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

# Design discharge for surface drainage, bunded field

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

- Asked in: 2079 Jestha, 4 marks
- Video: [surface drainage design | water balance equation | Irrigation numerical](https://www.youtube.com/watch?v=YYmufytG5M8) — Sudip Dhakal
- 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-discharge-bunded
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9

## Question

How to fix the design discharge for the surface drainage system in the bunded field?

## Short answer

Design dischargeQ = D × A, where drainage rateD is found by trial so that peak water depthh_peak = h0 + R –3D and duration above permissible depths stay within crop limits. Convert1 mm/day =0.1157 l/s/ha.

## Answer

In bunded rice fields the design discharge of the surface drain is fixed from the **drainage rate** that satisfies the tolerance of the crop to depth and duration of flooding.

1. **Design rainfall:** find the maximum 3-day rainfall $R$ for the chosen return period (5–10 years), using the frequency analysis or the mean annual value × growth factor.
2. **Initial depth:** take the standing water $h_0$ in the bunded field (40–50 mm).
3. **Crop tolerance:** fix the maximum depth $h_{max}$ and its permissible duration, and the depth $h_e$ and the duration $T_e$ for which it may persist (for example 300–400 mm for 1 day, 200–250 mm for 2–3 days).
4. **Water-level history:** assuming the rain falls uniformly in 3 days, ET and percolation neglected, and drainage $D$ constant:
   $$
   h_{peak}=h_0+R-3D, \qquad T_e=\left(3-\frac{h_e-h_0}{R/3-D}\right)+\frac{h_{peak}-h_e}{D}
   $$
   Find $D$ (mm/day) by trial so that the limits are just satisfied; convert: 1 mm/day = 0.1157 l/s/ha.
5. **Design discharge:**
   $$
   Q = D\,A
   $$
   with $D$ in l/s/ha and $A$ in ha (gives l/s). For large areas, a reduction factor for the travel time is sometimes applied.

## Check yourself

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

1. On what basis is design discharge fixed for bunded fields?
   A. The drainage rate satisfying crop tolerance to depth and duration of flooding
   B. Average daily rainfall
   C. Canal discharge
   D. Field area only
   Answer: A. The drainage rate D is chosen so rice depth-duration limits are not exceeded. (Key point: Basis of design discharge)

2. What are the peak depth and duration formulas?
   A. h_peak = h0 + R; T_e = R/D
   B. h_peak = h0 + R - 3D; T_e = (3 - (h_e - h0)/(R/3 - D)) + (h_peak - h_e)/D
   C. h_peak = R - D; T_e = 3
   D. h_peak = h0 - 3D
   Answer: B. Rain rises level at R/3 - D during 3 days, then it falls at D per day. (Key point: Peak and duration formulas)

3. How is D found?
   A. Directly from R/3
   B. By taking D = R
   C. By trial, so the limits are just satisfied, then converted by 1 mm/day = 0.1157 l/s/ha
   D. By neglecting tolerance
   Answer: C. T_e depends on D non-linearly, so trial is used. (Key point: Solving for D)

4. What is the design discharge?
   A. Q = D / A
   B. Q = R x A
   C. Q = h0 x A
   D. Q = D x A, with D in l/s/ha and A in ha
   Answer: D. Discharge equals drainage rate times area served, sometimes reduced for large areas. (Key point: Design discharge)

## Related in Irrigation and Drainage Engineering

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