Rapids, drop structures, causeways, siphons

1. Rapids

A rapid is a steep, long, rough-bedded channel (a chute) used to carry a drain down a steep slope over a long distance. Roughness elements (boulders, cross ribs) keep the velocity within the non-scouring limit. It ends in a stilling basin.

 drain \___rough bed___ \
                         \ chute (slope steep)
                          \____stilling pool__

2. Drop structures

A drop structure (stepped drain or cascade) lets the water fall vertically in one or more steps where the ground is too steep for a normal gradient. The jump forms in a lined pool at the foot, dissipating energy. It protects the drain from scour and erosion.

 ______
       |  step 1
       |____
            |  step 2
            |____ pool

3. Causeways

A causeway (drift or Irish bridge) is a road crossing built at or just above the stream bed, allowing floods to flow over the road. It may be unvented (a ford, with paving only) or vented (with pipes or small openings for low flows). It is cheap and used on low-traffic roads across wide, shallow streams whose floods are short and rare, but traffic is blocked during floods. It needs aprons and cut-off walls on both sides.

 road ---\_____ low-level slab _____/--- road
          =vents=  <-- low flow

4. Inverted siphon

An inverted siphon is a closed pipe or conduit that dips below a road, canal or valley and rises again, flowing full under pressure. It carries drainage water across depressions or under another structure where it cannot pass above. It needs a velocity of about 2 to 3 m/s to prevent silting and a silt trap/air vent.

 inlet \                  / outlet
        \________________/
          (pipe full, pressure flow)