---
title: "Catch water drains in hill roads"
subject: "Transportation Engineering I"
chapter: 5
key: "semester_6/transportation_engineering_i:catch-water-drain-hill"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/catch-water-drain-hill
site: BCT Engineering
---

# Catch water drains in hill roads

Transportation Engineering I · Chapter 5: Hill Roads

- Asked in: 2081 Chaitra; 2079 Jestha, 2 marks (asked 2 times)
- Video: [Functions and Design of Catchwater drains in hill roads, Requirement of catchwater drains in hills.](https://www.youtube.com/watch?v=6aZKcaD8fy0) — Transportation Engg Talks | Satish IITR
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/catch-water-drain-hill
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-5

## Question

Why are catch water drains necessary in hill roads? Define catch drain and its role in hill roads.

## Short answer

A catch water drain is a channel excavated above the cut slope to intercept hillside runoff before it reaches the road. It protects the cut face from erosion and saturation, reduces load on side drains, keeps the formation dry, and diverts water safely to natural streams.

## Answer

A **catch water drain** (catch drain) is a lined or unlined open channel excavated on the hillside above the top of a cut slope, roughly parallel to the road, which intercepts the surface runoff from the upper catchment before it reaches the road.

#### Role and necessity
- **Protects the cut slope** from erosion and from saturation by running water; the slope remains stable, which reduces landslides.
- **Reduces the load on the side drain** and cross-drainage structures, because they only have to carry the water from the road and the cut face.
- **Prevents water from entering** tension cracks at the top of the slope, which could trigger slips.
- **Keeps the pavement and formation dry**, protecting the subgrade strength and the road life.
- Diverts the water safely to the nearest natural stream or cross-drain through a stable outlet.

#### Practical details (IRC:SP:48 / NRS 2070)
- Placed about 5-10 m (at least 3-5 m) beyond the crest of the cut.
- Trapezoidal or rectangular, lined with stone masonry or concrete in soft soil; bed gradient about 1-6% so that it neither silts nor scours.
- The excavated material is placed as a bund on the downhill (road) side of the drain, and the outlets are protected by chutes.

## Check yourself

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

1. What is a catch water drain?
   A. A drain under the pavement
   B. A channel on the valley side
   C. A channel excavated on the hillside above the cut slope that intercepts upper catchment runoff before it reaches the road
   D. A pipe through the slope
   Answer: C. It runs roughly parallel to the road above the cut crest. (Key point: Definition)

2. How does it help the cut slope?
   A. It protects against erosion and saturation, reducing landslides
   B. It increases saturation
   C. It steepens the slope
   D. It adds weight on top
   Answer: A. Keeping running water off the slope maintains stability. (Key point: Role for slope)

3. What is the typical bed gradient and placement?
   A. Gradient over 30%, 1 m from the cut
   B. Zero gradient, on the road
   C. Bed gradient about 1-6% so it neither silts nor scours, placed about 5-10 m beyond the crest of the cut
   D. Any gradient, inside the cut
   Answer: C. A moderate gradient avoids silting and scouring. (Key point: Gradient and placement)

## Related in Transportation Engineering I

- [Surface water drainage in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-surface-drainage) — Transportation Engineering I, chapter 5, asked 2×
- [Necessity of catch drain, causeway, EDS](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Adrainage-elements-necessity) — Transportation Engineering I, chapter 4
- [Design and construction problems of hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-problems) — Transportation Engineering I, chapter 5, asked 11×
- [Typical cross-sections of hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-cross-sections) — Transportation Engineering I, chapter 5, asked 9×
- [Special structures in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-special-structures) — Transportation Engineering I, chapter 5
- [Short note: superelevation in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asuper-hill-roads) — Transportation Engineering I, chapter 5

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