Showing papers on "Wave flume published in 1994"

Hydraulics and sediment transport dynamics controlling step-pool formation in high gradient streams: A flume experiment

Abstract: A series of flume experiments was undertaken to determine the domain of flow conditions under which step-pool sequences are formed. This domain can be characterized by the ratio of average shear stress to the shear stress required to move the largest grain sizes (T/T cr) and the Froude number (Fr). Step spacing was correlated with the antidune wavelength and steps did not form when particle motion was continuous under high sediment transport rates.

Experimental Verification of Numerical Model for Nonlinear Wave Evolutions

Abstract: Data collected from measurements in a two-dimensional (2D) wave flume with a trapezoidal bar on a horizontal bottom are used to test the numerical model previously developed by the writers. The experiments were performed both for monochromatic and for random waves, with two different peak frequencies. Overall agreement between the computations and the measurements is quite satisfactory, indicating that the numerical model favorably predicts waveform evolutions resulting from shoaling over the upslope, near-resonant wave-wave interactions over the bar, and decomposition behind the bar.

Vertical structure of the flow due to waves and currents - Laser-doppler flow measurements for waves following or opposing a current

Abstract: Changes in the mean horizontal-velocity profile in combined wave-current motion, outside the bottom boundary-layer, are essential for the correct prediction of cross-shore sediment transport outside the surf zone and the transport of dissolved matter, e.g. Klopman (1992). To be able to verify mathematical and numerical models (see Klopman, 1992) experimental data is needed. This is especially the case with the vertical structure of the wave and turbulence Reynolds stresses where data is lacking. For this reason, a laboratory experiment was conducted to study the flow kinematics under combined wave-current motion, for waves propagating in the current direction and for waves opposing the current. The wave-current facility in which the tests have been carried out is equipped with two computer-controlled wave boards, one generating waves and the other absorbing waves. Both wave boards have active wave-absorption systems which eliminate spurious waves. A constant discharge was provided by a flow-circulation circuit. Special care was taken in the design of the inflow and outflow structures, in order to introduce the current smoothly into the channel and to minimize unwanted reflections of the waves at the inflow and outflow. Flow velocities were measured in one vertical cross-section of the channel, with two laserDoppler velocimetry (LDV) systems, mounted at a fixed distance above each other. Water surface elevations were measured with six resistance-type wave-height meters, and the discharge was measured with an electro-magnetic flow (EMF) meter. Tests were performed with mono-chromatic, bi-chromatic and random waves without current, following the current and opposing the current. Also a test series was performed for a steady current without waves.

Formation and Propagation of Steep Waves: An Investigative Experimental Interpretation

Abstract: From the end of the last century, many Authors have studied experimentally the formation and the propagation of steep waves on dry flume bottom. Up to the present time, however, some aspects of this phenomenon have not been sufficiently explained. The movement of the water immediately after rapid gate opening is mainly controlled by vertical acceleration due to gravity. It follows that the gradually-varied flow hypothesis is not valid in this initial phase and the motion is two-dimensional. Moreover, the type of gate and the velocity of opening influence the flow across the section and the wave celerity. Investigative experiments were conducted in a rectangular flume with the aim of studying in detail the flow through the gate section and the celerity of propagation of the wave immediately after the lifting of the gate. The tests were conducted at the Department of Hydraulic and Environmental Engineering at the University of Pavia by removing a top hinged sluice gate which dams in different sections a flume with horizontal bottom. A series of tests was carried out by using an adverse slope base at the downstream reach of the flume. The experiments were observed through the transparent walls of the flume by using three video cameras synchronized with the gate opening device. The video images, digitized directly by the video recorder, were processed and vectorized. In the gate section, the discharge showed a clear peak immediately after opening and thereafter it stabilized at the critical-state theoretical value. Other tests were conducted imposing a wave reflecting wall at the downstream section of the flume. The mixing of flows caused by the superimposition of waves travelling in opposite directions was studied by injecting a tracer. The experimental investigations were interpreted with the aid of a mathematical model which integrates the unsteady motion equations of a gradually varied free surface flow according to a finite difference scheme.

