---
title: "Tile size for laterals, 2 cm/day"
subject: "Irrigation and Drainage Engineering"
chapter: 9
key: "semester_6/irrigation_and_drainage_engineering:lateral-tile-size-2cmday-grade03"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/lateral-tile-size-2cmday-grade03
site: BCT Engineering
---

# Tile size for laterals, 2 cm/day

Irrigation and Drainage Engineering · Chapter 9: Water Logging and Drainage

- Asked in: 2079 Asoj, 4 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/lateral-tile-size-2cmday-grade03
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9

## Question

In a sub-surface drainage system, 200 m long laterals were laid out 50 m apart. The laterals have a grade of 0.3%. If the drainage coefficient of the area is 2 cm/day, what size of tiles would you recommend?

## Short answer

Use Manning's formula for full pipe flow. Drainage discharge is 2.31 L/s. Required diameter is 87 mm; recommend 100 mm standard tile with capacity 3.34 L/s.

## Answer

The tile must carry the drainage discharge from the area served by the lateral, flowing full under gravity. Manning's formula for a full circular pipe is used.

#### Data
- Area drained by one lateral = 200 m × 50 m = 10 000 m² = 1 ha
- Drainage coefficient = 2 cm/day = 0.02 m/day; grade $S$ = 0.003
- Manning's $n$ = 0.011 for clay/concrete tile (assumed)

#### Discharge
$$
Q = 0.02\times\frac{10000}{86400} = 0.00231\ \text{m}^3/\text{s} = 2.31\ \text{l/s}
$$

#### Diameter
For a pipe flowing full, $A=\pi D^2/4$ and $R=D/4$:

$$
Q=\frac{1}{n}\cdot\frac{\pi D^2}{4}\left(\frac{D}{4}\right)^{2/3}S^{1/2}
\quad\Rightarrow\quad
D^{8/3}=\frac{4^{5/3}\,nQ}{\pi\,S^{1/2}}
$$

$$
D=\left(\frac{4^{5/3}\times0.011\times0.00231}{\pi\times\sqrt{0.003}}\right)^{3/8} = 0.0871\ \text{m} = 87\ \text{mm}
$$

The next standard tile size is **100 mm**. Its full-flow capacity = 3.34 l/s (0.00334 m³/s) > 2.31 l/s, so it is safe (velocity 0.43 m/s).

**Answer: provide 100 mm diameter tiles.**

## Check yourself

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

1. What area does each lateral drain?
   A. 200 m x 50 m = 1 ha
   B. 200 m x 200 m = 4 ha
   C. 50 m x 50 m = 0.25 ha
   D. 200 m only
   Answer: A. Lateral length times spacing gives the area served. (Key point: Area served)

2. What is the discharge?
   A. 23.1 l/s
   B. 2.31 l/s (0.02 m/day x 10000 m2)
   C. 0.231 l/s
   D. 200 l/s
   Answer: B. Q = 0.02 x 10000/86400 = 0.00231 m3/s. (Key point: Discharge)

3. For a pipe flowing full, what are A and R?
   A. A = pi D^2/4 and R = D/2
   B. A = pi D and R = D
   C. A = pi D^2/4 and R = D/4
   D. A = D^2 and R = D/4
   Answer: C. Full circular pipe: hydraulic radius is area over perimeter = D/4. (Key point: Full-pipe Manning)

4. What tile size is recommended?
   A. 87 mm exactly
   B. 75 mm
   C. 200 mm
   D. 100 mm, next standard size above the computed 87 mm
   Answer: D. Computed D = 87 mm; the next standard size 100 mm has capacity 3.34 l/s > 2.31 l/s. (Key point: Size)

## Close variants set in other exams

- [Tile size (2 cm/day) and spacing (3 cm/day)](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alateral-tile-size-and-spacing-3cmday) — Irrigation and Drainage Engineering, chapter 9, asked 2×

## Related in Irrigation and Drainage Engineering

- [Tile drain spacing with drainage coefficient 0.012](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Atile-drain-spacing-drainage-coeff-0012) — Irrigation and Drainage Engineering, chapter 9