Showing papers on "Waves and shallow water published in 1997"

The "swan" wave model for shallow water

Abstract: The numerical model SWAN (Simulating WAves Nearshore) for the computation of wave conditions in shallow water with ambient currents is briefly described. The model is based on a fully spectral representation of the action balance equation with all physical processes modelled explicitly. No a priori limitations are imposed on the spectral evolution. This makes the model a third-generation model. In Holthuijsen et al. (1993) and Ris et al. (1994) test cases for propagation, generation and dissipation have been shown without currents. Current effects have now been added and academic cases are shown here. The model is also applied in a fairly academic case of a shallow lake (Lake George, Australia) and in a complex, realistic case of an inter-tidal area with currents (Friesche Zeegat, the Netherlands). The results are compared with observations. A new development to formulate the model on a curvi-linear grid to accommodate linkage to hydro-dynamic circulation models is presented and a first test is shown.

A Nonlinear Model of Internal Tide Transformation on the Australian North West Shelf

Abstract: A numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide. The model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf. The model is run using observed hydrographic conditions from the Australian North West Shelf and results are compared to current meter and thermistor observations from the shelf-break region. It is found from observations that the coefficient of nonlinearity in the K-dV equation changes sign from negative in deep water to positive in shallow water, and this plays a major role in determining the form of the internal tide transformation. On the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide, which distorts the density profile over a wave period. Both the model and observations show the formation of an initial shock on the leading face of the internal tide. In shallow water, the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves. After further propagation a second shock forms on the back face of the wave, followed by a packet of solitons. The inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion. Friction is found to be important in limiting the amplitudes of the evolving waves.

Tidally driven residual circulation in shallow estuaries with lateral depth variation

Abstract: Tidally driven residual circulation in shallow estuaries with lateral depth variation has been studied analytically using a two-dimensional, depth-averaged model. The solution is presented for a v-shaped channel. Exchange flow is found to be correlated with the topography. The magnitude of this exchange flow depends mainly on four parameters: the ratio between the minimum depth on the shoal and the maximum depth in the channel, the ratio between the tidal amplitude at the mouth and the mean depth, the ratio between the length of the estuary and the tidal wave length, and the ratio between the tidal timescale and the decay timescale due to friction. Generally, a net landward flow occurs over the shoals and is balanced by a return flow in the channel. The along-channel residual velocity changes nearly linearly across the estuary. The residual velocity decreases monotonically toward the head of the estuary. The transverse residual velocity is convergent in the channel and divergent on the shoals. The residual velocity is highly dependent on the length of the estuary. When the length of the estuary is much smaller than a quarter of the wave length, the residual velocity is relatively small. When the length of the estuary is about a quarter of the wave length, the magnitude of the residual velocity reaches its maximum. When the length of the estuary is much larger than a quarter of the wave length, the residual circulation approaches the pattern found in an infinite length estuary. On the basis of the analysis of the solution, we conclude that exchange flow generated by tides in a shallow estuary is the result of competition among several processes. An inward flux is caused by local nonlinearity, both in the bottom friction and from propagation of the tidal wave of finite amplitude. This inward flux is larger on the shoals and smaller in the channel owing to larger nonlinearity in the shallow water. The residual inward flow creates a setup (residual surface elevation) of water at the head of the estuary that produces a pressure gradient. This residual pressure gradient is approximately uniform across the estuary and drives an outward flow that is larger in the channel than on the shoals. In all of the parameter space the competition results in a net inflow on the shoals and a net outflow in the channel.

