Worldwide Measurements of Directional Wave Spreading

Abstract The directional spreading of waves is important for both theoretical and practical reasons. Enough measurements have now been made to draw conclusions about the behavior of wave spreading at sites in different climatic regimes. The measurements presented here of the directional spreading of fetch-limited waves agree in general with those of M. A. Donelan et al., but additional evidence is also found to support the conclusion of I. R. Young et al. that the spreading function is bimodal at high frequencies. The spreading factor ϕ is defined to be the square root of the in-line variance ratio defined by R. E. Haring and J. C. Heideman. This spreading factor gives an integrated measure of the degree of directional spreading in the wave spectrum and predicts the reduction in the in-line particle velocities under waves due to direction spreading. The value of ϕ is 1 for unidirectional waves and 0.707 for omnidirectional waves. For fetch-limited conditions, ϕ is essentially constant at 0.906. Results fr...

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