--- title: "Guide bank and launching apron, 5000 m3/s" subject: "Irrigation and Drainage Engineering" chapter: 6 key: "semester_6/irrigation_and_drainage_engineering:guide-bank-q5000-hfl254" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bank-q5000-hfl254 site: BCT Engineering --- # Guide bank and launching apron, 5000 m3/s Irrigation and Drainage Engineering · Chapter 6: River Training Works - Asked in: 2074 Bhadra, 5 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-bank-q5000-hfl254 - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6 ## Question A bridge is to be constructed across a river having the following hydraulic data: Maximum flood Discharge: 5000 m³/s; Highest flood level: 254.0 m; River bed level: 250 m; Average diameter of river sand: 0.25 m. Design and sketch a guide bank including launching apron to train the river. ## Short answer Lacey waterway W=336m. Upstream length 420m, downstream 84m, curved head radius 168m (120°). Pitching thickness 1.03m. Normal scour R=8.39m. Apron at shank: length 12.87m, launched thickness 1.54m (laid 2.29m). At nose: length 19.16m, same thickness. ## Answer #### Data and design basis - The average size of the river sand is taken as 0.25 mm (0.25 m would not be sand). - Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.25} = 0.88$ - Flood depth above bed: $h = 254 - 250 = 4.00$ m - Design discharge for scour/apron: $Q = 5000$ m³/s #### 1. Waterway and length of guide bund Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{5000} = 335.88$ m. Adopt $W \approx 336$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt $$ \begin{aligned} L_{us} &= 1.25W = 1.25\times 336 = 420\ \text{m} \\ L_{ds} &= 0.25W = 84\ \text{m} \end{aligned} $$ Curved heads: upstream radius $R_u \approx 0.5W = 168$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 84$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 168\times 2.094 = 352$ m, so total length on the upstream side $\approx 772$ m. #### 2. Thickness of stone pitching $$ t = 0.06\,Q^{1/3} = 0.06\times 5000^{1/3} = 0.06\times 17.10 = 1.03\ \text{m} $$ Adopt $t \approx 1.1$ 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: $$ R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{5000}{0.88}\right)^{1/3} = 8.39\ \text{m} $$ 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 | 16.77 m | 12.77 m | 19.16 m | | Shank (straight) | 1.5R | 12.58 m | 8.58 m | 12.87 m | Thickness: pitching $t = 1.03$ m, so the launched apron thickness is $1.5t = 1.5\times1.03 = 1.54$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.29$ m (about $2.25t$). #### Summary | Item | Value | |---|---| | Lacey waterway $W$ | 335.88 m (adopt 336 m) | | Length of guide bund (u/s) | 420 m; d/s tail 84 m | | Pitching thickness $t$ | 1.03 m | | Apron at shank: width $1.5D'$ / thickness | 12.87 m / 1.54 m launched (2.29 m laid) | | Apron at nose: width / thickness | 19.16 m / 1.54 m launched (2.29 m laid) | #### Sketches Plan of guide bunds: ```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 shank (straight portion): ```text top width 6 m ___________ <- HFL + 1.5 m / \ HFL ---/-------------\--- 2:1 slopes / pitching t \ bed__/__ filter ______ \___ \_ launching apron _/ (launches at 2:1) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For a design flood of 5000 m3/s, the guide bund design is based on the Lacey regime width. Which value is obtained? A. W = 4.75 Q^(1/3) = much smaller (about 81 m) B. W = 2.71 sqrt(Q) (about 192 m) C. W = 4.75 sqrt(Q) = 336 m D. W = 0.47 (Q/f)^(1/3) (about 8 m) Answer: C. Lacey's regime waterway is W = 4.75 sqrt(Q); it fixes the guide bund lengths and radii. (Key point: Lacey regime waterway) 2. What maximum scour depth below HFL is adopted for the upstream nose and for the straight shank? A. 2R at the nose and 1.5R at the shank B. 1.5R at the nose and 2R at the shank C. 2R at both D. 1.5R at both Answer: A. The nose (curved head) meets the attack of the flow, so it gets the deeper scour allowance 2R; the straight shank gets 1.5R. (Key point: Maximum scour factors) 3. How is the thickness of the launching apron obtained? A. Launched thickness equals t; laid thickness equals t B. Laid thickness 1.5t and launched thickness sqrt5 t C. Laid thickness is found from the slope 1:1 giving 2t D. Launched thickness 1.5t = 1.54 m; laid thickness T = sqrt5 t = 2.29 m by equating volumes Answer: D. The stones on the 2:1 slope length sqrt5 D' at thickness 1.5t come from a flat apron of width 1.5D'; equal volumes give T = sqrt5 t (about 2.25t). 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