Showing papers on "Waves and shallow water published in 1991"

Mean circulation and variability on the eastern Canadian continental shelf

Abstract: The circulation and variability on eastern Canadian continental shelves is reviewed with emphasis on horizontal and vertical structure in the water column. Mean and seasonal currents, sheared by stratification and friction, generally follow bathymetry and are driven primarily by surface wind stress and buoyancy fluxes. In certain near-resonant embayments, nonlinear interactions of the tide also make significant contributions to the mean and low-frequency currents. Direct forcing of low-frequency shelf circulation by offshore currents and pressure fields is of lesser importance according to recent observations and models. At higher frequencies, the wintertime response to atmospheric forcing occurs in three important frequency bands: surface wave (periods of 2–20 s), inertial (17–22 h) and synoptic or subtidal (2–10 days). Most of the subtidal energy in the subsurface pressure (SSP) field is attributable to direct local forcing by alongshore wind and to remote forcing by coastal-trapped waves (CTW). The subtidal current variability is less coherent with these sources primarily because of small scale circulations created by vortex stretching, topographic steering and scattering by the rugged shelf bathymetry. Bottom friction and scattering control the ratio of locally and remotely forced subtidal energy as a function of position on the shelf. Intermittent inertial currents in stratified shelf waters, often associated with mesoscale atmospheric forcing, exhibit a 180° phase change across the pycnocline and are inhibited near the coast. Surface waves generated by storm winds are strongly dissipated in shallow water, where they enhance bottom stresses that: (1) balance the surface wind stress; and (2) may contribute to the damping of inertial waves. Specific examples of these phenomena are drawn from observations on the Scotian Shelf and in the Gulf of Maine.

Entrainment and mixing in plane turbulent jets in shallow water

Abstract: Plane jets in shallow water are often encountered in geophysical flows and engineering applications. The present investigation shows that depending on the distance from the origin scaled by depth, the jets can be divided into three fields with distinct behaviour. A model is then presented allowing discussion of the conditions related to the observed increase or decrease of momentum flux in the vicinity of the orifice. The influence of skin-friction on spreading and entrainment is studied on the basis of existing experimental evidence including the one in the present work.

Within-lake patterns in depth penetration of emergent vegetation

Abstract: SUMMARY. 1. Within-lake relations of wave exposure (WE), and substratum softness (cone penetration depth; CPD) and organic content (loss on ignition; LOI), to water depth penetration of the emergent vegetation (DPE) was investigated in seven eutrophic lakes in southern Sweden, ranging in area from 1 to 46km2. 2. There was a positive relationship between WE and DPE within lakes. This relationship, however, only occurred for sites with relatively soft substrata, for which CPD and LOI were negatively related to both WE and DPE. 3. Analysis of aerial photographs revealed that expansion of the emergent vegetation towards open water, or recession from open water, was not related to wave exposure or water depth, except in one lake where expansion mainly occurred at high exposures. 4. For relatively static vegetation on soft substrata, regressions with CPD°.5 explained 62–88% of the within-lake variation of DPE. These regressions did not differ among lakes. Expanding and recessing vegetation were significantly dislocated towards shallower and deeper water, respectively, than predicted from the regression models for static vegetation. 5. Phragmites australis dominated at the lakeward edge of the emergent vegetation, followed in frequency by Typha angustifolia. More broad-leaved species were generally restricted to shallow water and very soft substrata. 6. The results suggest that changes in the distribution of emergent vegetation in moderately wave exposed eutrophic lakes can be predicted largely from substratum character and water depth.

Characteristics of Waves off the Mediterranean Coast of Egypt

Abstract: Analyses of all available wave records at Abu Quir, Egypt, collected between 1971 and 1987 are presented. Data for a 17 month period were recorded by the Offshore Pressure Operated Suspended wave recorder (OSPOS), and about 20 months of data were collected with the Cassette Acquisition System directional wave recorder (CAS). Severe wave conditions which might occur over a long period of time are given. It was found from this study, that the maximum wave height reaches about 4.00 m in winter, 3.50 m in spring, and 2.50 m in summer. The corresponding wave periods in these seasons are 14 sec. for winter, and 13 sec. for the other two seasons. The prevailing wave direction is from the WNW-NW sector, while a small amount of waves arrive from the NNE-NE sector. The average significant wave height is 0.50-1.00 m, and the average wave period is 7-8 sec. Percentage of occurrence of waves coming from different directions is also presented, which will be useful in the estimation of the net sediment transport at Abu Quir.

