Cross drainage works favourable in hilly areas

Definition (1 mark)

A cross-drainage structure is a structure built where an irrigation canal crosses a natural drain or stream, to carry the canal and the drainage water across each other without mixing or damage.

Types (2 marks)

1. Aqueduct (canal over drain, drain flows freely).
2. Siphon aqueduct (canal over drain, drain flows under pressure).
3. Super-passage (drain over canal, canal flows below with a free surface).
4. Canal siphon (drain over canal, canal under pressure).
5. Level crossing (canal and drain at the same level, with regulators).
6. Inlet and outlet (drain enters or leaves the canal).

Type favourable in the hills of Nepal (1 mark)

The aqueduct (a canal flume or trough carried over the drain) is the most favourable. In the hills, canals run along the hillside and cross many small, steep, deeply cut streams (khola) whose beds are well below the canal bed. An aqueduct:

• Passes sudden, high-velocity floods with boulders and debris freely under the structure.
• Needs no floor or cut-off in the drain bed, which is often rocky.
• Avoids the silting and choking of barrels that troubles siphon types.
• Is simple and cheap in stone masonry or RCC, and keeps the canal supply safe from floods.

Where a small gully or landslide channel crosses above a canal that is cut into the hillside, a super-passage (or a covered canal section) is used instead.