Showing papers on "Waves and shallow water published in 1998"

Evolution of nonlinear internal waves in the East and South China Seas

Abstract: Synthetic Aperture Radar (SAR) images from ERS-I have been used to study the characteristics of internal waves northeast and south of Taiwan in the East China Sea, and east of Hainan Island in the South China Sea. Rank-ordered packets of internal solitons propagating shoreward from the edge of the continental shelf were observed in the SAR images. On the basis of the assumption of a semidiurnal tidal origin, the wave speed can be estimated and is consistent with the internal wave theory. By using the SAR images and hydrographic data, internal waves of elevation have been identified in shallow water by a thicker mixed layer as compared with the bottom layer on the continental shelf. The generation mechanism includes the influences of the tide and the Kuroshio intrusion across the continental shelf for the formations of elevation internal waves. The effects of water depth on the evolution of solitons and wave packets are modeled by the nonlinear Kortweg-deVries (KdV) type equation and linked to satellite image observations. The numerical calculations of internal wave evolution on the continental shelf have been performed and compared with the SAR observations. For a case of depression waves in deep water, the solitons first disintegrate into dispersive wave trains and then evolve to a packet of elevation waves in the shallow water area after they pass through a “turning point” of approximately equal layer depths that has been observed in the SAR image and simulated by the numerical model. The importance of the dissipation effect in the coastal area is also discussed and demonstrated.

Shallow-Water Turbulence Modeling and Horizontal Large-Eddy Computation of River Flow

Abstract: By introducing the concept of “SDS (subdepth scale) turbulence” to model three-dimensional (3D) turbulence with length scales less than the water depth and treating it explicitly with a proper separate modeling, an SDS-2DH model has been developed to simulate the evolution of horizontal large-scale eddies in shallow water. Applying this model to river flows with transverse shear due to vegetation drag, the horizontal large-scale (HLS) eddies were found to dominate horizontal momentum mixing. The bottom friction and vegetation drag, acting as sinks of vorticity, play the key roles in the development of the horizontal large-scale eddies and in Reynolds stress generation. The SDS-2DH model can directly describe effects of flow geometry, such as vegetation layer width, on the large-scale eddy development and, hence, predicts turbulence-mixing better than the k-e model.

Internal wave reflection and mixing at Fieberling Guyot

Abstract: The structure of internal wave reflection off a sloping bottom on the steep flank of a tall North Pacific Ocean seamount is observed in year-long moored array records to differ substantially from the form predicted by linear theory, although linear theory accounts for several qualitative features of the process. This study documents new features of wave reflection as described below. Wave reflection is detectable as far as 750 m above the bottom. Motions are dominated by a single empirical mode whose phase structure obeys linear internal wave dispersion but whose amplitude decays with a scale comparable to the wavenumber. While the dominant mode has scales appropriate to the reflection of a first baroclinic mode wave incident from the open ocean, its decay from the bottom is such that current vectors in the vertical plane rotate clockwise in time when viewed with shallow water to the right. This flow resembles the lower half of the deepest cell pattern predicted by linear reflection from a uniform slope. At the local internal wave critical frequency, the dominant mode has nearly a vanishing wavenumber rather than the infinite wavenumber predicted by linear reflection. Reflected waves are aligned parallel to the bottom slope measured on wave spatial scales, rather than shorter ones. Wave reflection causes large, frequent density overturns, implying mixing. The rate and strength of these overturns imply a local vertical eddy viscosity of 2–6 × 10−4 m2/s over the bottom few hundred meters. The contribution of bottom-intensified mixing to the open deep ocean is roughly equivalent to that found in situ, although reflection from gentler slopes or at lower latitude may produce greater contribution from internal wave-reflection-induced mixing.

