---
title: "Launching apron thickness before launching"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:launching-apron-width-18-scour-31"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/launching-apron-width-18-scour-31
site: BCT Engineering
---

# Launching apron thickness before launching

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2070 Bhadra, 8 marks
- Video: [guide bank and launching apron design steps](https://www.youtube.com/watch?v=F_Q7A7EiTPI) — Competitors Academy
- 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-width-18-scour-31
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

The launching apron of a guide bank is laid in a width equal to 1.8 times the depth of scour below original bed. If a scour slope of 3:1 is to be maintained with thickness 1.5 t, find the thickness of apron before it gets launched. Draw neat sketch of designed structure.

## Short answer

Equate stone volume before and after launching: laid width 1.8D, launched slope length √10D, T×1.8D = 1.5t×√10D, so T = 2.64t. The apron is laid about 2.6 times the pitching thickness.

## Answer

A launching apron is laid flat at the toe of the pitching. When the bed scours, the stones slide down the scour slope and cover it. The thickness before launching follows from equality of the stone volume per metre run before and after launching.

#### Data
- Scour depth below original bed $= D$ (unknown, cancels out).
- Laid width of apron $= 1.8D$; scour slope $3H:1V$; launched thickness $= 1.5t$ ($t$ = thickness of pitching).

#### Calculation
Length of the scour slope: $\sqrt{D^2 + (3D)^2} = \sqrt{10}\,D = 3.162D$.

Volume per metre after launching: $V = 1.5t\times3.162D = 4.743\,tD$.

Volume per metre before launching: $V = T\times1.8D$, where $T$ is the thickness as laid.

$$
\begin{aligned} T\times1.8D &= 1.5t\times\sqrt{10}\,D \\ T &= \frac{1.5\sqrt{10}}{1.8}\,t = 2.635\,t \end{aligned}
$$

**Answer:** thickness of apron before launching $T \approx 2.64\,t$ (that is, about 2.6 times the pitching thickness).

#### Sketch
```text
 Before launching            After launching (3H:1V)
  pitching                     pitching
 ___\____ T                   ___\
      |<--1.8D-->|                 \  1.5t thick
 bed ____apron___| river bed        \ scour slope
                                     \__ depth D
```
For example, if $t = 1.0$ m the laid thickness is 2.64 m.

## Check yourself

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

1. How is the thickness before launching found?
   A. By equating the apron weight to the water weight
   B. By taking T = 1.5t
   C. By equating the stone volume per metre before and after launching
   D. By equating the widths
   Answer: C. Stone volume is conserved: T x laid width = launched thickness x slope length. (Key point: Principle)

2. For a scour slope of 3H:1V and scour depth D, the slope length is:
   A. 3D
   B. sqrt(D^2 + (3D)^2) = sqrt10 D = 3.162D
   C. sqrt5 D = 2.236D
   D. 4D
   Answer: B. Pythagoras on the 3:1 slope gives sqrt10 D; sqrt5 D is for a 2:1 slope. (Key point: Slope length)

3. With laid width 1.8D, launched thickness 1.5t, the laid thickness T is:
   A. T = 1.5 sqrt10 t/1.8 = 2.64t
   B. T = 1.5t
   C. T = 1.8 sqrt10 t/1.5 = 3.8t
   D. T = 2.25t
   Answer: A. T x 1.8D = 1.5t x 3.162D gives T = 2.635t. (Key point: Result)

4. Why does the scour depth D not matter in the final answer?
   A. D is assumed to be 1 m
   B. D is found from Lacey and is negligible
   C. D is replaced by t
   D. D appears on both sides and cancels
   Answer: D. Both volumes are proportional to D, so it cancels. (Key point: Effect of D)

## Related in Irrigation and Drainage Engineering

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