Hydrologic issues in arid, unsaturated systems and implications for contaminant transport

Abstract: Analysis of unsaturated flow and transport in arid regions is important, not only in water resource evaluation but in contaminant transport as well, partic- ularly in siting waste disposal facilities and in remediat- ing contaminated sites. The water fluxes under consid- eration have a magnitude close to the errors inherent in measuring or in calculating these water fluxes, which makes it difficult to resolve basic issues such as direction and rate of water movement and controls on unsaturated flow. The purpose of this paper is to review these issues on the basis of unsaturated zone studies in arid settings. Because individual techniques for estimating water fluxes in the unsaturated zone have limitations, a variety of physical measurements and environmental tracers should be used to provide multiple, independent lines of evidence to quantify flow and transport in arid regions. The direction and rate of water flow are affected not only by hydraulic head gradients but also by temperature and air pressure gradients. The similarity of water fluxes in a variety of settings in the southwestern United States indicates that vegetative cover may be one of the pri- mary controls on the magnitude of water flow in the unsaturated zone; however, our understanding of the role of plants is limited. Most unsaturated flow in arid systems is focused beneath topographic depressions, and diffuse flow is limited. Thick unsaturated sections and low water fluxes typical of many arid regions result in preservation of paleoclimatic variations in water flux and suggest that deep vadose zones may be out of equilib- rium with current climate. Whereas water movement along preferred pathways is common in humid sites, field studies that demonstrate preferential flow are restricted mostly to fractured rocks and root zones in arid regions. Results of field studies of preferential flow in humid sites, generally restricted to the upper 1-2 m because of shallow water tables, cannot be applied readily to thick vadose zones in arid regions.

Acoustic propagation through an internal wave field in a shallow water waveguide

Abstract: This paper addresses the problem of predicting and interpreting acoustic wave field properties in a stochastic ocean waveguide, for which the sound-speed variability within the water column is treated explicitly as a random process. It is assumed that the sound-speed distribution is composed of three components: a deterministic, time-independent profile and two stochastic components induced by internal wave activity. One random contribution represents a spatially diffuse Garrett–Munk field whose spectrum is constrained by the shallow water waveguide, while the second corresponds to spatially localized soliton packets. A high-angle elastic parabolic equation method is applied to compute single frequency realizations of the pressure field using this three-component representation of the sound-speed distribution. Ensemble-averaged transmission loss and scintillation index measures for the full pressure field and its modal components are estimated for different source depths and for both flat and sloping bott...

Why ‘very shallow’ lakes are more successful opposing reduced nutrient loads

Abstract: There are different approaches for classifying deep and shallow waters using physically and ecologically derived parameters. Nevertheless, transition states make it difficult to define border crossing points between the two types of limnetic ecosystems and to distinguish more precisely between different types of shallow, especially highly eutrophicated lakes. We contribute a detailed analysis of different characteristics of shallow waters from lakes in the Berlin/Brandenburg-region. In the catchment area of the river Dahme in Eastern Brandenburg (Scharmutzelseeregion) we find mainly shallow and highly eutrophicated lakes, dominated by Cyanobacteria. ‘Very shallow’ lakes of different morphometry and topography are compared with ‘medium shallow’ or deeper lakes in the region with similar loading characteristics for the following properties: morphometry, topography, theoretical retention time, mixing intensity, nutrient dynamics, external and internal loading, underwater light climate, zeu/Zmix, phytoplankton development and oxygen budget. We found that ‘very shallow’ lakes in the region are more efficient in converting the available phosphorus into phytoplankton biomass because of the constant and sufficient underwater light climate due to the favourable relation of zeu and Zmix We conclude that the regular mixing regime guarantees a stable and near optimum light/dark rhythm as well as higher heterotrophic activities, stimulating primary production up to the upper limit of algal development.

Coastal Tidally-driven Circulation and the Role of Water Exchange in the Linkage Between Tropical Coastal Ecosystems

