Appendix B. Description of Ocean Wave Spectra

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distribution of wave energy. The spectral shapes were derived from long-term

field wave measurements and depend on factors such as wind duration, the

distance or fetch over which the wind blows, and water depth.

Pierson-Moskowitz Spectrum

For fully-developed sea states in deep water where there is a local balance

between momentum transfer from the wind and wave breaking/nonlinear cross-

spectral energy transfer processes, Pierson-Moskowitz (1964)1 proposed the

following wave spectrum:

  f -4 

αg

exp -β  

S( f ) =

(B1)

 f  

(2π) f

  p 

where α = 0.0081 is Phillips' constant, g is the gravitational acceleration, β =

0.74, fp = g/2πU 19.5, and U 19.5 is the wind speed at 19.5 m above the mean sea

level.

Bretschneider Spectrum

The Bretschneider spectrum (Bretschneider 1959) has the same shape as the

Pierson-Moskowitz spectrum but is defined in terms of the significant wave

height, Hs, and spectral peak frequency, fp, instead of the wind speed. It can be

written as:

References cited in this appendix are listed in the References at the end of the main text.

Appendix B Description of Ocean Wave Spectra