(2) To evaluate the effect of diffraction on a directional wave spectrum, Goda, Takayama, and Suzuki

(1978) calculated diffraction coefficients for a semi-infinite breakwater and a breakwater gap by breaking the

spectrum into number of frequency (10) and direction (20 to 36) components and combining the result at

points in the breakwater lee. This produced an effective diffraction coefficient defined by

1

1

4

θ max

)

2

(II-7-1)

'

where *K*N is the diffraction coefficient for each frequency/direction component when acting as a

monochromatic wave, *M*0 is the zero moment of the spectrum, *df *and dθ are the frequency and direction

ranges represented by each component of the spectrum, *θ*max and *θ*min are the limits of the spectral wave

component directions, and *S(f,θ) *is the spectral energy density for the individual components. The spectral

frequency distribution they employed was similar to most typical storm spectra such as the JONSWAP

spectrum. The directional spread of the spectrum was characterized by a directional concentration parameter

directional spread is quite limited.

II-7-10

Harbor Hydrodynamics

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