Abstract: Water circulation and exchange processes in a shallow, semi-enclosed tropical bay were studied in southern Kenya (Gazi Bay) through measurements of tidal elevations, salinity, temperature, dissolved oxygen and current velocities at stations established in mangrove creeks, seagrass beds and coral reef zones. Occurrence of wide shallow entrance, lack of topographic controls (sills) and the orientation of the Bay entrance with respect to dominant tidal water circulation patterns, accounts for the high rates of exchange (60–90% of the volume per tidal cycle) between the inshore and offshore waters. High flushing rates are coupled with short residence times in the order of 3–4 h. The dominant water circulation driving force is the semi-diurnal tide, causing a strong reversing current in the mangrove creeks (0·6 ms−1) and low magnitude current in the seagrass and coral reef zones (<0·30 ms−1). Tidal asymmetry, characterized by stronger ebb flows than flood flows in the mangrove creeks, partly promotes the net export of organic matter to the seagrass beds. The brackish and turbid water plume in the mangrove creeks and south-western region of the Bay is trapped along the coast and in the mangrove swamp, and does not reach the coral reef. The freshwater influx via rivers and direct rainfall in the Bay accounts for a volume of 305 000 m3, of which 20% is lost as a result of enhanced evapotranspiration, which is also responsible for a salinity maximum zone (38) in the upper region of the Bay covered by mangroves.

High-frequency acoustic volume backscattering in the Georges Bank coastal region and its interpretation using scattering models

Abstract: High-frequency (120 and 420 kHz) sound was used to survey sound scatterers in the water over Georges Bank. In addition to the biological sound scatterers (the plankton and micronekton), scattering associated with internal waves and suspended sediment was observed. Volume backscattering was more homogeneous in the vertical dimension (with occasional patches) in the shallow central portion of the Bank where there is significant mixing. In the deeper outer portion of the Bank where the water is stratified, volume backscattering was layered and internal waves modulated the vertical position of the layers in the pycnocline. The internal waves typically had amplitudes of 5-20 m, but sometimes much higher. Species composition and size data from samples of the animals and suspended sediment used in conjunction with acoustic scattering models revealed that throughout the region the animals generally dominate the scattering, but there are times and places where sand particles (suspended as high as up to the sea surface) can dominate. The source of the scattering in the internal waves is probably due to a combination of both animals and sound-speed microstructure. Determination of their relative contributions requires further study.

On the crest instabilities of steep surface waves

Abstract: The forms of the superharmonic instabilities of irrotational surface waves on deep water are calculated for wave steepnesses up to 99.9% of the limiting value. It is found that as the limiting wave steepness is approached the rates of growth of the lowest two unstable modes (n=1 and 2) increase according to the asymptotic law suggested by the theory of the almost-highest wave (Longuet-Higgins & Cleaver 1994; Longuet-Higgins, Cleaver & Fox 1994; Longuet-Higgins & Dommermuth 1997). Moreover, each eigenfunction becomes concentrated near the wave crest, with a horizontal scale proportional to the local radius of curvature at the crest. These are therefore ‘crest instabilities’ in the original sense.Similar calculations are carried out for the normal-mode instabilities of solitary waves in shallow water, at steepnesses up to 99.99% of the limiting steepness. Similar conclusions are found to apply, though with greater accuracy.

Shallow-water flow around model conical islands of small side slope Part II Submerged

Abstract: Experiments have been conducted to study the unsteady wakes of submerged conical islands The islands used in the tests have side slopes ranging from 80 to 331° Experiments in a shallow-water channel with a steady, subcritical free stream showed vortex shedding to occur in the wake when the water depth above the island apex was relatively small Flow separation from positions near the island apex was found to be important in producing this unsteady wake As the water depth was increased the shedding was observed to become less vigorous and eventually stop All islands tested produced similar results with the angle of the island side slope exerting relatively little influence on the process The results of wind tunnel visualization studies, which used a rigid top plate to produce the effect of fluid depth, support the results from the water channel Pictures of the surface flow patterns produced on the islands by the wind action are presented Two-dimensional (2D) and three-dimensional (3D) shallow-wate

