Showing papers on "Waves and shallow water published in 1993"

Wave Breaking in Deep Water

Abstract: Every mariner is aware that dangerous large breaking water waves occur on the world's oceans. The scope of this review is somewhat greater. Wave breaking occurs at a large range of scales and we do not restrict ourselves to the deep ocean. "Deep water" in the context of water wave studies implies water sufficiently deep that the surface waves are unaffected by the direct effects of variations in bed topography. Thus even a small pond can support breaking deep-water waves. Shallow water breaking is reviewed in Peregrine (1983). Some comments on the visual aspect of breakers are in order, since direct observation still has a role to play in the study of this complex phenomenon. The most dramatic breakers are plunging breakers where the breaking commences by the wave overturning and forming a forward moving sheet of water which plunges down into the water in front causing splashes, air entrainment, and eddies. Although plunging breakers are common on beaches they are less common on deep water, so much so that some people have argued that they do not occur naturally. However, read Coles (1991) for a distillation of an experienced yachtsman's account of waves at sea. Most other breakers are described as spilling breakers. From their

Occurrence of Arctic Cod ( Boreogadus saida ) Schools and Their Vulnerability to Predation in the Canadian High Arctic

Abstract: We document the occurrence of large schools of Arctic cod (Boreogadus saida) in the Barrow Strait region of the eastern Canadian Arctic during the open water season. Schools were most frequently observed near shore, often in depressions inside bays. Schools ranged up to 130 O00 mz surface area and contained on the order of 4 X 108 fish, weighing 12 O00 tonnes. Evidence indicates that schools form before the arrival of predators, but when they occur in shallow water, they are often subjected to intense predation by thousands of seabirds and marine mammals, primarily black-legged kittiwakes (Rissa tridacfyla), northern fulmars (Fulmam glacialis), harp seals (Phoca groenlandicus), beluga (Delphinaptem Zeucas) and narwhal (Monodon monoceros). The reasons for such schooling behavior are unknown.

Carbonate Concepts from the Maldives, Indian Ocean

Abstract: A combination of ground truth, provided by a single exploration well, and geometries apparent on seismic lines, provides new insight on the geologic development of the Maldives. The single exploration well documents the occurrence of more than 2000 m of Tertiary carbonate rock overlying volcanic basement. Seismic lines demonstrate that the shallow water Paleogene part of the drilled section is generally horizontally bedded, whereas the overlying upper Oligocene-Miocene carbonates have a distinctly prograding pattern. Significantly, the prograding pattern is directed inward from the edges of the present platform rather than outward into the thousands of meters of Indian Ocean water depths. The punctuated lateral expansion of the carbonate platforms is therefore bilaterally opposing, i.e., away from either of the Indian Ocean edges of the platform and toward each other. The same opposing directions of expansion are apparent in Saya de Malha bank, where, unlike the Maldives, coalescence has been completed in the form of a large carbonate bank. In both cases, the opposing directions of progradation appear to be the result of a structural change in depositional architecture of the kind usually associated with the thermal contraction development of sag basins. Within this overall structural and stratigraphic framework, there is evidence that seismic stratigraphic relationships can vary considerably from atoll to atoll, indicating important local variations of the regional stratigraphy. A second significant depositional change resulted from the onset of Pliocene-Pleistocene glacial fluctuations in sea level.

Pressure-Forced Seiches of Large Amplitude in Inlets of the Balearic Islands

Abstract: Large-amplitude harbor seiches usually occur in summer in the Balearic Islands. A significant correlation between sea level oscillations and atmospheric pressure disturbances has been found, though a proved physical mechanism to account for this atmosphere-ocean interaction is still missing. Using a flat bottom, shallow water model, we show that a direct coupling between atmospheric pressure and the free mode of an inlet is unlikely but that an oceanic wave of atmospheric origin can act as an intermediate mechanism and adequately force the inlet by resonance. The phase relationship derived from this mechanism is in good agreement with observations, provided the whole spectrum of oceanic waves is in opposite phase to the generating atmospheric disturbance. We also show that the very large oscillations observed at Ciutadella (an elongated, shallow inlet in the west coast of Menorca, Balearic Islands) can be explained in terms of the particular shape of this inlet.

