Showing papers on "Wave flume published in 2002"

Sheet flow dynamics under monochromatic nonbreaking waves

Abstract: For the first time, detailed measurements of sediment concentrations and grain velocities inside the sheet flow layer under prototype surface gravity waves have been carried out in combination with measurements of suspension processes above the sheet flow layer. Experiments were performed in a large-scale wave flume using natural sand. Sand transport under high waves in shallow water is mainly contained within the so-called “sheet flow layer,” a thin layer (10–60 grain diameters) in which the volume concentration of sand decreases by an order of magnitude from a value near 0.6 at the stationary bed. The thickness of the layer varies over a wave cycle and the maximum thickness increases with increasing peak Shields stress. The concentrations within the sheet flow layer vary approximately synchronously with the orbital velocity measured by an Acoustic Doppler Velocimeter (ADV) located 0.1 m above the bed, with typical phase lags of 0–π/5. In contrast, the suspended sediment concentrations a few centimeters and higher above the bed exhibit larger phase lags. Grain velocities were successfully measured in the middle and upper portions of the sheet flow layer around the time of their maximums. These velocities increased weakly with elevation from approximately 50% to 70% of the velocity outside the wave boundary layer. The observations are compared to previous experimental work and are found to be mainly consistent with observations in steady unidirectional flows and in oscillating water tunnels (OWTs), although differences in the suspended sediment concentration and the total sediment transport rate are apparent. Observations are also compared to two very different models: a 1DV suspension model for oscillatory flow with enhanced boundary roughness and a two-phase collisional grain flow model for steady unidirectional flow. While the suspension model describes the velocity profile fairly well and the collisional model describes the concentration profile well, neither model accurately predicts both the velocity and the concentration and therefore the sediment flux over the full vertical extent of the sheet flow.

Suspended sediments under waves measured in a large-scale flume facility

Abstract: [1] Measurements of suspended sediment concentration, and the associated bedform morphology, were made beneath regular and irregular waves in a large-scale flume. The bed forms were measured using an acoustic ripple profiler, and the suspended sediments were measured using an acoustic backscatter system, together with pumped sampling. Using the measured bed form dimensions and the flow as inputs, standard approaches have been used to predict the reference concentration and vertical distribution of suspended sediment concentration. Nielsen's [1986] empirical expression for the reference concentration, C0, shows reasonable overall agreement with the present measurements in respect of the dependence of C0 on the Shields parameter (skin friction); but the formula somewhat overestimates the measured concentrations. To analyze the form of the measured mean concentration profiles, comparisons have been made with the simple one-dimensional (vertical) formulations, and extensions thereof, proposed by Nielsen [1992] on the basis of pure diffusion, pure convection, and combined convection and diffusion. It is concluded that in a near-bed layer of thickness about two ripple heights (i.e., the layer dominated by vortex formation and shedding above ripples), pure diffusion characterized by a height-independent sediment diffusivity provides a good representation of the measured profiles. Above this, Nielsen's [1992] convection-diffusion solution provides a better representation. It is shown, however, that by use of pure turbulent diffusion modeling concepts, the same profile can also be obtained by the use of a height-varying, “constant + linear,” sediment diffusion model. This diffusivity represents the enhanced mixing in the outer part of the oscillatory boundary layer caused by the breakdown of coherent vortex structures into random turbulence. The relative merits of convection and diffusion schemes are discussed.

Wave Overtopping of Marine Structures: utilization of wave energy

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Air bubble entrainment by breaking waves and associated energy dissipation

