Darbyshire and Draper (1963) presented widely used charts
(Fig. 14.14) for wave prediction, based on observations around Britain.
Figure 14.14 permits the determination of Hmaxand the period of waves in
deep water as functions of wind speed (km h^1 ), duration (h) and fetch
(km).Hmaxis the maximum wave height in a wave record of 10 min length.
As with the SMB method, wave height is found to be either fetch limited
or duration limited. Darbyshire and Draper found that the records col-
lected in coastal areas, where depth varied from 30 m to 45 m, showed
significant differences from those collected in oceanic waters. Figure 14.15
facilitates the determination of Hmaxand the period for coastal waters as
functions of wind speed (km h^1 ), duration (h) and fetch (km).
An alternative method of wave forecasting is based on the frequency
spectrum of the waves. In this method the irregular waves of the ocean
are assumed to consist of frequencies, f, ranging from 0 to infinity. If the
mean-square value of the wave amplitudes in the range f ∆f/2 and
f∆f/2 is a
2
( f ) , the energy per unit frequency centred at fis
E(f)
1
2
a^2 (^ f^ )^ /∆f. (14.46)
E(f) gives the energy spectrum showing the energy content at various
frequency components. The mean-square value of the wave amplitudes is
given by
596 WAVES AND OFFSHORE ENGINEERING
Fig. 14.13 Fetch graph for deep water (King, 1972)
