Showing papers on "Waves and shallow water published in 2007"

Soil water partitioning contributes to species coexistence in tallgrass prairie

Abstract: The majority of tallgrass prairie root biomass is located in the upper soil layers (0 � 25 cm), but species differences exist in reliance on soil water at varying depths. These differences have led to the hypothesis that resource partitioning belowground facilitates species co-existence in this mesic grassland. To determine if plant water relations can be linked to soil water partitioning as a potential mechanism allowing C3 species to persist among the more dominant C4 grasses, we measured differences in the source of water-use using the isotopic signature of xylem water, volumetric soil water content at 4 depths, and leaf water potentials. Data were collected for seven species representing C4 grasses, C3 forbs and C3 shrubs over three growing seasons at the Konza Prairie (Kansas, USA) to encompass a range of natural climatic conditions. C4 grasses relied on shallow soil water (5 cm) across the growing season and had midday leaf water potentials that were highly correlated with shallow soil water regardless of soil water availability at other portions of the soil profile (20, 40 and 90 cm). In contrast, C3 species only used shallow soil water when plentiful at this depth; these species increased their dependence on soil water from greater depths as the upper soil layers dried. Structural equation models describing plant water relations were very similar for the three C4 species, whereas a unique set of models and drivers were identified for each of the C3 species. These results support soil water partitioning as a mechanism for species coexistence, as C4 species in this grassland have relatively consistent dependence on water in shallow soil layers, whereas C3 species show niche differentiation in water use strategies to avoid competition with C4 grasses for water in shallow soil layers when this resource is limiting and leaf water stress is high.

Shoaling and shoreline dissipation of low?frequency waves

Abstract: The growth rate, shoreline reflection, and dissipation of low?frequency waves are investigated using data obtained from physical experiments in the Delft University of Technology research flume and by parameter variation using the numerical model Delft3D?SurfBeat. The growth rate of the shoaling incoming long wave varies with depth with an exponent between 0.25 and 2.5. The exponent depends on a dimensionless normalized bed slope parameter ?, which distinguishes between a mild?slope regime and a steep?slope regime. This dependency on ? alone is valid if the forcing short waves are not in shallow water; that is, the forcing is off?resonant. The ? parameter also controls the reflection coefficient at the shoreline because for small values of ?, long waves are shown to break. In this mild?slope regime the dissipation due to breaking of the long waves in the vicinity of the shoreline is much higher than the dissipation due to bottom friction, confirming the findings of Thomson et al. (2006) and Henderson et al. (2006). The energy transfer from low frequencies to higher frequencies is partly due to triad interactions between low? and high?frequency waves but with decreasing depth is increasingly dominated by long?wave self?self interactions, which cause the long?wave front to steepen up and eventually break. The role of the breaking process in the near?shore evolution of the long waves is experimentally confirmed by observations of monochromatic free long waves propagating on a plane sloping beach, which shows strikingly similar characteristics, including the steepening and breaking.

State of the art in the functioning of shallow Mediterranean lakes: workshop conclusions

Abstract: Studies on shallow lakes from the north temperate zone show that they alternate between clear and turbid water states in response to control factors. However, the ecology of semiarid to arid shallow Mediterranean lakes is less explored. Hydrological effects (e.g. water level fluctuations, water residence time) on major ions and nutrient dynamics and processes, and ecology of submerged macrophytes appear to have a crucial role for food webs in shallow Mediterranean lakes. Nutrient control may be of greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes. This will be relevant for the implementation of the European Water Framework Directive, and conservation and management of these ecosystems. Strong trophic cascading effects of fish resulting from dominance of omnivorous and benthivorous fish species, whose diversity is usually high, together with frequent spawning and absence of efficient piscivores, seem to be the reason for the lack of large-bodied grazers that could control phytoplankton. However, such effects may vary within the region depending on fish distribution and community. These factors need elaboration in order to allow shallow lake ecologists and managers to develop better restoration strategies for eutrophicated shallow Mediterranean lakes. Consequently, modifications for the implementation of the European Water Framework Directive for determining ecological status in shallow Mediterranean lakes appear to be necessary. Furthermore, the implications of climate warming may be even more challenging than in high latitude lakes since shallow lakes in the Mediterranean region are among the most sensitive to extreme climate changes. There is an urgent need for data on the ecology of shallow lakes in the region. An appeal is made for international cooperation, development of large-scale research and information exchange to facilitate this and a web-based discussion list has been implemented.

