• 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.

1. Falls in which D-Q relationship is maintained.

  • 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.

2. Falls which maintain fixed F.S.L. on the U/S side of the fall.

  • 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.

3. Combination of fall and regulator.

  • 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.

4. Miscellaneous type falls.

  • 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.