---
title: "Guide bund components and drawings, 6000 m3/s"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q6000-f11-hfl150"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q6000-f11-hfl150
site: BCT Engineering
---

# Guide bund components and drawings, 6000 m3/s

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2066 Bhadra (old course), 5+3 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-q6000-f11-hfl150
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design the following components of a guide bund for a river discharge of 6000 m³/s and silt factor as 1.1. Take HFL = 150 m, Bed level = 145 m. (i) Length of guide bund (ii) Thickness of pitching (iii) Width of launching apron (iv) Depth of launching apron. Using the design data, draw the following: A) Plan of guide bank B) Section at shank and C) Section at u/s curved head.

## Short answer

Lacey waterway W=4.75√Q≈368 m, upstream shank 1.25W=460 m, downstream 0.25W=92 m. Pitching t=0.06Q^(1/3)=1.09 m. Scour depth R=8.27 m. Apron: straight portion width 1.5×7.41=11.12 m, depth below bed 7.41 m; nose width 1.5×11.55=17.32 m, depth 11.55 m; laid thickness √5 t≈2.44 m.

## Answer

#### Data and design basis
- Silt factor given: $f = 1.10$
- Flood depth above bed: $h = 150 - 145 = 5.00$ m
- Design discharge for scour/apron: $Q = 6000$ m³/s

#### 1. Waterway and length of guide bund
Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{6000} = 367.93$ m. Adopt $W \approx 368$ m.
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 368 = 460\ \text{m} \\ L_{ds} &= 0.25W = 92\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 184$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 92$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 184\times 2.094 = 385$ m, so total length on the upstream side $\approx 845$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 6000^{1/3} = 0.06\times 18.17 = 1.09\ \text{m}
$$
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:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{6000}{1.10}\right)^{1/3} = 8.27\ \text{m}
$$
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 | 16.55 m | 11.55 m | 17.32 m |
| Shank (straight) | 1.5R | 12.41 m | 7.41 m | 11.12 m |

Thickness: pitching $t = 1.09$ m, so the launched apron thickness is $1.5t = 1.5\times1.09 = 1.64$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.44$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Lacey waterway $W$ | 367.93 m (adopt 368 m) |
| Length of guide bund (u/s) | 460 m; d/s tail 92 m |
| Pitching thickness $t$ | 1.09 m |
| Apron at shank: width $1.5D'$ / thickness | 11.12 m / 1.64 m launched (2.44 m laid) |
| Apron at nose: width / thickness | 17.32 m / 1.64 m launched (2.44 m laid) |

#### Sketches
A) Plan of guide bank:
```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 -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/
```
B) Section at the shank:
```text
           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)
```
C) Section at the u/s 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. Silt factor is given as 1.1. What is the normal scour depth R for Q = 6000?
   A. R = 0.47 (6000 x 1.1)^(1/3) = 9.8 m
   B. R = 0.47 (6000/1.1)^(1/3) = 8.27 m
   C. R = 4.75 sqrt(6000) = 368 m
   D. R = 0.47 x 6000/1.1 = 2564 m
   Answer: B. R = 0.47 (Q/f)^(1/3). (Key point: Silt factor)

2. With waterway W = 368 m, how are the upstream shank and downstream tail lengths chosen?
   A. Upstream shank about 1.25W = 460 m; downstream tail about 0.25W = 92 m
   B. Upstream shank about 0.25W; downstream tail about 1.25W
   C. Both equal to W
   D. Upstream shank about 3W; downstream tail about W
   Answer: A. 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. The thickness of stone pitching on the river-side slope is found from t = 0.06 Q^(1/3). For Q = 6000 m3/s it is about:
   A. 2.18 m (doubling the value)
   B. 0.55 m (halving the value)
   C. 1.09 m
   D. 4.6 m (using sqrt(Q) instead of Q^(1/3))
   Answer: C. Pitching thickness uses the cube root of Q: t = 0.06 Q^(1/3). (Key point: Thickness of stone pitching)

4. Lacey scour depth R = 8.27 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 = 12.4 m (forgetting to subtract the flood depth)
   B. Width = 2D' = 14.82 m
   C. Width = D' = 7.41 m
   D. D' = 1.5R - h = 7.41 m, so width 1.5D' = 11.12 m
   Answer: D. 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)

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