Dam types and forces on gravity dam

Dam

A dam is a barrier built across a river or stream to store water, raise its level or divert it. The stored water is used for hydropower, irrigation, water supply and flood control.

Types of dams

Examples in Nepal: Kulekhani (rockfill storage dam), Kaligandaki A (concrete gravity diversion dam).

Forces acting on a gravity dam

          crest
   ~~~~~ +---+  <- wave pressure Pw
   |     |   |\
 Pi ->   |   | \  <- wind
   |     | W |  \    W = self weight
 P1 ->   |   |   \   (+ EQ: aW horizontal)
   |     |   | W2 \
 Ps ->   |   |     \  ~~~~ tailwater
 (silt)  |   |      \  <- P2
         +---+-------+ toe
  heel   ^  ^  ^  ^  ^
         U (uplift)

1. Self weight (W): main stabilising force, $W = \gamma_c \times$ area, at the centroid.
2. Upstream water pressure: $P_1 = \frac{1}{2}\gamma_w h_1^2$ at $h_1/3$; plus water weight on a sloping upstream face.
3. Tailwater pressure: $P_2 = \frac{1}{2}\gamma_w h_2^2$ acting upstream, plus water weight on the downstream slope.
4. Uplift pressure (U): upward seepage pressure on the base, from $C\gamma_w h_1$ at heel to $\gamma_w h_2$ at toe; reduced by drainage galleries.
5. Silt pressure: $P_s = \frac{1}{2}\gamma_{sub}h_s^2K_a$.
6. Wave pressure: $P_w = 2\gamma_w h_w^2$ at $3h_w/8$ above water level.
7. Earthquake forces: inertia force $\alpha_h W$ on the dam and hydrodynamic water pressure (Westergaard/Zanger).
8. Ice and wind pressure: ice thrust in cold regions; wind usually neglected.