wind at a particular point is
1
a∂
∂
p
n 2 U*sin (14.45)where∂p/∂nis the pressure gradient normal to the isobar, Uis the wind
speed, (^) ais the density of air, *is the angular velocity of the Earth’s rota-
tion, and is the latitude.
In order to estimate the characteristic height of waves generated by
wind, the fetch Fhas to be estimated from the weather map (US Army,
1984). An approximate estimate of the fetch is the distance along the
isobar curve from the location to a position upwind at which the tangent to
the isobar deviates by about 15°. If the wind blows seaward from the coast,
then the fetch length is limited by the coast. Meteorological data provide
help in estimating the wind duration.
14.9.2 Significant height and period of the waves
Significant wave height, Hs, and the wave period, Ts, are the mean height
and period of the highest third of waves in a sample. Other characteristic
wave periods are the mean period of the waves whose troughs and crests
are above SWL and the wave period corresponding to that at which the
spectrum exhibits a peak.
Another measure of the wave period, called the zero-crossing period,
is obtained from the output of a wave record. It is determined by finding the
number of times, N, that the record trace crosses the still-water level as the
water level changes from the trough to the crest. The zero-crossing period,
Tz, is equal to the duration of the record divided by N. For engineering
applications (Tucker, 1963), Tzis nearly equal to the significant period Ts.
The significant wave height, Hs, is used as a design wave height in
coastal engineering practice. In the earliest method of wave forecasting,
known as the Sverdrup, Munk and Breitshneider (SMB) method (King,
1972) and applicable to deep water, HsandTsare related to the fetch F,
duration of the wind Tw, and acceleration due to gravity g. The relation-
ship is shown in Fig. 14.13 in non-dimensional form. The TminU/Fversus
gF/U^2 curve gives the minimum time, Tmin, required for the generation of
waves of maximum energy for the given fetch and wind speed. If the dura-
tion of wind, Tw, is greater than Tminthe generation of the wave is fetch
limited. In that case, Hsis found for the given fetch.
On the other hand, if TwTminthe production of waves is duration
limited. From the TminU/FversusgF/U^2 curve (Fig. 14.13) we can find F
corresponding to TwTminand from the value of Fthus determined, we
findHs. Figure 14.13 is also used to find Ts.