---
title: "Guide bund design, 4000 m3/s, d 0.1 mm"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q4000-hfl205-bed200"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q4000-hfl205-bed200
site: BCT Engineering
---

# Guide bund design, 4000 m3/s, d 0.1 mm

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2069 Bhadra, 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-q4000-hfl205-bed200
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Following hydraulic data near a proposed bridge site are obtained. Maximum discharge = 4000 m³/s; Highest flood level = 205.0 m; River bed level = 200.00 m; Average diameter of river bed material = 0.1 mm. Design the following components of a guide bund and neatly sketch it. (i) Length of guide bund (ii) Thickness of pitching of the slope (iii) Length of launching apron (iv) thickness of launching apron.

## Short answer

Lacey waterway W=300m. Upstream shank 375m, downstream tail 75m, curved head radius 150m (120°). Pitching thickness 1.0m. Normal scour R=9.07m. Apron at shank: length 12.91m, launched thickness 1.43m (laid 2.13m). At nose: length 19.71m, same thickness.

## 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: $h = 205.0 - 200.0 = 5.00$ 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:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4000}{0.557}\right)^{1/3} = 9.07\ \text{m}
$$
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 | 18.14 m | 13.14 m | 19.71 m |
| Shank (straight) | 1.5R | 13.61 m | 8.61 m | 12.91 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 | 12.91 m / 1.43 m launched (2.13 m laid) |
| Apron at nose: width / thickness | 19.71 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 D50 = 0.1 mm, Lacey's silt factor is:
   A. f = 1.76 x 0.1 = 0.18
   B. f = 1.76 / 0.1 = 17.6
   C. f = 1.76 sqrt(0.1) = 0.56
   D. f = 0.1 sqrt(1.76) = 0.13
   Answer: C. f = 1.76 sqrt(d_mm); fine sand gives a low f and deep scour. (Key point: Silt factor)

2. With waterway W = 300 m, how are the upstream shank and downstream tail lengths chosen?
   A. Upstream shank about 1.25W = 375 m; downstream tail about 0.25W = 75 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 = 4000 m3/s it is about:
   A. 1.9 m (doubling the value)
   B. 0.47 m (halving the value)
   C. 3.8 m (using sqrt(Q) instead of Q^(1/3))
   D. 0.95 m
   Answer: D. 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 = 9.07 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 = 13.61 m (forgetting to subtract the flood depth)
   B. D' = 1.5R - h = 8.61 m, so width 1.5D' = 12.91 m
   C. Width = 2D' = 17.22 m
   D. Width = D' = 8.61 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'. (Key point: Depth below bed and apron width)

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