Why a canal fall; site selection factors

Why a fall is constructed

The ground level of a command often slopes more steeply than the permissible bed slope of the canal. If the canal were laid at the steeper ground slope, its velocity would exceed the non-scouring limit and the bed and banks would erode. If it were laid at the permissible (flatter) slope, it would rise above the ground downstream and run in high, costly embankment (filling). A fall (drop) lowers the canal bed at one point in a controlled way and dissipates the surplus energy safely. It therefore:

1. Prevents scour by keeping the bed slope within the permissible value.
2. Avoids excessive filling and keeps the canal near balancing depth, so earthwork is economical.
3. Keeps the d/s FSL just above the ground level, so the command is maintained.
4. Can be combined with other works (bridge, regulator, outlet, flume or power plant).

Factors for the site selection of a fall

1. At the point where the canal would otherwise come into excessive filling, while the d/s FSL after the drop still stays above the ground level (so command is not lost).
2. Near a distributary off-take or a cross regulator, so that the structures can be combined.
3. Near a bridge or road crossing, so that one structure serves two purposes.
4. Good, stable foundation soil (rock or hard soil) with adequate bearing capacity; avoid loose or expansive soils.
5. Where the cost is lowest: a few moderate drops are usually more economical than many small ones.
6. At the head of a steep reach, so that the canal below has a flatter slope.
7. Away from bends or curves; the approach should be straight.
8. Close to the construction site and material sources, for lower cost.