---
title: "Launching apron, 7000 m3/s, HFL 114 m"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:launching-apron-q7000-hfl114"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/launching-apron-q7000-hfl114
site: BCT Engineering
---

# Launching apron, 7000 m3/s, HFL 114 m

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2063 Baisakh (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-q7000-hfl114
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design the length and thickness of launching apron for the straight portion of a guide bund in a river: Design flood = 7000 m³/s; Average diameter of river bed material = 1 mm; River bed level = 111.00; HFL = 114.00. Provide a neat sketch of designed apron.

## Short answer

f=1.76, t=1.15 m, R=7.45 m, D'=1.5R−3=8.17 m. Apron length=12.25 m, launched thickness=1.72 m (laid 2.57 m). Volume equality before and after launching on 2:1 slope used.

## 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: $h = 114.0 - 111.0 = 3.00$ m
- Design discharge for scour/apron: $Q = 7000$ m³/s

#### 1. Pitching thickness
$$
t = 0.06\,Q^{1/3} = 0.06\times 7000^{1/3} = 0.06\times 19.13 = 1.15\ \text{m}
$$

#### 2. Scour depth and launching apron
Lacey's normal scour depth:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{7000}{1.76}\right)^{1/3} = 7.45\ \text{m}
$$
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 | 11.17 m | 8.17 m | 12.25 m |

Thickness: pitching $t = 1.15$ m, so the launched apron thickness is $1.5t = 1.5\times1.15 = 1.72$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.57$ m (about $2.25t$).

**Answer (straight portion):** length of launching apron $=1.5D' = 12.25$ m, thickness $=1.5t = 1.72$ m launched (about 2.57 m as laid).

```text
        pitching
   ____/\__ 
  /\ apron laid: width 1.5D', T = 2.24 t
 bed ___________ river bed
   after launching: 2:1 slope, 1.5 t thick
```

## Check yourself

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

1. How is the flood depth h for the apron calculation found here?
   A. HFL - bed level = 114 - 111 = 3 m
   B. HFL + bed level = 225 m
   C. HFL / bed level
   D. 1.5 x HFL
   Answer: A. Flood depth above the bed is the difference between HFL and river bed level. (Key point: Flood depth)

2. For the shank, with R = 7.45 m, what is D'?
   A. 2R - h = 11.9 m
   B. 1.5R - h = 11.17 - 3 = 8.17 m
   C. R - h = 4.45 m
   D. 1.5R + h = 14.17 m
   Answer: B. Shank scour is 1.5R below HFL; subtracting the 3 m flood depth gives the depth below bed. (Key point: Shank scour and D')

3. What are the apron length and launched thickness?
   A. Length 1.5D' = 12.25 m; thickness t = 1.15 m
   B. Length sqrt(5)D' = 18.3 m; thickness 2.57 m
   C. Length 1.5D' = 12.25 m; thickness 1.5t = 1.72 m
   D. Length D' = 8.17 m; thickness 1.5t = 1.72 m
   Answer: C. The apron is laid 1.5D' wide and launches to a thickness of 1.5t, with t = 0.06 Q^(1/3) = 1.15 m. (Key point: Apron dimensions)

## 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, 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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