Journal of Natural Disaster Science

DRAG COEFFICIENT OVER WATER SURFACE UNDER THE ACTION OF STRONG WIND

Hisashi MITSUYASU
Professor
and
Tadao KUSABA
Research Associate, Research Institute for Applied Mechanics, Kyushu University

(Received 26 Oct., 1984 and in revised form 18 Feb., 1985)

Abstract

Experiments were done to determine the properties of the drag coefficient over a water surface under a high speed wind of up to U10≒35 m/s, where U10 was the wind speed at the height Z=10 m. The neutral drag coefficient, CD, over the water surface in the range of U10≦20 m/s can be given by the power law relation

CD = 1.77 x 10^(-3)U10^(-0.244) ,

or the linear form

CD = (1.29-0.024U10) x 10^(-3) ,

when wind waves are suppressed by adding a surfactant to the water. For a clean water surface on which the wind waves exist, the neutral drag coefficient, CD, in the range of 8 m/s≦U10≦35 m/s can be given by the power law relation

CD =2.60 x 10^(-4)U10^(0.654),

or the linear form

CD =(0.581+0.063U10) x 10^(-3) .

The latter form is very similar to the form obtained recently by Large and Pond (1981) using data recorded over the ocean. The neutral drag coefficient for a clean water surface in the range U10≦8 m/s does not differ much from that over a smooth water surface contaminated with surfactant.

Key words

storm surge, wind shear stress, drag coefficient