Improved bathymetric datasets for the shallow water regions in the Indian Ocean

Abstract: Ocean modellers use bathymetric datasets like ETOPO5 and ETOPO2 to represent the ocean bottom topography. The former dataset is based on digitization of depth contours greater than 200 m, and the latter is based on satellite altimetry. Hence, they are not always reliable in shallow regions. An improved shelf bathymetry for the Indian Ocean region (20°E to 112°E and 38°S to 32°N) is derived by digitizing the depth contours and sounding depths less than 200 m from the hydrographic charts published by the National Hydrographic Office, India. The digitized data are then gridded and used to modify the existing ETOPO5 and ETOPO2 datasets for depths less than 200 m. In combining the digitized data with the original ETOPO dataset, we apply an appropriate blending technique near the 200 m contour to ensure smooth merging of the datasets. Using the modified ETOPO5, we demonstrate that the original ETOPO5 is indeed inaccurate in depths of less than 200 m and has features that are not actually present on the ocean bottom. Though the present version of ETOPO2 (ETOPO2v2) is a better bathymetry compared to its earlier versions, there are still differences between the ETOPO2v2 and the modified ETOPO2. We assess the improvements of these bathymetric grids with the performance of existing models of tidal circulation and tsunami propagation.

Water waves and integrability

Abstract: Euler's equations describe the motion of inviscid fluid. In the case of shallow water, when a perturbative asymptotic expansion of Euler's equations is considered (to a certain order of smallness of the scale parameters), relations to certain integrable equations emerge. Some recent results concerning the use of integrable equation in modelling the motion of shallow water waves are reviewed in this contribution.

Retrieving key benthic cover types and bathymetry from hyperspectral imagery

Abstract: Bathymetry and the spatial distribution of benthic cover in coastal waters are of key importance in managing and monitoring our coastal water environments. Currently very little of the Western Australian shallow coastal water habitats are mapped, and for those maps that do exist, the spatial resolution generally is poor and the information is dated. Aircraft and space-borne hyperspectral sensors have been shown to be useful in imaging substrate features in shallow coastal waters. This paper describes a method for quantitatively estimating both bathymetry and benthic cover in shallow waters from hyperspectral imagery. The method incorporates a shallow water reflectance model, which accounts for the water column absorption and backscattering, water depth and substrate reflectance. The model was tested against simulated reflectance data, demonstrating the models' ability to retrieve appropriate fractional coverage of sediment, sea grass and brown algae for depths ranging from 1 - 12 m. The model was applied to a HyMap image encompassing a portion of the Jurien Bay Marine Park off the coast of Western Australia. The retrieved benthic cover products were compared to underwater video observations sampled within the image. The comparison shows the method's great potential for characterizing key aspects of marine ecosystems from remotely sensed hyperspectral data.

The Riemann problem for the shallow water equations with discontinuous topography

Abstract: We construct the solution of the Riemann problem for the shallow water equations with discontinuous topography. The system under consideration is non-strictly hyperbolic and does not admit a fully conservative form, and we establish the existence of two-parameter wave sets, rather than wave curves. The selection of admissible waves is particularly challenging. Our construction is fully explicit, and leads to formulas that can be implemented numerically for the approximation of the general initial-value problem.

