Therun-upof waves on the upstream dam face, i.e. the maximum
vertical height attained by a wave running up a dam face, referred to the
steady water level without wind action, depends primarily on the wave
height, the depth in front of the dam, the geometry and material of the
upstream face of the dam, and the approach conditions in the reservoir.
The wave run-up for a typical vertical face in deep water is equal to
Hd, but can attain values over 2Hdfor a smooth slope of 1 in 2 (ICE, 1996).
It may be appropriate to test several alternative wind/fetch directions
to determine the maximum value of significant design wave height. The
wind speed used in Fig. 4.1 should include adjustments for open water,
duration, direction, altitude and return period (see ICE, 1996) (the com-
bined effect of all adjustments is often close to 1). When determining the
final value of the freeboard all the surcharge components should be
summed; cross-reference can also be made to the last column of Table 4.1.
The probability of an extreme wind coinciding with the maximum
flood level in the reservoir is usually very small, and thus it may be
deemed unnecessary to superimpose all the extreme conditions for free-
board determination, particularly in the case of concrete dams. Equally, it
is a matter of engineering judgement of the possible damage from some
water passing over the dam and the cost of a large freeboard for a wave
run-up which will ultimately determine the adopted freeboard value. It is
generally accepted that well chosen grass on an embankment can with-
stand water velocities of up to 2 m s^1 for prolonged periods and up to
5ms^1 for brief periods (less than 1 h); reinforced grass waterways perform
substantially better (CIRIA, 1987).
4.5 Sedimentation in reservoirs
The assessment of economic viability, safety and cultural considerations as
well as the environmental and social impact assessment should form an
integral part of any large dam project; this assessment has many facets,
one of the most important being the estimation of the sediment deposition
in a reservoir and its ‘life’.
The loss of storage is only one deleterious effect of sedimentation in
reservoirs; others are increased flood levels upstream of the reservoir, ret-
rogression of the river bed and water levels downstream of the dam, the
elimination of nutrients carried by the fine sediments, the effect of sedi-
mentation on the reservoir water quality, etc. At present, many reservoirs
have a life expectancy of only 100 years. A useful life of a reservoir less
than, say, 200 years should certainly be a matter of concern, and one has to
consider whether the drastic environmental effects are outweighed by the
economic advantages during a relatively short effective life (see also
Section 1.2.4).
200 DAM OUTLET WORKS
