areas if they are not included in the investigated area (or it is necessary to extend the model over the whole neighboring coastal basin if these boundary conditions are not available) From the M2 velocity field solution, bottom friction coefficients are derived for the secondary astronomical constituents of the tidal spectrum, and accurate charts of the S2 and N2 tides are produced over the modeled area

Horizontal Round Buoyant Jet in Shallow Water

Abstract: A common outfall configuration in practice involves a buoyant discharge through a horizontal circular pipe into a shallow body of water. A detailed experimental study of the near‐field flow and mixing characteristics is presented. The proximity of the bed and the free surface has a significant influence on the flow pattern. Experimental results are adequately represented by length and buoyancy scales defined by source momentum and source buoyancy flux. Jet path, centerline dilution and horizontal and vertical half widths are compared with an integral model in an unconfined environment.

A shallow water spectral wave model

Abstract: A shallow water spectral wave prediction model based on a numerical solution of the radiative transfer equation is presented. The model is second generation and uses a simple yet effective representation for the nonlinear source term. In addition, the model pays particular attention to the shallow water processes of refraction, shoaling, bottom friction, and wave breaking. The flexibility of the model is demonstrated by an intercomparison with field data from a number of tropical cyclones. The turning winds from such storms provide a demanding test of model physics. The comparisons are in both deep and shallow water.

Sea surface sound : natural mechanisms of surface generated noise in the ocean

Abstract: Ocean Wave Dynamics.- Mechanisms of Wave Breaking in Deep Water.- Equilibrium Range Characteristics of Breaking Waves.- Experiments on Breaking Waves.- Wind Waves as a Coupling Process Between Air and Water Turbulent Boundary Layers.- On the Mechanics of Spilling Zones of Quasi-Steady Breaking Waves.- An Estimate of Wave Breaking Probability for Deep Water Waves..- Whitecap Coverage as a Remotely Monitorable Indication of the Rate of Bubble Injection into the Oceanic Mixed Layer.- Wave and Bubble Characteristics in the Surf Zone.- LDA as a New Tool to Detect Air-Sea Interaction Mechanisms.- Feasibility Study of At-Sea Measurement of Ocean-Air Interface Parameters Needed to Evaluate Ocean Surface Generated Noise.- Bubble Dynamics and Observations.- Bubble Noise Creation Mechanisms.- Bubble Dynamics in Oceanic Ambient Noise.- The Horizontal Structure and Distribution of Bubble Clouds.- On the Distribution of Bubbles Near the Ocean Surface.- Study of Micro-Bubbles in the North Sea.- Optical Microbubble Measurements in the North Sea.- Acoustical Estimates of Subsurface Bubble Densities in the Open Ocean and Coastal Waters..- Acoustic Measurements of Bubble Densities at 15-50 kHz.- Propagation Effects.- Propagation Effects Associated with Ambient Noise.- Low Frequency Wind Generated Ambient Noise in Shallow Water.- The Effect of Propagation Conditions on Wind-Generated Noise at Real Shallow Water Sites.- Simultaneous Measurements of Surface Generated Noise and Attenuation at the Fixed Acoustic Shallow Water Range "NORDSEE".- Wind and Wave Noise.- Mechanisms of Sound Generation at the Ocean Surface.- Ambient Noise Radiation by "Soliton" Surface Waves.- Ocean Noise Spectrum Below 10 hz - Mechanisms and Measurements.- Low Frequency Ocean Ambient Noise: Measurements and Theory.- Estimation of Source Characteristics from Underwater Noisefield Measurements.- Noise Generated by Motion of the Sea Surface - Theory and Measurement.- Observations of High Frequency Ambient Sound Generated by Wind.- On the Spectra of Wind Generated Sound in the Ocean.- Physical Mechanisms of Noise Generation by Breaking Waves - a Laboratory Study.- Audio Signature of a Laboratory Breaking Wave.- Noise Generation by Bubbles Formed in Breaking Waves.- Rain Noise.- Acoustic Emissions Associated with Drop Impacts.- The Sound Generated by Precipitation Striking the Ocean Surface.- Studies of Mechanisms Influencing Rain Noise.- Ice Noise.- Speculations on the Origin of Low Frequency Arctic Ocean Noise.- Observation of the Sound Radiated by Individual Ice Fracturing Events.- The Correlation of Mid-Frequency Pack Ice Noise with Environmental Parameters.- High Frequency Ambient Sound in the Arctic.- Arctic Ocean Noise Generation Due to Pack Ice Kinematics and Heat Fluxes.- Acoustic Ambient Noise in the Arctic Ocean Below the Marginal Ice Zone.- Ice Eddy Ambient Noise.- Panel Discussion Reports.- Breaking Waves.- Study of the Distribution of Bubbles and Turbulence In and Near a Breaking Wave.- Sources of Sound at the Ocean Surface Bubbles and Other Noise.- Wave and Turbulence Noise.- Precipitation Noise.- Ice Noise.

