--- title: "Guide bank design, 6500 m3/s" subject: "Irrigation and Drainage Engineering" chapter: 6 key: "semester_6/irrigation_and_drainage_engineering:guide-bank-bed105-depth5-q6500" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bank-bed105-depth5-q6500 site: BCT Engineering --- # Guide bank design, 6500 m3/s Irrigation and Drainage Engineering · Chapter 6: River Training Works - Asked in: 2077 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-bank-bed105-depth5-q6500 - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6 ## Question Design a guide bank for the weir site from the following data provided. Bed level of river = 105.00 m; Depth of water during high flood = 5 m; Discharge of river = 6500 m³/s. The value of Lacey's silt factor may be taken as 1. ## Short answer Waterway 383 m; upstream guide bund 479 m, downstream tail 96 m. Pitching thickness 1.12 m. Scour depth R=8.77 m. Nose apron width 18.81 m, shank apron 12.24 m; launched thickness 1.68 m, laid thickness 2.50 m. ## Answer #### Data and design basis - Bed level 105.00 m, flood depth 5 m, so HFL $=110.00$ m; top level of bank $=111.50$ m. - Silt factor given: $f = 1.00$ - Flood depth: $h = 5.00$ m - Design discharge for scour/apron: $Q = 6500$ m³/s #### 1. Waterway and length of guide bund Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{6500} = 382.96$ m. Adopt $W \approx 383$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt $$ \begin{aligned} L_{us} &= 1.25W = 1.25\times 383 = 479\ \text{m} \\ L_{ds} &= 0.25W = 96\ \text{m} \end{aligned} $$ Curved heads: upstream radius $R_u \approx 0.5W = 192$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 96$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 192\times 2.094 = 401$ m, so total length on the upstream side $\approx 880$ m. #### 2. Thickness of stone pitching $$ t = 0.06\,Q^{1/3} = 0.06\times 6500^{1/3} = 0.06\times 18.66 = 1.12\ \text{m} $$ Adopt $t \approx 1.2$ 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: $$ R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{6500}{1.00}\right)^{1/3} = 8.77\ \text{m} $$ 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 | 17.54 m | 12.54 m | 18.81 m | | Shank (straight) | 1.5R | 13.16 m | 8.16 m | 12.24 m | Thickness: pitching $t = 1.12$ m, so the launched apron thickness is $1.5t = 1.5\times1.12 = 1.68$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.50$ m (about $2.25t$). #### Summary | Item | Value | |---|---| | Lacey waterway $W$ | 382.96 m (adopt 383 m) | | Length of guide bund (u/s) | 479 m; d/s tail 96 m | | Pitching thickness $t$ | 1.12 m | | Apron at shank: width $1.5D'$ / thickness | 12.24 m / 1.68 m launched (2.50 m laid) | | Apron at nose: width / thickness | 18.81 m / 1.68 m launched (2.50 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. Bed 105.00 m, flood depth 5 m. What is the HFL and top level of the guide bank? A. HFL = 105 m; top level = 106.5 m B. HFL = 110 m; top level = 110 m C. HFL = 110 m; top level = 115 m D. HFL = 110.00 m; top level = HFL + 1.5 m = 111.50 m Answer: D. HFL = bed + flood depth; the bank crest includes 1.5 m freeboard above HFL. (Key point: Top level of bank) 2. For a design flood of 6500 m3/s, the guide bund design is based on the Lacey regime width. Which value is obtained? A. W = 4.75 sqrt(Q) = 383 m B. W = 4.75 Q^(1/3) = much smaller (about 89 m) C. W = 2.71 sqrt(Q) (about 218 m) D. W = 0.47 (Q/f)^(1/3) (about 9 m) Answer: A. Lacey's regime waterway is W = 4.75 sqrt(Q); it fixes the guide bund lengths and radii. (Key point: Lacey regime waterway) 3. With f = 1 and Q = 6500 m3/s, Lacey's R = 0.47 (Q/f)^(1/3) is about: A. 18.66 m B. 8.77 m C. 4.38 m D. 17.54 m Answer: B. R = 0.47 x 18.66 = 8.77 m; 17.54 m is 2R. (Key point: Scour depth with given f) 4. Lacey scour depth R = 8.77 m and flood depth h = 5.0 m. What is the apron horizontal width at the shank (scour 1.5R below HFL, launching slope 2H:1V)? A. Width = 1.5R = 13.15 m (forgetting to subtract the flood depth) B. Width = 2D' = 16.32 m C. D' = 1.5R - h = 8.16 m, so width 1.5D' = 12.24 m D. Width = D' = 8.16 m Answer: C. The apron must cover scour below the bed: D' = (factor x R) - h. At 2H:1V the apron width laid is 1.5D'. (Key point: Depth below bed and apron width) ## Related in Irrigation and Drainage Engineering - [Guide bank design, 4000 m3/s, d 0.40 mm](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aguide-bank-q4000-hfl205-bed202) — Irrigation and Drainage Engineering, chapter 6 - [Spur design for concave bank, 5000 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aspur-concave-bank-q5000) — Irrigation and Drainage Engineering, chapter 6 - [Guide bank and launching apron, 5000 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aguide-bank-q5000-hfl254) — Irrigation and Drainage Engineering, chapter 6 - [Weir design by Bligh's theory, 3200 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adiversion-weir-vertical-drop-bligh-q3200) — Irrigation and Drainage Engineering, chapter 5 - [Weir waterway and crest levels, 16000 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aweir-waterway-crest-q16000) — Irrigation and Drainage Engineering, chapter 5 - [Guide bund design, 4000 m3/s, d 0.1 mm](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aguide-bund-q4000-hfl205-bed200) — Irrigation and Drainage Engineering, chapter 6 ## Related in other subjects - [Gumbel design flood, factor of safety 1.3, 45-year life](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-25yr-life45-fos1-3) — Engineering Hydrology, chapter 6 - [Bridge site factors; river bank protection structures](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-river-bank-protection) — Transportation Engineering II, chapter 5 - [Slope-area flood discharge, Marin River at Kusuntar](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aslope-area-marin-kusuntar) — Engineering Hydrology, chapter 4 - [Gumbel design flood, safety factor, confidence limits](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-20yr-life35-rel87) — Engineering Hydrology, chapter 6