What is fall and its necessity

It is a structure constructed for the object of lowering the bed level of the canal suitable is called fall. change in elevation of water level converts potential energy into kinetic energy which increases the velocity of flow and as a result, it may damage the downstream of the canal by scouring.
The Natural surface may not always have uniform flow so the canal is constructed as per design conditions with a fixed slope given to the bed of the canal. If the natural surface falls rapidly then the canal bed goes high and ultimately canal comes filling. It increases the construction cost and after that, we need to bring the canal bed level down up to such a balancing depth. so here falls are necessary to construct.