Contents preface (VII) introduction 1—37

(ii) Reservoir and Tail-water Loads (W_w , W_w′, W₁, and W₁′)
These are obtained from tail-water curves and range of water surface elevations in reservoir obtained from reservoir operation studies. These studies are based on operating and hydrologic

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536 IRRIGATION AND WATER RESOURCES ENGINEERING
data such as reservoir capacity, storage allocations, stream flow records, flood hydrographs, and reservoir releases for all purposes. In case of low overflow dams, the dynamic effect of the velocity of approach may be significant and should, therefore, be considered. If gates or other control features are used on the crest, they are treated as part of the dam so far as the application of water pressure is concerned. In case of non-overflow gravity dams, the tail-water should be adjusted for any retrogression. Any increase in tail-water pressure due to curvature of flow in the downstream bucket of an overflow type gravity dam should also be considered in the design of gravity dams (4).

Reservoir water surface

h

Ww

Wc

D

^W1

u

Tail-water surface

W

^Ws

e

w

h

W1

p

b/2

b/2

p

p

p

U

Fig. 16.1 Usual loading combination for a gravity dam
(iii) Uplift Forces
Uplift forces (U) occur due to internal hydraulic pressures in pores, cracks, and seams within the body of a dam, at the contact between the dam and its foundation, and within the foundation. The distribution of internal hydrostatic pressure along a horizontal section through a gravity dam is assumed to vary linearly from full reservoir pressure at the upstream face to zero or tail-water pressure at the downstream face, and to act over the entire area of the section. The pressure distribution is also adjusted depending upon the size, location, and spacing of internal drains. Experimental and analytical studies indicate that the drains set in from the upstream face at 5 per cent of the maximum reservoir depth and spaced laterally twice that distance will reduce the average pressure at the drains to approximately tail-water pressure plus one-third the difference between reservoir water and tail-water pressures (3) (Fig. 16.2). It is assumed that uplift forces are not affected by earthquakes (2).