--- title: "Reservoir intake for an earth dam" subject: "Water Supply Engineering" chapter: 5 key: "semester_5/water_supply_engineering:reservoir-intake-earth-dam" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/reservoir-intake-earth-dam site: BCT Engineering --- # Reservoir intake for an earth dam Water Supply Engineering · Chapter 5: Intakes - Asked in: 2076 Chaitra, 1+7 marks; 2079 Bhadra, 8 marks (asked 2 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/reservoir-intake-earth-dam - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-5 ## Question What are the components of an intake? Describe with the help of neat sketches a (wet) reservoir intake for an earth dam, including its operation, characteristics and the factors to ensure stability and safety of the intake works. ## Short answer Intake components include ports, screens, well, gates, conduit, sump, footbridge, protection. For an earth dam, a wet intake tower is built upstream, connected by footbridge; outlet conduit passes under dam with cut-off collars. Ports at multiple depths allow selective withdrawal. Stability requires firm foundation, resistance to overturning/sliding/floatation, cut-off collars to prevent piping, and control house above flood level. ## Answer #### Components of an intake 1. **Intake opening / port** with a bell-mouth entry, placed at one or more levels. 2. **Coarse screen (trash rack)** with bars 20 to 50 mm apart to stop large floating objects, and a **fine screen** (6 to 12 mm mesh) to stop small debris. 3. **Intake well / tower / chamber** that houses the gates and valves and lets water settle. 4. **Sluice gates / valves** to regulate or cut off flow. 5. **Conveying (intake) pipe or conduit** from the well to the pump house or main. 6. **Sump or pump well** (for pumped schemes) and a **foot bridge** for access. 7. **Protection works**: pitching, groynes, wing walls, embankment, guard rails, and a **flushing / scour pipe** for removing silt. #### Wet reservoir intake for an earth dam In an earth dam the intake is not placed inside the embankment (which would cause leakage and settlement). Instead a **wet intake tower** is built in the reservoir upstream of the dam, near the upstream toe, connected to the crest by a **foot bridge**, and the **outlet conduit** (a pipe in a trench, or a tunnel through the abutment or under the base) passes under or beside the dam. ```text Foot bridge =======+ Earth dam HFL ~~~~~~~~~~~~~~~ | /\ NWL ~~~~~~~~~~~~[T] | / \ ports --> [ ]| / \ bed ///////////[ ]-+------\--///// conduit with cut-off collars ---> outlet / plant ``` **Operation and characteristics** - Ports at different depths (top, middle, bottom) with screens and sluice gates let the operator draw the best quality water at all reservoir levels. - Gates are lifted by stems from the service platform; the lowest port is above the silt level. - Water flows to the well, then through the outlet conduit to the downstream valve house and plant. #### Factors ensuring stability and safety of the intake works - The tower base is founded on a firm stratum, with a **foundation** wide enough to resist overturning and sliding under water pressure, wave and ice loads. - Check **stability against overturning, sliding and floatation** (empty well) and against the earthquake load, with the foot bridge as a hinged or sliding support. - The **conduit** through the dam has **cut-off collars** (anti-seepage collars) to stop piping and leakage along the pipe, and is laid on firm ground, not on fill. - Keep the gate and valve control house above flood level, and the tower top above the maximum water level plus wave height (freeboard). - Provide a **flushing pipe** to remove silt, a trash rack and a fine screen, and pressure-test the conduit. - Build the tower of reinforced concrete, or of masonry in waterproof cement mortar. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the intake tower not placed inside an earth dam embankment? A. It would stop flow B. Law forbids it C. It would cause leakage and settlement D. Earth cannot hold water Answer: C. Instead it stands in the reservoir upstream, with a conduit under or beside the dam. (Key point: Tower position) 2. What are cut-off collars on the conduit for? A. Filter silt B. Stop piping and leakage along the pipe C. Increase flow D. Support the foot bridge Answer: B. Anti-seepage collars prevent water travelling along the conduit. (Key point: Cut-off collars) 3. Which checks are made for the tower? A. Only evaporation B. Only taste C. Overturning, sliding, floatation and earthquake load D. Only colour Answer: C. The tower must be stable against water, wave, ice and seismic loads. (Key point: Stability checks) 4. Where should the tower top be? A. Above maximum water level plus wave height B. At LWL C. At bed level D. Below NWL Answer: A. This ensures safety against waves. (Key point: Freeboard) ## Related in Water Supply Engineering - [Wet (typical) reservoir intake](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awet-reservoir-intake) — Water Supply Engineering, chapter 5, asked 2× - [Reservoir intake and site selection factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areservoir-intake-site-factors) — Water Supply Engineering, chapter 5, asked 2× - [Intake components and site factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aintake-components-site-factors) — Water Supply Engineering, chapter 5 - [Wet and dry river intake](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awet-dry-river-intake) — Water Supply Engineering, chapter 5 - [Intake site factors; spring intake](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aspring-intake-site-factors) — Water Supply Engineering, chapter 5 - [Factors in constructing a spring intake](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aspring-intake-factors-constructing) — Water Supply Engineering, chapter 5 ## Related in other subjects - [Bridge site factors and components with sketches](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-components-sketches) — Transportation Engineering II, chapter 5 - [Types of cross drainage works with sketches](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acd-works-types-definition-sketches) — Irrigation and Drainage Engineering, chapter 8, asked 9× - [Safety precautions in electrical works](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aelectrical-safety) — Building Technology, chapter 12, asked 3× - [Earth dam with filter, seepage quantity](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aearth-dam-horizontal-filter-seepage) — Soil Mechanics, chapter 7