Showing papers on "Wave flume published in 1999"

Waves and currents over a fixed rippled bed: 3. Bottom and apparent roughness for spectral waves and currents

Abstract: This paper is the third in a series of three that presents the results of experiments designed to verify the use of a single bottom roughness length scale for waves and currents over a rough bed. While the first two papers concentrated on the bottom roughnesses experienced by monochromatic wave and current boundary layer flows, this paper presents the results of additional experiments that investigate the use of an equivalent wave representation to extend these results to spectral wave and current boundary layer flows. Spectral waves, simulated by five components, and currents were generated in a 20-m-long wave flume with a fixed rippled bottom. Attenuation due to bottom friction is determined from total attenuation measurements for individual wave components by removing the effects of sidewall dissipation and wave-wave interactions. These attenuation estimates are used to establish representative friction factors, which are used in conjunction with an existing eddy viscosity model to determine bottom roughnesses. The bottom roughnesses experienced by spectral waves (in the presence and absence of a current) match the bottom roughnesses for monochromatic waves. When these experimentally determined bottom roughnesses are used in conjunction with the eddy viscosity model, predictions of attenuation for individual wave components closely match measurements. When the wave boundary layer thickness is defined to be the height at which the predicted velocity deficit in the wave boundary layer is within 5% of the free stream velocity, excellent agreement is obtained between predicted and measured velocity profiles for currents in the presence of codirectional waves. Therefore these experiments show that a single bottom roughness, when used in conjunction with an equivalent wave representation, adequately characterizes both monochromatic and spectral wave-current boundary layer flows over a fixed rippled bed.

Development of two-dimensional numerical wave flume for wave interaction with rubble mound breakwaters

Abstract: The numerical wave flume VOFbreak2 for simulation of wave interaction with a rubble mound breakwater is presented. The key innovations are a porous flow model and wave boundary conditions. The porous flow is implemented using a Forchheimer model. At the boundaries waves are generated using a combined wave generation-absorption technique and are absorped using a numerical sponge layer.

Shoaling of Periodic Waves Over Barred-Beaches In a Fully Nonlinear Numerical Wave Tank

Abstract: A numerical wave tank based on fully nonlinear potential flow theory is used to calculate changes in local properties of periodic waves shoaling over barred-beaches (wave height, celerity, front-to-back asymmetry). Results show that strongly nonlinear wave decomposition phenomena occur in a modulation region beyond the bars. These are analyzed in detail and discussed in the paper. Received December 7, 1998; revised manuscript received by the editors May 7, 1999. The original version (prior to the final revised manuscript) was presented at the Eighth International Offshore and Polar Engineering Conference (ISOPE-98), Montréal, Canada, May 2429, 1998..

Interim development of a numerical wave flume for maritime structure design

Abstract: This paper describes interim development of a numerical wave flume for practical use in designing maritime structures. The flume is based on a 2-D numerical wave simulation using a NASA-VOF method particularly selected because it can reproduce highly nonlinear wave profiles against various structures, e. g., impact of a breaking wave on a vertical wall. During fiscal years 1998 to 2000, a cooperative research group has been tasked with this project, having almost completed the associated computer code in 1998 and studied application of the numerical wave flume for practical design of various types of coastal structures.

Impact loadings on vertical walls in directional seas

Abstract: Recent research under PROVERBS has demonstrated that wave impact loads can cause damage or failure of caisson or blockwork breakwaters. Research studies of wave loadings, pulsating or impact, have generally used only 2 - dimensional wave flume experiments, and so most design methods are strongly biased towards 2-d . This paper describes results from 3-dimensional wave basin tests by Universities of Naples, Sheffield, and HR Wallingford in the UK Coastal Research Facility (CRF). The aim of the study was to quantify effects on wave pressures or forces of oblique or short-crested wave conditions on simple vertical or composite breakwaters. This analysis has focussed particularly on wave impact loadings as earlier 2-d tests at Wallingford showed them to be potentially severe for some combinations of foundation level and relative wave conditions. New reduction factors are presented.

Wave forces on a vertical wave barrier

Abstract: This paper presents a comparison of measured and predicted wave forces on a vertical wave barrier, defined here as a thin impermeable vertical wall extending from above the water surface down to near mid-depth. Theoretical wave loads are computed using the eigenfunction expansion method. Measured wave loads are obtained from two sets of laboratory experiments, one conducted at the U.S. Naval Academy and the other conducted at the Oregon State University in a large wave flume. Results of this study suggest that the eigenfunction theory can predict wave loads to within 10% to 20% accuracy for a wide range of wave conditions, water depths, and wave barrier drafts.

