Showing papers on "Wave flume published in 2009"

Effectiveness of Small Onshore Seawall in Reducing Forces Induced by Tsunami Bore: Large Scale Experimental Study

Abstract: Tsunami force and pressure distributions on a rigid wall fronted by a small seawall were determined experimentally in a large-scale wave flume. Six different seawall heights were examined, two of which were exposed to a range of solitary wave heights. The same experiment was done without a seawall for comparison. The measured wave profile contained incident offshore, incident broken, reflected broken, and transmitted wave heights measured using wire resistance and ultrasonic wave gauges. Small individual seawalls increased reflection of the incoming broken bore front and reduced force on the rigid landward wall. These findings agree well with published field reconnaissance on small seawalls in Thailand that showed a correlation between seawalls and reduced damage on landward structures.

Numerical modeling of wave evolution and runup in shallow water

Abstract: Based on the Navier-Stokes (N-S) equations for viscous, incompressible fluid and the VOF method, 2-D and 3-D Numerical Wave Tanks (NWT) for nonlinear shallow water waves are built. The dynamic mesh technique is applied, which can save computational resources dramatically for the simulation of solitary wave propagating at a constant depth. Higher order approximation for cnoidal wave is employed to generate high quality waves. Shoaling and breaking of solitary waves over different slopes are simulated and analyzed systematically. Wave runup on structures is also investigated. The results agree very well with experimental data or analytical solutions.

Lagrangian Particle Method As Advanced Technology For Numerical Wave Flume

Abstract: The particle method, which is a solver of the Navier-Stokes equation without using a computational grid, has excellent robustness in analyzing a violent water-surface change accompanied with a fragmentation and coalescence of water. Thus the particle method is an optimum tool for the analysis of the process of wave breaking and runup. Here are outlined the calculation fundamentals of the particle method. The state-of-the-art of the particle method is briefly introduced, including highly precise particle methods, by improving momentum conservation in discretization of governing equations and the new methods for control of pressure fluctuation. Finally, we show as prospective studies on the particle method a few of the significant issues for promoting the substantial contribution of the particle method to a numerical wave flume, which is the computer-aided resistive design tool of coastal structures against wave action.

Studies on Resonant Water Motion Between a Ship and a Fixed Terminal in Shallow Water

Abstract: This work focuses on the hydrodynamical problem of a Liquid Natural Gas (LNG) carrier near a Gravity Based Structure (GBS) -type offshore terminal subject to incoming waves in medium deep to shallow water conditions. The work is restricted to 2D, and the ship is restrained from moving. The resonant behavior of the fluid in the gap between the ship and the terminal is investigated. The problem is investigated by means of a numerical model and model tests. Potential theory is assumed, and a linear as well as a nonlinear time-domain numerical wavetank based on a boundary element method with a mixed Eulerian-Lagrangian approach is implemented for this purpose. Model tests (near 2D) of a midship section near a vertical wall are carried out in a 26.5 m long and 0.595 m wide wave flume in model scale 1:70. In full scale the ship beam is 45 m and the ship draft is 12 m. The ship model is constructed in such a way as to avoid flow separation, i.e., no sharp corners. Several parameters are varied: water depth, wave period, and wave steepness. Wave elevation is measured at 12 locations.

Nonlinear Internal Wave Run-Up on Impermeable Steep Slopes

Abstract: Laboratory experiments were conducted to investigate the run-up of internal solitary waves (ISWs) on steep uniform slopes in a two-layered fluid system with a free surface. A 12 m long wave flume, which incorporated a movable vertical gate for generating ISWs, was used in the experiments. A steep uniform slope was modeled at one end of the flume. In the present study we looked at internal waves with small and large amplitudes using a steep uniform slope (from one of θ=30, 50, 60, 90, 120, and 130 deg) much longer than those previously published in the literature. Results collected from a wide range of experimental runs show that the run-up height depends on the planar slope, while the breaking depth is only antecedent to the amplitude of an ISW. The overall quantitative agreement with the linear feature aspects of the incident wave amplitude is encouraging.

