---
title: "Surface drainage system"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:surface-drainage-system"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/surface-drainage-system
site: BCT Engineering
---

# Surface drainage system

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2079 Ashwin, 5 marks
- Video: [Surface Drainage of Roads in Hindi ( Highway Engineering ) for Diploma, SSC JE, RRB JE](https://www.youtube.com/watch?v=TLfKbE8a8Ns) — IQ Cracker
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/surface-drainage-system
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Explain the surface drainage system.

## Short answer

Surface drainage removes rainwater from the road surface and surroundings before it enters the pavement. It uses camber, shoulder slopes, side drains, catch drains, and cross-drainage structures. Discharge is computed by the rational method and drains are sized by Manning's equation.

## Answer

**Surface drainage** is the system that removes rainwater from the road surface and its surroundings and leads it to a safe outlet before it can enter the pavement, pond on the surface or erode the road. It is the most important part of highway drainage.

#### Elements
1. **Camber (cross slope) on the carriageway** (about 2.5% on bituminous roads, steeper on earth roads) drains water quickly to the edges.
2. **Shoulders** with a cross slope of about 3 to 4% carry the water from the pavement to the side drain, away from the pavement edge.
3. **Side drains**: longitudinal open drains (triangular, trapezoidal or rectangular, lined or unlined) along the road on one or both sides. In urban areas, these are replaced by kerb, gutter, gully inlets and storm sewers.
4. **Catch drains** above cut slopes intercept water from the hill side.
5. **Cross-drainage structures** (culverts, causeways and bridges) pass water under the road at streams and at regular intervals.
6. **Outfall and disposal**: mitre drains, natural streams, and energy dissipation where the fall is steep.

```text
 catch drain
   \_/        hill
     \   cut slope
      \_ side drain
       \___ shoulder ___ carriageway ___ shoulder
                  camber (crown)
 slope of road -> side drain -> culvert -> stream
```

#### Design points
- Discharge by rational method: $Q=\dfrac{CIA}{360}$.
- Section by Manning's equation, with the velocity kept between the non-silting and non-scouring limits.
- Longitudinal gradient of side drains not less than about 0.5%.
- Lining where the velocity is high or the soil is erodible.

## Check yourself

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

1. Surface drainage aims to:
   A. Support the pavement
   B. Remove rainwater from the road to a safe outlet before it enters or erodes the road
   C. Lower the water table
   D. Reduce traffic
   Answer: B. It is the most important part of highway drainage. (Key point: Purpose)

2. What cross slope do shoulders carry?
   A. About 10%
   B. About 3 to 4%
   C. About 15%
   D. 0%
   Answer: B. Shoulders carry water from the pavement to the side drain. (Key point: Shoulder slope)

3. Discharge and section are found by:
   A. CBR method
   B. Rational method and Manning's equation
   C. Bernoulli only
   D. Darcy and Terzaghi
   Answer: B. Q = CIA/360, with Manning's for section capacity. (Key point: Design)

## Related in Transportation Engineering I

- [Design steps of surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-design-steps) — Transportation Engineering I, chapter 4, asked 3×
- [Surface versus sub-surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-vs-subsurface) — Transportation Engineering I, chapter 4, asked 2×
- [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×
- [Components of highway drainage system](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Adrainage-components) — Transportation Engineering I, chapter 4
- [Subsurface drainage and its importance](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asubsurface-drainage-define-importance) — Transportation Engineering I, chapter 4
- [Hill road drainage structures](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-drainage-structures) — Transportation Engineering I, chapter 5, asked 4×

## Related in other subjects

- [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
- [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
- [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
- [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