Acoustic backscattering at low grazing angles from the ocean bottom

Abstract: The measurement of acoustic backscattering at low grazing angles from the ocean bottom in a shallow water area near Charleston, South Carolina is described and the results of data analyses are compared with results reported earlier. It was found that, with fixed platforms resting on the bottom in shallow water at both test sites, interference in the form of backscatter from a rough sea surface is a significant problem. A modest frequency dependence and relatively strong azimuthal dependence for bottom backscattering strength were observed at both test sites. No strong relationship could be determined between bottom backscattering strength azimuthal dependence and bottom roughness characteristics.

Shallow-Water Waves. II: Data Comparisons

Abstract: Predicted spectral shapes, based on the theory derived in Part I (Resio 1987), appear to agree well with those measured under storm conditions. The results of this paper indicate that, under such conditions, a dynamic balance between wind input and nonlinear fluxes dominates the shape of a spectrum and via this balance also controls the total energy level and related energy losses in waves propagating into shallow water, even into depths less than 33 ft (10 m). The nonlinear flux estimates derived here also provide a quantitative means of evaluating energy losses, even in nonequilibrium conditions. This ability to quantify energy losses due to nonlinear fluxes will be particularly important in modeling waves in complex bathymetries, or in situations of mixed sea‐swell, or when wave generation is taking place at very oblique angles to the coast, and in very shallow water where the nonlinear fluxes can no longer maintain an equilibrium.

Sedimentologic approaches to paleobathymetry, with applications to the Merced Formation of Central California

Abstract: Sedimentologic evidence for paleobathymetty, like paleontologic evidence, is largely indirect in that it mostly reflects processes and conditions that are themselves partly depth-dependent. Sedimentologic approaches can be a useful adjunct to paleontology for interpreting paleodepths, particularly in relatively shallow water. Pleistocene deposits of the Merced Formation on the San Francisco Peninsula, California, provide an excellent opportunity to demonstrate the methods, value, and difficulties of the sedimentologic approach. Distinction between subaerial and subaqueous deposition can be drawn on the basis of eolian structure, paleosols, and other sedimentary features. The spacing of symmetric ripples relative to their grain size and lenticular laminae of heavy minerals may be useful for identifying deposition in very shallow water. The boundary between intertidal and subtidal deposits serves to establish ancient sea-level positions. The boundary is reflected by the upward transitionsfrom cross-bedded sand (nearshore facies) to planar-bedded sand (foreshore facies) in open-coast deposits, and from laminated sand andlor mud (subtidal channel bank facies) to bioturbated sand andlor mud (intertidal flat facies) in the deposits of coastal embayments. Where the physical setting (i.e., wave energy, sediment input, etc.) can be reasonably reconstructed, paleodepths in shelffacies are indicated by the maximum depth of cross-bedding and gravel deposits generated by the effects of shoaling waves, the contact between shelf sand and mud, and the deepest occurrence of shell lags. In Pleistocene shelf deposits of the Merced Formation on the San Francisco Peninsula, California, these depths are placed at 10 + 5 m, 60 + 10 m, and 100 + 20 m, respectively, based on their inferred depth on the modern central California shelf. At greater depths the sedimentologic approach loses resolution and reliability. Interpretations based on the distribution of turbidites, evidence of anoxic conditions, and calcium carbonate compensation depths and lysoclines are probably most useful for establishing paleodepth in a relative sense only. Integration of the sedimentologic and paleontologic approaches carries a high potential for improving our ability to determine paleobathymetry. INTRODUCTION

Lagrangian moments and mass transport in Stokes waves Part 2. Water of finite depth

Abstract: Some simple relations between the Lagrangian moments and cumulants in a steady finite-amplitude gravity wave on deep water are here generalized to water of finite depth. The first three Lagrangian moments are shown to be given in term of the mass-transport velocity U at the free surface, the potential and kinetic energy densities V and T, and the mean-square particle velocity on the bottom.A simple method of calculation is described, which exploits certain quadratic relations between the Fourier coefficients in Stokes's series. The ratio U/c and the associated Lagrangian skewness is calculated for periodic waves, as a function of the wave steepness and the mean water depth.For limiting waves, i.e. those with sharp crests, it is found that the most symmetric orbits, in the Lagrangian sense, occur not in very deep or very shallow water, but at one intermediate value of the ratio of depth to wavelength. When the depth parameter kd equals 1.93 the vertical displacement of a marked particle at the free surface is closely sinusoidal in the time t.

