---
title: "Classification of cross drainage per NRS"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:cross-drain-classification-suitability"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/cross-drain-classification-suitability
site: BCT Engineering
---

# Classification of cross drainage per NRS

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2067 Mangsir (old course), 8 marks; 2066 Magh (old course) (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/cross-drain-classification-suitability
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Classify cross drainage structures as per NRS. (Describe the suitability of construction of pipe, box, slab and arch culverts.)

## Short answer

NRS classifies: culverts (span ≤6m) - pipe, box, slab, arch; causeways/drifts; minor bridges (span >6m); major bridges. Pipe: small flow, flat terrain, sufficient cover. Box: moderate-large flow, weak foundation, high fill. Slab: up to 6m span, firm foundation. Arch: hill roads, stone available, strong foundation. Selection based on discharge, foundation, materials, cost.

## Answer

#### Classification
Cross-drainage structures carry streams and road-side drains across the road. As per NRS 2070 (Nepal Road Standard) and IRC practice, they are classified by span and discharge as:

1. **Culverts** (clear span up to about 6 m):
   - pipe culvert (single or multiple barrels),
   - box culvert (RCC, single or multi-cell),
   - slab culvert (RCC slab on masonry or concrete abutments),
   - arch culvert (masonry or concrete arch).
2. **Causeways / drifts** (submersible crossings): vented (with pipes) or unvented (fords, Irish bridges).
3. **Minor bridges** (span above about 6 m).
4. **Major bridges**, and **special structures** such as aqueducts and inverted siphons where canals meet the road.

#### Suitability
| Culvert | Suitable where | Notes |
|---|---|---|
| Pipe | Small, steady discharge; flat terrain; sufficient cover of 0.6 to 1.0 m or more; need for a quick, cheap crossing | Min. diameter about 600 mm (900 mm preferred). Multiple pipes for larger flows. Can choke with debris. |
| Box | Moderate to large discharge; soft or poor foundation; high embankment or low headroom; uncertain flow | RCC; rigid frame spreads the load over the base; also good in seismic areas; formwork is costly. |
| Slab | Small to medium span (up to about 6 m); firm foundation or rock; stable banks; little height available | Open waterway, easy to clean; needs abutments and wing walls. |
| Arch | Hill roads where stone is plentiful; high fill; strong abutments and good foundation; aesthetic needs | Masonry in compression, long life; slow to build; skilled labour needed. |

General rules: use a pipe where the flow is small, a slab where a clear opening is needed, a box where the soil is weak, and an arch where stone is available and the foundation is rock or hard soil. The culvert should follow the natural stream line, with minimum skew, and have a gradient that prevents silting.

## Check yourself

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

1. Culverts have a clear span up to about:
   A. 1 m
   B. 6 m
   C. 30 m
   D. 60 m
   Answer: B. Spans above about 6 m are minor bridges. (Key point: Culvert span)

2. Which condition best suits a box culvert?
   A. Soft or poor foundation, high embankment, uncertain flow
   B. Rock foundation with little discharge
   C. Very small flow in flat terrain
   D. Hill road with plentiful stone
   Answer: A. The rigid RCC frame spreads load over the base. (Key point: Box culvert suitability)

3. When is an arch culvert suitable?
   A. Soft soil and low headroom
   B. Hill roads with plentiful stone and strong abutments on good foundation
   C. Flat terrain with small flow
   D. Shallow wide streams
   Answer: B. Masonry arch acts in compression, needs skilled labour and is slow to build. (Key point: Arch culvert)

4. What is true of pipe culvert?
   A. Needs no cover
   B. Min diameter about 600 mm, cover 0.6-1.0 m, can choke with debris
   C. Used for large spans
   D. Open waterway easy to clean
   Answer: B. Pipes suit small, steady discharge; multiple pipes for larger flows. (Key point: Pipe culvert)

## Related in Transportation Engineering I

- [Road classification as per NRS 2070](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aroad-classification-nrs) — Transportation Engineering I, chapter 1, asked 13×
- [Cross drainage structures and types](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Across-drain-types) — Transportation Engineering I, chapter 4, asked 3×
- [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×
- [Classification of highway drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aclassify-highway-drainage) — Transportation Engineering I, chapter 4

## Related in other subjects

- [Classification for subgrade suitability](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asubgrade-suitability-classification) — Soil Mechanics, chapter 3
- [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×
- [Clogged drainage pipe soil volume](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclogged-drainage-pipe-soil-volume) — Soil Mechanics, chapter 6
- [Cross drainage works favourable in hilly areas](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acd-works-hilly-areas) — Irrigation and Drainage Engineering, chapter 8