Coupled boundary element and finite element model for fluid-filled membrane in gravity waves

Abstract: This paper describes a three-dimensional, coupled boundary element and finite element model for dynamic analysis of a fluid-filled membrane in gravity waves. The model consists of three components, describing respectively, the membrane deflection and the motions of fluids inside and outside the membrane. Small amplitude assumptions of the surface waves and membrane deflection lead to linearization of the mathematical problem and an efficient solution in the frequency domain. A finite element model, based on the membrane theory of shells, relates the membrane deflection to the internal and external fluid pressure. Two boundary element models, which describe the potential flows inside and outside the membrane, are coupled to the finite element model through the kinematic and dynamic boundary conditions on the membrane. As a demonstration, the resulting model is applied to evaluate the dynamic response of a bottom-mounted fluid-filled membrane in a wave flume. Previous two-dimensional numerical model results and three-dimensional laboratory data verify and validate the present three-dimensional model. Analysis of the computed membrane response and surface wave pattern reveals intricate resonance characteristics that explain the discrepancies between the numerical model results and the laboratory data.

A two-dimensional horizontal wave propagation and mud mass transport model

Abstract: The present study offers a two-dimensional horizontal wave propagation and morphodynamic model for muddy coasts. The model can be applied on a general three-dimensional bathymetry of a soft muddy coast to calculate wave damping, fluid mud mass transport and resulting bathymetry change under wave actions. The wave propagation model is based on time-dependent mild slope equations including the wave energy dissipation due to the wave-mud interaction of bottom mud layers as well as the combined effects of the wave refraction, diffraction and breaking. The constitutive equations of the visco-elastic–plastic model are adopted for the rheological behavior of fluid mud. The mass transport velocity within the fluid mud layer is calculated combining the Stokes’ drift, the mean Eulerian velocity and the gravity-driven mud flow. The results of the numerical model are compared against a series of conducted wave basin experiments, wave flume experiments and field observations. Comparisons between the computed results with both the field and laboratory data reveal the capability of the proposed model to predict the wave transformation and mud mass transport.

Analysis of Wave Reflection from Structures with Berms Through an Extensive Database and 2DV Numerical Modelling

Abstract: This paper analyses wave reflection from permeable structures with a berm, including reshaping cases. Data are obtained from recent wave flume experiments and from 2DV numerical simulations performed with the COBRAS-UC code. The objectives of this research were to identify the proper representation of the average structure slope to be included in the breaker parameter and to check the performance of the formula for the reflection coefficient developed for straight slopes by the Authors. Based on the observation that for reflection, differently from what happens for overtopping and run-up, the whole slope below sea water level (SWL) is important, the slope to appear in the breaker parameter is evaluated as a weighted average of the structure slope below the berm level and the average slope in the run-up/run-down area. The inclusion of this slope in the proposed formula allows to extend its prediction capacity to structures with a berm and a fair agreement with both experiments and simulations is obtained.

Nonlinear Wave Diffraction by Submerged Horizontal Circular Cylinder

Abstract: This paper presents an experimental and numerical study of 2-D wave diffraction by a fixed horizontal-axis submerged circular cylinder. The objective of the paper is to report and validate results from the fully nonlinear 2-D boundary-element method code CANAL; to assess the capability of the physical wave flume and the measurement methodology to study this type of problem; and to contribute to the understanding of the wave diffraction by a circular cylinder. Two test cases are presented, both in deep-water conditions. The cylinder axis submergence is 1 5r (r is the cylinder radius) for the first case, and 3r for the second, the latter corresponding to the experimental study presented in the paper. A discretization convergence evaluation is also presented for the second case. The good agreement between the numerical and the experimental results proves that this numerical model is able to accurately simulate this type of problems. No reflection was noticed on the incident wave side of the cylinder. It was found that nonlinear resonant interactions occur between the fundamental frequency and the harmonics on the transmitted wave side of the cylinder. Free waves were observed on this part of the wave flume.