Interannual sea level variations in the tropical Indian Ocean from Geosat and shallow water simulations

Abstract: Sea Level variations of the Indian Ocean north of 20°S are analyzed from Geosat satellite altimeter data over April 1985–September 1989. These variations are compared and interpreted with numerical simulations derived from a reduced gravity model forced by FSU observed winds over the same period. After decomposition into complex empirical orthogonal function the low-frequency anomalies are described by the first two modes for observations as well as for simulations. The sums of the two modes contain 34% and 40% of the observed and simulated variances respectively. Averaged over the basin, the observed and simulated sea level changes are correlated by 0.92 over 1985–1988. The strongest change happens during the El Nino 1986–1987, between winter 1986 and summer 1987 the basin-averaged sea level rises by ∼1 cm. These low-frequency variations can partly be explained by changes in the Sverdrup circulation. The southern tropical Indian Ocean between 10° and 20°S is the domain where those changes are st...

Topographic Upwelling off Southwest Nova Scotia

Abstract: The waters off Cape Sable in southwestern Nova Scotia have anomalously low temperatures and high nutrients during summer. Using a three-dimensional tidal model as well as field observations obtained between 1978 and 1985, a new mechanism is proposed to explain the origin of these anomalous properties. The numerical model predicts several areas of strong tidally induced residual upwelling and downwelling. On the submarine ridge off Cape Sable, upwelling occurs over the eastern flank and downwelling over the western flank. This upward vertical transport is very effective in supplying cold, saline, and nutrient-rich water from deep to shallow layers. The predicted upwelling and downwelling are induced by a tidal rectification process resulting from tidal currents flowing over complex bottom topography. Hence, the process is named “topographic upwelling and downwelling.” The upwelling (downwelling) is generated by the residual currents flowing from deep to shallow (shallow to deep) waters. This proce...

The North Atlantic circulation: Combining simplified dynamics with hydrographic data

Abstract: We estimate the time-averaged velocity field in the North Atlantic from observations of density, wind stress and bottom topography. The flow is assumed geostrophic, with prescribed Ekman pumping at the surface, and no normal component at the bottom. These data and dynamics determine velocity to within an arbitrary function of (Coriolis parameter)/(ocean depth), which we call the “dynamical free mode.” The free mode is selected to minimize mixing of potential density at mid-depth. This tracer-conservation criterion serves as a relatively weak constraint on the calculation. Estimates of vertical velocity are particularly sensitive to variations in the free mode and to errors in density. In contrast, horizontal velocities are relatively robust. Below the thermocline, we predict a strong 0(1 cm/set) westward flow across the entire North Atlantic, in a narrow range of latitude between 25N and 32N. This feature supports the qualitative (and controversial) conjecture by Whist (1935) of flow along the “Mediterranean Salt Tongue.” Along continental margins and at the Mid-Atlantic Ridge, predicted bottom velocity points along isobaths, with shallow water to the right. These flows agree with many long-term current measurements and with notions of the circulation based on tracer distributions. The results conflict with previous oceanographic-inverse models, which predict mid-depth flows an order of magnitude smaller and often in opposite directions. These discrepancies may be attributable to our relatively strong enforcement of the bottom boundary condition. This involves the plausible, although tenuous, assertion that the flow “feels” only the large-scale features of the bottom topography. Our objective is to investigate the consequences of using this hypothesis to estimate the North Atlantic circulation.