Soliton basis states in shallow-water ocean surface waves.

Abstract: The inverse scattering transform for the periodic Korteweg--de Vries equation is used to analyze surface-wave data obtained in the Adriatic Sea and a robust solition spectrum is found. While the solitons are not observable in the data due to the presence of energetic radiation modes, a new nonlinear filtering technique renders the solitons visible. Numerical simulations support the existence of solitons in the measurements and suggest that, as a wave train propagates into shallow water, the solitons grow at the expense of the radiation.

Violent water motion at breaking-wave impact

Abstract: A numerical study of water waves breaking against a vertical wall is described. The exact equations for free-surface potential flow are solved. Solutions for a simple overturning wave meeting a vertical wall placed at several positions give unexpectedly violent water motion. The face of the incident wave can converge towards a point on the wall causing very high pressure, acceleration and velocity. It seems likely that this particular type of flow corresponds to the most extreme conditions encountered in laboratories and on coasts despite the lack of any direct water impact. Details of the flow reveal that compressibility of the water is unlikely to be relevant for wave forces, but that scaling from laboratory to prototype should allow for wall roughness and for small waves riding on the incident wave.

Acoustic Variability Due to Internal Waves and Surface Waves in Shallow Water

Abstract: During a recent shallow water experiment, acoustic propagation of a continuous 400 Hz signal was measured over a 18.5-km range, from a fixed source to a broadside array of hydrophones. Transmission loss varied significantly over a wide range of time scales during certain periods, and was relatively uniform during other periods. During intermittent intervals, acoustic features appeared to propagate across the array of hydrophones, at a speed of about 0.75–1.0 m/sec. This speed is similar to the propagation speed of internal soliton waves, as predicted by internal wave theory.

Modelling shoaling directional wave spectra in shallow water

Abstract: This paper describes the development of a model for the shoaling and refraction of an incident directional spectrum over beach topography. The present model is limited to topography which varies only in the on- offshore direction, but no restriction is made on allowed angles of incidence with respect to the shore-normal direction. The model is verified in comparison to laboratory data for Mach reflection of cnoidal waves from a vertical plane wall. It is shown that the model provides a more accurate representation of the evolving wave field than does an earlier parabolic approximation, using the same laboratory data.

Nonlinear free surface waves due to a ship moving near the critical speed in a shallow water

Abstract: This paper describes two methods of solution to the nonlinear free-surface waves generated by a ship moving steadily with the trancritical speed in a shallow water. As a mathematical model, a nonlinear initial/boundary-value problem is formulated within the scope of potential theory. One method is based on matched asymptotic expansion techniques and the Kadomtsev - Petviashvili equation is obtained as the leading-order solution for a slender ship. The other one is based on classical Hamilton's principle and the finite element method is implemented for numerical calculations. In order to examine the effect of the tank width on the wave field and resulting hydrodynamic for the Series 60 ship model with Cb = 0.8 by these two different methods. For wider tanks, the pressure distribution on the free surface, equivalent to the ship model, is treated. The results obtained by two different methods are compared each other and with experimental measurements available. Also discussed are the appearance of stem waves at the tank wall and the evolution of the crestline of diverging waves in the wide tank.

Refraction of Sound by Islands and Seamounts

Abstract: We consider the propagation into shallow water of low-frequency and low-order acoustic modes trapped in the sound channel. The phase velocity slightly decreases and then increases with decreasing depth. This leads to an unusual pattern of wave refraction. Waves are “attracted” by islands and seamounts (rays turn toward shore) in “almost deep” water, and then are strongly “repelled” in shallow water. We examine the relative intensity of scattered arrivals in very long-range ocean transmissions. Forward scatter by islands and seamounts can give significant (order −10 dB) scattered arrivals, as can large-angle scatter (including backscatter) by large islands and by islands near the source or receiver. However, transmission of acoustic energy into the sea floor leads to loss of the scattered energy in the sound channel. Total reflection (no loss) is favored by glancing incidence on steep slopes of islands and seamounts with large compressional seismic velocities. We suggest a complementary relation between the intensities from an underwater explosion as recorded on axial hydrophones versus nearby land seismometers, with conditions favorable to reflection favoring hydrophones and vice versa. A preliminary attempt is made to compare measured and computed scattered intensities in the sound channel for operation WIGWAM (a deep-water nuclear explosion in 1955), operation CHASE, and the Perth to Bermuda transmission of 1960. Computed intensities are too low.