Submerged Plate Breakwater Composed of Horizontal Porous Plate And Slightly Inclined Solid Plate

Abstract: A coupled mechanism of the wave energy dissipation effects by a submerged porous plate and the blockage effects by a submerged solid plate has been investigated in the context of linear wave theory. A series of configurations of a multi-plate breakwater was studied parametrically, and an optimized system has been chosen as a new breakwater that is plausible and desirable in costal engineering practice. The optimum performance has been validated through experiments in a 2dimensional wave flume. The observed complicated wave deformations over, under and through the plates are discussed in detail. Wave reflection, transmission, dissipation coefficients, free-surface wave profiles and forces are calculated and discussed. A good use of the wave energy dissipation and blockage effects simultaneously stands on a resonance caused by the water wave interaction with the geometrical configuration of a system.

Low Frequency Waves in the Shoaling and Nearshore Zone

Abstract: It has been found by many researchers that low frequency (LF) waves dominate the wave energy spectrum in very shallow water. Given that many longshore and cross-shore morphological processes are located within this zone, LF waves play an important role in determining morphological change, especially dune erosion and overwash during storm events. The numerical model, XBeach [Roelvink et al. (2008)], has been developed to simulate such morphological processes which are influenced by LF waves. For hydrodynamics, it utilizes the wave forcing determined from a second generation wave module to drive linearized shallow water equations within a flow module. The objective of this thesis focuses on validating the hydrodynamics of XBeach with particular attention paid to the estimated LF waves. This is particularly important to correctly estimate morphological changes, as it is highly dependent on the accurate representation of waves and currents. In the validation study, XBeach is used to replicate the flume experiment of van Noorloos [2003] in which bichromatic and irregular wave conditions are imposed on a plane sloping beach. The advantage of using this experiment is that measurements have a high spatial resolution, allowing for decomposition into incoming and outgoing wave components. Group-varying time-averaged short wave parameters are used to investigate the accuracy of the short wave module in XBeach, while at the same time the relationship between the short waves and LF waves are determined by looking at the energy transfer to and from the LF components. The results confirm many previous findings, such as the plus 180 degree phase difference between the short wave envelope and the bound LF wave, the dominance of LF wave energy in shallow water and the ability of LF waves to extend their reach to the uppermost parts on a beach. Given the limitations of the assumptions based on linear wave theory, the short wave results from XBeach are quite good, especially when using a newly modified breaker parameterization of Battjes and Janssen [1978] by Roelvink [Pers. Com. 2009]. The main shortfall of the model is that it tends to overestimate the bound LF wave heights in shallow water for irregular waves. This is believed to be partly due to the overestimation of the energy and radiation stresses contained in the short waves in this region. In reality, as waves approach breaking, the non-linearities present in the waves increase, which, during breaking, effectively redistributes energy within the wave spectrum. Since frequency dependent shoaling of short waves is not currently enabled in XBeach, the energy within the HF band may tend to encourage continued shoaling of the LF waves in the surf zone. As such, it is recommended that the current wave action conservation scheme be improved to allow for either reduction or redistribution of the short wave energy during shoaling. Wave run-up as modelled with the linearized shallow water equations is shown to over-steepen the front of the LF wave in the swash zone. This effect is a likely result of the numerical scheme used, which tends to limit the maximum run-up height based on a minimum depth for which a computational grid cell is determined to be wet. In general, XBeach has shown that it is capable of modelling both quantitatively and qualitatively the short wave energy flux and LF wave shoaling and reflection quite will without inducing serious errors from the linear simplifications in the wave module. Wave-current interaction, which was not considered in the modelling, can most likely give even better results if it is successfully incorporated in the model.