Shallow Seismic Experiments Using Shear Waves

Abstract: During the summer of 1986, a series of seismo‐acoustic experiments was carried out in shallow water off the New Jersey shore. The purpose of these experiments was to measure the geoacoustic properties of the ocean sediments that comprise the upper few hundred meters of the sediment column. Seismic sources and receivers were deployed at or very near the bottom in order to excite shear waves in the sediment and minimize the effects of interference from waterborne propagation. The experiments were performed at several sites where prior field work had established physical properties and a detailed profile of the sediments. By using conventional air guns deployed in an unconventional way, strong interface and diving shear waves were generated; these data were inverted to obtain shear wave velocity as a function of depth. The inversion results were then compared with the predictions of a geoacoustic model that accounts for the effects of voids ratio, overburden pressure, and other physical parameters. The in si...

The energy balance of finite depth gravity waves

Abstract: The energy transfer equation, which describes the evolution of the surface wave spectrum, is solved numerically for different water depths, with an empirical expression for the bottom dissipation and explicit calculations of the resonant four-wave interactions. The resulting growth curves for total energy, peak frequency, etc., are explained from the balance between the source terms at the various stages of growth. The equilibrium which is reached between bottom friction and the resonant interactions on the forward face of the spectrum turns out to be important. The calculated results are briefly compared with recent shallow water wave experiments (TMA, Lake Marken) and with the Coastal Engineering Research Centre growth curves. Finally the results are compared with the SWIM study.

Benthic Communities in the South African Devonian

Abstract: The Lower Devonian clastic Bokkeveld Group of South Africa likely resulted from the vertical stacking of a number of wave dominated deltaic complexes. Concentrating on the lowermost of these deltaic cycles, comprising the Gydo and Gamka formations, it is possible to recognize a number of benthic invertebrate fossil communities. These can be correlated with the various depositional sub-environments within a delta complex. The deepest water environment was the prodelta-shelf area in which shale was deposited. This area was home to the most diverse fossil assemblage, dominated by thin-shelled, free lying brachiopods but which also contained infaunal bivalves, gastropods, trilobites, crinoids and hyoliths. Higher in the sequence the sediment becomes coarser grained and sandstone becomes interbedded with siltstone. In this delta slope environment thicker shelled brachiopods predominate and trilobites become progressively scarce. At the top of the deltaic pile shallow water depositional environments were developed, including distributary mouth bars and tidal flats. In such areas low diversity communities existed. Distributary mouth bars were dominated by large thick-shelled brachiopods fixed to the substrate by functional pedicles; tidal flats were dominated by infaunal bivalves and inarticulate brachiopods. Similar benthic assemblages to those detailed for the lowest deltaic cycle can be found in some of the overlying cycles but the topmost part of the Bokkeveld Group is apparently devoid of invertebrate remains. However, in the basal part of the overlying Witteberg Group a few representatives of the shallow water communities have been found.

Internal waves and mixing along the New England shelf-slope front

Abstract: A 48-day early spring data set from the outer continental shelf south of New England is used to describe the internal wave climate of the shelf-slope front. Current data from three near-bottom and one mid-depth instruments in water depths between 80 and 120 m indicate that internal waves provided a significant portion of the near-bottom current variance in the frontal zone. The data show that the internal wave energy was enhanced in the frontal zone relative to the shelf. At these depths, at least during the observational program, surface waves did not generally cause noticeable near-bottom water motions. Spectra describe an internal wave field of low vertical mode propagating perpendicular to the isobaths onto the shelf. This suggests significant refraction of the waves as they propagated onshore from the isotropic internal wave field of the continental slope. There appeared to be very little along-shelf variation in internal wave energy over distances on the order of 100 km. Onshore, however, there was a factor of two decrease in internal wave energy density over a distance of about 15 km. Assuming frictional dissipation within a bottom boundary layer, this decrease in energy translates into a friction velocity of about 2 cm s−1. Thus, internal wave dissipation could result in bottom stress levels comparable to values deduced for shallower waters under the influence of storm-driven surface waves. This dissipation rate is shown to produce a significant bottom mixed layer which can, under some circumstances, result in sizeable cross-frontal exchange of shelf and slope waters.