Carbon-isotope stratigraphy in shallow-water carbonates: implications for Cretaceous black-shale deposition

Abstract: A Barremian to Albian succession on Mount Kanala, part of a Tethyan isolated carbonate platform, was investigated for its δ13C variations. The limestone sequence is composed of a series of peritidal shallowing-upward cycles with clear petrographic evidence for strong early diagenetic overprinting related to repeated subaerial exposure. Despite significant impact of diagenesis, the observed changes in δ13C can be very well correlated with deep-water sections from different ocean basins and shallow water carbonate platforms in the Middle East. This lends further support to the applicability of δ13C variations for stratigraphic purposes in shallow-water limestones. Using the δ13C signal, time resolution in Lower Cretaceous platform carbonates can be significantly increased, independent of bio-zonations often hampered by ecological variability. Cyclostratigraphic analysis of the Aptian part of the section shows that strong positive excursions of the cumulative departure from mean cycle thickness of the peritidal shallowing-upward cycles coincide with global positive δ13C excursions. This, and the fact that positive shifts in the δ13C record are preserved within shallow water limestones, provide evidence that black-shale accumulation in the ocean basins occurred during sea-level rise and flooding of platform tops. Integration of carbon-isotope-, cyclo- and sequence-stratigraphic results from different carbonate platforms indicate that strong positive global δ13C shifts and concurrent organic-carbon burial during black-shale deposition are ultimately caused by rapid rises of eustatic sea level. Hence, the rate of change of eustatic sea level is considered to play a crucial role in black-shale accumulation in the global ocean basins during the Cretaceous.

Mapping Chicxulub crater structure with gravity and seismic reflection data

Abstract: The Chicxulub crater buried under the Yucat

Seasonal, horizontal, and vertical distribution of phytoplankton chlorophyll a in the northeast U.S. continental shelf ecosystem

Abstract: The broad scale features in the horizontal, vertical, and seasonal distribution of phytoplankton chlorophyll a on the northeast U.S. continental shelf are described based on 57,088 measurements made during 78 oceanographic surveys from 1977 through 1988. Highest mean water column chlorophyll concentration (Chlw,) is usually observed in nearshore areas adjacent to the mouths of the estuaries in the Middle Atlantic Bight (MAB), over the shallow water on Georges Bank, and a small area sampled along the southeast edge of Nantucket Shoals. Lowest Chlw «0.125 ug l-1) is usually restricted to the most seaward stations sampled along the shelf-break and the central deep waters in the Gulf of Maine. There is at least a twofold seasonal variation in phytoplankton biomass in all areas, with highest phytoplankton concentrations (m3) and highest integrated standing stocks (m2) occurring during the winter-spring (WS) bloom, and the lowest during summer, when vertical density stratification is maximal. In most regions, a secondary phytoplankton biomass pulse is evident during convective destratification in fall, usually in October. Fall bloom in some areas of Georges Bank approaches the magnitude of the WS-bloom, but Georges Bank and Middle Atlantic Bight fall blooms are clearly subordinate to WS-blooms. Measurements of chlorophyll in two size-fractions of the phytoplankton, netplankton (>20 um) and nanoplankton «20 um), revealed that the smaller nanoplankton are responsible for most of the phytoplankton biomass on the northeast U.S. shelf. Netplankton tend to be more abundant in nearshore areas of the MAB and shallow water on Georges Bank, where chlorophyll a is usually high; nanoplankton dominate deeper water at the shelf-break and deep water in the Gulf of Maine, where Chlw is usually low. As a general rule, the percent of phytoplankton in the netplankton size-fraction increases with increasing depth below surface and decreases proceeding offshore. There are distinct seasonal and regional patterns in the vertical distribution of chlorophyll a and percent netplankton, as revealed in composite vertical profiles of chlorophyll a constructed for 11 layers of the water column. Subsurface chlorophyll a maxima are ubiquitous during summer in stratified water. Chlorophyll a in the subsurface maximum layer is generally 2-8 times the concentration in the overlying and underlying water and approaches 50 to 75% of the levels observed in surface water during WS-bloom. The distribution of the ratio of the subsurface maximum chlorophyll a to surface chlorophyll a (SSR) during summer parallels the shelfwide pattern for stability, indexed as the difference in density (sigma-t) between 40 m and surface (stability 40. The weakest stability and lowest SSR's are found in shallow tidally-mixed water on Georges Bank; the greatest stability and highest SSR's (8-12:1) are along the mid and outer MAB shelf, over the winter residual water known as the "cold band." On Georges Bank, the distribution of SSR and the stability40 are roughly congruent with the pattern for maximum surface tidal current velocity, with values above 50 cms-1 defining SSR's less than 2:1 and the well-mixed area. Physical factors (bathymetry, vertical mixing by strong tidal currents, and seasonal and regional differences in the intensity and duration of vertical stratification) appear to explain much of the variability in phytoplankton chlorophyll a throughout this ecosystem. (PDF file contains 126 pages.)