Abstract: A simple mathematical model (air bubble model) that describes energy dissipation due to air bubble entrainment is proposed and applied to a series of laboratory experiments for plunging jet flows (steady) and surf zone waves (unsteady). This leads to a formulation wherein the rise velocity of bubbles is included. For the unsteady case, i.e. air entrainment by wave breaking, some parameters of the model have been estimated from the experiments and expressed in terms of local wave height and distance. Results obtained through the air bubble model are summarized in the following. Experiments in vertical circular plunging jets (steady) were performed for both in freshwater and seawater, which highlighted the distribution of void fraction, that follows closely analytical solutions derived by Chanson (1997). In addition, various properties of void fraction field were emphasized. Three scale models were used with freshwater for identical Froude numbers in the experiments, which highlighted significant scale effects when Weber number is less than 1000. Similar experiments were also performed with freshwater and seawater and the results showed lesser air entrainment in seawater plunging jets. The pseudo-bubble chord sizes obtained from the experimental data were in the range from less than 0.5 mm to more than 1.0 mm. The rate of energy dissipation due to entrained air was investigated by applying the air bubble model for three typical phenomena. The results demonstrated that the ratios of energy dissipation due to air bubble entrainment with respect to total energy loss were 25%, 1.4% and (2-4)% for hydraulic jump, 2-D vertical plunging jet and vertical circular jet, respectively. Experiments on unsteady air bubble entrainment by wave breaking were conducted in a wave channel. Maps of the evolution of the void fraction distribution in surf zone generated by various sizes breaking waves were presented. A significant fraction of the potential energy of entrained air was measured from the void fraction distributions provided by breaking waves. Measurements showed high void fraction up to 19% in plunging breakers at still water surface whereas 16% in spilling breakers. The ratio of energy dissipation due to entrained air to total energy loss was found (18-22)% and (17-19)% for spilling and plunging breaker, respectively. The characteristics of time averaged wave parameters (e.g. potential energy, kinetic energy, energy flux, radiation stress) for regular waves were discussed taking into account the air bubble effects. Analytical solutions were sought and explicit expressions were obtained for wave parameters under sinusoidal waves. Effects of the air entrainment on density, pressure and velocity fields were also discussed in detail. The conservation equations for energy and momentum were solved numerically using finite difference methods. Boundary conditions were used at the breaking point. Scale effects were discussed based on laboratory air entrainment in 2-D wave flume, which was believed to occur in small size models. The data were in good agreement with the basic assumption for vertical distribution of void fraction both in spilling and plunging breakers. The results of the air bubble model were compared with experimental data and found to give good agreement between them for the wave height and wave setup. Water level rise by entrained air was determined and found significant effects on surf zone hydrodynamics. In addition, wave run-up was measured and discussed.

Field observations of the effect of shear waves on sediment suspension and transport

Abstract: Field measurements of the effect of shear waves on sediment suspension and transport have been made on the seaward side of the crest of an intertidal bar using an array of pressure sensors, electromagnetic current meters and optical backscatter sensors mounted just above the bed. A tide with strong mean longshore currents (up to 0.77 m/s) was singled out for analysis, and during this tide shear waves contributed up to 93% of total velocity variance for far infragravity frequencies Suspension events in the time series occurred at incident wave frequency, but were modulated at shear wave frequency. The offshore phase of the incident waves was preferred for suspension. The most suspension occurred when the offshore directed backwash, the undertow, and the offshore travelling part of the shear wave combined to give strong seaward directed flows >0.5 m/s. Cross-shore sediment flux due to the shear waves was directed offshore, and accounted for up to 16% of the total cross-shore transport, and up to 37% of the oscillatory cross-shore transport. The mean component dominated the cross-shore transport, accounting for up to 69% of the total. In the longshore direction, shear waves dominated the oscillatory component of transport (up to 90%). This transport was in the opposite direction to the mean flow, but was small (up to 12% of the total) compared to the mean longshore sediment transport.

Physical model study of wave diffraction-refraction at an idealized inlet

Abstract: : ELECTRONIC FILE CHARACTERISTICS: 572 files; MS Word (DOC) and TXT PHYSICAL DESCRIPTION: 1 computer laser optical disc (CD-ROM); 4 3/4 in; 227 MB SYSTEMS DETAIL NOTE: IBM-clone PC-compatible ABSTRACT: This physical model study of wave refraction-diffraction at structures typically present at coastal inlets was conducted to provide data sets that would aid in the calibration and verification of numerical wave models The study was performed in the Coastal Inlet Research Program's (CIRP) idealized inlet experimental basin at the US Army Engineer Research and Development Center (ERDC), Coastal and Hydraulics Laboratory (CHL) Safe navigation, sediment transport into navigation channels, and shoreline erosion are all concerns at coastal inlets and are related to the transformation of waves as they change direction and height due to complex bathymetry and coastal inlet structures The idealized inlet physical model, created for inlet studies, provided a facility in which to make wave measurements of height and direction in enough detail to document wave diffraction and refraction Measurements of wave information included use of capacitive wave rods for wave height, acoustic-Doppler velocity sensors for wave direction, and new video-based wave direction measurement system Four idealized structural configurations were examined with two irregular Waves, 08 sec, 02 ft (61 cm), and 16 sec, 015 ft (46 cm), and one regular wave, 08 sec, 015 ft (46 cm) Structure 1 consisted of a shore-parallel breakwater with the wave generator creating shore-normal and 20-deg-angle waves Structure 2 was a typical dogleg jetty with shore-normal and 20-deg-angle waves Structure 3 was an unjettied inlet but included flood currents Structure 4 had parallel jetties at the inlet Structures 3 and 4 were concerned with wave transportation into the bay A total of 30 wave gauges were used to acquire data with 20 gauges placed on two movable racks for deployment into specified 7