Ocean Currents Over the Continental Slope

Abstract: Publisher Summary This chapter discusses the different aspects of ocean currents over the continental slope. Continental slopes underlie some 12% of the world ocean's surface area. Their importance to ocean circulation is out of proportion, since most of the major ocean currents flow over them and are in various ways affected by their topography. Superimposed on the typical upwelling regime are variations in the strength of the alongshore flow in response to changes of the wind stress. Coastal currents driven by density differences due to river runoff are common on continental shelves but reach to the upper slope only in a few places. Strong surface cooling, in the absence of significant salinity contrasts, produces cold and heavy coastal waters, the opposite of the coastal freshening case. Shallow water cools off rapidly, as its temperature also becomes uniform due to thermal convection, and tends to underflow less completely cooled deeper water. It is found that the fundamental slope effect examined above turns up in a different guise if the angular momentum balance of a fluid column fixed in space is considered instead of vorticity, allowing for external torques as well as the torque due to momentum advection.

Some characteristics of breaking waves

Abstract: In recent years there has been a surge in coastal engineering research devoted to various aspects of breaking waves including their kinematics at and after breaking. For a review of certain aspects of this field the interested reader is referred to Peregrine (1983) and Battjes (1988); in this discussion only certain publications pertinent to this investigation will be mentioned briefly. With the advent of laser-Doppler velocimetry (LDV) a number of investigators have measured the internal velocities of waves both before, at, and after breaking. For example, Nadaoka (1986) measured the velocities in the shoaling region under periodic breaking shallow water waves. This extensive study of the nearshore regions resulted in vector diagrams which described very well several spatial aspects of the flow shoreward of breaking. Skjelbreia (1987) also used LDV techniques to define the kinematic characteristics of breaking solitary waves. Measurements were made of the water particle velocities under spilling and plunging breaking waves both very near breaking and after breaking, close to the water surface and to the bottom. A high degree of reproducibility was possible with the laboratory wave generation system used so experiments were conducted at different locations with essentially the same wave; this will be discussed more fully later. Skjelbreia (1987) also presented vector diagrams of the velocities under plunging and spilling solitary breakers. These measurements when compared to those of Nadoaka (1986) raise several questions regarding similarities and differences between breaking oscillatory waves and waves of translation. In addition to detailed kinematic measurements, a macroscopic view of shoaling solitary waves was also taken by Skjelbreia (1987) yielding results on the variation of the wave height with distance both before and after breaking. Although there has been a considerable amount of work along these lines in the past, observations of the changes in the wave at and after breaking are still quite useful in developing an overall understanding of the breaking process.

Method for protecting a shallow water well

Abstract: Method of utilizing a jack-up well drilling rig in shallow water to drill an isolated well having an unsupported well conductor, and subsequently to attach a reinforcing frame near the water line and to drive pile through the frame into the ocean floor without moving the jack-up rig off location.

New Beach Ridge Type: Severely Limited Fetch, Very Shallow Water

Abstract: The southern end of Laguna Madre (Texas) north of the Rio Grande mouth, is marked by very shallow water, wide tidal flats, lunettes, islands made of beach ridges and lesser features. The number and variety of islands in the lagoon is remarkable. The lunettes ("clay dunes") are made primarily of quartz sand and coarse silt. They are commonly 15-35 feet (5-10 meters) high, irregular is shape and steep-sided. They were deposited from wide transport, and did not migrate. Those that are islands in the lagoon predate present position of sea level. Islands made of beach ridges were built from the lagoon side. Photo analysis, field work and granulometry all show that this sand was not moved into these ridges by Gulf of Mexico waves. Trenches in 12 beach ridges showed horizontal bedding but neither low-angle nor steep cross-bedding (hence quite unlike swash-built beach ridges). The ridges were built by wind-tide lag effects, not from the swash. Therefore these beach ridges are a new type, in addition to swash-built, eolian, and storm-surge ridges. Growth of the ridges appears to be over.

An Estimate of Wave Breaking Probability for Deep Water Waves

Abstract: Analytical expressions for the probabilities of both wave breaking and of the fractional energy dissipation losses generated by wave breaking, are derived on the basis of a joint distribution for wave frequencies and amplitudes. Direct effects of wind forcing on wave breaking are explicitly considered in the form of Phillips and Banner (1974) mechanism. The results are found to depend on the wave spectrum band width parameter θ and the characteristic frequency σ0 (definitions follow), and are not restricted to narrow-band spectrum cases. When the computed total energy dissipation losses (per average wave period) are used together with appropriate wave growth models to predict the fraction of wind momentum and energy fluxes transferred to water waves, the results appear to be in good agreement with their experimental counterparts. These expressions are also compared to other recent work on wave breaking and their similarities and differences are discussed.