Showing papers in "Ocean Wave Measurement and Analysis in 2002"

Extension and Modification of Discrete Interaction Approximation (DIA) for Computing Nonlinear Energy Transfer of Gravity Wave Spectra

Abstract: We examined the validity and the effectiveness of the DIA on the basis of the accuracy of the computations of the nonlinear energy transfer for various directional spectra. Then, we proposed new methods, an extended DIA (EDIA) and a modified DIA (MDIA). The optimum parameters included in EDIA and MDIA were properly adjusted by minimizing the errors between the exact values and their approximate values for various directional spectra. The efficiency and accuracy of the EDIA and the MDIA were examined for various directional spectra and computational conditions.

Extension of the Discrete Interaction Approximation for Computing Nonlinear Quadruplet Wave-Wave Interactions in Operational Wave Prediction Models

Abstract: This paper introduces a method to extend the Discrete Interaction Approximation for the computation of quadruplet wave-wave interactions in wind wave spectra. This is achieved in two ways. First, by introducing a generally shaped configuration between the four interacting wave numbers in a quadruplet for which the corresponding equations to compute these non-linear interactions in a wave spectrum are derived using the principle of detailed balance as starting point. Second, by adding additional wave number configurations. By using a kind of orthogonal expansion of generally shaped wave number configurations it is possible to approximate the exact solution to any desired degree of accuracy.

Neural Network Forecasting of Storm Surges along the Gulf of Mexico

Abstract: Accurate water level forecasts are of vital importance along the Gulf of Mexico as its waterways play a critical economie role for a number of industries including shipping, oil and gas, tourism, and fisheries. While astronomical forcing (tides) is well tabulated, water level changes along the Gulf Coast are frequently dominated by meteorological factors. Their impact is often larger than the tidal range itself and unaccounted for in present forecasts. We have taken advantage of the increasing availability of real time data for the Texas Gulf Coast and have developed neural network models to forecast future water levels. The selected inputs to the model include water levels, wind stress, barometric pressures as well as tidal forecasts and wind forecasts. A very simple neural network structure is found to be optimal for this problem. The performance of the model is computed for forecasting times between l and 48 hours and compared with the tide tables. The model is alternatively trained and tested using three-month data sets from the 1997, 1998 and 1999 records of the Pleasure Pier Station located on Galveston Island near Houston, Texas. Models including wind forecasts outperform other models and are considerably more accurate than the tide tables for the forecasting time range tested, demonstrating the viability of neural network based models for the forecasting of water levels along the Gulf Coast.

Wave Growth Limit in Shallow Water

Abstract: The wave growth limit in shallow water is studied by analyzing a) field data from two Dutch lakes, b) the wave growth curves according to Bretschneider and Young and Verhagen and c) the results of the numerical model SWAN. The uncertainty about the achievable wave height to water depth ratio appears to be considerable. SWAN tends to underestimate the wave growth limit and a strategy for further model improvement is pointed out.

Benchmark Cases for Tsunamis Generated by Underwater Landslides

Abstract: Three benchmark cases are proposed to study tsunamis generated by underwater landslides. Two distinct numerical models are applied to each benchmark case. Each model involves distinct center of mass motions and rates of landslide deformation. Computed tsunami amplitudes agree reasonably well for both models, although there are differences that remain to be explained. One of the benchmark cases is compared to laboratory experiments. The agreement is quite good with the models. Other researchers are encouraged to employ these benchmark cases in future experimental or numerical work.

Design Wave Evaluation for Coastal Protection Structures in the Wadden Sea

Abstract: The evaluation of design waves for coastal protection structures is enormously important for both the safety of the hinterland and economical use of public money. The recently gained advancements in mathematical modeling of waves allows its successful application even in areas with complex morphological structures like the Wadden Sea. A remaining major problem of coastal design wave modeling is still the evaluation of suitable boundary conditions for design conditions, particularly for offshore design waves. The following paper highlights procedures applied for the solutions of these problems in the Frisian Wadden Sea at the south North Sea coast which are partly also applicable at coastlines of any type.

Theory of Significant Wave Period Based on Spectral Integrals

Abstract: A formula for the significant wave period is derived. This containes the complex value integral of energy spectrum, and thus depends on the spectral shape asymetry. For the wind waves characterized by a Pierson-Moskowitz spectrum the spectral asymetry becomes remarkable, so that the significant wave period T 1/3 deviates from the mean period T m . The empirical ratio T 1/3 / T m of 1.2 is well interpreted by the proposed theory.