Swash zone sediment transport

Abstract: The coastal area is a busy area, many people live in these areas or use the coastal area for recreational purposes. The beach profile in the coastal area is continuously changing under the changing field conditions. These changes are induced by the changing sediment transports in the sea under changing field conditions. Several models have been developed to simulate and predict these changes in sediment transport and their related profile changes. However, these models fail in a region close to the shoreline, since the parameterized wave models (like Battjes Janssen, 1978) fail in this region. In this thesis a model has been developed to compute the sediment transport rates up to zero meter water depth. This Inner Surf Zone (ISZ) sediment transport model is based on the ISZ model of Aarninkhof, 2000, which models the wave height decay and the associated flow field up to zero meter water depth. The ISZ sediment transport model uses the energy approach of Bailard (1963;1966) for the sediment transport computations. According to Bailard the sediment transport is related to the work done by the fluid, the dissipated energy. The sediment transport is divided in two layers in the ISZ sediment transport model, an upper and a lower layer. In the lower layer suspended and bed load sediment transport is taken into account. In the upper layer only suspended sediment transport is computed, based on the shear stresses near the bottom. The sediment concentrations in the upper layer are assumed to decrease, when the water depth increases, resulting in a decreasing sediment transport in the upper layer. Implementation of this assumption by an empirical formula into the model works encouraging well. The model is calibrated against the tests of Koomans in the Scheldt flume (2000). The model is valid for spilling breaking and the computation area is determined by the non-linearity parameter of the waves. The non-linearity parameter depends on the wave period and so does the range of the model. Different field conditions have been investigated in this thesis. Changing transport rates for varying beach slope and sediment composition can be modelled encouragingly well by the developed ISZ sediment transport model. The ISZ sediment transport model requires adjustment when field conditions like wave height and wave period change, since the friction factor and the sediment concentrations change under changing field conditions. Based on the described changes in sediment transport in literature, under these changing field conditions, a recommendation is made how to adjust the model parameters. The friction and the bed/suspended load efficiency factors are used to represent the described changes in sediment transport under these changing field conditions.

Bar Formation Under Breaking Wave Conditions: A Laboratory Study

Abstract: An analysis of bar formation under breaking waves was carried out in a small-scale wave tank. Although the 1/100 size scale of the tank was unusual for sediment transport study, the results were in agreement with field studies concerning wave breaking criteria and the direction of sand transport. This size permitted accurate simultaneous measurement of the spatial and temporal variations of both the wave height and the bathymetric profile. This study is the extension of a previous one concerned with non-breaking conditions. The role of the nonlinear wave-wave interactions, reinforced under breaking waves, was displayed in the bar formation. Finally, it was suggested that a more realistic modeling of the mean flows distribution in the bottom boundary layer (difference in direction and intensity between the upper and the lower parts) owing to the superposition of the undertow and nonlinearity effects, will improve the modeling of bar formation.

Experimental study of reflection coefficient and wave forces acting on perforated caisson

Abstract: The reflection coefficient and the total horizontal forces of regular waves acting on the perforated caisson are experimentally investigated. The empirical relationship between reflection coefficient and the ratio of the total horizontal forces acting on the perforated caisson to those on solid vertical walls with the relative chamber width, relative water depth and porosity of perforated wall, etc. are given. Moreover, the results of the ratio of the total horizontal forces are also compared with formulas given by Chinese Harbour Design Criteria and Takahashi, which may be useful for the practical engineering application.

The SEAWOLF Flume: Sediment Erosion Actuated by Wave Oscillations and Linear Flow

Abstract: Sandia National Laboratories has previously developed a unidirectional High Shear Stress Sediment Erosion flume for the US Army Corps of Engineers, Coastal Hydraulics Laboratory. The flow regime for this flume has limited applicability to wave-dominated environments. A significant design modification to the existing flume allows oscillatory flow to be superimposed upon a unidirectional current. The new flume simulates highshear stress erosion processes experienced in coastal waters where wave forcing dominates the system. Flow velocity measurements, and erosion experiments with known sediment samples were performed with the new flume. Also, preliminary computational flow models closely simulate experimental results and allow for a detailed assessment of the induced shear stresses at the sediment surface.

Laboratory Investigation Of Sediment Resuspension By Waves At Full-scale

Abstract: Measurements of wave-induced flows, suspended sediments and bedforms have been obtained in random wave conditions in a large laboratory wave flume over beds of fine and medium sand. Selected results illustrating wave-induced flows and turbulence, bedform genesis and development, sediment resuspension and vertical suspended sediment concentration profiles are presented and are compared with existing empirical formulae and models.

