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

# Design criteria of surface drainage

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

- Asked in: 2070 Chaitra (old course), 5 marks
- Video: [Drainage Design Part 3 - Drainage Coefficient](https://www.youtube.com/watch?v=BKAgvcIW1rU) — Iowa NRCS Training Videos
- 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/surface-drainage-design-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9

## Question

Write down the design criteria of surface drainages.

## Short answer

Design uses a3-day rainfall of5–10 year return period, crop tolerance to flood depth/duration, and Manning’s formula withn=0.025–0.035. Velocity must be non-silting(≈0.3 m/s) and non-scouring(0.6–0.9 m/s); trapezoidal section, freeboard0.15–0.3 m, water level ≥0.3 m below field.

## Answer

The design of surface drains is based on the following criteria.

1. **Design rainfall:** the maximum 3-day rainfall of 5–10 years return period (taken as higher for main drains and outfalls, e.g. 10–25 years), based on frequency analysis and growth factors.
2. **Crop tolerance:** the permissible depth and duration of flooding of the crop. For rice: a maximum of 300–400 mm for 1 day and 200–250 mm for 2–3 days; the initial depth in the field is 40–50 mm.
3. **Drainage rate (coefficient):** found in mm/day or l/s/ha such that the depth–duration limits are not exceeded; ET and percolation are generally neglected.
4. **Design discharge:** $Q = D \times A$ for the area served.
5. **Hydraulic design:** Manning's formula $Q=\frac{1}{n}AR^{2/3}S^{1/2}$ with $n$ = 0.025–0.035 for earthen drains.
6. **Velocity limits:** non-silting (about 0.3 m/s) and non-scouring (0.6–0.9 m/s depending on the soil).
7. **Section:** trapezoidal; side slopes 1:1 to 2:1 as per the soil; economical bed width/depth ratio; free board 0.15–0.3 m.
8. **Bed slope:** following the natural ground slope (generally 1 in 500 to 1 in 2000 in the Terai).
9. **Water level:** the full-supply level of the drain is at least 0.3 m below the field level (or the outlets) so that the field drains freely.
10. **Outlet:** the lowest point of the system must be higher than the high flood level of the receiving river, or a flap gate is provided to prevent backflow.
11. Provision of structures (culverts, drops, check structures) and access for maintenance.

## Check yourself

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

1. What is the design rainfall for surface drains?
   A. Maximum 3-day rainfall of 5-10 years return period, higher for main drains
   B. Average daily rainfall
   C. Maximum 1-hour rainfall
   D. Annual rainfall
   Answer: A. Frequency analysis gives the 3-day rainfall, with larger return periods (10-25 years) for main drains. (Key point: Design rainfall)

2. Which velocity limits apply?
   A. Non-silting 2 m/s, non-scouring 5 m/s
   B. Non-silting about 0.3 m/s, non-scouring 0.6-0.9 m/s
   C. No limits
   D. Only a maximum of 0.3 m/s
   Answer: B. Velocity must be high enough to avoid silting and low enough to avoid scour of earth. (Key point: Velocity limits)

3. What is the rule for the drain full-supply level?
   A. 0.3 m above field level
   B. Equal to field level
   C. At least 0.3 m below field level or outlets
   D. At the river HFL
   Answer: C. This lets the field drain freely into the drain. (Key point: Drain water level)

4. What outlet condition is required?
   A. Outlet below river bed
   B. No condition
   C. Outlet blocked in floods
   D. Lowest point higher than river HFL, or a flap gate to prevent backflow
   Answer: D. Either the outlet is above the river HFL or a flap gate prevents backflow. (Key point: Outlet to river)

## Related in Irrigation and Drainage Engineering

- [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
- [Water logging remedies and surface drainage in Terai](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Awater-logging-causes-surface-drainage-terai) — Irrigation and Drainage Engineering, chapter 9
- [Internal/external drainage; water logging; Terai surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainternal-external-drainage-water-logging-terai) — Irrigation and Drainage Engineering, chapter 9
- [Layout planning of surface drainage system](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asurface-drainage-layout-planning) — Irrigation and Drainage Engineering, chapter 9
- [Undersluice functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aundersluice-function-criteria) — Irrigation and Drainage Engineering, chapter 5, asked 2×

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