---
title: "Launching apron, 5000 m3/s, HFL 225 m"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:launching-apron-q5000-d1mm-hfl225"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/launching-apron-q5000-d1mm-hfl225
site: BCT Engineering
---

# Launching apron, 5000 m3/s, HFL 225 m

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2064 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-q5000-d1mm-hfl225
- 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 = 5000 m³/s; av. dia. of bed material = 1 mm; HFL = 225.00 m; River bed level = 222.00 m.

## Short answer

With f=1.76, t=0.06×5000^(1/3)=1.03 m, R=0.47(5000/1.76)^(1/3)=6.66 m. Scour below bed D'=1.5R−3=6.98 m. Apron length=1.5D'=10.48 m, launched thickness=1.5t=1.54 m (laid √5 t≈2.29 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: $h = 225.0 - 222.0 = 3.00$ m
- Design discharge for scour/apron: $Q = 5000$ m³/s

#### 1. Pitching thickness
$$
t = 0.06\,Q^{1/3} = 0.06\times 5000^{1/3} = 0.06\times 17.10 = 1.03\ \text{m}
$$

#### 2. Scour depth and launching apron
Lacey's normal scour depth:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{5000}{1.76}\right)^{1/3} = 6.66\ \text{m}
$$
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 | 9.98 m | 6.98 m | 10.48 m |

Thickness: pitching $t = 1.03$ m, so the launched apron thickness is $1.5t = 1.5\times1.03 = 1.54$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.29$ m (about $2.25t$).

**Answer (straight portion):** length of launching apron $=1.5D' = 10.48$ m, thickness $=1.5t = 1.54$ m launched (about 2.29 m as laid).

## Check yourself

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

1. What silt factor is used for a 1 mm bed material?
   A. f = 1.76 sqrt(d in mm) = 1.76
   B. f = 1.0
   C. f = 0.47
   D. f = 1.76 x 1^2 = 3.52
   Answer: A. Lacey's silt factor is 1.76 sqrt(d_mm); for d = 1 mm it is 1.76. (Key point: Silt factor)

2. How is D' (scour depth below the bed) found for the shank?
   A. D' = 1.5R + h
   B. D' = 1.5R - flood depth h = 9.98 - 3.00 = 6.98 m
   C. D' = R - h
   D. D' = 2R = 13.3 m
   Answer: B. The 1.5R scour is measured from HFL, so the depth below bed is 1.5R minus the flood depth h. (Key point: Depth of apron below bed)

3. What is the horizontal length of the launching apron?
   A. 2 D' = 13.96 m
   B. sqrt(5) D' = 15.6 m
   C. 1.5 D' = 10.48 m
   D. D'/1.5 = 4.65 m
   Answer: C. The apron is laid with width 1.5 D', which is enough to cover the slope length sqrt(5) D' when it launches at 2:1. (Key point: Apron length)

4. What is the launched thickness of the apron and why is the laid thickness larger?
   A. Launched 1.5t; laid thickness is smaller because stone compacts
   B. Launched t = 1.03 m; laid thickness is 1.5t
   C. Launched 2.29 m; laid 1.54 m
   D. Launched 1.5t = 1.54 m; laid thickness is sqrt(5) t = 2.29 m because the same volume spreads over a longer slope
   Answer: D. The laid volume 1.5D' x T equals launched volume 1.5t x sqrt(5)D', so T = sqrt(5) t, about 2.25t. (Key point: Apron thickness)

## Close variants set in other exams

- [Launching apron, 8000 m3/s, HFL 128.5 m](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alaunching-apron-q8000-hfl1285) — Irrigation and Drainage Engineering, chapter 6, asked 3×
- [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×

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