Showing papers on "Wave flume published in 1995"

Sediment transport by waves and currents

Abstract: A bed-load model, based on a generalization of Bagnold's (1954) constitutive equations together with the assumption of similarity profiles for velocity and concentration, was proposed for sediment transport by waves by Sleath (1994). This model is now extended to wave/current flows and applied to three different situations: (a) oscillatory flows with superimposed currents, (b) net transport produced by progressive waves over a flat horizontal bed, and (c) longshore transport induced by waves approaching a beach. In all three cases the model gives reasonable agreement with experiment. In the case of longshore transport the model produces almost the same formula as that of the Coastal Engineering Research Center (1977), apart from additional terms for density and grain size.

Resonant generation of internal waves on the soft sea bed by a surface water wave

Abstract: The nonlinear response of an initially flat sea bed to a monochromatic surface progressive wave was studied using the multiple scale perturbation method. Two opposite‐traveling subliminal internal ‘‘mud’’ waves are selectively excited and form a resonant triad with the surface wave. The amplitudes of the internal waves grow on a time scale much longer than the period of the surface wave. It was found that the sea bed response is critically dependent on the density ratio of water and soil, depth of water, and depth and viscosity of the saturated soil. The result of instability analysis is in qualitative agreement with the result of a wave flume experiment.

Wind Effects on Runup and Overtopping

Abstract: The maximum distance a wave may travel up the face of a coastal structure, or rate of overtopping if runup exceeds structure crest elevation, are critical parameters in planning and design of a coastal structure. Runup and overtopping are usually estimated by empirical equations based on physical model studies that do not include the effects of strong onshore winds that are typically present during design storm conditions. While it is generally assumed that onshore winds will increase runup and overtopping over no-wind conditions, there is currently no means of accurately calculating effects of these winds on runup and overtopping. A joint research project by US Army Corps of Engineers and Texas A&M University (TAMU) is currently investigating wind effects on runup and overtopping of revetments and vertical walls through a series of physical model studies conducted in a combined wind/wave flume at TAMU. Initial tests measured runup and overtopping rates on a 1:3 smooth revetment for a range of incident monochromatic wave conditions, with wind speeds varying from no wind to maximum blower output. With the addition of wind, large increases in runup and overtopping were recorded over the no wind condition. The combined wind/wave spectrum recorded during tests with wind was then reproduced mechanically.

Wave-current interaction with mud bed

Abstract: A series of wave flume experiments for wave-current-mud system has been performed. The results reveal that the rate of wave attenuation increases in the opposing currents and decreases in the following currents. The results are the same with the waves on the fixed bed. Another important result is that the mud mass transport velocity increases in the opposing currents and decreases in the following currents. The visco-elastic-plastic model which was proposed by Shibayama and An (1993) for wave-mud interaction has further been extended to the wave-current-mud system by taking a wave deformation effect into considerations. The extended model succeeds in predicting the laboratory results. The model shows that due to pressure gradient in wave propagating direction, the largest mud mass transport velocity occurs in the strongest opposing current and the smallest one in the strongest following current.

Coastal engineering : waves, beaches, wave-structure interactions

Abstract: Part 1 Fundamentals: basic formulation of sea waves - regular wave theory, description of random waves wave interactions with structures and hydrodynamic forces - boundary-value problems on wave interaction with body, numerical analysis on the wave boundary-value problems, analysis for large bodies, wave forces on small bodies, wave transformation through permeable structures, vortex flow and fluid force generation of wave impact load waves, wave-induced currents and sediment transport - fluid motion and bottom shear stress in wave-current co-existing system, sediment movement and beach deformation, formulation of sediment transport rate, prediction of wave transformation and wave-induced current, prediction of topographic change caused by non-equilibrium suspended sediment transport. Part 2 Applications: structures for wave control - wave control mechanism, rubble mound breakwaters, composite breakwaters, submerged breakwaters with a wide crown width (artificial reef), low reflection structures, curtain-wall type breakwater, floating breakwater, membrane structures structure for controlling sediment movement - basic concept and structure for controlling sediment movement, beach deformation control by offshore detached breakwaters, control of beach deformation using groyne, control of beach deformation by submerged breakwater and artificial reef, control of beach deformation by sea dyke of gentle slope artificial beach nourishment, protection works against shoaling in harbours and navigation channels, structure for processing river mouth marine structures for ocean space utilization - offshore structures, piled pier structures, oil booms and silt curtains harbour tranquility - workable limit of a harbour - numerical methods for the prediction of a wave field in a harbour, numerical methods for the prediction of ship motions in a harbour, long-period ship motions, countermeasures for harbour tranquility, problems associated with relative motions of two adjacently-moored floating bodies fishery structures - classification and function of aquacultural propagation facilities, artificial fish reef (AFR), submerged moored artificial habitat, submerged buoy-cable system for shellfish farming, floating fishery cages.