Adriatic seiche decay and energy loss to the Mediterranean

Abstract: A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean. The free decay time of the dominant (lowest mode) seiche was determined from envelopes of handpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of decay time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2±0.5 d. This value is consonant with the observed width of the spectral peak. The decay caused by both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient k appearing in the linearized bottom stress term ρ0u (where u is the along-basin velocity and ρ0 the fluid density). The coefficient k is constrained by values obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven currents, with tides and with wind waves. Radiation is parameterized by the coefficient f appearing in the open strait boundary condition ζ =auh/c (where ζ is sea level, h is depth and c is phase speed). This parameterization of radiation provides results comparable to allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated runs of the model delineate the dependence of model free seiche decay time on k and a, and these plus the estimates of k allow estimation of a. The principle conclusions of this work are as follows. 1. (1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence. 2. (2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced by Croatian coastal orography. 3. (3) The free decay time of the 21.2 h Adriatic seiche is 3.2±0.5 d. 4. (4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godin's (1988) approximation to the quadratic bottom friction law ρ0CDu|u| using the commonly accepted drag coefficient CD = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with CD=0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least CD=0. In this case, bottom friction with no radiation leads to a damping time of 4.73 d, radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period.

Modeling tsunamis from earthquake sources near Gorringe Bank southwest of Portugal

Abstract: The Azores-Gibraltar fracture zone with the huge bathymetric reliefs in the area southwest of Portugal is believed to have been the source of large historic tsunami events. This report describes simulations of tsunami generation and propagation from sources near the Gorringe Bank. The well-documented 1969 tsunami event is examined both with a ray-tracing technique and with finite difference models based on various shallow water equations. Both methods show that the most likely source location is southeast of the Gorringe Bank near the epicenter location determined from seismic data. The tsunami source is calculated by formulas given by Okada [1985] for surface deformation of an elastic half-space caused by faulting. Observed wave amplitude and travel time and values computed from an initial wave field according to Okada [1985] formulas show acceptable agreement for most stations along the coast of Portugal and Spain. However, in order to explain a large primary wave with downward displacement observed on the coast of Morocco, an alternative source model with a larger area of downward displacement has been introduced. This also leads to a better overall fit with observed travel time. Implications for disastrous events, as the one in 1755, are also discussed. Linear hydrostatic shallow water models are used for most of the simulations, but the importance of nonlinearity and dispersion is examined with the Boussinesq equations. The sensitivity of the solution to changes in the location and the strength of the source is discussed, and a series of grid refinement studies are performed in order to assess the accuracy of the simulations.

Potential for deep sea invasion by Mediterranean shallow water echinoids: pressure and temperature as stage-specific dispersal barriers

Abstract: Hypotheses about the origin of the deep sea fauna often assume that the deep sea was first colonized by cold water animals migrating through isothermal water columns in polar seas. Deep water in the Mediterranean Sea has much warmer temperatures than comparable depths in the larger ocean baslns. Moreover, the entire water column may be virtually isothermal during the winter months, making oceanographic conditions in the Mediterranean analogous to those prevailing throughout most of the world ocean during the Mesozoic and Cenozoic. We investigated the physiological potential for deep sea invasion through a warm water column by studying the pressure and temperature tolerances of embryos and larvae of 3 species of shallow water Mediterranean echinoids, Paracentrotus lividus, Arbacia lixula, and Sphaerechinus granularis. Early life history stages of all 3 species tolerated pressures (to 150 atm) much higher than those experienced in the adult environment. Cold temperatures (<1O0C) exacerbated the adverse effects of pressure; larvae were more likely to survive at deep sea pressures and warm temperatures than at shallow water pressures and cold temperatures. Tolerances to high pressures and low temperatures increased with ontogeny and varied with species. In the Mediterranean, high pressures should be a more important limiting factor than low temperatures. Nevertheless, some species have physiological tolerances that should allow them to colonize bathyal depths. Absence of these shallow water species from such depths must be attributed to factors other than pressure and temperature. K E Y WORDS: Mediterranean . Echinoids Pressure . Barophysiology

Three-Dimensional Adjustment of Stratified Flow Over a Sloping Bottom

Abstract: : This study focused on understanding how advection of density within the bottom boundary layer influence the three-dimensional structure, evolution, and dynamics of both the bottom boundary layer and the overlying (interior) flow. Simple theories were developed and then tested and extended using both a numerical model and analysis of oceanic observations. Results indicate the simple theories are relevant to the ocean and that the bottom boundary layer plays an important role in the behavior of ocean currents, even when the boundary layer is thin compared to the current depth.