Bedforms in a Laboratory Wave Flume: An Evaluation of Predictive Models for Bedform Wavelengths

Abstract: Recent bedform dimensions measured in shallow waters in the nearshore zone (VINCENT and OSBORNE, 1993; OSBORNE and VINCENT, 1993; MARSH, 1996) compare poorly with published predictive models for bedform dimensions. A series of experiments were conducted in a large flume with a computer-controlled wave generator and a sand bed, using waves of various amplitudes and characteristics including waves from a field site and monochromatic waves. The ripple wavelengths were then compared to the wavelengths predicted by the models of NIELSEN (1981), GRANT and MADSEN (1982), MOGRIDGE et al. (1994) and WIBERG and HARRIS (1994), and to the semi-quantitative model of CLIFTON (1976). Under spectral waves from the field site the mean ripple wavelengths are anorbital remaining constant (within the scatter of the measurements) and showing none of the trends predicted by the models but falling between the dimensions predicted by NIELSEN (1981) for 'laboratory' and 'field' waves. Under monochromatic waves the ripples scaled with the wave orbital amplitude (λ= 0.4A 0 ) and were much closer to the model predictions. It is suggested that it is rather difficult to change the wavelength of ripples once they have formed. Field waves generally have a broad spectrum of frequencies (and hence of orbital excursions) so there is no length scale of sufficient dominance to force the bed to reform. With regular waves every orbital excursion is the same and the bed rapidly scales to this length. Our data suggest that bed form dimensions in an event may therefore be determined by the first waves capable of imposing their length scale on the bed, or by bed forms from an earlier event.

2DH Non-Linear Dispersive Wave Modelling and Sediment Transport in the Nearshore Zone

Abstract: A 2D-horizontal sediment transport energetics model is developed in this work for the evaluation of the wave-induced sediment transport. The time dependent energy equation is incorporated into a nonlinear dispersive wave model in order to simulate breaking wave propagation in the surf zone. Total immersed weight transport is related, through an energetics approach, to the total dissipated fluid power. Both the dissipation due to bed friction and, inside the surf zone, due to the wave breaking are considered. The methodology is applied to predict the longshore transport rate and to simulate the coastline evolution in beach nourishment scenario assuming a trapezoidal beachfill.

Hydraulic performance of a high mound composite breakwater

Abstract: The hydraulic performance of an innovative structure based on the concept of a high mound composite breakwater has been investigated in large-scale hydraulic model tests in the Large Wave Flume (GWK) of the Coastal Research Center (FZK), Germany. The experimental results concerning wave breaking, wave transmission and wave reflection are presented.

Numerical model of cnoidal wave flume

Abstract: The volume of fluid (VOF) method is used to set up a wave flume with an absorbing wave maker of cnoidal waves. Based on the transfer function between wave surface and paddle velocity obtained by the shallow water wave theory, the velocity boundary condition of an absorbing wave maker is introduced to absorb reflected waves that reach the numerical wave maker. For H/d ranging from 0.1 to 0.59 and T g/d from 7.9 to 18.3, the parametric studies have been carried out and compared with experiments.

Applicability of wave models in shallow coastal waters

Abstract: The wave propagation within coastal areas is strongly influenced by the coastal morphology with its islands, bars, shoals and channels. Predominant processes in this zone are shoaling, bottom friction, breaking, refraction, wind generation and to some extent diffraction of waves. The numerical formulation of these processes in standard wave models, like HISWA (HIndcast Shallow Water WAves, TU Delft), SWAN (Simulation WAves Nearshore, TU Delft) and MIKE 21 EMS (Elliptic Mild Slope, Danish Hydraulic Institute) were tested by comparing the simulation results with measurements in a wave tank and on site measurements at the North Frisian Coast of Germany. The numerical and experimental data measured in the wave flume are in very good agreement for all applied wave models proving the numerical formulation quality of bottom friction, shoaling and breaking. A comparison of numerical simulations results with SWAN and field data shows also a quite good agreement but revealed in some cases larger differences which may be contributed to the interaction of tidal currents and waves.

