coefficients for various angles of incidence. The latter proves to be 

 remarkably close to the corresponding results for a very wide shear 

 layer which is treated by LONGUET-HIGGINS and STEWART (1961). 



Coastal Engineering Significance. This paper has special importance 

 since it is one of the few papers to consider currents with length 

 scales much less than a wavelength. The results allow reflection to be 

 estimated; usually it is very small. 



13. FORRISTALL, G.Z., et al., "The Directional Spectra and Kinematics 

 of Surface Gravity Waves in Tropical Storm Delia," Journal of 

 Physical Oceanography , Vol. 8, No. 5, Sept. 1978, pp. 888-909. 



Keywords. Comparison of Theory and Measurement; Currents, Large-Scale; 

 Currents, Ocean; Observation; Spectra; Spectra, Directional; Wave 

 Energy; Waves, Storm; Waves, Wind. 



Discussion. Knowledge of the kinematics of the flow beneath surface 

 waves is vital for the design of offshore structures. Due to the tech- 

 nical difficulty of making pertinent measurements in storm conditions, 

 knowledge of the kinematics of storm waves has been based almost 

 entirely on theoretical considerations. Now measurements made with 

 electromagnetic current meters during Tropical Storm Delia have per- 

 mitted verification of the theories. 



There was considerable scatter between the measured velocities and 

 the predictions of unidirectional wave theories, with a clear bias 

 toward overprediction. Use of higher order and irregular unidirectional 

 theories did not substantially improve the comparisons. A good fit with 

 the data could, however, be obtained by using the concept of a 

 directional wave spectrum based on linear wave theory. 



The simultaneous wave and particle velocity measurements were used 

 to estimate the directional spectrum through an analysis procedure which 

 took into account the presence of a strong current. The directional 

 spectrum was also hindcast using a numerical model, and the comparison 

 of the hindcast with data was good. 



The fact that velocity spectra in confused storm seas can be 

 accurately calculated will be directly important in some design 

 problems. In other cases, it is necessary to know the probability 

 distribution of the extreme events. Using the assumption of a Gaussian 

 sea surface, it was possible to satisfactorily predict the distribution 

 of the magnitudes of velocity. All the comparisons lead to the 

 conclusion that a proper description of storm wave kinematics is 

 dependent on correctly accounting for the directional spreading of the 

 wave energy . (author's abstract) 



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