---
title: "Launching apron, 6000 m3/s, HFL 330 m"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q6000-hfl330"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q6000-hfl330
site: BCT Engineering
---

# Launching apron, 6000 m3/s, HFL 330 m

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2076 Bhadra, 6 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-hfl330
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Determine the length and thickness of launching apron for the straight portion of a guide bund in a river for the following data given below. Max. discharge = 6000 m³/s, Average dia. of bed material = 1.0 mm, Highest flood level (HFL) = 330 m and River bed level = 326 m.

## Short answer

Lacey silt factor f=1.76, normal scour R=7.07m. For straight portion, scour below bed D'=6.61m. Launching apron horizontal length 1.5D'=9.92m. Launched thickness 1.5t=1.64m (t=1.09m), laid thickness √5 t=2.44m.

## Answer

#### Data and design basis
- Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{1.0} = 1.76$
- Flood depth above bed: $h = 330 - 326 = 4.00$ m
- Design discharge for scour/apron: $Q = 6000$ m³/s

#### 1. Pitching thickness
$$
t = 0.06\,Q^{1/3} = 0.06\times 6000^{1/3} = 0.06\times 18.17 = 1.09\ \text{m}
$$

#### 2. Scour depth and launching apron
Lacey's normal scour depth:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{6000}{1.76}\right)^{1/3} = 7.07\ \text{m}
$$
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 $1.5D'$ |
|---|---|---|---|---|
| Shank (straight) | 1.5R | 10.61 m | 6.61 m | 9.92 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 $2.25t$).

**Answer (straight portion):** length of launching apron $=1.5D' = 9.92$ m (horizontal), thickness $=1.5t = 1.64$ m when launched (about 2.44 m as laid).

## Check yourself

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

1. For d = 1.0 mm, Q = 6000 m3/s, the normal scour depth R is found as:
   A. f = 1.0; R = 0.47 (6000)^(1/3) = 8.5 m
   B. f = 1.76 sqrt(1.0) = 1.76; R = 0.47 (6000/1.76)^(1/3) = 7.07 m
   C. f = 1.76; R = 0.47 x 6000/1.76 = 1602 m
   D. f = 1.76; R = 0.47 (6000 x 1.76)^(1/3) = 10.0 m
   Answer: B. Divide Q by f before taking the cube root: R = 0.47 (Q/f)^(1/3). (Key point: Silt factor and scour)

2. For the straight portion (shank), the maximum scour below HFL is taken as:
   A. 2R = 14.14 m
   B. R = 7.07 m
   C. 1.5R = 10.61 m
   D. 3R = 21.2 m
   Answer: C. The shank uses 1.5R; the nose uses 2R. (Key point: Straight portion scour)

3. Flood depth 4 m, shank scour 10.61 m below HFL. The horizontal apron length is:
   A. D' = 10.61 - 4 = 6.61 m; length 1.5D' = 9.92 m
   B. 10.61 m
   C. 1.5 x 10.61 = 15.9 m
   D. 6.61 m
   Answer: A. Only the scour below the bed counts; then the 2H:1V slope gives 1.5D'. (Key point: Apron length)

4. Pitching t = 1.09 m. What is the launched apron thickness?
   A. t = 1.09 m
   B. 2t = 2.18 m
   C. sqrt5 t = 2.44 m
   D. 1.5t = 1.64 m
   Answer: D. Launched thickness is 1.5t; 2.44 m is the thickness as laid, found by volume equality. (Key point: Apron thickness)

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