Performance of submerged active breakwaters in a hydraulic model

Abstract: A submerged active breakwater consists of a large buoyant cylinder that is held horizontal beneath the free surface of the water, by a spring and damper restraint system. The cylinder will be forced to oscillate in a certain mode, in response to an incident wave train. If properly "tuned", the cylinder can absorb a considerable fraction of the incident wave energy. Utilization of this concept may provide a number of potential benefits including; 1) a no loss to fish habitat; 2) a depth-independent materials cost; 3) a scheme easily adaptable to long term water level changes (such as those which occur naturally in the Great Lakes and those which are anticipated with sea level rise); and 4) the capability of adequately protecting a coastal area while maintaining boat access and water circulation. Knowledge of these devices however, is currently limited to performance in; 1) regular wave trains of narrow frequency bands, 2) zero angle of incidence between the wave crest and the structure, and 3) waves of small amplitude. Research evaluating the performance of submerged active breakwaters was performed in a two-dimensional wave flume in the Queen's University Coastal Engineering Research Laboratory (QUCERL). Both single cylinders and multi-cylinders placed in series were evaluated. Transmission coefficients in the range of 0.3 to 0.7 were measured over a broad range of conditions, indicating the possibility of these devices being used in prototype situations to achieve the benefits described above.

Laboratory investigation on wave reflection characteristics of suspended perforated pipe breakwater

Abstract: A laboratory investigation on the suspended perforated hollow cylinders in a single row was conducted in a two-dimensional wave flume, to study their hydraulic performance. The influence of depth of submergence, size of pipes, percentage of perforations and water depth on wave reflection characteristics have been investigated. From the investigation it was found that, as the relative depth of submergence increases, reflection increases. Water depth has insignificant effect on reflection coefficient Kr. The reflection coefficient Kr increases as incident wave steepness increases. For perforated pipes, size of the pipe has negligible effect on Kr. Wave period alone does not directly influence the reflection.

Long Waves in Flume Experiments

Abstract: This paper addresses the influence of long waves on the design wave height of structures in shallow water. Wave heights, wave periods, depths of water at the structure, time of wave measurement, length of the wave-guides were all varied in 29 series of twodimensional hydraulic model tests. The results indicated that long wave activity is an important design parameter for breakwaters in shallow water. The wave height at breaking and long wave reflection from the structure are the primary parameters influencing long wave activity.

Experimental Study of Wave-Absorbing Performance by Horizontal Punching Plates

Abstract: Wave absorbing system is needed at various kinds of wave basins (wave flume, towing tank, square tank) for the model test related to the ocean engineering. In this paper, the performance of wave absorbing system with new concept is estimated throughout the experiments. Herein, the wave absorbing system is designed by punching plate with a given porosity which is installed horizontally and submerged near the water surface. As the incident wave generated by a wave maker advances above a punching plate, the strong jet flow is formed near a hole of punching plate. As a result, wave energy is dissipated into heat energy, Systematic model tests were conducted at KRISO to verify the performance of the wave absorber using a punching plate. It was found that the reflection coefficient of wave absorber is deeply dependent on both the porosity and the submerged depth of a punching plate. Inclined installation of a punching plate shows better performance than a horizontal one within a certain inclined angle.

Simulation of Bichromatic Second-Order Stokes Waves In a Numerical Wave Flume

Abstract: The generation and propagation of second-order Stokes waves in a semi-infinite, narrow channel of uniform depth is simulated numerically. The wave motion is produced by the small-amplitude, bichromatic, oscillatory motion of a generic planar wavemaker. Both the firstand second-order problems are solved by a boundary element approach. A novel secondorder radiation boundary condition, previously developed for monochromatic waves, is extended herein to deal with a bichromatic wave system. The suitability of the second-order radiation boundary condition is verified by monitoring the wave profile and through energy considerations on the computational domain. The accuracy of the computed results is checked by comparing the time-domain results with published frequency-domain solutions and laboratory data. It is concluded that the present approach provides an accurate and efficient technique to simulate the generation and propagation of second-order bichromatic Stokes waves in a two-dimensional wave flume. Received April 13, 1998; revised manuscript received by the editors April 12, 1999. The original version was submitted directly to the Journal on April 13, 1998.