EroGRASS: Failure of grass cover layers at seaward and shoreward dike slopes. design, construction and performance

Abstract: A large number of the dikes in the North Sea and Baltic Sea regions are covered with grass that is exposed to hydraulic loading from waves and currents during storm surges. During previous storm surges the grass cover layers often showed large strength and remained undamaged. A clear physical understanding of the failure of grass cover layers due to different wave loads is therefore indispensable today, especially against the background of enhanced hydraulic impact due to climate change. The strength of the grass cover layer lies mainly in its ability to withstand three types of wave actions: • Wave impact due to wave breaking on the seaward slope • Wave run-up and run-down flow after wave breaking on the seaward slope • Down-slope flow on the landward slope caused by wave overtopping. The main objectives of this research project are therefore to perform large scale model tests to investigate in detail the failure of grass cover layers due to (i) wave impact, (ii) wave runup and run-down flow and (iii) wave overtopping. Wave impact as well as wave run-up and run-down flow may induce grass cover failure on the seaward dike slope. Wave overtopping causes failure of the grass cover at the dike crest and on the shoreward slope. Hence, this research project deals with the investigation of grass cover failure anywhere along a dike profile: seaward slope, dike crest and shoreward slope. It is envisaged that the proposed research and tests will improve the understanding of the failure of grass cover layers due to wave loading. To obtain the aforementioned research objectives, large scale model tests at a dike model have been performed in the Large Wave Flume of the Coastal Research Centre – a joint centre of the University of Hanover and the Technical University of Braunschweig, Germany. The dike model represents a typical sea dike. With exception of the seaward slope, it is comparable to typical cross sections of sea dikes built in The Netherlands, Germany and Denmark. This relatively steep seaward slope has been chosen to improve the generation of wave impact on the seaward slope. The crest height of the dike model is 5.8m above the bottom of the wave flume and the dike model consists of a sand core covered by a layer of clay and a grass layer. The 0.2m thick grass cover layer is constructed with grass sods that have been excavated at the existing Ribe sea defence in Denmark and transported to the Large Wave Channel in Hannover by trucks. The report presents a first reporting of the EroGRASS project including a description of the design and construction of the dike model in the Large Wave Flume, the measuring and observation techniques and the test programme together with examples of records from the performed tests. Focus of this report is put on providing a well-documented description of the aforementioned issues, whereas the data analysis and presentation of the results are not included in this report since these are in progress and will be reported later.

Two-dimensional physical modelling of sand filled geocontainers for coastal protection

Abstract: Recently, the installation and upgrade of many coastal structures using sand filled geotextile containers has occurred both within Australia and internationally. Within the limited amount of hydraulic scale modelling that has been undertaken for sand filled geocontainers, most studies have not been directly related to the fill volumes, masses, densities and dimensions of full sized prototype geocontainers. The Water Research Laboratory has recently undertaken a range of investigations of ELCO Solutions' geocontainers. Following measurements of these geocontainers in a full scale field application, detailed hydraulic modelling of various geocontainer applications (revetments and groynes) was conducted with model parameters consistent with the field measurements. This conference paper discusses the scale laboratory tests conducted in the three metre wide wave flume at WRL for geocontainer revetment structures, and the relevance of the recorded results for design and planning of future geotextile container structures. A revetment constructed from these units would typically be built at the back of the beach against a pre-existing dune, for protection of landward coastal assets. As the study was not site specific, generic water level and design wave combinations were investigated to cover a range of conditions likely to be encountered. Various combinations of wave height, wave period (both monochromatic and irregular) and water level conditions were tested against combinations of three revetment structure slopes and three foreshore slopes. The investigation yielded several important conclusions. As expected, the geocontainer stability was found to decrease with increasing wave height and period; however, increasing revetment slope steepness provided increased stability. Design guidelines to be used by coastal engineers when planning a revetment or seawall to be constructed from ELCO Solutions' geocontainers were generated for various real world conditions.

