Cross drainage structures and types

Cross-drainage structure

A cross-drainage structure is a structure built across or under the road to carry water from one side of the road to the other. It lets streams, gullies and the drainage collected from the road pass the alignment without damaging the road.

Types and application

The main types, in increasing order of discharge and span, are listed below (IRC/NRS 2070).

1. Pipe culvert

Concrete (Hume) or steel pipes laid under the embankment in one or more barrels. Minimum diameter 600 mm (900 mm preferred for ease of cleaning), minimum cover about 0.6 to 1.0 m. It is cheap and quick to build, but its capacity is limited and it may choke with debris.

 ====== road level ======
 ====== fill (cover) ====
    ___________
   (  pipe     )  -> flow
    -----------
 inlet headwall   outlet apron

2. Box culvert

RCC rectangular section, single or multiple cells. The top slab acts together with the walls and the base slab, so the load is spread over the soil. It suits soft ground and large flows and needs little cover.

 ====== road ======
 |  _____________  |
 | |      |      | |  <- 2 cells
 | |______|______| |
 |_________________|  base slab

3. Slab culvert

RCC slab deck on masonry or concrete abutments with wing walls. The waterway is open, so it is easy to clean. It is used where the bed is firm and the stream needs a clear span of up to 6 m.

   ____deck slab_______
  |                    |
  |abutment    abutment|
  |_____waterway_______|

4. Arch culvert

Masonry or concrete arch, which acts mainly in compression and so uses stone effectively, but needs strong abutments and a good foundation.

5. Causeway (drift) and Irish bridge

The road is carried across the stream at bed level, with or without small pipes (vented). The road is submerged during floods. It is used for broad streams that rise only for short periods and where a bridge is too costly. Protection works (aprons and cut-off walls) are needed.

 road ----\__________/---- road
    (dip)   vents  ==  low-water flow
     flood level ~~~~~~~~~~~~~

6. Minor bridge

Used when a culvert cannot pass the discharge; the length is above 6 m and up to about 60 m (IRC). It is designed for the HFL with adequate free board (soffit clearance) as per IRC/NRS bridge standards.

7. Aqueduct and siphon

An aqueduct carries a canal across a drainage channel or road; an inverted siphon carries drainage water under a canal or a road in a pipe running full.

Selection depends on discharge, stream slope, nature of the bed and foundation, height of embankment, availability of materials, and cost.