--- title: "Undersluice functions and design criteria" subject: "Irrigation and Drainage Engineering" chapter: 5 key: "semester_6/irrigation_and_drainage_engineering:undersluice-function-criteria" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/undersluice-function-criteria site: BCT Engineering --- # Undersluice functions and design criteria Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks - Asked in: 2062 Kartik (old course), 4 marks; 2068 Baisakh (old course), 2+2 marks (asked 2 times) - Video: [Part 20 ! Under sluice ! Divide wall ! Parts of diversion head work ! Scouring sluice !Divide groyne](https://www.youtube.com/watch?v=UOVHo7nn-6c) — NARAYANA COACHING CLASSES - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/undersluice-function-criteria - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5 ## Question Why is an undersluice provided in headworks? Write with explanation the major functions of the undersluice constructed in an irrigation headworks and mention its design criteria. ## Short answer It scours deposited silt in front of the head regulator, maintains a deep clear channel, passes low floods and surplus without losing pond level, and draws the silt-laden bottom layer away. Design: capacity ≥ twice canal discharge, crest at average river bed, velocity 2–3 m/s, floor designed for uplift and energy dissipation. ## Answer #### Why an undersluice is provided Silt tends to deposit in front of the canal head regulator because the velocity near the pocket is low. An **undersluice (scouring sluice)** is a set of gated bays at low level beside the head regulator. It is provided to keep a clear, deep channel in front of the regulator so that the canal gets silt-free water. #### Major functions 1. Scours out the silt deposited in the pocket in front of the head regulator, by opening the gates (the sluice acts like a drain for the pocket). 2. Maintains a well-defined channel from the river to the regulator, particularly in the dry season when the river flow is low. 3. Passes low floods (and the surplus) without dropping the weir shutters, so the pond level and the supply to the canal are kept. 4. Draws the bottom, silt-laden layer away from the canal (induced curvature, together with the silt excluder). 5. Passes part of the high flood and floating debris, which reduces the afflux and the load on the weir. #### Design criteria - **Capacity:** at least twice the canal discharge (to give a scouring velocity), the dry-season flow of the river, or 10-15% of the maximum flood, whichever is the greatest. - **Crest level:** at the average river-bed level (about 1.2-2.0 m below the canal head regulator crest). - **Waterway:** total width about 10-20% of the weir width; gate width 5-8 m with piers 1.5-2.0 m; at least 2 bays. - **Velocity:** through the sluices, 2.0-3.0 m/s, enough to carry the bed material. - **Discharge formula:** $Q = C\,L\,H^{3/2}$ (free flow, $C \approx 2.0$), or orifice flow when the gates are partly open. - **Floor and protection:** designed against uplift (Khosla) like the weir, with u/s and d/s aprons, cutoffs, and a stilling basin for the hydraulic jump. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is an undersluice provided? A. To store water in the pond B. To keep a clear, deep channel in front of the regulator so the canal gets silt-free water C. To raise the weir crest D. To pass fish Answer: B. Silt deposits in front of the regulator; the undersluice scours it. (Key point: Why provided) 2. What is the capacity criterion for the undersluice? A. Equal to canal discharge B. At most half the canal discharge C. At least twice the canal discharge, or dry-season flow / 10-15% of max flood, whichever is more D. Exactly 20% of the weir width Answer: C. It should pass at least twice canal discharge or the dry-season flow, whichever is greater. (Key point: Capacity) 3. Where is the undersluice crest set? A. At pond level B. 1.2-2.0 m above the regulator crest C. At the HFL D. At the average river-bed level, 1.2-2.0 m below the regulator crest Answer: D. The sluice crest is at river-bed level, below the regulator crest. (Key point: Crest level) 4. How does the undersluice help in the dry season? A. It maintains a well-defined channel from the river to the regulator B. It raises the pond C. It closes the canal D. It stops scour Answer: A. It keeps a defined channel when river flow is low. 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