Deterministic Reproduction of Nonlinear Waves

Abstract: A method for deterministic reproduction of non-linear long-crested waves has been implemented. The model is used for non-linear reproduction of measured wave time series from a model test programme in a wave flume. Regular waves, irregular waves and focused waves have been reproduced with the model. Based on measured surface elevation time series at one location in the flume the elevation time series and the kinematics have been reproduced at another location using both linear theory and the second order model. The numerical results have been compared with measurements and it is found that the second order model is able to reproduce the correct shape of the waves as they propagate in the flume — even when the waves are highly non-linear.Copyright © 2002 by ASME

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

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

Dynamic Earth Pressures on Deeply Embedded Large Cylindrical Structure under Random Waves and Mechanism Investigation on Structural Instability

Abstract: In order to investigate the mechanism of instability and failure of the deeply embedded large cylindrical structure under random waves, wave flume experiments with different wave conditions were carried out and the corresponding dynamic earth pressures on the structure were measured. By analyzing the experimental data both in the time and frequency domains and surveying the real state of structural instability, the mechanism of instability and failure of this type of structure is explored and interpreted in the paper. The present study demonstrates that dynamic change of material properties of foundation soil under random waves needs to be paid much attention in the mechanism analysis. The development of soften of mechanical strengths and inreversible plastic deformations of soil, which are induced by structural vibrations, can force the structure inclined, and this might lead the structure gradually to overturn due to long and continuous actions of random waves.

Experimental study on the effectiveness of TLDs under wave loading

Abstract: The objective of this study is to experimentally investigate the effectiveness of Tuned Liquid Dampers (TLDs) for suppressing the dynamic response of a platform structure subjected to wave loading and to explore the applicability of TLDs for suppressing the structural vibration of fixed offshore platforms The experimental model is scaled according to a full size platform by matching its dynamic properties Rectangular and circular TLDs of various sizes and water depths are examined The experiments were performed in a 2-D wave flume The effectiveness of TLDs is evaluated based on their response reduction By observing the performance and the behavior of TLDs through laboratory experiments, the effects of a number of parameters including container shape, container size, number of dampers, frequency ratio, mass ratio, and incident wave characteristics are investigated

Measurement of Physical Model Wave Diffraction Patterns Using Video

Abstract: : The complex interaction of surface waves with coastal inlet structures and inlet morphology is of significant importance to navigation channel operation and maintenance. Wave data in the vicinity of coastal inlets are limited. Where field wave data exist, the temporal and spatial coverage is inadequate to resolve the evolution of wave refraction and diffraction patterns of the free surface. To address the challenge of quantifying variations in wave direction in the coastal system, the US Army Engineer Research and Development Center applied video techniques in a physical model to obtain spatially and temporally dense measurements of wave direction. These measures are required to advance understanding of first-order inlet processes and to use the measurements in numerical simulation model development and verification. In this paper, detailed results are discussed for random and a single monochromatic wave experiments conducted for evaluation of wave diffraction patterns influenced by coastal structures and coastal inlet bathymetry.

Future Directions in the Study of Non-Conservative Water Wave Systems

Abstract: A few future directions and challenges in wave research are briefly discussed including Numerical Experiments as in the treatment of splashing waves, and topics in the Mechanics of Ocean Waves, especially in their non-linear, non-conservative reality. Breaking waves are featured.Copyright © 2002 by ASME

Dynamic behavior of bed material and local scour around a circular bridge pier under wave and current

Abstract: This study presents the dynamic behavior of bed material and local scour around a circular bridge pier under wave and current in a laboratory wave flume. Substantial bridge damages are considered to be occurred not only by the local scour but also by the dynamic instability of the bed material around the bridge pier under the persistent action by water waves. Observations were carried out on the local scour development under clear-water steady flow as well as under wave and current. Waves were generated from upstream side of bridge pier and from downstream side of bridge pier respectively. Observations were further carried out on the excessive pore-water pressure around a circular bridge pier. The results showed that the local scour depths found maximum in the downstream side of the pier for most of the cases under the coexistence of wave and current. The results also showed that the excessive pore-water pressure development decreased the effective stress of the bed material around the pier.

Vision and probrems for reflection of effects of long-period waves into design of coastal facilities

Abstract: The long-period waves have been researched in recent years because of its importance in the various problems of coastal engineering, such as long-period oscillation, long-period motions of moored vessels, sediment transport, wave-overtopping and stability of coastal structures. Importance of long period waves have been well recognized, however, its reflection of effects of the long-period waves into design of coastal facilities is not sufficient.In this paper, we review previous researches on the long period waves and consider the problems in the reflection of effects of long-period waves into design of coastal facilities.