Nonlinear coupling in waves propagating over a bar

Abstract: The degree of nonlinear coupling in a random wavefield propagating over and beyond a bar is examined using a physical wave flume as well as numerical simulations based on time-domain extended Boussinesq equations and their frequency-domain counter-part. The nonlinear phase speed is computed from the evolution of the nonlinear part of the phase function inherent in the frequencydomain model. Over the bar, the phase speeds of the higher harmonics are larger than the linear estimates due to the nonlinear couplings, resulting in virtually dispersionless propagation, while beyond the bar crest, nonlinear effects on the phase speed vanish rapidly, implying full release of bound harmonics. Quantitative measures of nonlinearity such as the skewness and asymmetry have also been determined. They have near-zero values in the deep-water region on either side of the bar and a pronounced peak over the bar. On the downwave side, the random wave field is found to be spatially homogeneous. This implies that it can be fully described by the energy density spectrum without additional phase information related to the bar location.

Multiple bar formation by breaker-induced vortices: a laboratory approach

Abstract: Using a two-dimensional wave flume and a two-videocamera system, mechanism and process of multiple bar formation were investigated in the light of breaker-induced vortices. A fixed-bed experiment revealed that (1) the vortex formed at the wave break point can be classified into three: oblique, A-type horizontal, and B-type horizontal vortices; (2) the vortex formed in the surf zone is always oblique; and (3) the location of a vortex reaching bottom can be described by wave properties and the bottom slope, and its water depth can be expressed by breaker height alone. A movable-bed experiment indicated that bars are initiated by the vortices reaching bottom in the surf zone; the number of bars formed coincides with the number of such vortices. Suspension of the bottom sediment due to vortex action and the mean-drift-velocity pattern can play an important role in multiple bar formation; mechanism for break-point bar formation is "convergence", while that for inner bar formation is "congestion". Two major modes for the development of multiple bars were found: simultaneous and successive: these depend on the interaction of vortex action and topography in the surf zone. Multiple-bar features such as trough spacing and crest depth could be explained from vortex features found through the fixed-bed experiment.

Numerical modelling of fluid flows with surface waves

Abstract: We consider some aspects of constructing the efficient computational algorithms for problems of wave hydrodynamics, including finite difference schemes for problems hi complete statement and approximate models of higher order in the shallow water theory. They are employed to solve nonconventional applied problems, using model test computational experiments. Particular attention has been given to the construction of the adaptive grids which are adjusted to the solution and to the form of a basin, as well as the formulation and realization of the so-called 'artificial' boundary conditions. 1. BASIC MATHEMATICAL MODELS One of the current methods for studying the surface gravitational waves is the method of direct numerical modelling of fluid motions. The waves are simulated either by approximate hydrodynamic shallow water models with various orders of approximation, viz. linear, nonlinear, and nonlinear dispersive models or the so-called complete models depending on the intervals of variation of the characteristic parameters. The difference between the complete and approximate models is that when deriving the complete models we do not make assumptions as to wavelengths, their amplitudes, the constancy of the fluid parameters in depth, etc. However, they are considered to be only conditionally complete because when constructing them we nevertheless make some other assumptions though not as restrictive as those for approximate models. Thus, the most commonly used model for potential fluid flows with a free boundary implies that the fluid is ideal and the flow is potential. The vortex flow model either may take or not take into account the viscosity of the medium, etc. The most important problem is to construct a hybrid mathematical model which can describe the wave transformation and choose automatically the adequate algorithm depending on the current characteristics of the wave and the water area where it is propagating. In order to construct an efficient hybrid model we must develop the criteria for choosing its components, algorithms for glueing the solution, and the boundary conditions that describe various physical phenomena being modelled, i.e. open boundaries, interaction with floating and anchored objects, the surf. We should take into account the requirements of adequacy the models, as well as the efficiency of computational algorithms which must be suitable for a priori and operative modelling. In order to correctly take into account the effects of dispersion and their interaction with the effects of nonlinearity we can employ the models which retain their dispersive properties in a wide range of wavelengths, whereas the form of their description and the assumptions made during their derivation allow us to use efficient stable algorithms in the two-dimensional statement of the problem. * The work was supported by the Russian Foundation for the Basic Research (93-01-01729). * Institute of Computational Technologies, the Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia

Wave velocity field measurements over a submerged breakwater

Abstract: The main focus of the experiment was to observe and measure large-scale vortices generated by breaking waves over a submerged breakwater. These flow structures are important in sediment transport due to their ability to trap sediment particles negating their normal settling velocity. The technique of Particle Image Velocimetry (PIV) was used to measure the spatial distribution of velocities at an instant; an approach which is essential for measuring coherent structures in the flow. Experiments were conducted in the 50m flume at The University of Florence which was fitted with a 1:100 beach, wave elevations, as well as velocity fields, were measured. The paper deals with the experimental details and a display of the velocity and vorticity maps obtained using PIV.

Sediment transport over ripples in waves and current

Abstract: Numerical simulations are presented for the flow and the dynamics of noncohesive sediment over 2D ripples in waves and current. For this purpose, a model based on a discrete vortex approach using the "cloud-in-cell" concept has been applied. The hydrodynamic model drives a Lagrangian model for the suspended sediment. The characteristic vorticity structures prevailing in the vicinity of ripples are accurately replicated and sediment suspension mechanisms are successfully described. Thus, the sediment transport resulting from asymmetrical waves and combined waves and current can be predicted. Especially, the wave-related component of the transport opposes the maximum free stream velocity and contributes significantly to the total transport. In the present work, the effect of the wave-induced drift is not included.

COMPARISON OF Two-DIMENSIONAL AND THREE-DIMENSIONAL BEACH PROFILES

Abstract: Scale effects are always a major concern in mobile-bed hydraulic-model studies. These scale effects can be minimized by using large models (i.e., almost prototype size), but large models require large and costly modeling facilities. Several large “super flumes” have been built for two-dimensional (2D) testing, but right now a comparable large “super basin” does not exist. Thus, beach processes can be tested in large 2D models, but not yet in three-dimensional (3D) models. The research in super flumes assumes that 2D-wave-flume experiments are representative of the 3D prototypes. Two aspects of that assumption are tested in this paper. Six sets of parallel tests were performed in both a 3D wave basin and a 2D wave flume. It was found that the time scales for morphological development are adequately modeled in the 2D flume, and that there are some differences in the resulting beach profile shapes. The 3D profiles correspond closely with the prototype observations; the 2D profiles are affected by exaggerated...

Vertical walls and low reflection alternatives - Results of wave flume tests

Abstract: Waves reflected from vertical breakwaters or other highly reflective structures forming port entrances can cause serious problems to vessel navigation. This often leads to port closures as vessels are unable or unwilling to enter or leave the port. This report describes all aspects of a 2D physical model study. This study was initiated to examine the reflection and overtopping performance of various 'low reflection' structures under irregular wave attack. It is these types of structures which can be used in the modification or construction of port entrances to reduce wave reflections, and so improve conditions for vessel navigation. Four main structures were tested consisting of a smooth vertical wall, single and double chamber wave screens and rock armour slopes. Various arrangements of these structures were tested to identify the optimum configurations. The study found that the use of single and double chamber wave screens is effective in reducing wave reflections and overtopping. From the analysis of these results, an empirical formula was derived which can be used to describe the reflection performance of such structures, within practical engineering limits. This report also describes a new technique to examine the effect of 'low reflection' structures on wave conditions. This was carried out through the analysis of local wave steepness. Previously a description of the reflection performance and overtopping of a structure was used in the assessment of the likely effect of wave reflections, and hence the relative changes in wave height, on vessel navigation. However, vessel navigation problems are not only related to wave height as the wave period is also significant in the creation of hazardous conditions. This new technique enables both relative changes in wave height and wave period to be considered, allowing a better assessment of the effect on vessel navigation to be made.