Eigenfunction analysis of water wave propagation down a wave flume

Abstract: An eigenfunction expansion method is used to analyze the generation and propagation of linear water waves down a wave flume consisting of a wavemaker, an abrupt expansion, (possibly) a finite width vertical porous breakwater spanning the channel, and a fully absorbing ending wall. Progressive wave modes, as well as vertical evanescent modes (which exist near the wave-maker, the expansion section, and the upstream and downwave faces of the porous wall), are considered. Moreover, in order to satisfy the lateral boundary conditions along the flume walls, a transversal variation of the velocity potential is included allowing the propagation of an obliquely incident wave train, scattered modes at the same frequency, and additional evanescent modes in the transversal direction. Thus, the wave solutions are expanded in eigenseries in two orthogonal directions, one over the depth, and the other across the channel cross-section. Laboratory experiments were conducted to verify the reflection and transmission proces...

Wind/wave interactions in the surf zone

Abstract: Wind/wave interactions in the surf zone are studied using a wave tank and environmental wind tunnel. The wind simulation is achieved over a relatively short fetch using accelerated growth techniques at a scale of roughly 1:100. Waves are scaled at approximately 1:50, and consequently there is some scaling mis-match between the wind and wave simulations. Results show that wind has a significant effect on the breaking of the waves. Both breaker location and breaker type are shown to be affected by the wind. Results are in agreement with those of Douglass (1989 & 1990), who used a wind/wave flume to simulate the prototype conditions, but made no attempt to correctly simulate the turbulence in the air flow. The main findings, are that onshore winds promote spilling waves and increase the surf zone width, whereas offshore winds promote plunging waves, decreasing surf zone width. Hot-film measurements of the air flow over the waves show that there exists significant differences between the air flow structure of offshore and onshore winds over the surf zone. Under offshore winds, the surf zone exerts a large drag on the air flow, dramatically increasing turbulence intensities aerodynamic roughness z0, and friction velocity, u*, near the point of wave breaking. Under onshore winds the air flow is less affected and at the point of wave breaking, z0 for onshore winds is an order of magnitude lower than the value under offshore winds. Phase-averaging techniques indicate large wave-induced perturbations to the mean velocity over the waves, and these are present to heights of up to 5 or 6 times the breaker height over the point of wave breaking. Spectra indicate that for onshore winds large wave-frequency fluctuations are present at the shore. Additionally, studies of particle motion offshore of the surf zone indicate wind effects on the drift velocities of suspended particles, although the precise nature of the wind effect was not clear.

A Second-Generation Wave Model for Coastal Wave Prediction

Abstract: A second-generation numerical wave model, WINDWAVE, has been developed for wave prediction in coastal regions. The modal is based on the numerical integration of the energy balance equation for waves in water of finite depth. Three Source terms are included in the physical representations of the model. These terms describe wave growth due to the wind, nonlinear wave-wave interactions and dissipation due to bottom friction in shallow water. The Source term for wave-wave interactions is based on a simple, but directionally-responsive, formulation given by Young (1988). The model has been tested against the SWAMP (1985) results and for real winds for storms in the Irish Sea and elsewhere.

Breaking wave propagation model based on the turbulent eddy viscosity concept and the Boussinesq equations

Abstract: A turbulent eddy viscosity model based on analytical expressions for the energy dissipation in a bore is presented to predict wave conditions in the surf zone. The computational model, based on the Boussinesq equations, has been tested for a bar‐trough beach in random waves, and the results are compared with wave flume measurements. The root‐mean‐square (rms) values of the wave heights in the breaker zone are well predicted, but detailed features of the setup are not accurately presented by the model.

Simulation of fully nonlinear irregular waves in a 3-d numerical wave tank

Abstract: This paper presents the result of a simulation of fully nonlinear irregular waves in a truncated three-dimensional wave tank. A Stokes third-order-like irregular wave is used as the incident input wave at the inflow boundary. The Sommerfield/Olanski open boundary condition is applied and the sponge layer beach is added to partly damp the wave near the down tank to ensure the non- reflection condition from the outflow boundary. The exact nonlinear free surface condition is satisfied while the simulation is continued for the repeating period of the incident irregular wave of 128 seconds. The wave elevation recorded at a few locations along the tank is used to estimate the amplitude spectrums, energies and skewness. It is found from the study that the energy dissipates and the skewness reduces as the wave progresses along the tank. In solving the boundary integral equation, a robust time domain higher- order boundary element method is employed.

