Spatial growth of mechanically generated water waves under the action of wind has been measured in a laboratory wind-wave flume both for pure water and for water containing a surfactant (sodium lauryl sulphate, concentration 2.6 × 10 −2 %). I n the latter case, no wind waves develop on the surface of the mechanically generated waves as well as on the still water surface for wind speeds up to U 10 ≈ 15 m/s, where U 10 is the wind velocity at the height Z = 10 m. Therefore we can study the wind-induced growth of monochromatic waves without the effects of co-existing short wind waves. The mechanically generated waves grew exponentially under the action of the wind, with fetch in both cases. The measured growth rate β for the pure water can be fitted by β/ f = 0.34( U * /C) 2 0.1 [lsime ] U * /C [lsime ] 1.0, where f is the frequency of the waves, C is the corresponding phase velocity, and U , is the friction velocity obtained from vertical wind profiles. The effect of the wave steepness H / L on the dimensionless growth rate β/ f is not clear, but seems to be small. For water containing the surfactant, the measured growth rate is smaller than that for pure water, but the friction velocity of the wind is also small, and the above relation between β/ f and U * /C holds approximately if the measured friction velocity U * is used for the relation.
SYNOPSISModel experiments were made on the wave force acting against a wall, when oscillatory waves progressed towards the sloping beach and impinged against the wall. The total wave force F was measured by a newly designed instrument and a non-dimensional wave force coefficient,Cw=F/pgH0Wd (H0: deep water wave height, W: width of the wall, d: water depth at the wall),was related to the beach slope, the relative depth at the wall and the deep water wave steepness. Other informations presented include the standard devitation of the wave force, a relationship between the wave impulse and the momentum of the impinging wave, and also a relationship between the total wave force and the wave pressure at the foot of the wall.
The results of the author's recent studies on the growth of the spectrum of wind-generated waves have been used for determining a simple form of fetch- limited wave spectrum which can be used for p...
Systematic measurements of the surface drift current, the wind profile over the water surface and the wave spectra have been made for (i) pure wind-waves, (ii) a coexisting system of wind-waves and swell propagating against the wind, and (iii) a coexisting system of wind-waves and swell propagating in the direction of the wind. The surface drift current is gradually intensified by the swell propagating against the wind when the swell steepness increases. The maximum increase of the surface drift velocity caused by the opposing swell is about 46 % of the surface drift velocity for pure wind-waves at the same wind speed. Such a phenomenon was not observed when the swell was propagating in the direction of the wind.
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