12. the stable oxygen isotope signal in shallow-water, upper-slope sediments off the great barrier reef (hole 820a)1

Abstract: Oxygen-isotope ratio measurements are presented for the planktonic species Globigerinoides ruber collected from shallowwater, upper-slope sediments from Holes 820A and 820B in 280 m of water, on the seaward edge of the Great Barrier Reef. Correlation of the Site 820 isotope curve with deep-sea reference curves of the Pacific Ocean (Core V28-238, Hole 677A, Hole 607A) permits the definition of isotope stages 1 to 19 in the top 145 m of Holes 820A and 820B. However, paleontological data indicate that stages 4 and 7 might be missing and that two hiatuses occur at a depth of 8.05 to 12.1 and 34.55 to 35.8 mbsf. Using deep-sea Hole 677A as a reference for ice-volume variations, we determine the difference in isotopic signature between it and Site 820. We propose that this difference is a regional signal representing a progressive 4°C increase in surface-water temperature at Site 820. The proposed temperature change was initiated at about 400 k.y. and corresponds to a change from high-to-low frequency variations in Pleistocene isotope signals. We postulate that these changes may have catalyzed the growth of the Great Barrier Reef. The shift also coincides with changes in seismic character and some physical and chemical sediment characteristics.

Mathematical and numerical modelling of shallow water flow

Abstract: This paper deals with shallow water equations. We discuss the mathematical model, the admissible boundary conditions, some popular numerical methods in the specialized literature, as well as we propose new approaches based on fractional step and finite element methods.

Complex effective depth of the ocean bottom

Abstract: The effective depth approximation is extended to allow for energy loss on reflection at an attenuating, low shear speed, solid ocean bottom. The resulting complex effective depth leads to a novel and efficient algorithm for finding the exact complex eigenvalues for the normal modes in isovelocity water over an attenuating fluid or solid bottom while avoiding numerical searches. The method gives rapid normal mode solutions for shallow water propagation including the effects of leaky modes.

Nonlinear evolution of surface gravity waves over an uneven bottom

Abstract: A unified theory is developed which describes nonlinear evolution of surface gravity waves propagating over an uneven bottom in the case of two-dimensional incompressible and inviscid fluid of arbitrary depth. Under the assumptions that the bottom of the fluid has a slowly varying profile and the wave steepness is small, a system of approximate nonlinear evolution equations (NEEs) for the surface elevation and the horizontal component of surface velocity is derived on the basis of a systematic perturbation method with respect to the steepness parameter. A single NEE for the surface elevation is also presented. These equations are expressed in terms of original coordinate variables and therefore they have a direct relevance to physical systems. Since the formalism does not rely on the often used assumptions of shallow water and long waves, the NEEs obtained are uniformly valid from shallow water to deep water and have wide applications in various wave phenomena of physical and engineering importance. The shallow- and deep-water limits of the equations are discussed and the results are compared with existing theories. It is found that our theory includes as specific cases almost all approximate theories known at present.

Experimental investigation of sediment effect on acoustic wave propagation in the shallow ocean

Abstract: In shallow water the sediment layers have very strong effects on the propagation of acoustic waves. An effort to study the effects of the sediment has been made using 50‐ to 600‐Hz continuous wave acoustic propagation data taken by Carey [‘‘Experimental verification and application of bottom shear modulus profile (BSMP) method,’’ Oceans ’91 Proceedings (1991)] at the Atlantic margin coring project (AMCOR) borehole 6010 off the coast of New Jersey combined with sediment properties measured at that site by Yamamoto et al. of the University of Miami Geoacoustic Laboratory using the bottom shear modulus profiler (BSMP) method. Excellent agreement was found between the model and data indicating the acceptability of BSMP sediment values as input for acoustic propagation studies. The introduction of shear or tangential stresses in the model was found to have no effect upon which modes propagated but only on their modal intensity. The higher the order of the mode the greater the penetration in the seafloor and th...

Crop Water Use from Shallow, Saline Water Tables

Abstract: The effect of shallow, saline water tables on the irrigation requirements of corn, alfalfa, and winter wheat was determined using lysimeters in western Colorado. The crops were grown with no water table and with water-table depths of 0.60 and 1.05 m supplied with water having electrical conductivities of 0.66, 3.0, and 6.0 dS/m. The portion of total seasonal evapotranspiration supplied from shallow groundwater was strongly affected by water-table depth and, for corn and wheat, slightly affected by salinity of the water in the saturated zone. Results suggest that surface irrigations may be reduced when shallow water tables are present.

