Showing papers on "Swell published in 1972"

Field observation of nearshore circulation and meandering currents

Abstract: Field observations were made of wave-induced nearshore circulations and meandering longshore currents on an undulatory surf-zone bed, under the action of uniform incident waves. Circulations were associated with normal-wave incidence; meandering currents were associated with oblique-wave incidence. The transport in the observed circulations generally agreed with Bowen's (1967) linear theory based on the concept of radiation stress (Longuet-Higgins and Stewart, 1962, 1964), provided that a friction coefficient C = 0.014 was assumed. The longshore current near the shore line moved from a shoal to a depression as predicted, but this movement was also directed from an area of high waves to one of low waves, which is different from the case of a circulation driven by nonuniform breaker heights on the bar. Spilling breakers over a shoal underwent greater energy dissipation than plunging breakers in the rip current. Observed streamlines were narrow in the outflow and broad in the inflow, a characteristic that was probably associated with a nonlinear mechanism arising from a steep depression in the rip channel, as previously explained by Arthur (1962). These circulations were pulsational, unlike the circulations of a steady-state solution. Occasional strong outflows at beat frequencies caused water to escape from the circulation. For a given surf-zone undulation, breaking over the inner bar was essential to the formation of a circulation, and the intensity of breaking, controlled by tide, corresponded with a proportionally stronger circulation. Thus, circulations were generally stronger during low tide than during high tide. Low rip-current velocities at high tide fluctuated with incoming swells, whereas high velocities at low tide tended to fluctuate at surf beat frequencies. In proportion to increasing rip velocities, the rip pulsation tended toward lower intervals. Mean surface slopes caused by wave set-up and set-down agreed with trajectories of neutral-density balls released in the circulation. Meandering currents associated with oblique-wave incidence could be explained as a combined effect of circulation cells and parallel longshore flows.

Use op crenulate shaped bays to stabilize coasts

Abstract: Crenulate shaped bays are the rule rather than the exception on coastal margins of oceans, inland seas or lakes where sedimentary beaches exist between headlands. They have a particular orientation to the swell or resultant wave energy vector, such that the straight tangent section is downcoast and the curved portion upcoast. The latter is a logarithmic spiral at all stages of development of the bay. When fully stable, that is no littoral drift taking place, the constant of the log-spiral equation has a specific relationship to the approach angle of the waves to the headland alignment. In this condition it is shown that diffraction and refraction are involved when waves sculpture the curved beach in the lee of the upcoast headland. A further ratio to identify stable bays appears to be the ratio of indentation length to clearance between headlands. The application of crenulate shaped bays to stabilization of a reclaimed shoreline suffering strong littoral drift on Singapore Island is described.

Case study of duration-limited wave spectra observed at an open ocean tower

Abstract: A sequence of double-peaked open-ocean wave spectra was observed simultaneously with eddy correlation estimates of momentum flux and atmospheric stability over a 15-hour interval at Argus Island tower (near Bermuda) on March 22 and 23, 1967. The wave spectra were associated with a generating sea and advecting swell in the fetch of the warm sector of an advancing cyclone. The equilibrium range constant for these wave spectra was estimated at (7.8 ± 1.6) × 10−3. The Miles-Phillips exponential growth parameter was in reasonably good agreement with other field investigations, and observations support predicted amplification factors given by Phillips (1966) over the range 20 ≤ C/U* ≤ 32 where C is wave phase speed and U* is the friction velocity. Two observations of both linear and exponential growth parameters associated with spectral frequencies of 0.13 and 0.14 Hz were available to compare predicted and observed wave growth, with good results after correction for residual wave background. Observed temporal overshoot-undershoot of a particular wave component is interpreted in terms of an energy balance between wind input, wave-wave interactions, and wave breaking as a wave component evolves to its equilibrium condition. An example of spectral growth associated with advecting lower-frequency swell illustrates wave-induced fluctuations associated with momentum transfer from the sea to the atmosphere. Peaks in the horizontal and vertical wind-velocity spectra and the cospectra and quadrature spectra associated with wave spectral peaks are clearly evident when ocean swell propagates through the observation area.

Atmospheric Gravity Waves from Winds and Storms

Abstract: Atmospheric gravity waves were explored on the leeward side of the north-south trending Continental Divide in Colorado by using an array of electronic microbarographs and recording anemometers. Observations of low-velocity gravity waves in a two-to-three octave region for wave periods of 3–24 min were made. These waves are apparently caused by locally generated signals from upper tropospheric winds, jet streams, weather fronts, thunderstorms, and severe weather, with shear the principal mechanism. Lee waves and moderate-to-severe turbulence were frequently observed in conjunction with the appearance of gravity waves generated at mountain-top level. A unique wave source is identified due to the interaction of down-slope winds on the leeward side of a mountain and an inversion-layer boundary. Weather fronts on some occasions seem to provide a passive boundary layer for the production of waves, but in other cases they may actively generate waves. Readings on summer thunderstorms indicate that an ear...