The Daytona Beach Large Wave Event of 3 July 1992

Abstract: The fundamental mechanisms behind the anomalous large wave which struck Daytona Beach on 3 July 1992 are addressed. Although the wave was originally believed to have resulted from a submarine landslide, both physical data and anecdotal accounts are presented which link the wave to a southward propagating squall system. The wave appears to be an example of general type of forced long wave motion which may be called a `squall-line surge', in which a resonant coupling occurs between the moving pressure front and free surface wave, resulting in potential large wave amplitudes. Historical precedent is found for the generation of such waves; two notable examples are compared with the wave which impacted Daytona Beach.

Waves Propagating on an Adverse Jet

Abstract: A laboratory investigated was conducted of the propagation of waves on an adverse three-dimensional jet. The problem which was simulated was the ebb-tide flow from a tidal inlet into a constant depth ocean and the effect of such a flow on an incoming waves. The following major conclusions may be drawn from this study: 1. A slat type wave machine which permits flow through the wave generator plate appears to work well for such a study of wave-current interactions. The performance of the generator is described reasonably well by an available theory. 2. The jet discharge from a channel located asymmetrically in the flume appears to agree reasonably well with results for a two-dimensional symmetric turbulent jet. 3. The experimental arrangement used controls the jet discharge by the use of the training wall, and in this manner allows for the isolation of certain aspects of he problem. 4. Even fro small ratio of the average velocity at the channel exit to the wave celerity, i.e., less than 10%, the incident wave height with the adverse current is more than twice what it is without the current. 5. In the region seaward of the channel exit a wave maximum occurs caused primarily by the effects of refraction. 6. The wave height decreases significantly as waves propagate up the channel for the case with an adverse current. This effect apparently is not due to friction. 7. There is a significant lateral variation of wave height across the flume caused by the jet which may be due to both wave refraction and the shoreward velocities induced by the effects of the entertainment of the ambient fluid by the jet which discharges from the channel exit.

Kinematics of Wave Overtopping on Marine Structure

Abstract: Kinematics of transient wave overtopping on coastal breakwaters has been studied both experimentally and numerically. For the laboratory experiments, solitary waves with moderate wave amplitude are used as the incident waves. The wave profiles are obtained by resistance type wave gauge. The two dimensional water particle velocities are measured by a portable, four-beam, fiber optic Laser Doppler Velocimeter (LDV) system. For the numerical analysis, potential flow theory and Boundary Element Method (BEM) are used for analyzing the wave field induced by the coastal structure. It is found that the numerical results compared well with the experimental data in the case where the numerical model is valid. Some interesting experimental observations are also presented.

Wave Runup and the Coastal Watertable

Abstract: Wave run-up beyond the shoreline on a sandy beach causes infiltration and consequently a lifting of the water table. This is a significant factor affecting the coastal groundwater hydrology. The water table over-height due to pure wave forcing without tidal effect is investigated through laboratory experiments with regular waves in a wave flume. Different beach materials with different permeabilities are considered. The asymptotic inland over-height shows strong correlation with the wave run-up height, the combined wave parameter (HoLo)/D which is linked to the run-up height, and the surf similarity parameter. However, the effect of sand sizes, which were tested in this study, on the over-height is little apparent.

Experimental Study for Directional Wave Force on a Column

Abstract: Real sea wave is quite random in amplitndes, periods and propagating directions. Offshore sea waves, therefore, have characteristics of “directional randomness”. Directional random waves in the offshore consist of the component waves each propagating in different directions in shallow area. Some coastal structures constructed in deep water area should be designed for directional random waves. The experiment for the directional wave force are done. The target structure is a column, which is the simplest component in coastal structures. The wave force on a column in directional seas is compared with that in uni-directional waves.