An Application of Wavelet Transform Analysis to Landslide-Generated Impulse Waves

Abstract: This paper presents an analysis of applying continuous wavelet transform spectrum analysis to the results of laboratory measurement of landslide-generated impulse waves. As the measured results are understandably unsteady, nonlinear, and non-stationary, the application of time-localized wavelet transform analysis is shown to be a suitable as well as useful approach. The analysis on correlating the time-frequency wavelet spectrum configurations in connection with the ambient parameters that drive the impulse wave process, with respect to time and space, leads to interesting and stimulating insights not previously known.

Space-Time Imaging of Shoaling Waves and Surf

Abstract: : A fundamental barrier to consequential evaluation of modem, very capable shoaling wave and surf models has been the inability to provide high-quality ocean data with which to test model results. This paper describes a development intended to satisfy this need by providing space-time visible images of the nearshore from which three parameters crucial to such evaluations are simultaneously retrieved. These fields are the wave spectrum, bathymetry and currents. A panchromatic digital framing camera has been mounted on a small aircraft and used to collect time series of images of waves as they shoal and break. The camera system is controlled by a computer-driven turret which provides accurate location and pointing angles so that the images can be mapped to the mean water level on a common geodetic reference surface. This effectively separates space and time variations associated with the waves. The resulting time series imagery can be mapped and displayed much like a movie taken from a sky hook. These data are used with algorithms to retrieve the ocean parameters of interest, specifically the wave spectrum, water depth and currents. The 3-D frequency-wavenumber spectrum is calculated in sub-regions of the nominal 2 km scene, and the theoretical dispersion relation for linear gravity waves is fit to the spectrum, with the local water depth and current as free parameters.

Regional Wave Transformation and Associated Shoreline Evolution in the Red River Delta, Vietnam

Abstract: Several stretches of shoreline in the Red River Delta, Vietnam, are suffering from severe erosion, especially an area in the southern part of the delta known as Hai Hau Beach. Commonly found explanations for this erosion is the construction of a large dam upstream the Red River Delta and the closing of a river branch that used to supply sediment to the beach. However, in this study it is shown that the prevailing wave climate during the winter monsoon in combination with the complex topography of the delta create longshore transport gradients that promotes erosion at Hai Hau Beach. A newly developed two-dimensional random wave transformation model based on the mild slope equations was employed for a 20-year long time series of hindcast waves to compute representative nearshore wave properties for the Red River Delta shoreline. These properties were utilized to compute the longshore sediment transport rate along the delta from which shoreline evolution was inferred.

Modelling the Effects of Structures on Nearshore Flows

Abstract: The effects of defense structure on nearshore water flows are analyzed by means of an efficient and robust flow solver. This is both flexible enough to incorporate bathymetric data typically collected in field surveys and accurate enough to allow for a detailed description of important flow features. Such a tool is believed to be capable of providing useful information for design activities of coastal structures like submerged breakwaters.

Using Wave Statistics to Drive a Simple Sediment Transport Model

Abstract: Because both contaminant and nutrient cycles in the Laurentian Great Lakes depend on particle behavior and movement, sediment transport is a critical component of many of the water quality models being developed to understand and manage this important resource. To avoid complicated models that cannot be supported by the available field data, we have used observation-based, empirical analysis as the basis for developing methods of predicting sediment resuspension from relatively simple measurements of the surface wave field. Our modeling is based on data obtained from instrumented tripods designed to measure near-bottom hydrodynamic and sedimentological conditions for extended periods of time. Because of the long duration of the deployments, it usually is impractical to both sample and record the data at the high frequency that would be needed to resolve the effects of individual surface waves. Instead, we have used a system of burst sampling, in which we sample the sensors at high frequency during a period of time that is repeated at an interval appropriate for the deployment duration. Rather than record the individual samples during the burst, we record only statistics obtained from the individual samples. Our results show that simple representations of the surface wave field obtained from the burst statistics can be used to model sediment transport in wave-dominated environments. We also show that once the model parameters are determined, the forcing wave conditions can be derived from other sources, including wind-driven wave models, with comparable success.

Kinematics and Transformation of New Type Wave Front Breaker over Submerged Breakwater

Abstract: This study presents the discovery of a new type of wave breaking over a submerged breakwater. Our findings were made through laboratory experiments. In this new type, wave breaking first occurs at a wave's front part, not at the top of the wave crest. Next, the wave crest passes over the breakwater's crown without breaking. In this study, we call this wave breaker a wave front breaker , and the process of wave transformation and the conditions for the wave front breaker to occur are clarified. The wave kinematics of the wave front breaker is also investigated by PIV and LDV measurements. Consequently, it is shown that the occurrence of the wave front breaker strongly depends on the incident wave length, and the wave transmission coefficients of the wave front breaker become larger. It is also found that an inverse layer of the mass transport velocity exists near the seabed against the main region of the mass transport velocity. Furthermore, the direction of the mass transport velocity near the toe in the inverse layer is onshore, while that of the maximum acceleration is offshore and the value of the acceleration increases as the measuring points get closer to the seabed and away from the toe.