Nutrient and plankton distribution near a shelf break front in the region of the Bass Strait cascade

Abstract: Nutrient enrichment of surface water to the east of a shelf break front at the eastern end of Bass Strait occurs in winter. This area of enrichment is more than 100 nautical miles (E-W) by 150 nm (N-S). From east of Banks Strait (40o 45'S,148oE), some of the nutrient-rich water is carried northwards with the northward flow of Bass Strait water which later forms the well-known 'cascade' below the warmer waters of the Tasman Sea. In September 1984, the chlorophyll a concentration increased along the line of this northward flow, producing a maximum off the Victorian coast near where the cascade occurs. In contrast to nutrient and chlorophyll a distributions, zooplankton biomass (dry weight) was higher in the shallow water of Bass Strait than over the continental slope. This suggests that the plankton growth observed in shallow Bass Strait waters in late winter had ceased by September, but was continuing to the north-east and over the slope in waters with a shallow mixed depth. We propose that the northward flow of water along the shelf break maintains plankton in a nutrient-rich environment, so that they continue to grow until they are carried below the photic zone by the cascade.

Determination of geoacoustic parameters of the ocean bottom : data requirements

Abstract: An important problem in ocean acoustics is the determination of the acoustic parameters of the ocean bottom sediment layers. A variety of inverse methods has been proposed in the literature for obtaining these quantities from measurements of the acoustic field in the water column using either narrow‐band sources. The ability of some of the perturbative methods to yield accurate estimates of the unknown parameters is investigated. For shallow water experiments, it is shown that a full wave method that uses the complex pressure field as data is nonlinear, the nonlinearity increasing with frequency and waveguide thickness. Methods that use modal eigenvalues as input data are only weakly nonlinear and can successfully yield estimates with acceptable resolution if the experiment is performed over a number of frequencies. In the case of deep water experiments, however, the experimental configuration can be so arranged as to make methods based on full wave inversion only weakly nonlinear.

Long wave interaction with steeply sloping structures

Abstract: A fully nonlinear model for free surface potential flows is used to analyze the transformation of solitary waves above a mild slope, from intermediate to shallow water (shoaling, overturning), and to study the interaction of these waves with coastal structures located in the shallow area. Computations include wave runup, overturning and reflection form steep slopes or vertical wall, and from a combination of a slope and a submerged breakwater. Results are compared with other numerical, analytical and experimental results. Effects of the submerged breakwater, of making horizontal velocities more uniform over depth and of reducing wave runup on the slope, are further detailed.

Forces on vertical walls due to obliquely-incident waves

Abstract: The force exerted on a vertical wall due to obliquely-incident waves has been analyzed in this paper from the calculation of short-crested waves system by a numerical scheme. The numerical model preserves the wave elevation in an implicit function form for the dynamic and kinematic boundary equations on water surface, the accuracy is then improved. It is found that the maximum loading in onshore direction may be caused by obliquely-incident waves as the water is greater than intermediate depth. However, the onshore loading does not necessarily be the greatest force for such instance. The greatest force could be produced by the offshore loading under steeper normal standing waves. While the wall is in the shallow water region, the greatest force is occurred in onshore loading at crest by normal standing waves. This research also shows good comparisons from some experimental results.

A Summary of DREA Observations of Interface Waves at the Seabed

Abstract: During the past ten years or so, DREA scientists have observed interface waves in shallow water in the frequency range 2 Hz - 20 Hz over a variety of seabed types: sedimentary rock (chalk), unconsolidated sediments (sand and gravel), and crystalline bedrock (granite). In this paper we summarize our interface wave experiments and discuss how we used the observations to improve our understanding of low frequency acoustic propagation in shallow water. We have found that (a) the analysis of interface wave propagation is a good means of estimating seabed geo-acoustic parameters, including shear speed; and (b) over crystalline bedrock seabeds, interface waves in the frequency band 3 Hz - 6 Hz can be the dominant mode of propagation to ranges of 40 km or more.

Extension of the Boussinesq Equations to Include Wave Propagation in Deeper Water and Wave-Ship Interaction in Shallow Water

Abstract: Mathematical short wave models based on the Boussinesq equations have been shown to be capable of reproducing the combined effects of most of the wave phenomena of interest to the coastal engineer for a relatively low cost. Today, the following phenomena can be taken into account in the most advanced numerical wave models: o Shoaling, refraction, diffraction o Partial reflection from breakwaters o Irregular wave trains o Directional wave input o Wave-wave and wave-current interaction. In this presentation the standard Boussinesq equations will be extended for two different purposes: o To simulate irregular wave propagation from deep to shallow water o To simulate wave-ship interaction in shallow water.