Random wave kinematics and coastal structures

Abstract: A linear theory-based method for the prediction of wave kinematics in front of coastal structures for the general case of obliquely-incident waves is described. The method uses empirical equations to determine the reflection coefficient and wave phase shift spectra for given incident wave structure characteristics. Surface elevations, water particle velocities and bed shear stresses were obtained from a wide range of experiments conducted in the UK Coastal Research Facility involving steep and very non-linear incident waves. Wave behaviour at the structures varied from heavy breaking of steep incident waves onto sloping structures to high reflection of less steep waves from a vertical wall. Comparison of predicted and measured kinematics shows that the linear method yields good engineering estimates of the spectra and time series of surface elevation and water particle velocity. The results from no-structure experiments show that a 10% overprediction in velocity is inherent in the application of linear wave theory to the very non-linear wave conditions involved. Predicted near-bed horizontal velocities can be used to obtain estimates of bed shear stress which compare favourably with measured bed shear stresses.

Three-Dimensional Model of Waves Passing Submerged Porous Structures

Abstract: In this paper a three-dimensional model of waves passing submerged porous structures is presented. The boundary element method is used to describe flows in the porous structure and waves passing above the structure. Linear wave theory is used to describe the wave fields, and the theory of waves in the porous structures proposed by Sollitt and Cross (1972) is used for the porous structures. The present effort is to combine a wave model and a porous wave model for three-dimensional problems. A simple grid generation program is also developed to facilitate the numerical computation. The numerical model is first verified by the results of waves passing submerged impermeable structures. Then the model is used to study three-dimensional effects of submerged porous structures on passing waves. Wave profiles along the centerline of the structure in the wave field are used to compare with two-dimensional results. The results show that the present model can duplicate the cases of waves passing submerged impermeable structures. For the cases of submerged porous structures, the results show that in comparison with the two-dimensional problem there exist diffraction effects due to the submerged structures. Since the structures are submerged the incident waves are transmitted over the structure while waves are partially reflected. Due to the submerged structures the transmitted waves generate focusing behind the center of the structures. Three-dimensional wave patterns are presented, and characteristics of transmitted waves are also investigated.

Numerical prediction of wave transformation, velocity, and bottom stress in the inner surf and swash zone

Abstract: A physical model study was conducted in a narrow wave flume to verify a previously developed numerical model for predicting the hydrodynamic response to irregular waves on a rough impermeable slope. In the physical model study, one case was run using a narrow banded TMA spectrum and a rough impermeable composite slope. The composite slope consisted of a 1:10 slope fronted by a 1:35 slope. The incident and reflected waves were resolved at 13 locations along the 1:35 slope using a three gage array. In addition, the free surface was measured along the 1:10 slope at 15 locations. A vertical stack of three capacitance sunup gages was used to measure the waterline oscillation along the 1:10 slope. Also, velocities were measured using a laser Doppler velocimeter (LDV) at 11 horizontal locations along the 1:10 slope at several elevations. A logarithmic profile was shown to exist for many phases of the onshore and offshore flows in the surf zone. Inside the swash zone, the logarithmic profile was more clearly established during the offshore flow. The measured and computed hydrodynamic responses were compared to evaluate the capability of the model in predicting the flow on a rough slope in the swash zone. The incident wave time series at two locations was specified at the seaward boundary of the numerical model. The measured and computed time series of both the reflected wave time series at the seaward boundary and the water line oscillation on the slope were plotted. The ability of the model to predict the detailed time varying quantities as well as the general wave characteristics of wave reflection, sunup, and flow velocity on the rough impermeable slope indicate that the model could be used to predict the swash zone hydrodynamics. The model showed a strong sensitivity to changes in the friction factor along the 1:10 slope. Improvements in the estimation of the bottom friction factor are needed to better predict the bed stress in the swash zone.

A study on breaking of multi-directional random waves due to a submerged breakwater

Abstract: The objective of this study is to investigate experimentally the breaking limit of waves due to a submerged breakwater in a multi-directional random wave field. Experiments have been conducted in a 3-D wave basin equiped with a multi-directional random wave generator. In this study, a special type of wave gages has been employed to record the water surface fluctuations at wave breaking with favorable accuracy. This study investigates the influence of incident wave conditions and wave directionality parameter Smax on the wave breaking limit. An equation has been introduced to predict the wave breaking limit in regular wave field using the least square technique applied to the results. This equation has been revealed to work well over a wide domain of wave conditions ranging from crossing waves to multi-directional random waves.

Wave Breaking, Infragravity Waves, and Sediment Transport in the Nearshore

Abstract: : The long-term goals of this research are to understand and model the spatial and temporal transformation of wave breaking in the surf zone of natural beaches, and to predict the effect of wave breaking on the forcing of mean and oscillatory flow, sediment transport, and the evolution of large scale topography.