Louisiana barrier islands and their importance in wetland protection : Forecasting shoreline change and subsequent response of wave climate

Abstract: The role that barrier islands play in mitigating the wave climate in lower energy, bay or lagoonal environments has not yet been addressed in detail. With the exception of one study in which a shallow water wave prediction model (HISWA) (LIST et. aI., 1992) was applied to idealized barrier-bay configurations, the critical linkages among barriers, wave energy transmission into bays, regenerated local waves, and subsequent wave climate have not been made. In Louisiana, barrier disintegration is rapid over the short-term (10 2 years) and the mere potential for impacts of barrier loss on the bay wave climate is highly significant. Because of a paucity in scientific data which could be utilized to address this issue, there remains a significant debate as to the value of barrier islands in mitigating wave climate in the bays and along fringing marshes. In this paper we present historical shoreline change data which are used to predict the rapid disintegration of a section of barrier island coast along central Louisiana (Isles Dernieres) and resultant forecasted increase s in wave energy in the adjacent bays. The methods associated with shoreline, bathymetric and wave energy forecasting are briefly presented as an example of a larger, ongoing project regarding the feasibility of large-scale barrier island restoration in Louisiana. A brief overview of the magnitude and causal mechanisms associated with wetland loss are provided in addition to the implications associate d with barrier island loss and subsequent detrimental impacts on fringing marshes. The example data set presented here indicates that the role of Louisiana's barrier islands comprising the Isles Dernieres in mitigating the wave climate in their adjacent bays and fringing marshes appears critical. Considering only fairweather conditions, the data indicate that the bays adjacent to the Isles Dernieres could experience an increase in wave height of 700% if the barrier chain is reduced to shoals. Although large-scale barrier island restoration will greatly reduce wave energy in Louisiana's bays and along fringing marshes, additional devices capable of absorbing wave energy around portions of the fringing marshes will likely require construction. This may occur in areas where the fetch permits regeneration of incident waves that have propagated across the Louisiana shelf, or locally genera ted higher frequency waves.

Experiments on Nonlinear Wave Groups in Intermediate Water Depth

Abstract: Evolution of nonlinear wave groups of various initial envelope shapes is studied experimentally in a wave tank. Experiments are performed for different values of the water depth in the tank. The experimental results are compared with the calculations based on the cubic Schrodinger equation. Qualitatively different results are obtained for water depth values corresponding to different signs of the nonlinear term coefficient in the model equation. In relatively shallow water, the demodulation of wave groups is observed, while in a deeper water the maximum wave height increases along the tank due to the focusing, as expected.

Simulation of water waves generated by a potential debris avalanche in Montserrat, Lesser Antilles

Abstract: The evolution of the volcano activity in Montserrat could lead to the collapse of a portion of the lava dome in the Tar River Valley and to a sudden entry of debris avalanche into the Caribbean Sea. The impact of a debris avalanche with a volume of 40× 10 6 m 3 into the sea and the generated tsunami have been simulated numerically by a mixture model solving the 3D Euler's equations. The mixture is composed of sediments considered as an homogeneous fluid of density 2 and of water. Numerical tests show that the generated waves are sensitive to both initial impact velocities and avalanche fronts of the landslide. The water surface and velocities calculated by the 3D mixture model are used as input data in a non-linear shallow water model, to calculate tsunami propagation along the coasts of Montserrat. The hydraulic risks in Montserrat are roughly assessed for a tsunami generated by a mass of 40×10 6 m 3 entering the sea.