Generation of edge waves in shallow water

Abstract: 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.

Electromagnetic fields generated by a three dimensional global ocean circulation

Abstract: A simplified form of the motional induction equation is used to calculate the dominant three-dimensional (3-D) electromagnetic (EM) fields generated by a specified steady 3-D global ocean circulation. The EM calculations require, at most, vertical integrations and do not require running a global 3-D model. Two cases for ocean bottom conductivity are considered: an electrically insulating ocean bottom and a high-conductance sediment layer. The approximations are discussed, and the solutions are plotted for various depth levels. Many aspects of the dominant ocean-generated EM fields (particularly the electric currents near the sea surface and the magnetic fields) are shown to be insensitive to ocean bottom conductivity. Other aspects (particularly the horizontal electric field in shallow water) are very sensitive. We perform a global integration to estimate the role of the “nonlocal” electric currents. We find that the importance in including these nonlocal currents when making EM field estimates is the same or less than that for including a model for the bottom conductance. Hence the simple EM estimates from one-dimensional integrations are not improved in globally integrated models until these models include a realistic model for bottom conductivity.

Salinity Intrusion In The Fly River Estuary, Papua New Guinea

Abstract: Intensive field and model studies were undertaken into the dynamics of the Fly River estuary, Papua New Guinea. The estuary has three dominant channels forming a shallow, fan-shaped delta, and receives a mean freshwater discharge of approximately 6,000 m 3 s -1 with little seasonal variation. The estuary is vertically well-mixed in salinity by strong tidal currents. The saline water is distributed unevenly between the channels. Model studies verified by field data suggest that this due to the dynamics of the estuary which are controlled by shallow water frictional effects that generate higher tidal harmonics, the shoaling of the tidal wave from the funnel shape of the estuary, a low value of the bottom friction coefficient resulting from the presence of fluid mud, and the along-channel water surface gradient. This gradient is in turn controlled by two dominant forcings, namely the freshwater discharge and the dominant offshore trade wind. This gradient is also modulated by the spring-neap cycle of the tidal currents which controls the low-frequency friction coefficient. The absence of strong cross-channel salinity gradients and of axial convergence zones is attributed to enhanced horizontal mixing by the lateral velocity shear due to the sinuosity of the thalweg meandering between numerous islands and shoals.

Modeling the environmental influence on the vertical directionality of ambient noise in shallow water

Abstract: The vertical directionality of acoustic ambient noise has been a subject of much interest in the past. It is a well-defined physical quantity that can be measured experimentally with a vertical array. It possesses certain deterministic features that can be modeled theoretically with environmental acoustic and source data. Ambient noise in shallow waters, including its vertical directionality, is not very well known and is also difficult to model/predict. This is because the acoustic environment varies with time and is location dependent. Thus arises the question, how does the vertical directionality of the ambient noise depend on the acoustic environments (found in typical coastal waters)? Due to the shallow water depth, it is noted that sound (noise) propagation can be significantly influenced by the bottom. The degree of bottom interaction will depend on the sound-speed profile in the water column: whether it is downward refractive or not. Bottom attenuation will in turn determine how far the sound will...

The dynamic coupling of a third-generation wave model and a 3D hydrodynamic model through boundary layers