Some qualitative aspects of nonlinear wave radiation in a surf zone

Abstract: Photographs are presented which show a series of small waves produced in a surf zone behind the front of each breaking swell. They probably represent solutions to the Korteweg deVries (cnoidal-wave) equation, and their generation is favored by bottom topography not typical of most surf zones.

Surface Curvature of Wind Waves Observed from Different Angles.

Abstract: The curvature distribution of the wind-disturbed water surface in a laboratory tank is observed from different angles with an optical instrument. The distribution is generally skewed with greater radius of curvature at steeper viewing angles from the normal to the mean water surface. As the wind velocity increases, the average radius of curvature decreases: rapidly at low wind velocities when waves are effectively excited by wind, and gradually at high wind velocities when waves approach saturated state.

Taylor-Görtler vortices expected in the air flow on sea surface waves—II

Abstract: The instability of Taylor-Gortler vortices which are expected in the air flow on water waves was studied in part I, under the assumption that the curvature around the crest or the trough of water waves, where the instability was expected to take place first, was constant, namely that the characteristics of the vortices were affected little by the local change of the curvature along the direction of the progress of water waves (the direction ofx-axis) However, the curvature actually varies from positive to negative, or vice versa. In order to study this effect, the instability of Taylor-Gortler vortices is examined with respect to the range of the part of a constant curvature, in the model in which the curvature is positive constant near the trough and negative constant near the crest, and zero in the intermediate regions, respectively. It is shown that as the region of the constant curvature becomes narrower, the instability tends to weaken. For the same example with part I, namely, when the wind of 12.2 m s−1 is blowing over swells of 15 m in wavelength, if the range of constant curvature near the trough is taken as a quarter of one wave length, the critical wave height becomes 0.96 m instead of 0.50 m, and conversely, the wave length and the height of center of the vortex become 11.9 m and 2.1 m instead of 24 m and 3.7 m, respectively. Further, using the energy equations, quantitative estimates are performed of the intensity of the vortices which develop when the wave height of the swell is 1.05 m in the above described example, and also of the influence of the vortices upon the wind profile when the equilibrium state is reached. When the vortices are generated and grow to attain to an equilibrium state interacting with the mean flow, the maximumx-component of velocity in the vortices is about 1.04 m s−1. Consequently, the wind profile undergoes a considerable distortion from the logarithmic one near the level of 2 m height. This distorted wind profile has a form similar to those sometimes observed above the sea surface.

A Theoretical Model of Primary Frequency Microseisms

Abstract: Summary A comprehensive mechanism for making quantitative estimates of the seismic response associated with quasi-monochromatic system of pressure waves in shallow water is proposed. Based on generalized linear wave theory, it applies to low-frequency components in the microseism spectrum. A model of pressure response function is derived by considering the agitated surface of open sea with wave trains converging towards the shore-line from scattered sources. As expected, this favours low-frequency swell and is able to isolate energy associated with a specified frequency component. Lastly, an expression for the local energy balance in the seismic wave guide is deduced and the computed results from this theory is tabulated with those from recent measurements. Broadly speaking the two are sufficiently in agreement except at very low frequency range of the spectrum.

Impact Pressure acting on Bow of Large Full Ships

Abstract: The impact pressure acting on bow of large and full ship may be predicted by the following considerations. The ship motions are mainly due to the waves longer than ship but scarcely affected by the shorter waves. Therefore, the ship cruising in the ocean is oscillated violently by the swell (longer waves) and the velocity of the bow motion relative to wave surface becomes very large. Impacts on bow occur when short waves are superposed to the longer waves and collide with such oscillating ship. If the angle between the stem and the wave surface of the short waves is small, large impact pressure is produced. The slope of stem and bluntness of bow are considered to be main parameters controlling the amount of pressure and its frequency.At Nagasaki Experimental Tank, impact pressures on the model are measured in shorter waves superposed on longer waves or using models oscillated with frequency and amplitude corresponding to the condition in longer waves.

Period by the wave-group method

Abstract: The wave-group period, T , is obtained by a eimple manual procedure from the periodic wave groupff'and sequences that appear in strip-chart records. A dozen or more values are commonly derived from a 20-minute record. When plotted as a time-series, T values form patterns that represent individual wave trains, each generated by a synoptic weather event. Swell trains and wind-wave trains can be distinguished by their characteristic period distributions. For swell trains, the mean wave-group frequency, f , obtained from the best-fit line to the time-series plot of f_ values (reciprocal T values), is equivalent to the frequency of maximum energy density, fm, obtained from spectral analysis. The distance and time of origin for a swell train can be determined from the time rate of change of f„, and with the use of weather maps the generating area can be identified and the deep-water arrival direction of the swell train at the wave gage determined. The zero-line crossing period, T , represents an integration of the waves from all wave trains present and has no synoptic significance.

Ocean swell magnetic field simulation

Abstract: Magnetic field components generated by ocean swell correlate with other swell-induced ocean parameters Simulated magnetic fields are related to other measured ocean parameters through transfer functions Simulation criteria are continuous power spectral density functions and realistic time variation of waveforms of the simulated field