Estimating the compressional and shear wave speeds of a shallow water seabed from the vertical coherence of ambient noise in the water column

Abstract: Due to the multiple bottom reflections encountered in shallow water environments, the spatial structure of the ambient noise field depends strongly on the geoacoustic properties of the seabed, which are invariant over time scales associated with most measurements. The vertical directionality and coherence are relatively stable features of the noise that are determined primarily by the seabed, rather than temporal variations in the surface source distribution. In this paper, estimates of the compressional and shear wave speeds are determined from ambient noise measurements over shear supporting seabeds. Using a model of wind-generated noise over an elastic seabed, it is shown that the noise is sensitive to the compressional and shear wave speeds in the upper few meters of seabed. An inversion procedure is developed based on a matched field of the complex, broadband coherence from a single hydrophone pair. Using ambient noise data from two shear supporting sites, compressional and shear wave estimates are obtained that are in good agreement with independent surveys. For one site where the bedrock is exposed, a half-space model of the seabed yields reasonable estimates of the seabed parameters. For the other site, the presence of a thin sedimentary layer results in a low estimate from the half-space model. However, when the layer is included in the model, the estimates of the underlying bedrock are in good agreement with a seismic survey.

The depth distribution of Ostracoda from the Greenland Sea

Abstract: Sixteen box core samples of Recent sediment collected in three transects across the East Greenland shelf and slope were analysed for Ostracoda. The samples which range in depth from 274 m to 3355 m, yielded a total of 52 species belonging to 25 genera. No less than 26 of the species also occur in the adjacent Scoresby Sund fjord complex. The fauna represents an interesting mixture of high latitude shallow water Arctic species and others known from bathyal and abyssal depths in the North Atlantic, including some pandemic deep sea species. Many species occur in shallower water here than in the North Atlantic due to the colder water. The fauna comprised of three associations (Shelf/Upper Slope; Slope: Lower Slope/Abyss). A marked faunal turnover occurs at the Upper Slope. The study raises questions concerning the constancy and universality of the relationship between certain ostracod species and water masses.

The Persistence of Anthropogenic Turbidity Plumes in a Shallow Water Estuary

Abstract: Increased turbidity and light attenuation (Kd) in the suspended sediment plumes created by hydraulic clam dredging were examined in Chesapeake Bay, MD, U.S.A. Turbidity andKdvalues were measured along transects in areas with and without dredge plumes. The turbidity andKdof individual plumes were tracked as they returned to background levels using both Lagrangian and Eulerian techniques. Existing aerial photographs and a geographical information system (ARC/INFO) were used to examine plume sizes and dredge boat locations in relation to bathymetry. Hydraulic clam dredging produced plumes with significantly higher turbidity and light attenuation compared to background values. Plume characteristics were determined primarily by bottom sediment type and water depth. The greatest increase in turbidity and light attenuation occurred when dredges operated in shallow water (<1·0 m) where bottom sediments had increased amounts of silt and clay. Plume turbidity andKddissipated exponentially over time. The initial change in concentration of suspended sediments was rapid as the coarse sediments settled to the bottom. The rate of plume decay slowed as diffusion and resuspension acted on the finer sediments remaining in suspension. Some Lagrangian rates of plume dissipation were faster than Eulerian rates due to resuspension in shallow waters. Examination of aerial photographs indicated that 72% of the dredge boats digitized in the Chester River were operating in less than 2 m water. The area of a plume measured per boat in the Chester River was highly variable, ranging from 0·01 to 0·64 m2.

