(f) In this equation, *α *is the scaling parameter, *γ *the peak enhancement factor, *f*p the frequency at the

spectral peak, *U*10 the wind speed at the elevation 10 m above the sea surface, *F *the fetch length. Figure II-1-

38 qualitatively illustrates the relationship between JONSWAP and PM spectra. The JONSWAP spectrum

can also be fitted mathematically to observed spectra by iteratively solving for d, γ, fm , and σ.

(g) A six-parameter spectrum developed by Ochi and Hubble (1976) is the only wave spectrum which

exhibits two peaks (Figure II-1-39), one associated with underlying swell (lower frequency components) and

the other with locally generated waves (higher frequency components). It is defined as

λj

4λj % 1

4

ω0*j*

4

2

j

2

4λj%1

ω0*j*

4

1

exp &

(II-1-156)

4

Γ(λj)

ω

4 j'1

4λj%1

ω

where *H*s1, *ω*01, and *λ*1 are the significant wave height, modal frequency, and shape factor for the lower-

frequency components while *H*s2, *ω*02, and *λ*2 correspond to the higher frequency components (Figure II-1-39).

The value of *λ*1 is usually much higher than *λ*2. For the most probable value of *ω*01, it can be shown that *λ*1

= 2.72, while *λ*2 is related to *H*s in feet as

&0.027*H*s

λ2 ' 1.82 *e*

(II-1-157)

II-1-90

Water Wave Mechanics

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