Langmuir turbulence in shallow water. Part 1. Observations

Abstract: During extended deployment at an ocean observatory off the coast of New Jersey, a bottom-mounted five-beam acoustic Doppler current profiler measured large-scale velocity structures that we interpret as Langmuir circulations filling the entire water column. These circulations are the large-eddy structures of wind-wave-driven turbulent flows that occur episodically when a shallow water column experiences prolonged strong wind forcing. Many observational characteristics agree with former descriptions of Langmuir circulations in deep water. The three-dimensional velocity field reveals quasi-organized structures consisting of pairs of surface-intensified counter-rotating vortices, aligned approximately downwind. Maximum downward velocities are stronger than upward velocities, and the downwelling region of each cell, defined as a pair of vortices, is narrower than the upwelling region. Maximum downward vertical velocity occurs at or above mid-depth, and scales approximately with wind speed. The estimated crosswind scale of cells is roughly 3–6 times their vertical scale, set under these conditions by water depth. The long axis of the cells appears to lie at an angle ∼10 ◦ –20 ◦ to the right of the wind. A major difference from deep-water observations is strong near-bottom intensification of the downwind ‘jets’ found typically centred over downwelling regions. Accessible observational features such as cell morphology and profiles of mean velocities, turbulent velocity variances, and shear stress components are compared with the results of associated large-eddy simulations (reported in Part 2) of shallow water flows driven by surface stress and the Craik–Leibovich vortex forcing generally used to represent generation of Langmuir cells. A particularly sensitive diagnostic for identification of Langmuir circulations as the energy-containing eddies of the turbulent flow is the depth trajectory of invariants of the turbulent stress tensor, plotted in the Lumley ‘triangle’ corresponding to realizable turbulent flows. When Langmuir structures are present in the observations, the Lumley map is distinctly different from that of surface-stress-driven Couette flow, again in agreement with the large-eddy simulations (LES). Unlike the LES, observed velocity fields contain two distinct and significant scales of variability, documented by wavelet analysis of observational records of vertical velocity. Variability with periods of many minutes is that expected from Langmuir cells drifting past the instrument at the slowly time-varying crosswind velocity. Shorter period variability, of the order of 1–2 min, has roughly the observed periodicity of surface wave groups, suggesting a connection with the wave groups themselves and/or the wave breaking associated with them in high wind conditions.

Real-time Breaking Waves for Shallow Water Simulations

Abstract: We present a new method for enhancing shallow water simulations by the effect of overturning waves. While full 3D fluid simulations can capture the process of wave breaking, this is beyond the capabilities of a pure height field model. 3D simulations, however, are still too expensive for real-time applications, especially when large bodies of water need to be simulated. The extension we propose overcomes this problem and makes it possible to simulate scenes such as waves near a beach, and surf riding characters in real-time. In a first step, steep wave fronts in the height field are detected and marked by line segments. These segments then spawn sheets of fluid represented by connected particles. When the sheets impinge on the water surface, they are absorbed and result in the creation of particles representing drops and foam. To enable interesting applications, we furthermore present a two-way coupling of rigid bodies with the fluid simulation. The capabilities and efficiency of the method will be demonstrated with several scenes, which run in real-time on today's commodity hardware.

Three‐dimensional shoaling of large‐amplitude internal waves

Abstract: [1] The three-dimensional (3-D) shoaling of large-amplitude internal waves (LAIW) is studied in the framework of a fully nonlinear nonhydrostatic numerical model. The vertical fluid stratification, parameters of the propagating waves and bottom topography were taken close to those observed in the northern part of the Andaman Sea. It was found that three-dimensional evolution of LAIWs propagating from the deep part of a basin onto the shelf differs from two-dimensional shoaling in many ways largely because of the process of wave refraction developing in the areas of local bottom elevations or depressions. In the 3-D case the wave refraction produces concave and convex fragments of the wave fronts which may lead to the transverse redistribution of energy along the wave. Results demonstrate that concave wave fragments work as optical lenses focusing the wave energy to the centers of curvature. This process is especially important for LAIWs in shallow water zones where wave amplitudes are close to the saturation level. In general, the wave refraction leads to more fast wave breaking than that in the 2-D case. As a results, it should be expected to find localized regions of higher levels of water mixing and turbulence in the vicinity of local banks and headlands where LAIWs produce concave patterns. The areas of local bottom depressions, on the contrary, should be considered as potential places with lower level of background mixing.

