--- title: "Guide bund with plan, section, nose; 2500 m3/s" subject: "Irrigation and Drainage Engineering" chapter: 6 key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q2500-bed755" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q2500-bed755 site: BCT Engineering --- # Guide bund with plan, section, nose; 2500 m3/s Irrigation and Drainage Engineering · Chapter 6: River Training Works - Asked in: 2081 Chaitra, 8 marks - 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-q2500-bed755 - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6 ## Question Design and sketch (plan, section at straight portion, section at nose) guide bunds including launching apron, for a bridge site with following hydraulic data: Designed discharge = 2500 m³/s; H.F.L = 78.8 m; river bed level = 75.5 m; and D50 of bed material = 0.1 mm. ## Short answer Lacey waterway 238 m; upstream guide bund 298 m, downstream tail 60 m. Pitching thickness 0.81 m. Normal scour depth R=7.75 m. Nose apron width 18.31 m, shank apron 12.50 m; launched thickness 1.22 m, laid thickness 1.82 m. ## Answer #### Data and design basis - Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.1} = 0.557$ - Flood depth above bed: $h = 78.8 - 75.5 = 3.30$ m - Design discharge for scour/apron: $Q = 2500$ m³/s #### 1. Waterway and length of guide bund Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{2500} = 237.50$ m. Adopt $W \approx 238$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt $$ \begin{aligned} L_{us} &= 1.25W = 1.25\times 238 = 298\ \text{m} \\ L_{ds} &= 0.25W = 60\ \text{m} \end{aligned} $$ Curved heads: upstream radius $R_u \approx 0.5W = 119$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 60$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 119\times 2.094 = 249$ m, so total length on the upstream side $\approx 547$ m. #### 2. Thickness of stone pitching $$ t = 0.06\,Q^{1/3} = 0.06\times 2500^{1/3} = 0.06\times 13.57 = 0.81\ \text{m} $$ Adopt $t \approx 0.9$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter). #### 3. Scour depth and launching apron Lacey's normal scour depth: $$ R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{2500}{0.557}\right)^{1/3} = 7.75\ \text{m} $$ Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$. | Portion | Scour factor | Scour depth below HFL | Depth below bed $D'$ | Apron length $1.5D'$ | |---|---|---|---|---| | Nose (u/s curved head) | 2.0R | 15.51 m | 12.21 m | 18.31 m | | Shank (straight) | 1.5R | 11.63 m | 8.33 m | 12.50 m | Thickness: pitching $t = 0.81$ m, so the launched apron thickness is $1.5t = 1.5\times0.81 = 1.22$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 1.82$ m (about $2.25t$). #### Summary | Item | Value | |---|---| | Lacey waterway $W$ | 237.50 m (adopt 238 m) | | Length of guide bund (u/s) | 298 m; d/s tail 60 m | | Pitching thickness $t$ | 0.81 m | | Apron at shank: width $1.5D'$ / thickness | 12.50 m / 1.22 m launched (1.82 m laid) | | Apron at nose: width / thickness | 18.31 m / 1.22 m launched (1.82 m laid) | #### Sketches Plan: ```text U/S D/S bridge / weir axis Shank (straight) | ________________________________ | ___ / 1.0-1.5 W upstream \ | / \ tail \________________________________/=|=\___/ 0.25 W curved head | R, sweep 120 deg <-- waterway W --> ________________________________ | ___ / \ | / \ \________________________________/=|=\___/ ``` Section at the straight portion (shank): ```text top width 6 m ___________ <- HFL + 1.5 m / \ HFL ---/-------------\--- 2:1 slopes / pitching t \ bed__/__ filter ______ \___ \_ launching apron _/ (launches at 2:1) ``` Section at the nose (upstream curved head): ```text Section at u/s curved head (nose) ______ <- HFL + 1.5 m / \ top 6 m, side 3:1 (flatter) HFL ----/---------\---- / pitching t \ bed ___/_____________\__ \_ apron: 2.0R scour, 1.5D' wide _/ ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. HFL = 78.8 m and bed = 75.5 m. The flood depth h is: A. 78.8 m B. 3.30 m C. 2.5 m D. 154.3 m Answer: B. h = HFL - bed level = 3.30 m; it is subtracted from the scour below HFL to get depth below bed. (Key point: Flood depth) 2. With waterway W = 238 m, how are the upstream shank and downstream tail lengths chosen? A. Upstream shank about 0.25W; downstream tail about 1.25W B. Both equal to W C. Upstream shank about 3W; downstream tail about W D. Upstream shank about 1.25W = 298 m; downstream tail about 0.25W = 60 m Answer: D. Garg's rule: upstream shank 1.0 to 1.5 W and downstream tail 0.2 to 0.5 W; the upstream side is longer to guide flow smoothly into the waterway. (Key point: Length of guide bund) 3. What radii and sweep are used for the curved heads? A. Upstream R about 0.5W with sweep 120-145 deg; downstream R about 0.25W with sweep about 45 deg B. Upstream R about 0.25W, 45 deg; downstream R about 0.5W, 120 deg C. Both R equal to W with sweep 180 deg D. Upstream R about 2W with sweep 30 deg Answer: A. The upstream head is larger and sweeps more to guide the flow smoothly toward the waterway. (Key point: Head radius and sweep) 4. Lacey scour depth R = 7.75 m and flood depth h = 3.3 m. What is the apron horizontal width at the nose (scour 2.0R below HFL, launching slope 2H:1V)? A. Width = 2.0R = 15.5 m (forgetting to subtract the flood depth) B. D' = 2.0R - h = 12.21 m, so width 1.5D' = 18.31 m C. Width = 2D' = 24.42 m D. Width = D' = 12.21 m Answer: B. The apron must cover scour below the bed: D' = (factor x R) - h. At 2H:1V the apron width laid is 1.5D'. 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