---
title: "Water logging remedies and surface drainage in Terai"
subject: "Irrigation and Drainage Engineering"
chapter: 9
key: "semester_6/irrigation_and_drainage_engineering:water-logging-causes-surface-drainage-terai"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/water-logging-causes-surface-drainage-terai
site: BCT Engineering
---

# Water logging remedies and surface drainage in Terai

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

- Asked in: 2075 Bhadra, 3+3 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/water-logging-causes-surface-drainage-terai
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9

## Question

Explain causes and remedial measures of water logging in the agriculture land and write the method as well as assumption adopted to design the surface drainage in Terai region of Nepal.

## Short answer

Waterlogging results from canal seepage, flat terrain, monsoon rain, and poor drainage. Remedies include lining, surface/subsurface/vertical drainage. Surface drainage design for Terai paddy assumes bunded fields, design 3-day rainfall, crop tolerance limits, neglects ET/percolation, finds drainage rate by trial, and sizes drains with Manning’s equation ensuring freeboard and velocity limits.

## Answer

#### Causes of water logging
- **Seepage from unlined canals** and over-irrigation raise the water table.
- **Flat land and poor natural drainage** in the Terai, with blocked or obstructed drains (roads, embankments).
- **Heavy monsoon rain** with flooding from the rivers, on an impervious or clayey subsoil.
- Shallow impervious layer; absence of proper drains and poor maintenance.

#### Remedial measures
1. **Prevention:** lining of canals and water courses, controlled irrigation and good water management, cropping pattern suited to the water availability.
2. **Surface drainage:** improving natural drains and constructing artificial open drains.
3. **Subsurface drainage:** tile or pipe drains, mole drains.
4. **Vertical drainage:** tube wells to lower the water table (Terai has a good aquifer) and to give irrigation water.
5. Interceptor drains near canals, land levelling and the use of deep-rooted crops.

#### Method and assumptions for surface drainage design in the Terai
The method is based on the drainage rate for bunded paddy fields:
1. Take the design 3-day rainfall for a return period of 5–10 years (mean annual maximum × growth factor).
2. Assume a **bunded field** with the initial water depth $h_0$ = 40–50 mm and no inflow from outside.
3. Rice tolerance: a maximum depth (300–400 mm) for 1 day and depth above a limit (200–250 mm) for 2–3 days.
4. ET and deep percolation are neglected (safe side); the rain falls uniformly over 3 days and drainage is constant.
5. Find the drainage rate $D$ (mm/day, then l/s/ha) satisfying the depth–duration limits by trial.
6. Design discharge $Q = D \times A$; the drains are sized by Manning's formula with $n$ = 0.025–0.035, non-silting and non-scouring velocities, and free board 0.15–0.3 m.
7. The drains follow the natural slope and discharge to a natural drain or river at a level lower than the field outlets.

## Check yourself

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

1. Which is a key cause of water logging in the Terai?
   A. Flat land, poor natural drainage and obstructed drains with heavy monsoon rain
   B. Steep slopes with fast runoff
   C. Very deep water table and sandy soil
   D. Lack of irrigation
   Answer: A. Flat topography and blocked drains with heavy monsoon on clayey subsoil leave excess water without an outlet. (Key point: Causes in the Terai)

2. Which measure is a vertical drainage remedy?
   A. Canal lining
   B. Tube wells that lower the water table and give irrigation water
   C. Mole drains
   D. Open field drains
   Answer: B. Vertical drainage means pumping groundwater from tube wells, suitable in the Terai's good aquifer. (Key point: Remedial measures)

3. Which assumptions are adopted for the surface drainage design?
   A. Unbunded field with inflow from outside
   B. ET and percolation fully included
   C. Bunded field with initial depth 40-50 mm, ET and percolation neglected, rain uniform over 3 days, constant drainage
   D. Rain falls in one hour
   Answer: C. The method assumes a bunded field and neglects ET and percolation (safe side) with uniform 3-day rain. (Key point: Assumptions of surface drainage design)

4. Where should the drain discharge?
   A. To a canal at a higher level
   B. Into the field itself
   C. Above the field outlets
   D. To a natural drain or river at a level lower than the field outlets
   Answer: D. Drains follow natural slope and outfall below the field outlets so the fields drain freely. (Key point: Drain level)

## Related in Irrigation and Drainage Engineering

- [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
- [Water logging: causes, effects, remedies](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Awater-logging-causes-effects-remedies) — Irrigation and Drainage Engineering, chapter 9, asked 13×
- [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