Late Miocene–Pleistocene sequence stratigraphy and mass-movements on the western Barents Sea margin

Abstract: By sequence stratigraphic analysis of seismic data from the western Barents Shelf, nine sequences from late Miocene to Pleistocene are discussed. The sequences reflect a generally aggradational and progradational period of the shelf, and by using global eustatic curves they are tentatively proposed to be deposited from 5.5 to 0.8 Ma. In the eastern part of the studied area, incised valleys and progradation of sediments reflects shallow shelf conditions with possible sand prone coastal sediments. To the west these shallow water sediments have prograded into deeper water, dominated by mud rich sediments. Large parts of the sequences were eroded by grounded glaciers during the last 0.8 Ma, and only the lowermost sequence plus one younger sequence have the transgressive or highstand deposits preserved. Close to the onset of glaciation by grounded glaciers on the Barents Shelf, a huge slide occurred on the continental slope and rise, covering an area of about 12,000 km 2 and displacing about 5100 km 3 of the previously deposited sediment. The slide is characterized by imbricated blocks and more chaotic areas, probably representing slumped materials and debris or mud flow deposits. It mainly affected the parts of the late Miocene to Pleistocene sequences deposited as slope fans, or the deepest parts of the progradational wedge. High accumulation rates and shallow gas in the sediments are suggested as the main triggering agents of the slide. The onset of glaciation on the shelf may have been an additional triggering factor. The slide scar seems to have been infilled mainly by turbidites. They were transported, at least in part, through one or two large canyons and generated slope fan deposits, thus decreasing the relief of the Barents Sea slope. This slope fan infilling was probably contemporaneous to the first period of glaciations on the shelf areas.

Nonlinear, dispersive, shallow-water waves developed by a moving bed

Abstract: Landslides and avalanches plunging into lakes or reservoirs located in mountainous regions can generate large waves which can result in loss of life and significant property damage. As is the case with tsunamis generated in the ocean by underwater seismic activity, landslide induced water waves result from the motion of a solid boundary of the fluid. The present study deals with the mathematical modelling of water waves developed in a channel by a moving bed. A set of depth-averaged governing equations is derived to predict the evolution of the free surface resulting from a predetermined bed motion. These equations, which constitute a generalization of the Boussinesq system for waves over a flat bed, include both nonlinear and dispersive effects. Numerical solutions are obtained by using the finite difference method coupled with a Flux Corrected Transport (FCT) algorithm. The resulting model is used to predict the waves resulting from simple bed motions.

Physical and biological observations of coastal squirts under nonupwelling conditions

Abstract: Offshore-directed plumes of inshore water, marked by a shallow mixed layer, low salinity, and low temperature during winter, were observed south of the Hokitika Canyon on the South Island west coast, New Zealand. Hydrographic and satellite observations are presented which show that plume time scales were of the order of a few days, and across-shelf length scales were approximately 50 km. The low-salinity plumes were found to be associated with the southern edge of an offshore-directed jet that followed the Hokitika Canyon bathymetry. Decay of the plumes was rapid and appears to be associated with mixing of plume water into offshore surface waters. The occurrence of the plumes is nonseasonal, and they do not appear to be associated with upwelling. Observations suggest that strong southward flow around the head of the Hokitika Canyon, perhaps as a consequence of coastal-trapped wave activity, is a possible mechanism for plume formation. Biologically, the plumes were important because they stabilized the water column and prevented the mixing of phytoplankton below the photic zone. Although the plumes were found to transport productive nearshore waters offshore, the major impact of the plumes was to lower total water column chlorophyll a in outer shelf regions. The relationship between the Hokitika plumes and the filaments and squirts identified off the Californian coast is discussed, and it is concluded that the plume dynamics resemble those of the squirts found inshore of the seasonal filaments off California. The results of this study indicate that squirts not only influence biological variability in shelf waters but are important mechanisms for the across-shelf exchange of coastal and oceanic waters.

