Short note: aqueducts and inverted siphon

Aqueduct

An aqueduct is a structure that carries a canal or open channel over a natural drainage, river, or road. The canal bed is above the High Flood Level (HFL) of the drainage. The canal flows in a trough supported on piers or abutments, and the drainage water passes below, like a bridge for water. Where a road meets an irrigation canal, an aqueduct (canal over road) lets the road pass freely beneath.

Types: (i) a canal trough of RCC/steel on piers (like a bridge); (ii) a siphon aqueduct, where the HFL of the drain is above the canal bed, so the drain flows under pressure through barrels beneath the canal.

   canal trough ===============   (canal over drain)
   ============================
   |pier|     drain flows     |pier|
   |____|~~~~~~~~~~~~~~~~~~~~~|____|

Inverted siphon

An inverted siphon is a closed conduit (pipe or barrel) that runs below an obstacle: the water goes down, runs full under pressure, and rises again on the other side. It is used where a drain, stream or canal must cross a road, canal or valley and there is not enough head to pass above it. In road drainage, a drain that passes beneath a canal or road at a level below its hydraulic gradient line forms an inverted siphon.

  inlet |\               /| outlet
        | \_____________/ |    <- pipe full, under
        |  (below road/canal)    pressure

• Velocity should be 2 to 3 m/s to prevent silting.
• Needs a silt trap and an air vent.
• Head loss must be checked.

An aqueduct is chosen when the canal bed is well above the HFL of the drain. An inverted siphon is used when the obstacle (canal or road) lies across the water's path at or above its water level, so the water must pass beneath it under pressure.