Comparisons of Wave Measurements from the Eurorose Fedje Experiment

Abstract: The objective of the EuroROSE project was to combine area covering ground-based remote-sensed wave and current data with high resolution numerical forecast models to provide now- and forecasts for coastal marine operators. The first experiment to test and demonstrate the system took place on the coast of Norway, north of Bergen in March 2000. Qualitative and quantitative intercomparisons of the wave measurements and wave model products from this experiment are presented. These include measurements using the WERA HF radar, the WaMoS X-band radar, a directional waverider and output from the WAM wave model. Comparisons are made of the full directional spectra and of various derived parameters. This is the first ever intercomparison between HF and X-band radar wave measurements and between either of these and WAM.

Boussinesq Waves on Vertically Sheared Currents

Abstract: Boussinesq-type equations with second order dispersion axe derived for an arbitrary distribution of vorticity. The governing equations have the velocity at an arbitrary depth as a dependent variable and terms involving vorticity are kept as integrals. Linear dispersive properties are shown to be accurate to the order of dispersion when compared to the exact solution. Current input conditions (from a large scale hydrodynamic model) and the scheme for the numerical model are obtained in order to later test model results against measurements from an experiment in a wave flume for the case of waves propagating against a vertically sheared current.

System Identification Techniques for the Modelling of Irregular Wave Kinematics

Abstract: This paper presents the development of a parametric model linking the free-surface elevations with the orbital velocities field under an irregular wave. The system identification procedures are applied to estimate the parameters of the model based on data recorded in the wave tank. The free-surface time series are taken as input data and the output data are components of the particle velocity vector. A linear time-invariant model with the static nonlinearities at the input side incorporated is assumed. In the study different system identification procedures are applied for two cases—firstly when orbital velocities are modelled at points that are always submerged in the water and, secondly, in the surface zone where measurement points are not continuously submerged. This paper demonstrates the results of modelling the horizontal component of the orbital velocity in comparison with wave kinematics data taken in a wave flume using Laser Doppler Velocimetry specifically designed to give accurate measurements also in the vicinity of the mean water level. Comparison between the modelled and observed velocity time series presents a sufficiently good agreement to prove the effectiveness of the applied approach. The SI techniques applied to determine from the free-surface records the orbital velocities as well as other hydrodynamic parameters such as particle acceleration and forces may be very important and useful in engineering and oceanography.

A curvilinear Boussinesq model and its application

Abstract: In this paper, a curvilinear Boussinesq model is further improved in order to be used in more practical applications. Firstly, the energy dissipation due to wave breaking is considered by introducing an eddy viscosity term into momentum equations. Secondly, the slot technique is introduced into the curvilinear model for simulations of wave run-up/run-down in swash zones. Thirdly, Smagorinsky subgrid lateral turbulent mixing is utilized to account for the effect of the resultant eddy viscosity on the wave-generated underlying flow, in addition, a TMA spectral wave maker is implemented in the model for simulations of irregular waves. Finally, the model is used to simulate wave propagation in Ponce de Leon Inlet. Model/data comparisons show the fully nonlinear Boussinesq model give accurate results for nonlinear wave transformation over irregular bathymetry. It is also shown that the curvilinear model is an efficient model for wave computations in large computational regions with complex geometry.

Nonlinear Wave Interaction and Its Applications to the Analysis of Ocean Waves

Abstract: Interactions among free waves result in bound waves, which may significant affect resultant wave properties in an ocean wave field, especially when waves are steep. Since the measurements record the resultant properties, the presence of bound waves makes it inaccurate to analyze the measurements of steep ocean waves using linear spectral methods. To overcome this difficulty, Hybrid Wave Models (HWM) developed recently, separate bound waves from free waves in the decomposition of an irregular wave field as well as the prediction of its resultant properties. To ensure the convergence, the HWMs selectively use the conventional and phase modulation approaches to address the nonlinear interactions between free waves of different frequency ratios. The models are able to decompose a wave field accurately and hence can predict the wave properties accurately and deterministically based on the time-series measurements. Examples of their applications to the analyses of laboratory and field measurements are given to demonstrate the usefulness of HWMs.

