Showing papers on "Wave flume published in 1984"

A Model for Offshore Sediment Transport

Abstract: Observation of the two-dimensional breaking of random waves on a beach suggests that under conditions of active surf an important mechanism in the process of offshore sediment transport is the transport by the undertow or return flow, induced by the breaking of waves. It is found that a model incorporating this mechanism exclusively is able to describe the local sediment transport and the resulting bottom variation of a beach under random wave attack to a first approximation. A laboratory verification is made based on measurements of both the dynamics of the water motion and the bottom profile. Finally, a realistic equilibrium state is shown to result from the model

A simplified model for longshore sediment transport

Abstract: An energetics-based longshore sediment transport model is developed which takes the form of a modification to the wave power equation. Instead of being constant, the wave power coefficient is a function of the breaker angle and the ratio of the orbital velocity magnitude and the sediment fall velocity. This modification extends the range of application of the wave power equation to include both field and laboratory conditions.

Depositional effects of offshore breakwater due to onshore-offshore sediment movement

Abstract: Physical parameters controlling the development of tombolo behind offshore breakwaters are clarified on the basis of the results obtained by laboratory tests. Especially, the depositional effects are evaluated by taking a permeable type structure into account. They are wave characteristics, sediment size, the water depth where the structures are to be placed and its dimension. The amount of sand deposition behind the structures is presented by non-dimensional parameters such as the relative distances from the initial shoreline to the structure and to the breaking point of waves, the ratio of the gap width to the wave length in deep water and the ratio of the length of the offshore breakwater to the wave length in deep water.

Sediment Transport due to Waves and Tidal Currents

Abstract: The paper reviews existing methods for the prediction of sediment transport under the action of combined waves and currents, and presents the results of an experimental programme aimed at verifying some of the main assumptions implicit in such methods. A logarithmic layer is confirmed to exist in the turbulent oscillatory boundary layer, and bed shear stresses and roughness lengths are compared favourably with theoretical estimates. Field observations of sediment transport under combined waves and tidal currents are shown to be predicted with the use of an appropriate power law.

The probability characteristics op waves and wave pressures at a vertical breakwater

Abstract: In this paper, under the condition of waves in front of a breakwater being not broken, studies were made on the characterises of probability distribution of waves and wave pressures, the regularity of the spectral component attenuation with depth and the constitution of the high frequency band of wave pressure spectrum. The distributions of wave heights in front of a vertical breakwater, the range of wave pressure fluctuation at different subsurface levels, and the wave periods have shown that they are practically invariable with depth and can be determined theoretically. The spectral constitution of wave pressure field and the regularity of attenuation of spectral components were analyzed at the vertical breakwater, and a new expression describing the equilibrium range of wave pressure spectrum was obtained.

The interaction of small and finite amplitude long waves and currents

Abstract: The interaction of waves and currents is important for many engineering problems. For example, when considering forces on marine structures, the velocity and acceleration field must be defined, and thus the manner in which a current interacts with small and finite amplitude waves must be understood. When the current is large and oblique to the waves, the direction of the force on an offshore structure may change significantly with depth introducing a torsional moment. Wave refraction and the concomitant attenuation or amplification of waves are also affected by offshore currents. An example is the effect on incident waves of offshore currents induced by the discharge of cooling water from coastal-sited power plants. This current can modify the direction and magnitude of approaching waves, and by these changes the breaking waves at the shore and the nearshore sediment transport associated with these waves may be changed. A number of theoretical studies have been conducted on various aspects of wave-current interactions; see Peregrine (1976). One theoretical study, Thomas (1981), will be used in this investigation. Careful experiments in this area are limited; several are: Iwagaki and Asano (1980), Sarpkaya (1957), and Thomas (1981). Each of these has given attention to certain aspects of small amplitude wave-current interactions. The experiments are difficult to conduct because of the problems inherent in introducing waves into a flume with a steadyuniform current or conversely a current into a wave tank with permanent waves. Certain features of these experimental problems can be seen through the following two examples. If a plunger-wave machine were used and located at one end of a flume in which a steady current is flowing, although the waves would be developing as they interact with the current, the previously steady current would be changed to an unsteady one by the periodic blockage of the flow by the plunger. If the waves are generated at one end of the tank and allowed to develop, and a current is introduced from the bottom of the tank, this current must expand to the full depth of the flow; hence, the waves propagate on a developing current. Therefore, comparisons to theory are, to some extent, difficult to realize, because the theory generally assumes wavecurrent interactions when each is fully developed.