This type of measurement gives values of the current due to the 

 wind. Linear perturbation theory indicates that the advection of the 

 wave by wind-drift is odd (depends on wind direction) while the initial 

 effects of the air are even (independent of wind direction). Thus, it 

 is possible to separate these effects at first order by forming the sum 

 and difference of the speeds of downwind- and upwind- traveling waves. 



The advection effect of wind drift is found to increase less 

 rapidly than linearly with the air friction velocity u*. Since Wu 

 (1975) showed that surface drift is proportional to u^, this implies 

 that wind drift becomes thinner for high winds. 



The effect of inertial pressure on the speeds of 9.8-centimeter 

 waves, particularly windspeed dependence, was well predicted by using 

 values of the air roughness length deduced from several measurements. 

 For longer waves it was much smaller, possibly due to a reduction of the 

 mean air flow near the surface by wave drag. 



The interaction between wave components due to finite-amplitude 

 effects leads to an increase in phase speed. However, measurements show 

 that at constant windspeed each component increased in magnitude with 

 fetch, but its phase speed decreased. 



Coastal Engineering Significance . These experimental measurements 

 indicate the importance of allowing for current and wind profiles in 

 work with surface waves being actively generated by wind. The decrease 

 of phase velocity with fetch is opposite to all present theoretical 

 results. These results are most relevant to interpreted "remote 

 sensing" measurements. 



48. PRANDLE, D., and WOLF, J., "The Interaction of Surge and Tide in 

 the North Sea and River Thames," Geophysical Journal of the Royal 

 Astronomical Society, London, England, Vol. 55, No. 1, Oct. 1978, 

 pp. 203-216. 



Keywords. Bottom Friction; Comparison of Theory and Measurement; Cur- 

 rents, Tidal; Equations of Motion; Interactions, Long Waves; Numerical 

 Model; Observation; Statistics; Wave Height; Waves, Long. 



Discussion. Tide surge interaction is examined in three ways. The 

 first is a statistical analysis and uses surges which can be identified 

 propagating along the whole east coast of the British Isles (approxi- 

 mately 1000 kilometers). There is clear indication of an interaction 

 when the rate of amplification is plotted for positive and negative 

 surges occurring at different phases of the tide. Also, a comparison of 

 measurements at Lowestoft and London gives an indication that amplifica- 

 tion is linearly dependent on surge and amplitude. 



52 



