---
title: "Guide bund: length, curved head, apron; 4500 m3/s"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q4500-hfl154"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q4500-hfl154
site: BCT Engineering
---

# Guide bund: length, curved head, apron; 4500 m3/s

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2072 Asoj, 5 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-q4500-hfl154
- 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 launching apron of a guide bund to train a river with the following data. Design Flood Discharge: 4500 cumecs; Bed Level of river: 150.00 m; HFL: 154.00 m; Av. dia. of river bed material: 0.1 mm.

## Short answer

Lacey waterway W=319m. Upstream shank 399m, downstream tail 80m, curved head radius 160m (120°). Pitching thickness 0.99m. Normal scour R=9.43m. Apron at shank: length 15.22m, launched thickness 1.49m (laid 2.21m). At nose: length 22.30m, 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: $h = 154 - 150 = 4.00$ m
- Design discharge for scour/apron: $Q = 4500$ m³/s

#### 1. Waterway and length of guide bund
Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{4500} = 318.64$ m. Adopt $W \approx 319$ m.
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 319 = 399\ \text{m} \\ L_{ds} &= 0.25W = 80\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 160$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 80$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 160\times 2.094 = 334$ m, so total length on the upstream side $\approx 733$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 4500^{1/3} = 0.06\times 16.51 = 0.99\ \text{m}
$$
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:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4500}{0.557}\right)^{1/3} = 9.43\ \text{m}
$$
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 | 18.87 m | 14.87 m | 22.30 m |
| Shank (straight) | 1.5R | 14.15 m | 10.15 m | 15.22 m |

Thickness: pitching $t = 0.99$ m, so the launched apron thickness is $1.5t = 1.5\times0.99 = 1.49$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.21$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Lacey waterway $W$ | 318.64 m (adopt 319 m) |
| Length of guide bund (u/s) | 399 m; d/s tail 80 m |
| Pitching thickness $t$ | 0.99 m |
| Apron at shank: width $1.5D'$ / thickness | 15.22 m / 1.49 m launched (2.21 m laid) |
| Apron at nose: width / thickness | 22.30 m / 1.49 m launched (2.21 m laid) |

## Check yourself

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

1. For a design flood of 4500 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 78 m)
   B. W = 2.71 sqrt(Q) (about 182 m)
   C. W = 0.47 (Q/f)^(1/3) (about 8 m)
   D. W = 4.75 sqrt(Q) = 319 m
   Answer: D. Lacey's regime waterway is W = 4.75 sqrt(Q); it fixes the guide bund lengths and radii. (Key point: Lacey regime waterway)

2. The upstream curved head radius is:
   A. About 0.5W = 160 m with sweep 120 deg
   B. About 0.25W = 80 m with sweep 120 deg
   C. About 2W = 640 m
   D. About 0.5W with sweep 45 deg
   Answer: A. Upstream head R about 0.5W; the 0.25W radius is for the downstream head. (Key point: Radius of curved head)

3. Lacey scour depth R = 9.43 m and flood depth h = 4.0 m. What is the apron horizontal width at the nose (scour 2.0R below HFL, launching slope 2H:1V)?
   A. Width = 2.0R = 18.86 m (forgetting to subtract the flood depth)
   B. Width = 2D' = 29.74 m
   C. D' = 2.0R - h = 14.87 m, so width 1.5D' = 22.3 m
   D. Width = D' = 14.87 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)

4. How is the thickness of the launching apron obtained?
   A. Launched thickness equals t; laid thickness equals t
   B. Launched thickness 1.5t = 1.49 m; laid thickness T = sqrt5 t = 2.21 m by equating volumes
   C. Laid thickness 1.5t and launched thickness sqrt5 t
   D. Laid thickness is found from the slope 1:1 giving 2t
   Answer: B. The stones on the 2:1 slope length sqrt5 D' at thickness 1.5t come from a flat apron of width 1.5D'; equal volumes give T = sqrt5 t (about 2.25t). (Key point: Launched and laid apron thickness)

## Close variants set in other exams

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