Performance of a Horizontally Scanning Doppler Sonar Near Shore

Abstract: In August 1990, tests were performed to investigate the usefulness of a horizontally scanning Doppler acoustic technique in shallow water. Comparisons of radial velocity estimates from a vertical fan beam versus a horizontally aimed pencil beam indicate no degradation attributable to multiple reflections from the surface and bottom. Further tests, in which ping-to-ping phase-coherent means are examined, indicate negligible stationary backscatter from the bottom. Tests in which the acoustic beams were directed shoreward indicate that an extremely dense bubble cloud formed by plunging breakers produces an impenetrable “wall” at the breakpoint, at acoustic frequencies near 195 kHz. Useful velocity estimates (one component) are obtainable everywhere seaward of the breakpoint of the incoming surf. The spatially extensive velocity estimates offered by this technique provide enormous potential for the study of horizontal currents and wave-current interactions in shallow water.

Observations of currents and density structure across a buoyant plume front

Abstract: Observations of the Mobile Bay, Alabama, plume during a flood event in April 1991 reveal significant differences in the current field on either side of a front associated with the buoyant plume. During a strong southeasterly wind, turbid, low salinity water from Mobile Bay was pushed through an opening in the west side of the ebb-tidal delta and moved parallel to the coast. A stable front developed between the low salinity water of the buoyant plume (11‰) and the high salinity coastal water (>23‰) that was being forced landward by the prevailing winds. Despite the shallow water depth of 6 m, measurements of currents, temperature, and salinity show large shears and density gradients in both the vertical and the horizontal directions. At a station outside of the buoyant plume, currents at 0.5 m and 1.5 m below the surface were in the same direction as the wind. Inside the plume, however, currents at 0.5 m below the surface were parallel to the coast, 45°, off the direction of the wind and the magnitude was 45% larger than the magnitude of the surface currents outside the plume. Beneath the level of the plume, the currents were identical to the wind-driven currents in the ambient water south of the front. Our observations suggest that the wind-driven surface currents of the ambient water converged with the buoyant plume at the front and were subducted beneath the plume. The motion of the ambient coastal surface water was in the direction of the local wind stress, however, the motion of the plume had no northerly component of motion. The plume also did not show any flow toward the front, suggesting a balance between the northerly component of wind stress and the southerly component of buoyant spreading. In addition, the motion of the plume did not appear to affect the motion of the underlying ambient water, suggesting a lack of mixing between the two waters.

Interaction of surface waves with vorticity in shallow water.

Abstract: Vortical flows in shallow water interact with long surface waves by virtue of the nonlinear terms of the fluid equations. Analytical formulas are derived that quantify the spontaneous generation of such waves by unsteady vorticity as well as the scattering of surface waves by vorticity. In a first Born approximation the radiated surface elevation is linearly related to the Fourier transform of the vorticity. The «dislocated» wave fronts that are analogous to the Aharonov-Bohm erect are obtained as a special case

Round buoyant jet entering shallow water

Abstract: This paper focuses on the mixing processes that develop when a jet outfall enters a shallow body of water. With the main purpose of enhancing predictive one-dimensional integral models, a number of non-buoyant and buoyant round jet situations were experimentally investigated with laser Doppler velocimetry and a thermistor system being used to measure velocity and temperature patterns, respectively. Results are presented which show that the dilutions of both non-buoyant and buoyant jets are significantly reduced when the receiving water is shallow. Other mixing features associated with buoyant jets entering shallow water are discussed including; the application of scaling relationships in collapsing integral parameters, possible mathematical relationships which may be used in describing these parameters, the role played by the Coanda effect in influencing the path of the jet and reducing dilution, and, the significance of the differences between the velocity and temperature patterns.

Wave Set-up and Wave-generated Currents on Coral Reefs

Abstract: Coral reefs are a unique environment and wave action is one of the factors determining that environment. Reef-tops exposed at low tide are essentially horizontal platforms and waves propagating over such reefs at high tide are usually shallow water ones. Both set-up and wave-induced flow are generated by waves breaking at or near the reef-edge. Laboratory experiments using both two and three dimensional reef models show that the set-up and flow both increase with increasing off reef wave height. The depth over the reef is also important with set-up being greatest at small depths and flow greatest at larger depths. At small submergence both set-up and flow are controlled by reef topography.