Abstract: A third-generation wind wave model based on the energy balance equation taking into account the effects of time-varying currents and coupled dynamically with a semi-implicit three-dimensional hydrodynamic model incorporating the influences of time- and space-varying vertical eddy viscosity, bottom topography and wave-current interactions is presented in this paper. The wave model is synchronously coupled with the three-dimensional hydrodynamic model through the surface atmospheric turbulent boundary layer and the bottom boundary layer. The theory of Janssen (1991) (in Journal of Physical Oceanography 21 , 1631–1642) is used to incorporate the effects of waves on the surface boundary layer, while the theory of Grant and Maddsen (1979) [in Journal of Geophysical Research (Oceans) 84 , 1797–1808], which was used by Signell et al. (1990) (in Journal of Geophysical Research 95 , 9671–9678) on the bottom boundary layer for constant waves, is modified for the inclusion of time-varying waves. The mutual influences between waves and currents are investigated through an idealized continental shelf case and hindcastings of storm events in the sea area adjacent to Hong Kong in the northern South China Sea. Calculations are compared with other computed results and observations. Calculations show that the wave-dependent surface stress incorporated in the three-dimensional hydrodynamic model has significant impact on water surface velocities and surface elevations (over 10% higher). The inclusion of wave-dependent bottom stress also shows some effects; however, in the presence of the wave-dependent surface stress, its effect on surge levels becomes negligible. The effect of currents on waves amounts to the reduction of the significant wave height by about 8% and less for wave mean periods. However, the inclusion of the wave-dependent bottom stress in the three-dimensional hydrodynamic model has little effect on wave characteristics whether or not the wave-dependent surface stress is considered although it does reduce the bottom flow velocities, especially in shallow water regions. © 1997 Elsevier Science Ltd

Effects of internal waves and turbulence on a horizontal aperture sonar

Abstract: Random variability in the water column will affect the operation of a horizontal aperture sonar. Two sources of variability in shallow water are turbulence and internal waves. In a modeling study, the effects of turbulence and internal waves on a shallow-water imaging system are compared. The operational principles of a large aperture imaging system are first reviewed. A shallow-water internal wave model is developed by modifying the Garrett-Munk model. The internal waves are assumed to dissipate and drive the small-scale turbulence. The two phenomena are predicted to have markedly different effects on a system. Turbulence has short spatial correlation scales whose primary effects will be manifested in the variance of the acoustic phase. By contrast, internal waves will have much larger scattering but also a longer correlation scale. The primary acoustic quantity of interest for internal waves is shown to be the curvature of the phase as observed along the aperture. Properties of shallow-water internal waves are shown to preclude the use of standard acoustic calculations based on the Markov approximation. Using archival environmental data, sample calculations are presented for the site of a planned August 1996 experiment.

Shallow water wave modelling with nonlinear dissipation

Abstract: In this paper a new shallow water wave model is described which uses nonlinear dissipation derived from turbulent diffusion as damping mechanism. The source functions of the model are presented in detail. Analytical results of the dynamical equation for simple cases illustrate basic features of the model. Academic test runs in deep and shallow water are performed. The designed cases are identical to the ones used in previous wave model intercomparison studies and thus allow comparison with other wave models. Results of a hindcast of a North Sea storm event illustrate the model behaviour in nonuniform real shallow water systems. In this case we can compare with field data and with the community wave model WAM cy. 4, which has been run parallel to our model. Our study shows that the concept of wave modelling with nonlinear dissipation is consistent with common knowledge of wave evolution in oceanic and shelf sea applications.

Use of lidar technology for collecting shallow water bathymetry of Florida Bay

Abstract: Due to an accelerated decline in water quality, Florida Bay is the focus of an inter-agency restoration program involving a modeling effort to define water circulation patterns both internally and between its surrounding waters. Models such as these require adequate resolution of the Bay's morphologic features which are characterized by extensive shallow water networks of mud banks, cuts, and basins. However, the information necessary to resolve the complex bathymetry does not exist on current NOAA navigation charts. The Bay's expansive shallow water characteristics renders much of it inaccessible by conventional waterborne survey methods. Obtaining this information requires an alternative survey technology capable of covering large shallow water areas and producing high resolution bathymetric data. During the spring of 1994 the SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) system was employed by NOAA to test its ability to resolve the complex shallow water bathymetry for a test area in central Florida Bay. Approximately 13 km of area was surveyed with a total surveying time of 12 hours. The data set presented here demonstrates that airborne lidar bathymetric technology such as SHOALS can be a valuable and cost effective tool for surveying large shallow water areas, without damage to the environment, that are otherwise inaccessible by conventional methods.