Field Signatures of the SE-Asian Mega-Tsunami along the West Coast of Thailand Compared to Holocene Paleo-Tsunami from the Atlantic Region

Abstract: The Andaman-Sumatra Tsunami of Dec. 26, 2004, was by far the largest tsunami catastrophe in human history. An earthquake of 9 to 9.3 on the Richter scale, the extension of waves over more than 5000 km of ocean and run-ups up to 35 m are its key features. These characteristics suggest significant changes in coastal morphology and high sediment transport rates. A field survey along the west coast of Thailand (Phuket Island, Khao Lak region including some Similan Islands, Nang Pha mangrove areas and Phi Phi Don Islands) seven to nine weeks after the tsunami, however, discovered only small changes in coastal morphology and a limited amount of dislocated sediments, restricted to the lower meters of the tsunami waves. This is in striking contrast to many paleo-tsunami’s events of the Atlantic region. Explanations for this discrepancy are sought in: a. Mechanics of the earthquake. A rather slow shock impulse on the water masses over the very long earthquake zone, b. Shallow water in the earthquake zone, and c. Bathymetry of the foreshore zone at the impacted sites. Shallow water west of Thailand has diminished wave energy significantly. The differences in geomorphological and sedimentological signatures of this tsunami compared with many paleo-tsunami worldwide makes it unsuitable to be used as a model for old and future tsunami imprints by an event of this extreme energy and extension.

Tsunami and Nonlinear Waves

Abstract: Propagation.- Waves in shallow water, with emphasis on the tsunami of 2004.- Integrable Nonlinear Wave Equations and Possible Connections to Tsunami Dynamics.- Solitary waves propagating over variable topography.- Water waves generated by a moving bottom.- Tsunami surge in a river: a hydraulic jump in an inhomogeneous channel.- On the modelling of huge water waves called rogue waves.- Numerical Verification of the Hasselmann equation.- Source & Run up.- Runup of nonlinear asymmetric waves on a plane beach.- Tsunami Runup in Lagrangian Description.- Analytical and numerical models for tsunami run-up.- Large waves caused by oceanic impacts of meteorites.- Retracing the tsunami rays.- Modeling and visualization of tsunamis: Mediterranean examples.- Characterization of Potential Tsunamigenic Earthquake Source Zones in the Indian Ocean.- Erratum.

Increasing the Existence of Very Shallow-Water LIDAR Measurements Using the Red-Channel Waveforms

Abstract: Mapping shallow-water bathymetry with acoustic techniques is complicated and expensive. The environmental parameters in shallow-water ( 2 m) whose shape is invariant with respect to the water depth and (2) waveforms in shallow-water depths that show a change in shape as a function of the depth in the water column. The data for this study are from the US Geological Survey LIDAR surveys of Lake Tahoe, CA, and Lake Michigan, using a SHOALS-400 LIDAR system

The influence of geomorphology and sedimentary processes on shallow-water benthic habitat distribution: Esperance Bay, Western Australia

Abstract: The mapping of seabed environments is fundamental to successful fisheries management and environmental monitoring, however, there is an emerging need to better characterise habitats based upon appropriate physical parameters. In this study, relationships between seabed geomorphology and the distribution of benthic habitats were examined using multibeam sonar, underwater video, predicted wave energy, and sediment data for Esperance Bay, part of the Recherche Archipelago. This shallow (<50 m), high energy, biogenic sediment dominated environment is located in temperate southwestern Australia. Exposure to wave energy appears to determine the distribution of unconsolidated substrate, and is the most useful regional scale predictor of rhodolith and seagrass habitats. Although they are intermittently smothered by mobile sediments, limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. The distribution of rhodolith beds is related to poorly sorted sediments that contain high gravel, mud, and CaCO3 percentages. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised sediment transport and accumulation play a major role in increasing the diversity of inner shelf benthic habitats. This highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats.

