Short note: culverts

A culvert is a short cross-drainage structure with a clear span of up to about 6 m (IRC/NRS 2070) that carries a small stream or the water from side drains below the road embankment. Larger structures are called bridges.

Components

• Waterway (barrel / opening), abutments and piers, deck slab or arch, wing walls and return walls, foundation and bed protection (apron), headwalls, parapet and wearing surface.

Types

1. Pipe culvert: Hume (RCC) or steel pipes, single or in rows. Cheap and easy; for small discharge.
2. Box culvert: RCC rectangular cells; for larger discharge and weak soil.
3. Slab culvert: RCC slab on abutments; open waterway.
4. Arch culvert: stone or concrete arch; for hill roads.

 road surface =====================
 ====deck slab=====================
 |abutment|    waterway   |abutment|
 |________|_______________|________|
          apron / bed protection

Location and design points

• Place along the natural stream line, at right angles to the road if possible.
• Provide a bed gradient of 0.5 to 2% to avoid silting and scour.
• Find the design discharge by the rational method or the Dickens or Ryves empirical formulas, with a return period of about 25 years for culverts (and a check for a higher flood).
• Provide the required minimum cover, inlet and outlet protection, and a free board.
• Culverts are spaced more closely on hill roads (every 100 to 250 m) to avoid long drain runs.

Culverts are generally cheaper and simpler than bridges, but they must be cleaned regularly to avoid blockage by debris.