---
title: "Launching apron, 8000 m3/s, HFL 128.5 m"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:launching-apron-q8000-hfl1285"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/launching-apron-q8000-hfl1285
site: BCT Engineering
---

# Launching apron, 8000 m3/s, HFL 128.5 m

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2062 Kartik (old course), 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/launching-apron-q8000-hfl1285
- 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. Design flood = 8,000 m³/s; average diameter of river bed material = 1 mm; river bed level = 125.00; highest flood level = 128.50.

## Short answer

f=1.76, t=0.06×8000^(1/3)=1.20 m, R=7.79 m, D'=1.5R−3.5=8.18 m. Straight-apron length=12.27 m, launched thickness=1.80 m (laid √5 t=2.68 m).

## 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: $h = 128.5 - 125.0 = 3.50$ m
- Design discharge for scour/apron: $Q = 8000$ m³/s

#### 1. Pitching thickness
$$
t = 0.06\,Q^{1/3} = 0.06\times 8000^{1/3} = 0.06\times 20.00 = 1.20\ \text{m}
$$

#### 2. Scour depth and launching apron
Lacey's normal scour depth:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{8000}{1.76}\right)^{1/3} = 7.79\ \text{m}
$$
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 | 11.68 m | 8.18 m | 12.27 m |

Thickness: pitching $t = 1.20$ m, so the launched apron thickness is $1.5t = 1.5\times1.20 = 1.80$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.68$ m (about $2.25t$).

**Answer (straight portion):** length of launching apron $=1.5D' = 12.27$ m, thickness $=1.5t = 1.80$ m launched (about 2.68 m as laid).

## Check yourself

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

1. Which formula gives Lacey's normal scour depth R?
   A. R = 0.47 (f/Q)^(1/3)
   B. R = 4.75 sqrt(Q)
   C. R = 0.06 Q^(1/3)
   D. R = 0.47 (Q/f)^(1/3)
   Answer: D. Lacey's scour depth is 0.47 (Q/f)^(1/3), here 7.79 m. (Key point: Scour depth formula)

2. With R = 7.79 m and h = 3.5 m, what is D' for the shank?
   A. 1.5R - h = 11.68 - 3.5 = 8.18 m
   B. 2R - h = 12.1 m
   C. 1.5R = 11.68 m
   D. 1.5R + h = 15.2 m
   Answer: A. Scour measured below HFL is 11.68 m; remove the flood depth to get depth below bed. (Key point: Depth below bed)

3. On what slope does the apron launch and what width is laid?
   A. 1H:1V slope; laid width D'
   B. 2H:1V slope; laid width 1.5D' = 12.27 m
   C. 3H:1V slope; laid width 3D'
   D. 2H:1V slope; laid width 2D'
   Answer: B. A 2:1 slope has length sqrt(5)D'; a laid width of 1.5D' holds the stone needed to cover it. (Key point: Apron slope and width)

## Close variants set in other exams

- [Launching apron, 7000 m3/s, HFL 114 m](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alaunching-apron-q7000-hfl114) — Irrigation and Drainage Engineering, chapter 6, asked 3×
- [Launching apron, 5000 m3/s, HFL 225 m](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alaunching-apron-q5000-d1mm-hfl225) — Irrigation and Drainage Engineering, chapter 6, asked 3×

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