---
title: "Gabion guide bund and apron, 3000 m3/s"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-gabion-q3000"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-gabion-q3000
site: BCT Engineering
---

# Gabion guide bund and apron, 3000 m3/s

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2071 Magh, 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-gabion-q3000
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design the length, radius of curved head, length and thickness of gabion slope pitching and gabion launching apron of a guide bund to train a river with the following data. Design Flood Discharge: 3000 cumecs; River Bed Level: 240.0 m; HFL: 245.0 m; Average dia. of river bed material: 0.1 mm.

## Short answer

Lacey waterway W=260m. Upstream length 325m, downstream 65m, curved head radius 130m (120°). Gabion pitching 1.0m thick. Normal scour R=8.24m. Apron at shank: length 11.04m, launched thickness 1.30m (laid 1.93m). At nose: length 17.22m, same thickness. Gabion apron uses 2.0m (two 1m layers).

## 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: $h = 245 - 240 = 5.00$ m
- Design discharge for scour/apron: $Q = 3000$ m³/s

#### 1. Waterway and length of guide bund
Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{3000} = 260.17$ m. Adopt $W \approx 260$ m.
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 260 = 325\ \text{m} \\ L_{ds} &= 0.25W = 65\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 130$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 65$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 130\times 2.094 = 272$ m, so total length on the upstream side $\approx 597$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 3000^{1/3} = 0.87\ \text{m}
$$
Gabion slope protection: use gabion boxes (1.0 m thick) in one layer, so adopt $t_g = 1.0$ m (next standard size above 0.87 m) over a 0.15 m gravel filter.

#### 3. Scour depth and launching apron
Lacey's normal scour depth:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{3000}{0.557}\right)^{1/3} = 8.24\ \text{m}
$$
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 | 16.48 m | 11.48 m | 17.22 m |
| Shank (straight) | 1.5R | 12.36 m | 7.36 m | 11.04 m |

Thickness: pitching $t = 0.87$ m, so the launched apron thickness is $1.5t = 1.5\times0.87 = 1.30$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 1.93$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Lacey waterway $W$ | 260.17 m (adopt 260 m) |
| Length of guide bund (u/s) | 325 m; d/s tail 65 m |
| Pitching thickness $t$ | 0.87 m |
| Apron at shank: width $1.5D'$ / thickness | 11.04 m / 1.30 m launched (1.93 m laid) |
| Apron at nose: width / thickness | 17.22 m / 1.30 m launched (1.93 m laid) |

#### Gabion details
- Slope pitching: gabion boxes $2\times1\times1$ m laid in one layer (1.0 m thick) on a 0.15 m geotextile or gravel filter, wire mesh 8x10 cm, galvanised and PVC coated.
- Launching apron: launched thickness $1.5t = 1.30$ m; laid thickness $\sqrt5\,t = 1.93$ m, so adopt **2.0 m** (two layers of 1 m gabion boxes or four layers of 0.5 m mattress) over a width $1.5D'$ = 11.04 m (shank) and 17.22 m (nose).
- Gabion boxes are flexible, so they follow the scour hole and launch without breaking.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Computed pitching t = 0.87 m. What gabion thickness is adopted?
   A. 1.0 m, the next standard gabion size above 0.87 m in one layer
   B. 0.5 m mattress
   C. 0.87 m exactly
   D. 3.0 m in three layers
   Answer: A. Gabion boxes come in standard sizes, so the next size above the computed value is adopted. (Key point: Gabion pitching thickness)

2. For a design flood of 3000 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 69 m)
   B. W = 2.71 sqrt(Q) (about 148 m)
   C. W = 4.75 sqrt(Q) = 260 m
   D. W = 0.47 (Q/f)^(1/3) (about 7 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)

3. Why is a gabion launching apron suitable?
   A. They are rigid and resist all scour
   B. They are lighter than water and float
   C. They remove the need for a filter
   D. Gabions are flexible, follow the scour hole and launch without breaking
   Answer: D. Flexibility lets the wire boxes settle into the scour hole while staying intact. (Key point: Why gabions for apron)

4. Laid thickness sqrt5 t = 1.93 m. What is adopted?
   A. 1.30 m
   B. 2.0 m (two layers of 1 m boxes or four of 0.5 m mattress)
   C. 1.0 m
   D. 3.0 m
   Answer: B. Round the laid thickness 1.93 m up to a practical 2.0 m of gabion layers. (Key point: Apron adopted thickness)

## Close variants set in other exams

- [Guide bund: length, curved head, apron; 4500 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aguide-bund-q4500-hfl154) — Irrigation and Drainage Engineering, chapter 6, asked 2×

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