Environmental Engineering Reference
In-Depth Information
Figure 13.1.
Method for calculating effective fetch (adapted from Saville et al., 1962).
13.1.3.1
Fetch
In reservoirs, fetches are limited by the land surrounding the body of water. The shorelines
are irregular and an effective fetch is calculated from:
xcosa
cos a
i
i
F
(13.1)
i
where a
i
angle between the central radial from the dam and radial i; x
i
length of pro-
jection of radial i on the central radial.
A trial and error approach should be used to select the critical position on the dam and
direction of the central radial to give the maximum effective fetch.
The radials spanning 45° on each side of the central radial should be used to compute
the effective fetch. Figure 13.1 shows an example.
13.1.3.2
Design wind
Design wind estimates should be obtained from the Bureau of Meteorology or equivalent
organisation. The critical wind duration depends on the fetch; longer durations are criti-
cal for longer fetches. The estimates should allow for local topographic effects.
USBR (1981) indicate that for maximum freeboard moderate winds should be used, i.e.
winds that can reasonably be expected to occur concurrent with maximum reservoir flood