The circular wind/wave facilities at the University of Heidelberg

Abstract: At the University of Heidelberg circular wind/wave flumes are used to study small-scale air sea interaction processes under a wide range of conditions. The outer diameter of the large flume is 4 m and the channel width is 30 cm. Wind in the range of 0‐12 m/s is generated by a rotating paddle wheel. The unique circular design yields to homogeneous quasi-infinite fetch conditions. Since the flume is gas tight and built from transparent PVC, it is suitable for experiments with various chemical species including acid gases and surface active materials. The flume accommodates a wide variety of techniques for noninvasive sensing of waves, water flow, air flow, and gas exchange. A brief summary of experiments performed within the last twenty years is given.

The large scale dolos flume study

Abstract: In 1993, the U.S. Army Corps of Engineers, Waterways Experiment Station, Coastal Engineer Research Center conducted the Large Scale Dolos Flume Study (LSDFS) in order to investigate the structural response of concrete armor units. The study was primarily carried out in the large wave flume at the O.H. Hinsdale Wave Research Laboratory, Oregon State University. Over 300 model dolos units with a mass of 26 kg and a waist ratio of 0.32 were used. The units were cast from concrete. The instrumented dolosse were fitted with surfacemounted strain gages then subjected to a wide range of wave loading conditions. This strain gaging and the state-of-the-art data acquisition system increased the signal-to-noise ratio so that accurate measurements of static, quasi-static, wave-induced hydrodynamic, and unit-to-unit impact loading could be recorded. The LSDFS included a standard calibration series, static ramp tests, dry-land impact tests, and regular and irregular wave flume tests. Hydrodynamic instrumentation in the flume tests consisted of a very dense array of wave gages, current meters, runup-rundown gages, pore pressure transducers, hydrophones, digital video, and still photography in the nearshore zone. This paper presents previously unpublished details of the LSDFS specifically pertaining to instrumentation, calibration, dry-land impact tests, and some preliminary results of impact response captured in the flume tests.

On the generation and propagation of waves in 2d numerical wave tanks

Abstract: In the perspective of a time domain simulation of fully nonlinear motions of free floating bodies, a study is conducted on the characteristics of generated waves in numerical wave tanks. Under the hypothesis of a perfect fluid flow, the adopted mathematical model is first presented and widely discussed. The main features of generated waves are then reported and discussed both from a physical and a numerical point of view.

A numerical model of beach change due to sheet-flow

Abstract: First a discussion is made on the relation between the time-average of a Boussinesq-type equation and the mean flow equation. Then nonlinear wave transformation on a sloping bed is computed by a set of Boussinesq equations including a breaker-induced energy dissipation term. Comparisons are made between the computations and laboratory measurements for cross-shore distributions of the wave height and mean water elevation, and for time-histories of the near-bottom velocity near and after breaking. Undertow current velocity in the nearshore zone is calculated by a semi-empirical formula and is also compared with measurement data. A beach profile change model is set up by combining the Boussinesq-type equations, a sediment transport rate formula for the sheet-flow proposed by Dibajnia and Watanabe, which incorporates the asymmetric orbital velocity due to wave nonlinearity as well as the undertow current, and a sediment mass conservation equation proposed by Watanabe et ah, which includes the effect of local bottom slope. The validity of the model is examined through the comparisons of the computed transport rate distributions and beach profiles with the laboratory data obtained in large wave flume experiments.

Potential flow of fluids

Abstract: Reflection and the transmission of solitary waves on a two layer fluid over a small step some recent advances on wave effects on large offshore structures nonlinear mechanics of gravity waves on deep water - on the Nonlinear Schrodinger Equation prediction of wave breaking processes at the coastline wave breaking simulation second order wave leads in 3D bodies in regular waves recent mathematical developments of potential theory applied to diffraction of water waves. (Part contents).

