---
title: "Surface water drainage in hill roads"
subject: "Transportation Engineering I"
chapter: 5
key: "semester_6/transportation_engineering_i:hill-surface-drainage"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/hill-surface-drainage
site: BCT Engineering
---

# Surface water drainage in hill roads

Transportation Engineering I · Chapter 5: Hill Roads

- Asked in: 2075 Baisakh; 2064 Shrawan (old course), 8 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/hill-surface-drainage
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-5

## Question

Explain with sketches how the surface water is collected and disposed off in hill roads. What are the special problems in drainage of surface water in hill roads?

## Short answer

A catch drain above the cut intercepts hillside runoff, which is led to a side drain and then through culverts across the road. Downstream chutes discharge water into natural streams. Special problems include high-velocity scour, sediment blockage, limited bench space, and water-triggered landslides.

## Answer

#### How surface water is collected and disposed of
Rain falling on the hill above the road and on the carriageway is collected before it can enter the road structure and then carried across or away from it.

1. **Catch water drain** (uphill, 5-10 m above the cut) intercepts the water from the upper hillside and leads it to a natural stream or cross-drain.
2. **Side drain** (V or trapezoidal, lined) along the hill-side edge of the road collects water from the cut slope and the carriageway (camber 2.5-3% towards the hill, or crown with cross-fall).
3. **Cross drainage** - culverts and small bridges take the water of the side and catch drains across the road at close intervals (about 50-100 m or less, and at all natural nallahs).
4. **Outlet** - the culvert outlet is connected to a **chute/cascade, drop wall, or stilling basin** to discharge into the natural gully without scouring the fill slope.
5. **Valley side** - berms and kerbs on high fills guide water to outlets; the road surface is kept sealed.

```text
 UPHILL ~~~~~
   catch drain =====>--+
        \              |
   cut slope           v
   ----side drain--> culvert ---> chute
   =======road=======      \
        fill slope          \__ stream
```

#### Special problems of surface-water drainage in hill roads
- Steep slopes give **high velocity** and scouring of drains, fill slopes and outlets; lining and drop structures are needed.
- **Concentrated flow and heavy rainfall** in short time; many natural streams and gullies to cross.
- Water entering cracks and cut faces causes **landslides, slips and erosion**.
- Large **sediment/debris load** blocks drains and culverts; frequent cleaning is needed.
- **Limited space** for drains on narrow benches; difficult disposal because no place to discharge safely.
- **Snow melt and springs** add flow in cold areas.
- The cross-drainage works are many, costly and difficult to build on steep terrain.

## Check yourself

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

1. What is the role of the catch water drain in collecting surface water?
   A. It collects water from the carriageway
   B. It intercepts water from the upper hillside and leads it to a natural stream or cross-drain
   C. It carries water under the road
   D. It drains the valley side
   Answer: B. The uphill catch drain keeps upper catchment water out of the road structure. (Key point: Collection sequence)

2. How is water from the culvert outlet discharged without scouring the fill slope?
   A. Directly on the fill slope
   B. Straight onto the carriageway
   C. Through a chute, cascade, drop wall or stilling basin into the natural gully
   D. Into the side drain again
   Answer: C. These structures dissipate energy and carry water safely to the gully. (Key point: Cross drainage and outlet)

3. Why do steep slopes cause a special problem for surface drains?
   A. High velocity scours drains, fill slopes and outlets, so lining and drop structures are needed
   B. Water stands still in drains
   C. Drains never silt
   D. Flow is always small
   Answer: A. High velocity causes scour, hence lining and drop structures. (Key point: Special problem: velocity)

4. What problems arise from sediment/debris and water entering cracks?
   A. Drains remain clean
   B. Water in cracks stabilises slopes
   C. Nothing
   D. Debris blocks drains and culverts needing frequent cleaning, and water in cracks triggers landslides and erosion
   Answer: D. Debris loads and infiltration into cracks cause blockages, slips and erosion. (Key point: Debris and landslides)

## Related in Transportation Engineering I

- [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
- [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×
- [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×
- [Collection and disposal of surface water](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-collection) — Transportation Engineering I, chapter 4, asked 2×
- [Special considerations in hill road design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-special-considerations) — Transportation Engineering I, chapter 5, asked 5×
- [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

- [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
- [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
- [Types of cross drainage works with sketches](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acd-works-types-definition-sketches) — Irrigation and Drainage Engineering, chapter 8, asked 9×
- [Hill climbing and its problems](https://www.bctengineer.com/question.md?key=semester_6%2Fartificial_intelligence%3Ahill-climbing-problems-solutions) — Artificial Intelligence, chapter 3