--- title: "Guide bund purposes and design components" subject: "Irrigation and Drainage Engineering" chapter: 6 key: "semester_6/irrigation_and_drainage_engineering:guide-bund-purpose-design-components" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-purpose-design-components site: BCT Engineering --- # Guide bund purposes and design components Irrigation and Drainage Engineering · Chapter 6: River Training Works - Asked in: 2065 Kartik (old course), 2+8 marks; 2063 Asoj (old course), 8 marks (asked 2 times) - Video: [Design of Guide Bank River Training Structure | Irrigation Engineering | IOE TU](https://www.youtube.com/watch?v=3p4xQ418oOk) — Parash Joshi - Civil Construction and Tutor - 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/guide-bund-purpose-design-components - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6 ## Question What are the purposes of guide bunds? Explain with plan and sectional views at critical locations design of components of guide bunds (design criteria). ## Short answer Guide bunds guide river flow axially through the waterway, prevent flanking and eddies, and minimize afflux. Design: waterway W = 4.75√Q; upstream shank length 1–1.5W, downstream 0.2–0.5W; top width 6 m, freeboard 1.5 m; pitching thickness t = 0.06Q^(1/3); launching apron width 1.5D' based on scour depth R = 0.47(Q/f)^(1/3), max 2R at nose. ## Answer A **guide bund** is a pair of embankments, built along both banks upstream (and a short length downstream) of a bridge or barrage, which guide the river through the opening without allowing it to outflank the structure. #### Purposes 1. Confine and guide the river axially through the waterway. 2. Prevent the river from attacking the approach embankments and abutments (flanking). 3. Prevent the formation of eddies and cross-currents near the structure. 4. Keep the scour holes and deposits away from the structure. 5. Maintain a smooth, symmetrical entry of flow, so the afflux is minimum. #### Design criteria (Lacey and Garg) **Plan** - Waterway $W = 4.75\sqrt{Q}$. - Length of upstream shank: $1.0$ to $1.5\,W$; downstream tail: $0.2$ to $0.5\,W$. - Upstream curved head: radius about $0.5W$ and sweep $120^\circ$ to $145^\circ$; downstream curved head: smaller radius, sweep about $45^\circ$. ```text U/S D/S ___________________________ | ____ / shank 1.0-1.5 W \ | / \ tail \____________________________/=|=\____/ 0.2-0.5 W curved head axis <------ waterway W ------> ``` **Cross-section at the shank** - Top width 6 m (3 to 6 m) and top level $= $ HFL $+$ 1.5 m free-board. - Side slopes 2:1 on the river side (pitched) and 2:1 on the country side. - Pitching thickness $t = 0.06\,Q^{1/3}$ on a filter 0.15 to 0.30 m thick. **Launching apron** - Scour depth $R = 0.47(Q/f)^{1/3}$; maximum scour below HFL is $2R$ at the nose and $1.5R$ at the shank. - Depth of apron below bed $D' = $ (scour below HFL) $-$ (flood depth). - Width $= 1.5D'$ and thickness $= 1.5t$ when launched on a 2:1 slope (laid thickness about $2.25t$). ```text Section at shank Section at nose ____6 m___ ___6 m___ / \ HFL+1.5 / \ HFL/____________\ HFL/___________\ / pitching t \ / \ _/_______apron____\_ _/____apron 2R_____\_ ``` At the nose the scour is deeper (2R), so the pitching continues around the head and the apron is wider and thicker; sections are flatter (3:1) at the head. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a purpose of guide bunds? A. Prevent flanking of approach embankments and give a smooth axial flow with minimum afflux B. Raise scour near the abutments C. Increase eddies near the structure D. Store water Answer: A. Guide bunds confine the river axially, stop it attacking the approaches, and avoid eddies and cross-currents. (Key point: Purpose) 2. Which plan dimensions are recommended? A. Head sweep 10 degrees B. Waterway sqrt(Q); shank 5W C. Waterway 4.75 sqrt(Q); u/s shank 1 to 1.5W; d/s tail 0.2 to 0.5W; u/s head radius about 0.5W, sweep 120-145 degrees D. Shank 0.2W; tail 1.5W Answer: C. These are the Lacey and Garg criteria for the plan. (Key point: Plan dimensions) 3. What is the cross-section at the shank? A. Top width 1 m, level at HFL, slopes 1:1 B. Top level at bed level C. Top width about 6 m, top level HFL + 1.5 m, side slopes 2:1 D. Top width 20 m, level HFL - 1 m Answer: C. A 6 m (3 to 6 m) top at HFL plus 1.5 m free-board with 2:1 slopes is standard. (Key point: Cross-section) 4. How is the launching apron sized? A. D' = 2R; width D'; thickness t/2 B. Width 5D'; thickness 4t C. Depth D' = scour below HFL - flood depth; width 1.5D'; launched thickness 1.5t D. D' = flood depth only Answer: C. The apron depth is measured below the bed, with width 1.5D' for 2:1 launching and thickness 1.5t. (Key point: Launching apron design) 5. Why is the nose treated more heavily than the shank? A. The nose has less flow B. It is cheaper C. Scour is deeper there (2R against 1.5R), so apron is wider and thicker with flatter 3:1 slopes D. The shank has deeper scour Answer: C. The head faces the river attack and deeper scour, hence stronger protection. 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