Classification of falls

The falls can be classified into the following four categories according to the approach conditions.

1. Falls which maintain relation between depth and discharge.

2. Falls which maintain level of U/S water constant.

3. Combination of fall and regulator.

4. Miscellaneous falls.

In such falls there is neither drawdown nor heading up of water on U/S side.
The Depth-discharge relationship for the channel is maintained.
Trapezoidal notch falls and low crested rectangular notch falls, fall under this category.

Siphon fall or Siphon spillway high crested weir fall are the falls which fulfil this condition.
Such falls cause silting on U/S side. Such falls are necessary under following circumstances.
(i) When a sub-channel has to take off at some distance U/S of the fall.
(ii) When Hydro-electricity is being generated using drop in the fall.

In such falls the level of water on U/S side, can be varied at will.
The regulation is done with the help of vertical needles, horizontal stop logs and sluice gates.
They are used as regulators also.
They are installed at locations where some sub-channel is to take off from the parent channel.
And also fall is to be provided on the parent channel.
The falls of this category are exclusively the rectangular notch falls fitted with sluice gates.

The falls of this category suit to some specific conditions.
Well or cylindered fall is one such fall.
It consists of a vertical well connected to D/S side with the help of a horizontal pipe.
Water enters the vertical well from U/S side and escapes into Tail water on D/S side.
The energy is dissipated by turbulences in the well.
They are quite suitable for low discharges but large drops.
Fall may be inform of a steep sloped channel or pipe.
The U/S water surface and D/S water surface are connected by steeply sloped open channel or closed pipe.