Cross-shore profile modelling under random waves

Abstract: This paper presents part of the test results conducted in the Large Wave Flume (LWF) for 2D beach profile response under random wave input as well the numerical modelling effort to simulate the laboratory data. The tests were conducted with various input wave spectra, initial profiles and for both erosional and accretional cases. The numerical model is a 2DV(two dimensional vertical) beach profile model. It couples a sediment transport model with a random wave model. The sediment model is a modified version of the SBEACH model developed by Larson and Kraus (1989) for regular waves. The input wave condition is a time-series of irregular waves simulated from a given wave height probability density function, here selected as the Rayleigh distribution. The final profile is then computed from the cumulative changes due to each randomly occurring individual wave.

A model for cross shore sediment transport

Abstract: In this paper a mathematical model for the computation of the cross shore sediment transport for random waves is presented. The model consists of the hydrodynamic, the suspended sediment transport and bed load modules. The suspended sediment concentration is computed with the use of the wave-averaged convection diffusion equation. The vertical structure of both the wave-induced current and suspended sediment concentration are modelled with the use of quasi-3d techniques. Mechanisms for the offshore transport (undertow) and the onshore transport (Lagrangian transport, transport due to wave asymmetry) are included. The model is tested against an experiment with random waves at prototype scale.

Analysis of practical rubble mounds

Abstract: This paper describes results from an unusual research project completed under Topic 3R2 of the European Union's MAST research project G6-S Coastal Structures. In this project, data were collected from the major European hydraulics laboratories on the hydraulic and structural responses of example rubble mound breakwaters and sea walls that each laboratory had previously studied in wave flume or wave basin tests. The main responses considered here are: a) Main armour stability, given by measurements of armour movement and/or displacement under wave action. b) Wave overtopping, described by the number of waves passing over the structure crest, or by the mean overtopping discharge; The paper describes some of the analysis of armour stability and hydraulic performance of these structures, and explores the potential to develop general conclusions from ad hoc studies. This paper develops some of the analysis described initially within the G6-S project by Allsop & Franco (1992), but also re-considers and revises some of the early analysis and initial conclusions.

Mud transport and muddy bottom deformation by waves

Abstract: The present study focuses on numerical simulation of wave height decay and muddy bottom deformation under wave action. Experiments have been performed to determine the vertical distribution of the water content ratio of mud. Using the experimental results, a simulation model has been set up to estimate change in the water content ratio in the mud bed. Also, a simple one-dimensional numerical model has been developed to predict the muddy bottom deformation under waves as well as the wave height decay. Wave flume experiments have been carried out and the changes in the bottom topography and wave height have been compared with the calculations.

Mean flux in the free surface zone of water waves in a closed wave flume

Abstract: This paper presents some results of research relating to the theoretical predictions of mass-transport velocity within the free surface zone of water waves in intermediate water depth. The theoretical results are compared with measurements made in a wave flume. The theoretical estimate of a mean drift has allowed for a better estimation of the return flow in the wave flume. Examples of such estimation are given and graphically presented in the paper. Finally, the stability of the obtained mean velocity profiles throught the experiments is examined.

A vorticity-based model for gas transfer under breaking waves : Special topic on : Ocean-atmosphere interactions

Abstract: This paper presents a new renewal model for gas transfer under the influence of breaking waves. It is proposed that the renewal rate is proportional to the vorticity of the waves at the water surface. Constants were evaluated from experimental data obtained in a wave flume at the Laboratory of Harbour Works Athens. Experiments on oxygenation due to breaking waves on a uniformly sloping beach and on a rubble mound breakwater of the S-type were performed. The water was chemically deoxygenated and dissolved oxygen (D.O.) concentration was followed over time in characteristic locations. Experimental data showed that the transfer velocity increased with increasing wave height for waves of the same frequency. Experiments with waves of the same wave height but increasing wave frequency showed also an increase in transfer velocity. The one-dimensional transport equation was used for the determination of the transfer coefficients. Preliminary analysis of the data indicated that the transfer coefficients varied almost linearly with the vertical wave velocity at the water surface. A rather good linear correlation was obtained for the breaking wave data, with a much higher slope as compared to the case of non-breaking waves. Further, an additional positive influence of the wave steepness on the dimensionless transfer coefficient was shown. However, in both of these correlations there was a distinct difference between sets with different wave frequencies. For the vorticity-based model presented in the paper no such difference appears between different sets of data. Two equations, with high correlation coefficients, are obtained, one for the breaking waves on the sloping beach and one for the breaking waves on the breakwater. The breakwater data give lower transfer velocities as compared to the sloping beach data for the same wave characteristics.