Effects of hydrodynamics on phosphorus concentrations in water of Lake Taihu, a large, shallow, eutrophic lake of China

Abstract: To understand the effect of hydrodynamical process on water phosphorus concentration, wind, wave, and several water quality indices were observed in Meiliang Bay, a shallow and eutrophic bay locates in north of Lake Taihu. During the 7 day observation period, wind speed and significant wave height were recorded more than 3 h per day, and water samples were collected in five water-depth layers once a day. Hydrodynamical disturbance had no significant correlationship with the water quality at the top layer when the significant wave height was smaller than 30 cm, but it significantly increased suspended solids (SS) concentration of the bottom water layer. Concentrations of nutrients showed no positive correlationship with SS concentration in the water body. Intensive sediment resuspension may not have occurred when the hydrodynamic stress on sediment was only a little higher than the critical stress for sediment resuspension. A new method for confirming the critical stress for intensive sediment resuspension and nutrient release still needs to be developed. The range of the water quality indices was quite high during the seven days of observation. High variation seems to be a common character of large shallow lakes like Taihu.

Frequency dependence and intensity fluctuations due to shallow water internal waves.

Abstract: A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30–200Hz on the New Jersey continental shelf.

Excitation of microseisms

Abstract: [1] Excitation of microseisms is generally considered to be due to pressure change at ocean bottom, for which Longuet-Higgins derived his celebrated formula in 1950. Use of this formula is an approximation, however. Comparison with a more rigorous normal-mode formula shows that this conventional approach is acceptable for ocean depths less than 1 km but fails in deep oceans. On the other hand, there seems to be a multitude of evidence that source region for double-frequency microseim is near the coast and thus is generally in shallow water. An evidence from buoy data for nonlinearity in ocean waves is presented to support this view. If a source region is in shallow water, use of the Longuet-Higgins pressure formula at ocean bottom for the excitation of microseisms is justified, although one should pay attention to ocean depths very carefully.

Mathematical Justification of a Shallow Water Model

Abstract: The shallow water equations are widely used to model the flow of a thin layer of fluid submitted to gravity forces. They are usually formally derived from the full incompressible Navier-Stokes equations with free surface under the modeling hypothesis that the pressure is hydrostatic, the flow is laminar, gradually varied and the characteristic fluid height is small relative to the characteristics flow length. This paper deals with the mathematical justification of such asymptotic process assuming a non zero surface tension coefficient and some constraints on the data. We also discuss relation between lubrication models and shallow water systems with no surface tension coefficient necessity.

Scholte-wave tomography for shallow-water marine sediments

Abstract: SUMMARY We determine the 3-D in situ shear-wave velocities of shallow-water marine sediments by extending the method of surface wave tomography to Scholte-wave records acquired in shallow waters. Scholte waves are excited by air-gun shots in the water column and recorded at the seafloor by ocean-bottom seismometers as well as buried geophones. Our new method comprises three steps: (1) We determine local phase-slowness values from slowness-frequency spectra calculated by a local wavefield transformation of common-receiver gathers. Areal phase-slowness maps for each frequency used as reference in the following step are obtained by interpolating the values derived from the local spectra. (2) We infer slowness residuals to those reference slowness maps by a tomographic inversion of the phase traveltimes of fundamental Scholte-wave mode. (3) The phase-slowness maps together with the residuals at different frequencies define a local dispersion curve at every location of the investigation area. From those dispersion curves we determine a model of the depth-dependency of shear-wave velocities for every location. We apply this method to a 1 km2 investigation area in the Baltic Sea (northern Germany). The phase-slowness maps obtained in step (2) show lateral variation of up to 150 per cent. The shear-wave velocity models derived in the third step typically have very low values (60–80 m s−1) in the top four meters where fine muddy sands can be observed, and values exceeding 170 m s−1 for the silts and sands below that level. The upper edge of glacial till with shear-wave velocities of 300–400 m s−1 is situated approximately 20 m below sea bottom. A sensitivity analysis reveals a maximum penetration depth of about 40 m below sea bottom, and that density may be an important parameter, best resolvable with multimode inversion.

