introduced to cover malfunction of the gates. A further component may be
required to account for the effect of impulse waves generated by shore
instabilities (rock falls, landslides, snow avalanches etc.); for the hydraulics
of these waves and their effect on the dam see, e.g., Huber (1997), Vischer
and Hager (1998) and ICOLD (2002). In embankment dams the total free-
board must also include an allowance for the settlement of the dam and
foundations (Chapter 2). Thus freeboard determination involves engin-
eering judgement, statistical analysis, and consideration of the damage that
would result from the overtopping of a dam.
Seiches, periodic undulations of the reservoir, are usually ignored,
particularly in medium-sized reservoirs, and their effect is included in a
safety margin added to the other freeboard components. Thomas (1976)
mentions seiches up to 0.5 m high in some of the very large reservoirs.
Wind set-up(or wind tide (Roberson, Cassidy and Chaudry, 1998)), s
(m) results from the shear induced by continuous wind (or regular gusts in
one direction). Its value will depend on the reservoir depth, dr(m), wind
fetch,F(km) (the maximum free distance which wind can travel over the
reservoir), the angle of the wind to the fetch, , and the wind speed U
(km h^1 ) measured at a height of 10 m. The Zuider Zee equation can be
used as a guide:
sU^2 Fcos/(kdr) (4.9)
where kis a constant – about 62 000; using m units throughout and
U(ms^1 ) equation (4.9) can be written as
sk 1 Fcos (4.9a)
wherek 1 2 10 6.
To produce this set-up the wind must blow for a certain time; the
shorter the fetch and higher the wind speed the shorter the time. Typical
values are 1 h for a fetch of 3 km, 3 h for F20 km, 8 h for F80 km and
U40 km h^1 , or 4 h for U80 km h^1. It must also be remembered that the
wind speed at 10 m above the surface of a new reservoir will be higher than
that recorded over the original topography. The ratio of wind speed over
water to speed over land will be about 1.1–1.3 for effective fetches of 1–12 km.
Allowances for wave height and the run-up of wind-generated waves
are the most significant components of freeboard.
As a basis for a wave heightcomputation,H(m) (crest to trough) can
be estimated from:
H0.34F1/20.76 0.26F1/4 (4.10)
(for large values of fetch (F20 km) the last two terms may be neglected).
U^2
gdr
198 DAM OUTLET WORKS
