--- title: "Dam types and forces on gravity dam" subject: "Hydro Power" chapter: 4 key: "semester_6/hydro_power:dam-types-forces" source: https://www.bctengineer.com/q/semester_6/hydro_power/dam-types-forces site: BCT Engineering --- # Dam types and forces on gravity dam Hydro Power · Chapter 4: Dam Engineering - Asked in: 2078 Chaitra, 1+4+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/dam-types-forces - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-4 ## Question What do you mean by dam? Enlist types of dam based on various criteria. Write the possible forces acting on gravity dam with a neat sketch. ## Short answer A dam is a barrier storing or diverting water. Types include storage/diversion, overflow/non-overflow, concrete/earthfill, gravity/arch/embankment. Forces on gravity dams: self weight, water pressure, uplift, silt, wave, earthquake, ice and wind. ## Answer #### 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 | Criterion | Types | |---|---| | Function | Storage dam, diversion dam (weir/barrage), detention (flood) dam, debris dam, coffer dam | | Hydraulic design | Overflow dam (spillway section), non-overflow dam | | Material | Rigid: concrete, masonry, steel, timber; Non-rigid: earthfill, rockfill | | Structural action | Gravity, arch, buttress, embankment (earth/rockfill) | | Height | Low (up to 30 m), medium (30–100 m), high (> 100 m); ICOLD defines a "large dam" as one 15 m or more high | | Purpose | Single purpose, multipurpose | Examples in Nepal: Kulekhani (rockfill storage dam), Kaligandaki A (concrete gravity diversion dam). #### Forces acting on a gravity dam ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a dam? A. A barrier across a river to store, raise or divert water B. A tunnel for water C. A power house D. A canal Answer: A. A dam is a barrier across a stream used for storage, diversion, power, irrigation and flood control. (Key point: Dam definition) 2. By material, which are non-rigid dams? A. Steel and timber B. Earthfill and rockfill C. Arch and buttress D. Concrete and masonry Answer: B. Rigid dams are concrete, masonry, steel, timber; non-rigid are earthfill and rockfill. (Key point: Classification by material) 3. By structural action, which list is correct? A. Storage, diversion, detention B. Overflow, non-overflow C. Gravity, arch, buttress, embankment D. Low, medium, high Answer: C. That is the structural-action criterion; the others are function, height and hydraulic design. (Key point: Structural classification) 4. In the force list, which is true about tailwater pressure? A. It is ignored B. It acts downstream C. It increases uplift D. It acts upstream, opposing water pressure, and the water on the downstream slope adds weight Answer: D. Tailwater pressure opposes the reservoir thrust. 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