Waves in shallow water, with emphasis on the tsunami of 2004

Abstract: This conference was organized in response to the 2004 tsunami, which killed nearly 300,000 people in coastal communities around the Indian Ocean. We can expect more tsunamis in the future, so now is a good time to think carefully about how to prepare for the next tsunami. With that objective, this paper addresses three broad questions about tsunamis.

Generalized Shallow Water Wave Growth from Lake George

Abstract: Probably the best set of observations of wave growth in shallow water presently available has been acquired in 1996 by Young and Verhagen in Lake George, Australia. These observations were taken during nominally ideal conditions of a constant wind, in fetch-limited conditions, over water with a constant depth (wind from the North or South, along the main axis of the elongated lake). However, we found a north-south stratification in the data that Young and Verhagen ignored. This stratification suggests that the wave growth was affected by the tapering of the lake in southern-wind conditions but not in northern-wind conditions (when the upwind lake is wider). This interpretation was supported by computations with a numerical wave model. We therefore removed the southern-wind data and reanalyzed the northern wind data to reformulate the growth curves for the significant wave height and peak period. For young sea states (relatively deep and transitional water depths), we find considerably higher (by 30%)—but ...

Evolution of solitary waves and undular bores in shallow-water flows over a gradual slope with bottom friction

Abstract: This paper considers the propagation of shallow-water solitary and nonlinear periodic waves over a gradual slope with bottom friction in the framework of a variable-coefficient Korteweg-de Vries equation. We use the Whitham averaging method, using a recent development of this theory for perturbed integrable equations. This general approach enables us not only to improve known results on the adiabatic evolution of isolated solitary waves and periodic wave trains in the presence of variable topography and bottom friction, modeled by the Chezy law, but also importantly, to study the effects of these factors on the propagation of undular bores, which are essentially unsteady in the system under consideration. In particular, it is shown that the combined action of variable topography and bottom friction generally imposes certain global restrictions on the undular bore propagation so that the evolution of the leading solitary wave can be substantially different from that of an isolated solitary wave with the same initial amplitude. This non-local effect is due to nonlinear wave interactions within the undular bore and can lead to an additional solitary wave amplitude growth, which cannot be predicted in the framework of the traditional adiabatic approach to the propagation of solitary waves in slowly varying media.

Effect of a Shallow Water Obstruction on Long Wave Runup and Overland Flow Velocity

Abstract: A study is presented to examine the one-horizontal dimension effect of a shallow shelf obstacle on nonlinear long wave runup. Due to the large horizontal-vertical aspect ratio of this problem, it is not well suited for experimental analysis, and therefore this study is purely numerical. Simulations are performed for various incident wave conditions, obstacle height and widths, and final beach slopes. Many of the setups involve breaking, either through approaching the obstacle as a large breaking bore, incipient breaking on top of the obstacle, or breaking during the beach uprush. The general conclusion of this study is that, for highly nonlinear waves wave height/shelf water depth 0.5, the obstacle will always act to reduce the runup and the maximum overland velocity. However, for very small obstacle lengths, particularly for extremely large waves, this reduction may be practically inconsequential. Interestingly, for weakly nonlinear waves 0.1, due to front steepening over the obstacle, greater overland velocities can result from increasing obstacle length. Consistent with previous studies, it is found that the final beach slope is of primary importance for determining the runup.

