Showing papers on "Wave flume published in 1988"

Regular water wave kinematics

Abstract: A proper knowledge on the water wave kinematics is important from several points of view. The need to accurately determine the wave forces on offshore structures has initiated the series of studies reported in this paper.

Wave-induced flow and nearshore suspended sediment

Abstract: It has been realized for nearly one hundred years that the transport of sediment is related to the characteristics of a wave, in particular its shape. Cornish (1898) noticed that the shoreward velocity associated with a wave crest was more effective at moving coarse sediment than was the seaward velocity associated with the wave trough. Cornish's observation was consistent with the theory of Stokes (1847), which predicts the onshore velocity associated with the wave crest is stronger and of shorter duration than the offshore velocity associated with the wave trough. This horizontal asymmetry of the cross-shore flow, which is a reflection of the wave shape, is known as velocity skewness. It has been suggested that "the existence of the beach depends on small departures from symmetry in the velocity field balancing the tendency for gravity to move material offshore"(Bowen, 1980). Although the concept of velocity skewness has been incorporated into detailed predictors of sediment transport (Bowen, 1980; Bailard and Inman, 1981) it is only one of many facets that needs to be understood in order to make the accurate prediction of sediment transport realizable. A comprehension of sediment transport is hampered by both an incomplete knowledge of the hydrodynamics and a lack of instrumentation to directly measure instantaneous sediment concentration and the accurate prediction of sediment transport is probably the most enigmatic problem in coastal engineering. Occasionally, suspended sediment concentration has been inferred from in situ pumps and hand-held tubes, but these methods lack the temporal and spatial resolution necessary to elucidate the details of the interaction between the waveinduced flow and the sediment. Recently, a miniature optical backscatter sensor (MOBS), which provides a time series of suspended sediment concentration at a "point", was developed by Downing et al. (1981). During a recent field experiment a vertical array of 5 of these optical backscatter sensors and a colocated flow meter was deployed close to the sea bed. These colocated measurements provide a unique opportunity to investigate the response of near-bed suspended sediment concentration to the wave-induced flow.

Sediment transport in case of irregular non-breaking waves with a current

Abstract: Investigation to sediment transport rate in case of waves and currents, tests executed in the wave-current flume of the laboratory of Fluid mechanics of TU Delft. Sediment transport was measured using sand of 100 micron; comparison to similar tests with 200 micron sand.

Kinematics and return flow in a closed wave flume

Abstract: Stokes (1847) showed that finite amplitude progressing waves cause a net drift of fluid, in the direction of wave motion, which occurs in the upper portion of the water column. In a closed wave flume this drift must be accompanied by a return flow toward the wave generator to satisfy the conservation of mass. This study presents Eulerian velocity and water surface measurements soon after the onset of wave motion from 12 locations in a large scale flume. Waves with .67 < kh < 2.29 and .09 < H/h < .39 were produced in a water depth of 3.5 meters. Superimposing the return flow theory of Kim (1984) with seventh order stream function theory is shown to improve the velocity predictions. The measured return flows are a function of time and depth and agree with Kim's theory as a first approximation. The mean water surface set-down agrees with the theory of Brevik (1979) except for the nearly deep water waves.

Bed load transport due to non-linear wave motion

Abstract: The bed load transport rate due to wave motion is measured in a wave flume. The modified stream function theory of the author ( Tanaka (1988) ) is applied to the formulation of the sediment transport rate in order to include the non-linearity. The proposed formula predicts well except near the surf zone where the effect of the acceleration plays an important role.

Surging Characteristics of Spilling Zones of Quasi-steady Breaking Water Waves

Abstract: Wave breaking on a wide range of length scales is a commonly observed feature of the wind-driven ocean surface. Despite the importance of breaking waves in various interfacial processes of basic and technological significance, our detailed knowledge of the attendant hydrodynamics is relatively sparse, both observationally and theoretically.

Kinematics of breaking waves in coastal regions

Abstract: Waves breaking on various slopes in a wave flume are examined. Plunging and spilling breakers are considered. The parametric results show the consistency of the measurement and the independence of scale. A method is given for predicting the maximum breaking height for a wave of known period in a known depth. The velocity is measured to the crest of the wave and comparisions with numerical and analytical solutions demonstrate the shortcomings of many of the established methods of predicting wave kinematics.

Comparison of a numerical model for longshore sediment transport with measurements

Abstract: The numerical model TRADEF simulates morphological changes due to longshore currents induced by breaking waves. The numerical results are compared with measurements of bed evolutions surveyed in a wave basin with movable bed for two different offshore wave conditions. The main features of bottom changes could be well simulated by computation for the highest offshore wave height whereas discrepancies occur for the little one. These results are discussed and improvements are proposed; introduction of a turbulence model, modification of the transport formula.