Application of Permeable Groins on Tourist Shore Protection

Abstract: The Northwestern region of Egypt is characterized by its virgin nature with its all-original rural conditions without human interference. During the last few decades, the beach strips that overlook the waterfront have witnessed an urbanization revolution. The majority of the carried out installations fall within one single sector of investment, which locally called Tourist Village. Human intervention in the coastal balance, by installation of different kinds of measures in the near shore zone, causes more diverse side effects and environmental deterioration. The unlucky shores subjected to erosion; one of these shores is located at about 140 km to the west of Alexandria and extends to about 1100 m. The shoreline change simulation for this shore is calibrated and modeled by GENESIS. The suggested protection measure was permeable groins. These groins, with its minimal effect on the adjacent shores, verify the requirements of Northwestern coastal protection policy. The proposed permeable groin is composed mainly of supported piles as main elements. In between each two piles, a wave screen is mounted. It is characterized by less water blocking, so it permits water flow exchange, and exerting slightly less total hydrodynamic force compared to a conventional pile groin. A construction system is designed to effectively reduce the installation time and in turn low cost. The modeling results and estimated cost show that the deterioration in the shoreline could be controlled with minimal side effects on the adjacent shores and with valued low cost.

Frequency Properties of the Rainflow Filter—Some Examples in Oceanography

Abstract: The rainflow filter is a nonlinear filter which can be used to eliminate small-amplitude oscillations in a signal. An advantage of the rainflow filter, well known in applications to fatigue, is that a considerable reduction of data is made, while the calculated total damage is not affected. An approximation of the filter in the form of a transfer function can be obtained from spectral simulations. To validate the so derived filter, the capability of filtering out small cycles is examined. Further, we pay attention to calculation of distributions for characteristic wave parameters. These methods need as input a spectral density, where high frequencies often are cut off. Numerical examples indicate that the rainflow filter is advantageous to use when filtering records of sea-elevation.

Depth-Limited Wave Breaking for the Design of Nearshore Structures

Abstract: Four laboratory datasets acquired in a directional wave basin and in a large-scale flume are re-analyzed to provide practical advice to coastal engineers in charge of estimating significant wave heights in the nearshore zone as part of a coastal structure design. An estimation of the total water depth including wave set-up and long wave activity is proposed. The underestimation of the significant wave height by using a spectral approach is stressed and rules of thumb are suggested to correct it.

Sediment Transport Related to Potential Sand Mining Offshore New Jersey

Abstract: Interest has developed to use borrow material from resource areas located on the inner continental shelf offshore New Jersey for beach replenishment purposes. A study was conducted to quantify the potential impacts of the offshore dredging and consequent beach nourishment on the local sediment transport patterns. In order to determine the offshore sediment transport patterns at the borrow locations, sediment transport rates were determined by coupling the results of spectral wave modeling with ambient current measurements. Having determined the sediment transport rates and directions at the borrow locations, and by knowing the volume of sediment to be mined, estimates of the borrow location recovery time were determined. These recovery time estimates were utilized to determine the time scale of influence on wave transformation, nearshore sediment transport, and re-mining frequencies.

A Low-Cost Wave-Sediment-Towing Tank

Abstract: The design and construction of a small wave-sediment tank, built and instrumented for under $10,000, is described. The tank is equipped with a piston-type wavemaker, capable of producing both small-amplitude and solitary waves. The middle section of the tank contains a sediment bed appropriate for wave-soil-structure interaction and sediment transport experiments. Wave energy is absorbed by a gravel beach located at the opposite end of the tank from the wavemaker. Instrumentation includes three capacitive wave height gauges. The wavemaker is controlled by a desktop computer, which also provides data-logging capability. Applications of this wave-sediment tank to research into grounded ship migration and barge added-mass are presented.

Simulation of Sediment Suspension Using Two-Phase Approach

Abstract: A dilute sediment transport model based on the two-phase mass and momentum equations is introduced with appropriate closures on the fluid turbulence. Due to the presence of the sediment phase, an important damping mechanism in the fluid turbulent kinetic energy equation is derived and modeled. The proposed model is solve both analytically and numerically to study the sediment transport experiment in a steady uniform open channel flow. In the analytical approach, we made additional approximations in order to obtain simple solutions. The analytical solution show clear improvement, which is due to a better modeling on the additional damping mechanism in the fluid turbulent kinetic energy equation, on the calculated concentration profile as compare with the solutions from the Rouse formula. A numerical model, which solves the complete dilute two-phase equations, is also developed. The accuracy of the numerical is checked with the experimental data. With an appropriate closure on the particle stress, the numerical model can be extended to solve the sheet-flow problems in the future.