Spatial variation of wave group statistics and representative wave-heights of swell

Abstract: This study derives a practical equation that can describe the weakly nonlinear waves having an arbitrary spectral form and propagating unidirectionally from deep to shallow water, and carries out the numerical simulation using the equation. Based on the simulated results, some investigations are made on the characteristics of spatial variation of wave group and representative wave-heights in the propagation process.

The effect of shear wave attenuation on acoustic bottom loss resonance in marine sediments

Abstract: Acoustic propagation experiments in shallow water over thinly-sedimented bedrock sometimes show high propagation loss, with resonant-like structure, in the 10 to 100 Hz band. This can be interpreted with the aid of plane-wave reflection models that associate high bottom reflection loss at certain combinations of frequency and grazing angle with shear-wave resonances in the sediment layer. In this paper, the seabed is modelled as a thin layer of elastic-solid sediment overlying a hard rock substrate, with plane interfaces at layer boundaries. The bottom reflection loss resonances are explained by both full wave-theoretical reflection loss calculations and a simplified coherent ray reflection loss model. For the ray model, two simplifying assumptions are applied: (1) there is no conversion of energy between compressional waves and shear waves at the water-sediment boundary, and (2) sediment shear waves are attenuated so strongly that they traverse the sediment layer once up and down only. The paper investigates the effect of sediment shear-wave attenuation on bottom reflection loss. We find that the resonance frequencies are controlled by the shear speed and sediment layer thickness, and that the amount of bottom reflection loss is sensitive to the shear-wave attenuation.

Change in Molluscan Assemblages in the Holocene Accompanied by the Formation of Futtu Spit, Boso Peninsula

Abstract: Five marine molluscan assemblages are identified in the present shallow sea bottom around the Futtu Spit on the west coast of the Boso Peninsula and in the Holocene marine sediments below the spit.(1) A tidal flat assemblage characterized by Crassostrea gigas, Batillaria multiformis and Cyclina sinensis is present in the sediments on Pleistocene bedrock. (2) An inner-bay mud-bottom assemblage composed of Raetellops pulchellus, Macoma tokyoensis and Dosinella penicilata is found in the mud sea bottom deeper than 12m around the spit and in the muddy sediments below the spit from 20m to 30m in depth, which filled the depression of the basement. (3) An inner-bay sand-bottom assemblage dominated by Musculus senhousia and Tapes philippinarum is distributed in the sand sea bottom from 0 to 12m in depth to the north of the spit. M. senhousia often forms mats and covers the sea bottom. (4) An inland-sea sand-bottom assemblage consists of Mactra chinensis, Phacosoma japonicum and other outer sea molluscans. It is present in the sand bottom with a depth shallower than 15m and in the sand sediments which form the spit. (5) An inland-sea sand-and-mud bottom assemblage composed of fragments of Kellia porculus and Tonna luteostoma is found only in the westernmost core below 10m.These molluscan assemblages change with change in the sedimentary facies. From 10, 000y. B. P. to 5, 000y. B. P., the sea level rose rapidly, and the Pleistocene bedrock was submerged. The tidal flat assemblage occurred on tidal flats overlain by mud, and the inner-bay mud-bottom assemblage was present in the mud sediments. From 5, 000y. B. P. to the present, the sea level has dropped slightly and the spit has prograded westward. The inland-sea sand-bottom assemblage was present in the sand sediments which form the spit. This assemblage is also found in the shallow water to the south of the spit. After the spit was formed, the inner-bay sand-bottom assemblage appeared in the shallow water to the north of the spit.

Finite element modeling of shallow water wave equations

Abstract: The purpose of this paper is to introduce the major potentials and perspectives of applications of finite element analysis in solving the problems of shallow water wave equations. One‐dimensional and two‐dimensional shallow water wave equations will both be incorporated into the modeling procedures. For one‐dimensional flows, the models will cover the typical single channels, confluence channels system, division channels system, and natural river systems. As far as two‐dimensional flows are concerned, the overland flows are investigated. The simulation results are compared with the data obtained by physical modeling and field observation and with the results of other existing literature. The models were found to be very feasible in modeling the complex flow fields of shallow water wave equation problems.

Propagation of Sound in Shallow Water

Abstract: In shallow-water propagation any ray (except a direct one in the case of a homogeneous medium) undergoes one or more reflections from the bottom. At relatively long ranges the sound field is mainly due to the multiple reflections of the rays from the bottom. The theory of sound propagation in shallow water will be presented below, assuming constant water depth.