An Experimental Investigation into Nonlinear Resonant Wave Interactions in the Ship Model Basin

Abstract: The experiment was executed for studying the properties of tertiary wave interactions in a ship model basin. Long-time evolution of resonant wave was observed by linear array of wave gauges located along the direction of propagation of the wave. The behavior of tertiary resonance was found to depend on the amplitudes of two principal waves as well as their wavelengths. The comparison was made in fair agreement with the theoretical prediction after Zakharov equation.

About the energy dissipation over barred beach

Abstract: Since wave energy dissipation in the surf zone is a stochastic process closed mathematical formulations cannot be expected. The dissipation was computed using several analytical and/or empirical approaches and compared with prototype measurements in the Big Wave Flume (GWK) in Hannover as well as with field measurements from the west coast, of the Island of Sylt/North Sea. Generally good agreements were found for moderate energy dissipation conditions (spilling-breaker), whereas in the case of plunging breakers, however, the fitting is not solved satisfactory.

The turbulent flow fields and vortex structures inside surf zone

Abstract: In order to understand the characteristics of turbulence due to wave breaking, flow visualization techniques are used to observe the external and internal flow structures. Two typical spilling and plunging breakers are made in wave flume. 2W Argon-Ion laser, and 120W ultraviolet lamps are used to brighten the tracing particles respectively. In this situation, not only the external structures of wave breaking can be visualized but also the instantaneous velocity of particles in the internal flow field will be outlined. Moreover, it is valuable to point out that the three dimensional flow structures inside surf zone are also found in our elaborated experimental studies.

Transformation of shallow water wave spectra

Abstract: Investigations are presented, on some effects of nonlinearity in the motion of shallow water wave spectra. The waves were generated, mechanically in a laboratory wave flume with fixed bottom. Essential differences with the linear dispersion relation are found, showing vanishing dispersivity of higher frequency spectral components in strongly nonlinear spectra. The mean frequency increases with decreasing water depth. The relation of the peak frequency to the mean frequency varied in the experiments from 0.9 to 0.5, for deep to shallow water wave spectra respectively.

Second-order analysis of shallow-water wave spectra

Abstract: This paper presents a study based on a second-order interpretation of wave «energy» spectra, which may explain the build-up of low-and high-frequency peaks for wave spectra measured in shallow water. A comparison between this interpretative model and laboratory data, which were obtained in a bidimensional wave flume, is also given in the paper.

Laboratory Measurement of Spatial and Temporal Suspended Sediment Concentration Under Waves.

Abstract: : This study examines the feasibility of obtaining spatial and temporal suspended sediment concentrations in the laboratory through the use of high-speed filming and image analysis. A series of filming experiments was conducted in a laboratory wave flume to investigate filming parameters and to determine optimum conditions for filming high quality footage of sediment transport. A relatively simple procedure is used to map the contours of light intensity returned from digitized 16mm film images and to relate these contours to the concentration of suspended sediment. Improvement of this nonintrusive measurement technique could provide researchers with the capability to perform quantitative measurements of sediment transport at the microscale. Keywords: Image analysis, Suspended sediment, Sediment concentration.

Towards modelling coastal sediment transport

Abstract: Sediment transport data from the field and laboratory tests are used to gain insight into two fundamental questions. Firstly: What is the relative importance of coexisting waves and currents for the resulting sediment transport? Secondly: Is the influence of grain size as strong as traditional models predict, or is it as weak as the empirical CERC-formula indicates? Wave tank data reveal that the oscillatory velocity will in most cases determine the direction as well as the magnitude of the shore normal sediment transport, and wave flume data on shore normal transport as well as field data on littoral drift show weaker grain size dependance than traditional sediment transport models predict. It is suggested that wave dominance as well as weak grain size dependence are manifestations of the fact that the dominant transport mechanisms are often more organised than the diffusion process on which many traditional models are based.

Estimates op gross-shore bedload and bed changes

Abstract: Four series of tracer studies employing radioisotopic sand, tracked remotely from land, were carried out at the IBW PAN's coastal research station at Lubiatowo. Incipient motion of single grains with velocity Vcr1 has agreed with Komar and Miller's (1974) formula, and has been 5...8 times smaller than voro, the threshold velocity of water inducing bulk motion_of sediment. The ratio of average nearbed water velocity, v, to_the effective speed of tracer sand, in the inshore zone, va, is estimated about 100...1,000. The cross-shore sediment transport rates measured are ten times smaller than those predicted by Pruszak's (1987) energetics formula, the discrepancy being due to the real field phenomena of mixing and overburdening in a thick bed layer. Our wave flume and wave tank studies with movable bed point to the importance of initial slope in shore evolution. The laboratory findings on bed variability are extended by our 1987 field data obtained with an ultrasonic setup. Intensive cyclic depth changes are acoorapanied by varying transport rates computed from continuity equation.