---
title: "Short note: Design of drainage of irrigated land"
subject: "Irrigation and Drainage Engineering"
chapter: 9
key: "semester_6/irrigation_and_drainage_engineering:drainage-irrigated-land-design-note"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/drainage-irrigated-land-design-note
site: BCT Engineering
---

# Short note: Design of drainage of irrigated land

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

- Asked in: 2063 Baisakh (old course), 4 marks
- Video: [Drainage Requirements of Irrigated lands](https://www.youtube.com/watch?v=JYQxjNSRVcw) — Coordinator cas
- 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/drainage-irrigated-land-design-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9

## Question

Write a short note on design of drainage of irrigated land.

## Short answer

Design requires collecting hydrologic and soil data, determining drainage coefficient, and setting permissible water table depth below root zone plus capillary rise. Then select surface/subsurface systems, size drains using Manning's and Donnan's/Hooghoudt's formulas, ensure safe velocities, and provide outlets with structures and maintenance plans.

## Answer

Drainage of irrigated land is designed to keep the water table below the root zone and to remove excess surface water, so that water logging and salinity do not develop.

#### Design steps
1. **Collect the data:** rainfall, irrigation intensity and efficiency, soil permeability $K$, depth of the impervious layer, ground levels, the water table depth and the outlet level.
2. **Find the drainage requirement (rate $q$):** the part of the rainfall and irrigation water to be removed in a fixed time (for example 1–2 % of annual rainfall in 24 h), or a drainage coefficient in mm/day (or l/s/ha) for the paddy field from the crop tolerance.
3. **Fix the water table level:** the highest permissible water table is below the root zone plus capillary rise (about 1.5–1.8 m below ground).
4. **Choose the system:** surface drains (open field, collector and main drains) for excess rain water; subsurface (tile) drains for lowering the water table; tube wells where suitable.
5. **Subsurface design:** drain depth 1.5–2.5 m and spacing from Donnan's/Hooghoudt's formula
   $$
   S=\sqrt{\frac{4K(b^2-a^2)}{q}}
   $$
   Size the lateral by Manning's formula for a pipe flowing full.
6. **Surface drain design:** discharge $Q=D\times A$; Manning's formula for the section; velocities within the non-silting and non-scouring limits; free board 0.15–0.3 m.
7. **Layout and outlets:** field, collector and main drains lead to a natural outlet higher than the high flood level, or provided with a flap gate; add structures and an operation and maintenance plan.

## Check yourself

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

1. What is the purpose of drainage of irrigated land?
   A. Keep the water table below the root zone and remove excess surface water to prevent waterlogging and salinity
   B. Increase irrigation intake
   C. Raise the water table for crops
   D. Store water for dry seasons
   Answer: A. Drainage prevents waterlogging and salinity. (Key point: Purpose)

2. How is the highest permissible water table fixed?
   A. At ground level
   B. Below root zone plus capillary rise (about 1.5-1.8 m below ground)
   C. Equal to the drain depth exactly
   D. 0.1 m below ground
   Answer: B. The root zone plus capillary rise sets the limit. (Key point: Water table level)

3. Which drains suit which purpose?
   A. Tile drains for rain runoff; surface drains for water table
   B. Only tube wells always
   C. Surface drains for excess rain water; subsurface tile drains for lowering the water table
   D. Surface drains only for lowering the water table
   Answer: C. Surface drains remove excess rain water, tile drains lower the water table. (Key point: System choice)

4. Which formulas size subsurface drains?
   A. Darcy's formula for lateral size only
   B. Lacey's formula for spacing
   C. Kennedy's formula for spacing
   D. Donnan's/Hooghoudt's formula for spacing; Manning's formula for lateral size
   Answer: D. Spacing S = sqrt(4K(b^2 - a^2)/q) and Manning for the full-flowing pipe. (Key point: Design tools)

## Related in Irrigation and Drainage Engineering

- [Necessity and methods of drainage of irrigated land](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adrainage-necessity-methods) — Irrigation and Drainage Engineering, chapter 9, asked 3×
- [Reservoir capacity with irrigated areas](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Areservoir-capacity-areas-79j) — Irrigation and Drainage Engineering, chapter 2
- [Siphon design: canal 18, drainage 60](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asiphon-q18-drain60-lacey) — Irrigation and Drainage Engineering, chapter 8
- [Design criteria of surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asurface-drainage-design-criteria) — Irrigation and Drainage Engineering, chapter 9
- [Design discharge for surface drainage, bunded field](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asurface-drainage-design-discharge-bunded) — Irrigation and Drainage Engineering, chapter 9
- [Surface drainage design, 40 ha Terai field](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asurface-drainage-40ha-terai) — Irrigation and Drainage Engineering, chapter 9

## Related in other subjects

- [Design steps of surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-design-steps) — Transportation Engineering I, chapter 4, asked 3×