Evolution of solitary waves and undular bores in shallow-water flows over a gradual slope with bottom friction

Abstract: This paper considers the propagation of shallow-water solitary and nonlinear periodic waves over a gradual slope with bottom friction in the framework of a variable-coefficient Korteweg-de Vries equation We use the Whitham averaging method, using a recent development of this theory for perturbed integrable equations This general approach enables us not only to improve known results on the adiabatic evolution of isolated solitary waves and periodic wave trains in the presence of variable topography and bottom friction, modelled by the Chezy law, but also, importantly, to study the effects of these factors on the propagation of undular bores, which are essentially unsteady in the system under consideration In particular, it is shown that the combined action of variable topography and bottom friction generally imposes certain global restrictions on the undular bore propagation so that the evolution of the leading solitary wave can be substantially different from that of an isolated solitary wave with the same initial amplitude This non-local effect is due to nonlinear wave interactions within the undular bore and can lead to an additional solitary wave amplitude growth, which cannot be predicted in the framework of the traditional adiabatic approach to the propagation of solitary waves in slowly varying media

Environmental implications of stratification and turbulent mixing in a shallow lake basin

Abstract: The extent of stratification and vertical mixing in the water column (7–11 m deep) was investigated over an offshore reef in the western basin of Lake Erie. Measurements reveal that the vertical transport of oxygen and heat is controlled by the complex interaction of several physical mechanisms. Generally, when the wind speed (W) was >7 m·s–1 and the air was cooler than the water (Tair Tw) occurs every year. Results indicate that there is strong probability of hypoxia due to stratification (i.e., when diffusivities < 10–6 m2·s–1) and sediment oxygen demand (i.e., 0.1–1.0 g·m–2·day–1) during these periods. The environmental implications of stratification to water quality and its effects...

Characteristics of oceanic impact‐induced large water waves—Re‐evaluation of the tsunami hazard

Abstract: The potential hazard of a meteorite impact in the ocean is controversial with respect to the destructive power of generated large ocean waves (tsunamis) We used numerical modeling of hypervelocity impact to investigate the generation mechanism and the characteristics of the resulting waves up to a distance of 100150 projectile radii The wave signal is primarily controlled by the ratio between projectile diameter and water depth, and can be roughly classified into deep-water and shallow-water impacts In the latter, the collapse of the crater rim results in a wave signal similar to solitary waves, which propagate and decay in agreement with shallow-water wave theory The much more likely scenario for an asteroid impact on Earth is a relatively small body (much smaller than the water depth) striking the deep sea In this case, the collapse of the transient crater results in a significantly different and much more complex wave signal that is characterized by strong nonlinear behavior We found that such waves decay much more rapidly than previously assumed and cannot be treated as long waves For this reason, the shallow-water theory is not applicable for the computation of wave propagation, and more complex models (full solution of the Boussinesq equations) are required

Hydrodynamic characteristics of a fringing coral reef on the east coast of Ishigaki Island, southwest Japan

Abstract: To investigate the characteristics of currents on a fringing coral reef, a field survey was conducted, mostly under weak wind conditions in summer, on the east coast of Ishigaki Island, southwest Japan, which is encompassed by well-developed fringing reefs. For the same study period, numerical simulations of the current were also performed using a shallow water turbulent flow model with high accuracy reef bathymetry data, which were estimated from high-resolution imagery obtained from satellite remote sensing. The numerical simulation results showed good agreement with the observed data and revealed that the currents have an appreciable magnitude of tide-averaged velocities, even during neap tides, which are governed mostly by wave set-up effects. The results also indicated that temporal variations in velocity and water surface elevation during a tide cycle in the reef exhibit highly asymmetrical patterns; in spring tides especially, the velocities around channels indicate rapid transitions over a short period from peak ebb flow to peak flood flow. The simulations also indicated that a big channel penetrating deeply into the reef attracts the tide-averaged mean flow, even from distant areas of the reef.