Wave phase shift at coastal structures

Abstract: Wave reflection from coastal structures is defined by the magnitude and phase of the reflected wave. Both properties have a profound impact on the wave kinematics and coastal processes in front of the structure. This paper focuses on the phase shift on reflection. Using a large experimental data set, involving normally incident and obliquely incident regular and irregular waves, it is shown that the phase is uniquely determined by a nondimensional number χ\d3 defined by structure slope, water depth at the structure toe, wave period, and angle of incidence. A theoretical method for predicting phase shift based on matching the equation for linear long waves on a sloping beach with an equation for flat-bed standing waves seaward of the structure toe yields good estimates of the phase shift at low values of χ\d3. A second method based on integration of the shallow water wave number over the slope is considered and not recommended. Example cases are presented that demonstrate the practical importance of the phase shift in determining the kinematics in front of the structure.

An application of the plaster dissolution method for quantifying water velocity in the shallow hyporheic zone of an Appalachian stream system

Abstract: 1. A method for quantifying interstitial water velocity based on the dissolution rate of plaster of Paris standards was developed as part of a study of vertical, longitudinal (1–4 order sites) and seasonal variation in the biotic and physical characteristics of the shallow hyporheic zone (0–30 cm) of a headwater stream system in West Virginia, U.S.A. 2. A calibration model was developed using a water velocity simulation tank to relate mass loss of plaster standards to water velocity and temperature. The model was then used to calculate water velocity through artificial substrata embedded in the shallow hyporheic zone of four stream reaches based on in situ mass loss of plaster standards. 3. Water velocity in the hyporheic zone increased with stream order, was highest in early spring and winter during high stream base flows, and decreased with depth into the substratum. There was a strong interaction between depth and season: during periods of high stream discharge, water velocity through the upper level of the shallow hyporheic zone (0–10 cm into the substrate) increased disproportionately more than velocity at greater depths. Mean interstitial velocity in March ranged from 0 cm s–1 in the lowest level (20–30 cm) to 3.5 cm s–1 at the upper level (0–10 cm) at the first-order site, and from 2.5 cm s–1 (20–30 cm) to 9.5 cm s–1 (0–10 cm s–1) at the fourth-order site. Gradients in stream discharge and sediment permeability accounted for treatment effects. 4. Use of calibrated data improved the ability to resolve among-season differences in interstitial water movement over the use of uncalibrated mass loss data. For some applications of the plaster standard method, empirical calibration may not be necessary.

Resonant interaction between an atmospheric gravity wave and shallow water wave along Florida's west coast

Abstract: On 25 March 1995, a large solitary wave, seemingly from nowhere, washed ashore along the normally tranquil Gulf Coast of Florida from Tampa Bay to south of Naples. On this Saturday morning, many be...

An investigation of ocean tides derived from along-track altimetry

Abstract: With the availability of a long data record of accurate sea surface height measurements, it is now possible to estimate the ocean tide along the ground track of TOPEX/POSEIDON (T/P). This has been done from over 4.5 years of data using both response and harmonic analyses. These estimates agree well with each other, and with other gridded models over both long and short wavelengths in deep water, for those tidal components whose alias frequencies are separable by the Rayleigh criterion. Comparisons of along-track (AT) estimates to current tide models show shorter wavelength features not present in dynamical and empirical global models. AT estimates follow the general trends of empirical models far from sharp topographical changes, but near sharp changes, they tend to follow dynamical model results. Error estimates show that the T/P data are not significantly worse in shallow water than in deep water, suggesting that there is accurate information about tides in shallow water in the T/P data. Tides at crossover locations are improved by computing estimates with both ascending and descending track data. Possible techniques to improve AT tidal estimates between crossover points are discussed. AT tidal estimation can be useful for studying local regions where resolution is more important than regular spacing, for studying tidal interactions over sharp topography, and for extending tidal models from deep to shallow waters through assimilation into a dynamical model.

