---
title: "Collection and disposal of surface water"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:surface-drainage-collection"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/surface-drainage-collection
site: BCT Engineering
---

# Collection and disposal of surface water

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2075 Baisakh; 2064 Poush (old course), 4 marks (asked 2 times)
- Video: [BQS601 Surface Water Drainage P1](https://www.youtube.com/watch?v=ZArlOCJKA7c) — PROFESOR MADYA DR ASMAH ALIA MOHAMAD BOHARI
- 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-collection
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Explain how the surface water is collected and disposed-off in rural and urban roads.

## Short answer

Rural roads: camber and shoulders direct water to side drains, catch drains intercept hill water, mitre drains divert at intervals, culverts/bridges cross streams. Urban roads: kerb and gutter collect water, gully inlets feed underground storm sewers with manholes, outfall to river or soak pit. Both use rational method and Manning's equation.

## Answer

Surface water must be collected from the pavement and the area around it and taken to a safe outlet.

#### Rural roads
1. **Camber and shoulders**: the cross slope (about 2.5% for bituminous, 3 to 4% for gravel/earth) sends water from the carriageway to the shoulders, and the shoulder slope (3 to 4%) sends it to the side drain.
2. **Side (longitudinal) drains**: open earth, stone-pitched or lined drains along both sides in cutting, or on the hill side. In embankment, water flows down the side slopes or into drains at the toe.
3. **Catch (intercepting) drains**: placed above the cut slope to stop hill water entering the road.
4. **Mitre / turnout drains** lead water away from the side drain into the natural ground at intervals (every 100 to 250 m or so, depending on gradient).
5. **Cross-drainage works**: culverts, causeways and minor bridges carry the collected water across the road at natural streams and at fixed spacing.
6. **Disposal**: at stable natural outlets, with energy dissipation if the fall is steep.

```text
 catch drain   ____ cut slope
 ----\_/-----\      \ side drain
              \______\___ road ___/ shoulder
                 --> culvert --> natural stream
```

#### Urban roads
1. **Kerb and channel (gutter)**: the road is bounded by kerbs; water flows along the gutter at the edge of the road.
2. **Gully inlets / catch basins**: grated inlets set in the gutter at regular spacing take water into underground pipes. A silt trap catches sand and debris.
3. **Underground storm sewers / box drains** carry the water, with manholes every 30 to 90 m for inspection.
4. **Outfall** into a river, a larger storm drain or a soak pit. Footpath drains join this system, and open drains are often covered with slabs.

```text
 footpath |kerb| road ---> gutter -> [gully inlet]
                                        |
                              storm sewer (pipe) ===> outfall
```

In both cases the design uses the rational method for runoff and Manning's equation for the capacity of the drains.

## Check yourself

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

1. In rural roads, water from the carriageway goes first to:
   A. Storm sewer
   B. Shoulder, then side drain
   C. Gully inlet
   D. Manhole
   Answer: B. Camber sends it to the shoulders, and the shoulder slope leads it to the side drain. (Key point: Rural collection path)

2. In urban roads, water from the gutter enters underground pipes via:
   A. Mitre drains
   B. Gully inlets (catch basins)
   C. Catch drains
   D. Causeways
   Answer: B. Grated gully inlets at regular spacing lead water to storm sewers; a silt trap catches debris. (Key point: Urban collection)

3. What do mitre (turnout) drains do?
   A. Collect water above cut slopes
   B. Lead water from the side drain to natural ground at intervals
   C. Carry water across the road
   D. Lower the water table
   Answer: B. They divert water away from the road at intervals (about 100-250 m). (Key point: Mitre drains)

## 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×
- [Catch water drains in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acatch-water-drain-hill) — Transportation Engineering I, chapter 5, asked 2×
- [Control of sub-surface water and seepage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asubsurface-drainage-measures) — Transportation Engineering I, chapter 4, asked 15×

## Related in other subjects

- [Rural and urban scheme layout diagram](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arural-urban-scheme-diagram) — Water Supply Engineering, chapter 1, asked 6×
- [Selecting load centre and transformer, rural/urban](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aload-centre-transformer-rural-urban) — Transmission and Distribution System Design, chapter 7, asked 3×
- [T/F: thermal limit design urban, losses rural](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-thermal-limit-urban-loss-rural) — Transmission and Distribution System Design, chapter 7, asked 3×
- [Septic effluent disposal, high water table/rocky](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-effluent-high-water-table-rocky) — Sanitary Engineering, chapter 9