Rundown Velocity Along the Slope of a Breakwater with Accropode Cover Layer

Abstract: Rundown velocity along the slope of a breakwater is quite an important parameter in breakwater design. Literature review on rundown velocity measurements indicates that a few small scale studies have been attempted in the past, mostly along beach slopes. This paper details the experimental investigations conducted at large wave flume (GWK), Franzius Institute, Hannover, Germany, in regard to rundown velocity along the slope of a rubble mound breakwater with an accropode armour layer. As the wave flume facilitates generation of wave heights in the range of 0.20 to 2.0 m with wave period ranging between 3.0 and 12.0 seconds, measurements on rundown velocity for the near prototype conditions were possible

Rundown Velocity Along the Slope of a Breakwater with

Abstract: Rundown velocity along the slope of a breakwater is quite an important parameter in breakwater design. Literature review on rundown velocity measurements indicates that a few small scale studies have been attempted in the past, mostly along beach slopes. This paper details the experimental investigations conducted at large wave flume (GWK), Franzius Institute, Hannover, Germany, in regard to rundown velocity along the slope of a rubble mound breakwater with an accropode armour layer. As the wave flume facilitates generation of wave heights in the range of 0.20 to 2.0 m with wave period ranging between 3.0 and 12.0 seconds, measurements on rundown velocity for the near prototype conditions were possible. Two methods were adopted to determine rundown velocity viz., (i) with a float and (ii) with a wave gauge. The results on the variation of rundown velocity with Iribarren Number and wave steepness are presented in the form of non-dimensional graphs and discussed. The studies indicated that for identical wave input parameters float method predicts a higher value of rundown velocity compared to wave gauge method. For instance, with a wave height of 0.6 m and wave period of 5.8 seconds, float method predicts a rundown velocity of 1.7 m/sec as against 1.24 m/sec predicted by wave gauge. The experimental results strongly predict the dependency of rundown velocity on wave period, in addition to wave steepness and Iribarren Number The trend curves of rundown velocity show the existence of an upper boundary below which all trend curves lie regardless of the wave period. Present experimental investigations provided valuable information about the magnitude of rundown velocity along breakwater slope which hitherto was not available to the engineers dealing with breakwaters.

Protection of outfall structures

Abstract: Stability of rock on horizontal bottoms and steep slopes subjected to wave attack has been a subject of investigation in the past. But the stability of rock on gentle slopes has not been investigated systematically. The objective of this research is to investigate the relations between the different variables involved. A possible application is the protection of outfall structures. Due to the lack of information on this subject, relations derived for stability of rock on horizontal bottoms subjected to wave attack were applied. Application of these theories imply a destabilization of the rock by orbital velocities causing shear stresses at the bottom. Orbital velocities were calculated along the profile of the structure with the linear wave theory and substituted in the stability relations for horizontal bottoms according to the theories of Ranee & Warren and Jonsson / Sleath, respectively. The results of the calculations were expressed in the stability variable H/ADn 5 0 versus the relative waterdepth, h/H. This was done for regular as well as for irregular waves. The calculations showed an increase in the stability for increasing values of the wave steepness. Also, the deeper the water the higher the stability values. Experiments were conducted in the large wave flume of the Laboratory of Fluid Mechanics at the Delft University of Technology. The model consisted of an impermeable 1:25 slope, on which several materials were tested. Regular and irregular waves were applied and for various conditions the wave heights and bottom velocities along the test slope were measured. The experimental results were compared with the calculations. For regular waves it appeared that for h/H values larger than one the calculations describe the stability of the rock quite well. For h/H values smaller than one the calculations are not adequate to describe the stability of the rock. The location of maximum attack was around h/H = 1. For irregular waves the location of attack was not that clear. The damage was not as concentrated and more spread out. The location of maximum attack was around h/Hs = 1. For both regular as irregular waves the general tendency could be described by the calculations but the 'plunging' effect o f the more curl-shaped waves with lower values of the wave steepness resulted into a more severe attack on the structure. For irregular waves more experiments have to be conducted for the slope section where waves are not yet broken. This to confirm or reject the theories derived applied to horizontal bottoms for the stability of rock on gentle slopes attacked by irregular waves. To investigate whether the computer simulation ODIFLOCS, can be used to simulate wave motion on gentle slopes, a comparison was made with the measurements in the experimental model. ODIFLOCS proved not to be suitable for simulation of wave motion on gentle slopes, mainly due to short comings of the numerical scheme used by ODIFLOCS which was developed for 'short' steep slopes.