The motion of an intense vortex near topography

Abstract: The initial value problem for the motion of an intense, quasi-geostrophic, equivalent-barotropic, singular vortex near an infinitely long escarpment is studied in three parts. First, for times small compared to the topographic wave timescale the motion of the vortex is analysed by deriving an expression for the secondary circulation caused by the advection of fluid columns across the escarpment. The secondary circulation, in turn, advects the primary vortex and integral expressions are found for its velocity components. Analytical expressions in terms of integrals are found for the vortex drift velocity components. It is found that, initially, cyclones propagate away from the deep water region and anticyclones propagate away from the shallow water region. Asymptotic evaluation of the integrals shows that both cyclones and anticyclones eventually propagate parallel to the escarpment with shallow water on their right at a steady speed which decays exponentially with distance from the escarpment. Secondly, it is shown that for times comparable to, and larger than, the wave timescale, the vortex always resonates with the topographic wave field. The flux of energy in the topographic waves leads to a loss of energy in the vortex and global energy and momentum arguments are used to derive an equation for the distance (or, equivalently, the vortex velocity) of the vortex from the escarpment. It is shown that cyclones, provided they are initially within an O(1) distance (here a unit of distance is dimensionally equivalent to one Rossby radius of deformation) from the escarpment, drift further away from the deep water (i.e. toward higher ambient potential vorticity), possibly crossing the escarpment and accumulate at a distance of approximate to 1.2 on the shallow side of the escarpment. For distances larger than 1.2 there is essentially no drift of the vortex perpendicular to the escarpment. Anticyclones display similar behaviour except they drift in the opposite direction, i.e. away from the shallow water or toward lower ambient potential vorticity. Third, the method of contour dynamics is used to describe the evolution of the vortex and the interface representing the initial potential vorticity jump between the shallow and deep water regions. The contour dynamic results are in good quantitative agreement with the analytical results.

On Modelling Wind-Waves in Shallow and Fetch Limited Areas Using the Method of Holthuijsen, Booij and Herbers

Abstract: A parametric discrete spectral model, MIKE 21 NSW, based on the equations presented by HOLTHUIJSEN, BOOIJ and HERBERS, 1989 (HBH) is used to compute the growth of wind-waves in the shallow and fetch limited waters off the coast of Lolland, Denmark. A simple procedure is used to obtain the HBH wind-wave growth coefficients from empirical deep water wave growth formula without the need of tuning. Simulations are carried out to assess the influence of wind-wave growth source functions, bottom friction, wave breaking and assumed spectral shape. Results indicate that for this limited fetch (about 20 km) in relatively shallow water (depths about 3-4m near measurement locations), the locally generated significant wave heights are not sensitive to bottom friction and depth induced wave breaking. Of the 5 wave growth formulas investigated, the best results were obtained using HBH coefficients corresponding to the SPM (1984) or the KAHMA & CALKOEN (1994) formulas. The results show that the method performs much better for calculating waves in shallow and fetch limited situations than a direct calculation with the SPM (1984) shallow water formula.

Biomass and photosynthetic responses to irradiance by a shallow and a deep water population of Thalassia testudinum on the west coast of Florida

Abstract: Short shoot density and blade width were significantly lower while chlorophyll a levels were significantly higher in a deep (6 to 8 m) Thalassia testudinum bed when compared to a (1 m) shallow bed. Although the above and below ground biomass was significantly higher in the shallow bed, there were no differences in the ratios of above to below ground portions of the cores or individual unit plants. No significant patterns in compensation point (I c ), saturation irradiance (I k ), and linear slope of the curve (α) were evident for plants from the deep and shallow water beds. The findings suggest an irradiance-depth limitation and that T. testudinum does not photoacclimate through shifts in P-I factors.

Temperature Behavior of a Natural Shallow Water Body during a Summer Period

Abstract: For many biological processes, the daily and seasonal behavior of the temperature of the water layer close to the water surface is very important. An example is the development of larvas of mosquito species which develop in small still water bodies. An outdoor experiment during a summer period of four months has been executed in a small water body situated within a bog area, in order to obtain insight into the daily and seasonal temperature behavior of this water body. The measurements show that during daytime a strong thermal stratification occurs in the water body and that during nighttime a well mixed layer develops growing from the water surface. In the upper water layer a well developed daily temperature cycle is present. This cycle is strongly reduced by depth. On average, the daily maximum water temperature excursion from the mean agree with the daily maximum air temperature excursion from the mean. The range between maximum and minimum water temperature correlate well with the daily global irradiation.

