--- title: "Design steps of a silt excluder" subject: "Irrigation and Drainage Engineering" chapter: 5 key: "semester_6/irrigation_and_drainage_engineering:silt-excluder-design-steps" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/silt-excluder-design-steps site: BCT Engineering --- # Design steps of a silt excluder Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks - Asked in: 2076 Bhadra, 6 marks - Video: [Design of Silt Excluder | Irrigation Engineering | Most asked problem | TU.PU.KU.POU](https://www.youtube.com/watch?v=YpeCaBZKNtc) — Sudip Dhakal - 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/silt-excluder-design-steps - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5 ## Question Describe briefly the design steps of a silt excluder with neat sketches. ## Short answer Fix crest levels, set tunnel discharge (20% of canal), velocity 2.5–3 m/s. Compute area, height, width, number of tunnels (2–4) fitting the undersluice bay. Length equals head regulator width plus transition; check hydraulics with orifice formula; design roof, floor, and entrance/exit transitions. ## Answer A silt excluder is a set of tunnels built under the canal head regulator in the river pocket that passes the silty bottom water to the river through the undersluices. #### Design steps 1. **Fix the crest levels.** Undersluice crest level (river-bed level) and the head-regulator crest level, normally 1.2-2.1 m higher. The difference gives the available tunnel height (minus roof slab thickness 0.2-0.3 m). 2. **Discharge through the tunnels.** Take 20% of the canal discharge (Garg), or the silt-carrying capacity needed. 3. **Tunnel velocity.** Fix 2.5-3 m/s (non-silting, non-erosive). Required area $A = Q_t/V$. 4. **Height of tunnels** $h = $ (regulator crest level $-$ undersluice crest level) $-$ roof slab thickness. **Total tunnel width** $= A/h$. 5. **Number and width of tunnels.** Adopt 2-4 tunnels with 0.5-0.75 m thick piers so that the total width equals the bay of the undersluice. 6. **Length of tunnels** equals the width of the head regulator plus the transition, with a gently sloping floor to improve flow. 7. **Hydraulic checks.** The head causing flow comes from the pond to the tailwater in the river (difference of levels), checked by the orifice formula $Q = C_dA\sqrt{2gH}$. 8. **Structural design.** Roof slab, piers and floor are designed for water load; the tunnel floor, piles and cut-offs are checked for uplift. 9. **Entrance and exit.** Wing walls with smooth transitions, so that the silt does not enter the canal; d/s outlet to the sluiceway with an apron. ```text SECTION pond level ~~~~~~~~~~~~~~~~~~~~~~~~~~~ regulator crest ____________ (roof of tunnel) ---> clear water | roof slab |--> canal silty water ---> [ tunnel ]---> to river undersluice crest level ____________ ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What discharge is taken through the excluder tunnels? A. About 20% of canal discharge B. 100% of canal discharge C. 50% of canal discharge D. 1% of canal discharge Answer: A. About 20% of canal discharge is used. (Key point: Tunnel discharge) 2. How is tunnel height found? A. Equal to canal depth B. Regulator crest minus undersluice crest, minus roof slab thickness C. Twice the pier thickness D. Pond level minus HFL Answer: B. h = (regulator crest - sluice crest) - roof slab thickness. (Key point: Tunnel height) 3. What velocity range is fixed in the tunnels and why? A. 0.3-0.5 m/s to settle silt B. 5-8 m/s to scour piers C. 2.5-3 m/s so it is non-silting and non-erosive D. Any velocity Answer: C. 2.5-3 m/s keeps silt moving without erosion. 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