Environmental Engineering Reference
In-Depth Information
Table 13.2.
Wind relationships - water to land (USBR, 1981).
Effective fetch (Fe) (km)
0.8
1.6
3.2
4.8
6.4
8 (or more)
over water
over land
Wind velocity ratio
1.08
1.13
1.21
1.26
1.28
1.30
Figure 13.2.
Wave heights and minimum duration wind (Saville et al., 1962; USBR, 1981). Note 1 mile
per hour
1.6 km per hour, 1 foot
0.3 metres.
level. If the rate of flood rise is rapid, the high wind velocity associated with the design
storm may be applicable, but for a slower rate of flood rise a lower wind velocity would be
appropriate. In many cases, a wind velocity of the order of 1 in 10 year event is appropriate.
For the normal freeboard calculation, maximum expected wind velocities, duration and
direction should be used. The values should exceed 1 in 100 year winds. The estimates
should be adjusted over land to over water wind velocity by applying the correction factors
listed in Table 13.2.
The design wind velocity and duration are determined iteratively, by using Bureau of
Meteorology estimates of wind speed vs duration and applying these to wind velocity vs
fetch vs minimum duration shown in Figure 13.2.
13.1.3.3
Wave height
For the estimation of the minimum freeboard the significant wave height in metres (Hs), which
is the average of the highest one-third of the waves in the wave spectrum, should be used. Hs
can be estimated from Figure 13.2. The wave period (T) can be estimated from
Figure 13.3
.
The deepwater wave length in metres can then be computed from:
L .56T
2
(13.2)
in which T
wave period in seconds.