Experimental and computational activities at the oregon state university nees tsunami research facility

Abstract: A diverse series of research projects have taken place or are underway at the NEES Tsunami Research Facility at Oregon State University. Projects range from the simulation of the processes and effects of tsunamis generated by sub-aerial and submarine landslides (NEESR, Georgia Tech.), model comparisons of tsunami wave effects on bottom profiles and scouring (NEESR, Princeton University), model comparisons of wave induced motions on rigid and free bodies (Shared-Use, Cornell), numerical model simulations and testing of breaking waves and inundation over topography (NEESR, TAMU), structural testing and development of standards for tsunami engineering and design (NEESR, University of Hawaii), and wave loads on coastal bridge structures (non-NEES), to upgrading the two-dimensional wave generator of the Large Wave Flume. A NEESR payload project (Colorado State University) was undertaken that seeks to improve the understanding of the stresses from wave loading and run-up on residential structures. Advanced computational tools for coupling fluid-structure interaction including turbulence, contact and impact are being developed to assist with the design of experiments and complement parametric studies. These projects will contribute towards understanding the physical processes that occur during earthquake generated tsunamis including structural stress, debris flow and scour, inundation and overland flow, and landslide generated tsunamis. Analytical and numerical model development and comparisons with the experimental results give engineers additional predictive tools to assist in the development of robust structures as well as identification of hazard zones and formulation of hazard plans.

Toe structures of rubble mound breakwater: Stability in depth limited conditions

Abstract: This thesis is about the stability of toe material for rubble mound breakwaters in depth limited conditions. The present equation, Van der Meer 1998, gives results for depth limited conditions but is not validated. The empirical equation is based on physical model tests done by Gerding 1993. The Van der Meer equation implies deep water and breaking waves on the structure slope. For shallow water conditions this assumption is not valid. Waves start breaking at the fore shore slope and toe which results in a different hydrodynamical wave load at the toe. Toe material is exposed to waves and starts behaving as armour rock. The uncertainties, introduced by shallow water situation are investigated in this research. The objective for this thesis is finding a more reliable design equation in this situation. Fore shore slope and wave steepness are considered of influence. The research is done by performing scale model tests in a two dimensional wave flume. The observations from the experiments and the analysis of the performed dataset gave following conclusions: Fore shore slope is strongly influencing toe stability. This is not only valid in shallow water but also in deep water. In shallow water, wave steepness influences toe stability as well. This is not proven for deep water. Very shallow water shows different hydrodynamic behaviour. Wave breaking occurs at the fore shore. The toe structure is attacked by breaking or already broken waves. Although a reduced wave height reaches the toe, damage is larger because the toe is exposed to turbulent wave attack. A new design equation for very shallow water is suggested in which fore shore slope and wave steepness are included. This is an empirical relation, using dimensionless relations like the Hudson stability number and a new damage number in percentages.

Maximum wind effect on wave overtopping of sloped coastal structures with crest elements

Abstract: Wave flume experiments were carried out on wind affected overtopping of rough and smooth sloping coastal structures with crest elements in the low overtopping regime (q* = q/ gHm0 < 2 x 10). The wind influence was modelled by a paddle wheel, a technique which has been shown to be reliable in assessing the maximum effect of wind on overtopping. Rough and smooth sloped coastal structures with slopes of 1:2 and 1:1.5 were tested. The crest of the coastal structure was modelled as a sharp crest using vertical crest elements of varying height. The results show that the mean overtopping discharge for rough and smooth sloping coastal structures under wind influence is in the range of 1.26.3 times the mean overtopping discharge without wind (qw/q 1.2 and 6.3) and thus in the same range as previously published for vertical structures.

A Lagrangian Finite Element Analysis of Gravity Waves in Water of Variable Depth

Abstract: The paper is concerned with the problem of gravitational wave propagation in water of variable depth. The problem is formulated in the Lagrangian description, and the ensuing equations are solved numerically by a finite element method. In computations a convecting mesh that follows the material fluid particles is used. As illustrations, results of numerical simulations carried out for plane gravity waves propagating over bottoms of simple geometry are presented. For parameters typical of a laboratory flume, the transformation of a transient wave, generated by a single movement of a piston-like wave maker, is investigated. The results show the evolution of the free-surface elevation, displaying steepening of the wave over sloping beds and its gradual attenuation in regions of uniform depth.

