---
title: "Internal/external drainage; water logging; Terai surface drainage"
subject: "Irrigation and Drainage Engineering"
chapter: 9
key: "semester_6/irrigation_and_drainage_engineering:internal-external-drainage-water-logging-terai"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/internal-external-drainage-water-logging-terai
site: BCT Engineering
---

# Internal/external drainage; water logging; Terai surface drainage

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

- Asked in: 2073 Magh, 3+3+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/internal-external-drainage-water-logging-terai
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9

## Question

Explain about internal and external drainage system. Also explain the causes and remedial measures of water logging in the agriculture land and write the method as well as assumptions adapted to design the surface drainage in Terai region.

## Short answer

Internal drainage (field to collector) handles on-farm excess; external drainage (main drain to river) removes it from the area. Waterlogging from canal seepage, poor drainage, heavy rain is remedied by lining, drains, and vertical pumping. Surface design assumes bunded paddy, design 3-day rainfall, tolerance limits, and uses trial drainage rate for Manning’s sizing.

## Answer

#### Internal and external drainage
- **Internal drainage** is the system inside the command area or farm boundary: the field drains and the lateral and collector drains (open or tile) that collect excess water from the fields and carry it to the boundary of the project area.
- **External drainage** is the system outside the farm/project area: the main drains, outfall channels and improved natural streams that carry the collected water to the final outlet (river), together with the drains and flood protection works that deal with runoff entering from the catchment outside the project.

```text
 field --> field drain --> collector --> main drain --> river
 |<--- internal drainage --->|<--- external drainage --->|
```

#### 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, and a 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 (the Terai has a good aquifer) and to supply 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 an 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 the drainage rate is constant.
5. Find the drainage rate $D$ (mm/day, then l/s/ha) that satisfies 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 a free board of 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. What is internal drainage?
   A. The system inside the command area: field, lateral and collector drains up to the project boundary
   B. Main drains and outfall channels outside the project
   C. Only tube wells
   D. Flood protection works
   Answer: A. Internal drainage collects water from fields and carries it up to the boundary of the project area. (Key point: Internal drainage)

2. What does external drainage consist of?
   A. Field drains only
   B. Main drains, outfall channels and improved natural streams to the final outlet, with flood protection
   C. Tile drains inside fields
   D. Canal distributaries
   Answer: B. External drainage lies outside the project area and carries collected water to the river. (Key point: External drainage)

3. Which is a preventive measure?
   A. Increasing canal seepage
   B. Blocking natural drains
   C. Lining of canals and controlled irrigation
   D. Over-irrigation
   Answer: C. Lining and good water management stop the water table rising. (Key point: Water logging remedies)

4. Which assumption is used in Terai surface drainage design?
   A. Design 1-hour rainfall of 100 year return period
   B. Rice tolerates unlimited flooding
   C. Drainage rate varies with time
   D. Design 3-day rainfall of 5-10 year return period, rice tolerance limits, ET and percolation neglected
   Answer: D. The drainage rate is found by trial so rice tolerance limits are met under the 3-day design rain. (Key point: Design assumptions)

## Related in Irrigation and Drainage Engineering

- [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
- [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

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