Wave loads on inclined cylinders due to random waves

Abstract: The aim of the present study is to investigate the wave induced pressures and forces exerted on inclined circular cylinders due to the action of random waves. The test cylinder is of 20 cm diameter consisting of different individual segments, in which one of the segment was mounted with pressure transducers around its circumference. The tests were carried out in a wave basin with random waves described by three different standard spectra. The sectional force time history is obtained by integrating the pressure time histories. Both spectral and statistical approaches have been applied to investigate the effect of inclination of the cylinder on the wave forces. The salient results of the present study are reported in this paper.

Experimental Study of Internal Waves Using Particle Image Velocimetry

Abstract: Oceanic internal waves cause significant currents over large depths centred about the density difference between upper and lower layers of warm and cold water produced by the thermocline. Model waves are produced in a laboratory wave flume using a saline solution and fresh water as the two fluids. Elevation records from the model reveal a modulation in regular waves connected with the buoyancy frequency. Velocity measurements of the flow field show that the linear theory is adequate to predict the flow of both small and medium amplitude waves.

Propagation and Breaking of Wave Groups.

Abstract: : It is well known that long waves and currents in the cross-shore and long- shore directions are generated due to the process of wave breaking. Numerous authors have worked on the mechanisms for the generation of long waves; all these studies show that wave groupiness is a primary factor in the generation of long waves in the offshore as well as in the nearshore region. To predict the long waves, a comprehensive model is required which can predict the time varying wave height and frequency.

Sediment concentrations and Sediment transport in case of Irregular breaking waves

Abstract: Coastal changes occur mostly as a result of changes in sediment transport along the coast. If at cross-section A, the sediment transport is for any reason larger (or smaller) than at cross-section B, accretion (or erosion) will take place in between the two cross-sections. For prediction of coast-lines in the future, the prediction of the net sediment transport is therefore essential. Various models, such as that of Bijker, Van Rijn, Nielsen, Engelund & Hansen and Ackers & White are available to predict the sediment transport by knowledge of wave height and current strength. The reliability of these models is unknown because data under field conditions are scarce. Only few relations between sediment transport, current velocity and wave height are known. For these reasons a laboratory study was carried out to extend the knowledge of the basic phenomena in morphological processes. The study contains experiments in which sediment concentrations and fluid velocities have been measured in case of irregular breaking waves alone and in combination with a current. Chapter 2 deals with the sediment transport basics. Two types of sediment transport, the longshore and the cross-shore sediment transport are discussed and the objectives of the present experiments are presented. In Chapter 3 the experimental set up is described. The measured parameters, methods and instruments are discussed. The experimental programme of the series A and the series B I and B2 are presented. Chapter 4 covers the experimental results from test series A. The wave characteristics, fluid velocities and the influence of these parameters on the sediment concentration, sediment load and sediment transport are studied. Chapter 5 deals with the experimental results from test series B I and B2. The distribution of the sediment concentrations, fluid velocities, sediment loads and sediment transport rates over a sand bar are studied. In Chapter 6 a comparison is made between the measurements and the sediment transport models by Van Rijn and Bijker. Transport rates, concentration profiles and velocity profiles are compared. In Chapter 7 a list of conclusions and recommendations is presented.

곡면 해양구조물에 작용하는 동파력 고찰

Abstract: This study is to investigate the dynamic pressure caused by breaking waves on a recurved offshore structure. A physical modelling was performed in a two-dimensional wave flume. The measuments from the physical modelling were compared with several known equations. The shock and secondary pressures were found to be dependent on water depth, breaking wave height and the size of the air pocket. The maximum pressure was recorded near the still water level and the secondary pressures near the recurved section showed negative values. The study concluds that the maximum shock pressures on the recurved structure were found to be less than those experienced in a vertical offshore structure.