The initialization of nonlinear water waves in a numerical wave flume

Abstract: This study investigates the initialization of nonlinear free-surface simulations in a numerical wave flumeDue to the mismatch between the linear input wavemaker motion and the kinematics of fully nonlinear waves,direct numerical simulations of progressive waves,generated by a sinusoidally moving wavemaker,are prone to suffering from high-frequency wave instability unless the flow is given sufficient time to adjustA time ramp is superimposed on the wavemaker motion at the start that allows nonlinear free-surface simulations to be initialized with linear inputThe duration of the ramp is adjusted to test its efficiency for short waves and long wavesNumerical results show that the time ramp scheme is effiective to stabilize the wave instability at the start of the simulation in a wave flume

Sediment transport and beach morphodynamics induced by long waves

Abstract: New laboratory data are presented on the influence of long waves on sediment transport in the surf zone. Due to the very significant difficulties in isolating the morphodynamic processes induced by long waves in field conditions, the laboratory study was designed practically to measure the net sediment transport rates, and gradients in sediment transport, arising from the interaction between long waves and short waves in the surf zone. The bathymetric evolution of model sand beaches, with dB50B = 0.2 mm, was observed under monochromatic short waves, long-wave short-wave combinations (free long waves), and bichromatic wave groups (forced long waves). The beach profile change and net cross-shore transport rates, Q(x), were extracted and compared for conditions with and without long waves. The experiments include a range of wave conditions, e.g. high-energy, moderate-energy, low-energy waves, and the beaches evolve to form accretionary, erosive, and intermediate beach states. Hydrodynamic measurements were made to identify the influence of long waves on short waves and to determine the correlation between surf zone bars and standing long waves. A shallow water wave model was modified for this application to surf zone morphodynamics and compared to both hydrodynamics and measured sediment transport. This data clearly demonstrate that free large-amplitude long waves influence surf zone morphodynamics not only under accretive conditions, by promoting onshore sediment transport, but also under erosive conditions, by decreasing offshore transport. For the dominant berm-bar feature, the strong surf beat induces offshore transport in the inner surf zone and onshore transport around the outer surf zone and throughout the shoaling zone. In contrast, forced (bound) long waves and wave groups correlated with bichromatic short wave groups play a pronounced role under erosive conditions, increasing offshore sediment transport across the whole beach profile. For accretionary conditions, only a very narrowbanded wave group promotes onshore sediment transport across the whole beach profile, while broader banded wave groups again promote offshore transport. The modified numerical model of Li et al. (2002) provides good predictions of the standing long wave pattern for the long-wave short-wave combinations, but generally poor agreement for the bichromatic wave groups. Similarly, this model performs poorly in terms of predicting the net sediment transport for all waves, even after optimising the sediment transport coefficients. This is because the model cannot predict the correct hydrodynamics around the breakpoint position and does not correctly represent net sediment transport mechanics. Overall, the model does not correctly predict the trends in beach profile evolution induced by the long waves and wave groups. Further, there is little evidence that the long wave nodal structure plays a dominant role. The influence of the free long waves and wave groups is consistent with the concept of the Gourlay parameter, H/wBsBT, as a dominant parameter controlling net erosion or accretion. Free long waves tend to reduce H/wBsBT, promoting accretion, while wave groups tend to increase H/wBsBT, promoting erosion.

Experimental study on scouring and silting stability of the man-made beach in Dongjiang Harbor Area of Tianjin Port

Abstract: Experiments are carried out to study the sedimentation of the man-made beach in the east coast of Tianjin Port using the whole tidal current and sediment physical model and wave flume model.Since the coastal area of the project is a typical flow mud transfer coast,sedimentation in the intertidal zone of the hydrophilic section(about 5cm a year) is inevitable.Under the condition of sheltering,the man-made beach is basically stable under a 10-year wave.But under a 50-year wave,the deformation of the beach profile will occur,with the loss quantity in single width of about 3.3 m~3/m.