EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-1-39. Definition sketch for Ochi-Hubble spectrum (Ochi and Hubble 1976)
(f) In this equation, α is the scaling parameter, γ the peak enhancement factor, fp the frequency at the
spectral peak, U10 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
ω0j
4
2
j
2
Hs j
4λj%1
ω0j
4
1
exp &
E(ω) '
(II-1-156)
4
Γ(λj)
ω
4 j'1
4λj%1
ω
where Hs1, ω01, and λ1 are the significant wave height, modal frequency, and shape factor for the lower-
frequency components while Hs2, ω02, and λ2 correspond to the higher frequency components (Figure II-1-39).
The value of λ1 is usually much higher than λ2. For the most probable value of ω01, it can be shown that λ1
= 2.72, while λ2 is related to Hs in feet as
&0.027Hs
λ2 ' 1.82 e
(II-1-157)
II-1-90
Water Wave Mechanics
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