Effect of Directional Spreading and Spectral Bandwidth on the Nonlinearity of the Irregular Waves

Abstract: Wherever one stands, deep water, intermediate or shallow water, in extreme conditions the nonlinearity in the wave kinematics is large and has a strong influence on the design parameters. Simple models of the wave kinematics have been studied based on hypotheses of narrowband and unidirectionality. Obviously a real sea is neither narrowbanded nor unidirectional and the width of the spectral density and the directional spreading influence the nonlinear characteristics of the waves (skewness, asymmetries,...). A second order directional irregular wave model is used to simulate time series of the free surface elevation. Based on a large simulated data base, a parametric study of the influence of the spectral width and directional spreading is led on several wave characteristics sensitive to nonlinearities (skewness, wave and crest heights distributions, steepness of maximum crest). Three typical situations are analysed which correspond to extreme situations in long and short fetch wind sea. The validity of the simplified assumptions of narrowband and unidirectionality is then discussed.

Sedimentology of shallow, hurricane-affected lagoons : Grand Cayman, British West Indies

Abstract: Frank Sound and Pease Bay are small narrow (~4 km long and <1 km wide) shallow water (15-20 m average depth) lagoons (< 05 m deep) located on the exposed-windward margin of south coast of Grand Cayman Collectively, the Rubble and Knob, Bare Sand, and Thalassia and Sand zones form 95-97% of the substrates in these lagoons The Bare Rock Zone, Coral Knolls, and Shoreline Zone are restricted to small areas in both lagoons Between 1985 and 1992, the area covered by Thalassia expanded by colonizing the Bare Sand Zone During fair-weather conditions, onshore wave energy is dampened by the reef Nevertheless, waves that cross the reef are sufficient to maintain lagoonal circulation and "normal" marine conditions Under these conditions, there is limited sediment production (eg, Thalassia epibionts, bioerosion, green algae), active bioturbation, and expansion of the Thalassia banks Sediment transportation is minimal At the height of a hurricane, waves and currents pass over the reef and produce a turbulent sediment laden currents that cross the lagoons As the current loses energy, deposition produces a sediment wedge that grades from boulders-cobbles near reef crest, to pebbles and coarse sand ~80 m from the reef crest, to fine sand 500-600 m from the reef crest The sediment is poorly sorted and typically has a unimodal grain size distribution Silt and clay sized sediment is rare Skeletal constituents indicate that most sediment originates in the fore-reef and shelf environs As the storm wanes, water that was piled in the lagoons during the storm starts to drain out through topographically controlled mega-rip currents These high velocity currents destroy biota, uproot Thalassia, and strip sediment to reveal bare rock substrates

The Response of Suspended Particulate Material to Upwelling and Downwelling Events in Southern Lake Michigan

Abstract: Time series measurements of water temperature, beam attenuation, and current velocity were made at three stations located in 28, 58, and 100 m of water off the east coast of Lake Michigan from late August until mid-October 1995. When combined with water intake records and wave measurements our observations show that local resuspension is not responsible for maintaining the benthic nepheloid layer (BNL) during the stratified period. Resuspension of bottom material by surface wind waves occurs in shallow water (13 m), but this material is not transported offshore into the BNL during downwelling events. During upwellings material may be transported onshore from the BNL into shallow water, but additional material is required to produce the concentrations observed at the inner stations. Resuspension by internal waves may be the source of this additional material. Changes in the vertical structure of the benthic nepheloid layer appear to account for most of the changes in suspended particulate material at the two offshore stations. These changes are probably due to a combination of internal wave action and regional changes in current patterns.