---
title: "Guide bund design, 4000 m3/s, f 1.1"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q4000-f11-depth5"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q4000-f11-depth5
site: BCT Engineering
---

# Guide bund design, 4000 m3/s, f 1.1

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2075 Bhadra, 8 marks; 2075 Baisakh, 8 marks; 2064 Jestha (old course), 8 marks (asked 3 times)
- 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-q4000-f11-depth5
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Neatly sketch a guide bund and design the following components of a guide bund for a river discharge of 4000 m³/s, flood height 5.0 m and silt factor 1.1: (i) length of guide bund (ii) thickness of pitching (iii) width of launching apron (iv) depth of launching apron.

## Short answer

Using Lacey's theory: waterway W = 4.75√4000 = 300 m, upstream guide bund length 1.25W = 375 m, downstream tail 75 m. Pitching thickness t = 0.06 × 4000^(1/3) = 0.95 m. Scour depth R = 0.47(4000/1.1)^(1/3) = 7.23 m; maximum scour at nose 2R = 14.45 m, shank 1.5R = 10.84 m. Apron width 1.5D': nose 14.2 m, shank 8.8 m; launched thickness 1.43 m.

## Answer

#### Data and design basis
- Silt factor given: $f = 1.10$
- Flood depth: $h = 5.00$ m
- Design discharge for scour/apron: $Q = 4000$ m³/s

#### 1. Waterway and length of guide bund
Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{4000} = 300.42$ m. Adopt $W \approx 300$ m.
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 300 = 375\ \text{m} \\ L_{ds} &= 0.25W = 75\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 150$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 75$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 150\times 2.094 = 314$ m, so total length on the upstream side $\approx 689$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 4000^{1/3} = 0.06\times 15.87 = 0.95\ \text{m}
$$
Adopt $t \approx 1.0$ 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:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4000}{1.10}\right)^{1/3} = 7.23\ \text{m}
$$
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 | 14.45 m | 9.45 m | 14.18 m |
| Shank (straight) | 1.5R | 10.84 m | 5.84 m | 8.76 m |

Thickness: pitching $t = 0.95$ m, so the launched apron thickness is $1.5t = 1.5\times0.95 = 1.43$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.13$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Lacey waterway $W$ | 300.42 m (adopt 300 m) |
| Length of guide bund (u/s) | 375 m; d/s tail 75 m |
| Pitching thickness $t$ | 0.95 m |
| Apron at shank: width $1.5D'$ / thickness | 8.76 m / 1.43 m launched (2.13 m laid) |
| Apron at nose: width / thickness | 14.18 m / 1.43 m launched (2.13 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 Q = 4000 m3/s, what waterway and upstream shank length are adopted?
   A. W about 300 m; upstream shank 375 m (1.25 W)
   B. W about 150 m; shank 150 m
   C. W about 300 m; shank 75 m
   D. W about 600 m; shank 300 m
   Answer: A. W = 4.75 sqrt(4000) = 300 m, and the upstream shank is taken as 1.25W = 375 m with a tail of 0.25W = 75 m. (Key point: Waterway and length)

2. How is the pitching thickness found?
   A. t = 0.06 Q^(1/3) = 0.95 m
   B. t = 0.47 (Q/f)^(1/3)
   C. t = 1.5 D'
   D. t = 4.75 sqrt(Q)
   Answer: A. Pitching thickness is 0.06 times the cube root of Q, which is about 0.95 m (adopt 1.0 m). (Key point: Pitching thickness)

3. What is the normal scour depth R, and the maximum scour at the nose?
   A. R = 7.23 m; nose scour 1.5R
   B. R = 5 m; nose scour 10 m
   C. R = 14.45 m; nose scour 2R
   D. R = 7.23 m; nose scour 2R = 14.45 m below HFL
   Answer: D. R = 0.47 (4000/1.1)^(1/3) = 7.23 m; the nose takes 2R below HFL and the shank 1.5R. (Key point: Scour depth)

4. At the nose, how do depth D' below bed and apron width follow?
   A. D' = 14.45 m; width 1.5D' = 21.7 m
   B. D' = 14.45 - 5 = 9.45 m; width 1.5D' = 14.18 m
   C. D' = 9.45 m; width = D'
   D. D' = 5 m; width 7.5 m
   Answer: B. Depth below bed is scour below HFL minus the flood depth (5 m), and width is 1.5D' for a 2:1 launching slope. (Key point: Apron depth and width)

5. What is the launched thickness of the apron?
   A. t = 0.95 m
   B. 0.5 t
   C. 2 t = 1.9 m
   D. 1.5 t = 1.43 m
   Answer: D. The apron thickness when launched is 1.5 times the pitching thickness. (Key point: Launched apron thickness)

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