• It is a low height weir, whose D/S face is either vertical or nearly vertical.
  • In this spillway the flow drops freely from the crest.
  • The under side of the falling nappe is sufficiently ventilat to prevent a pulsating jet.
  • The crest is also sometimes extend in the form of an over hanging slab on D/S side so as to keep the nappe of falling water away, from coming in contact with the D/S surface of the spillway.
  • The falling jet of water hits the D/S floor almost vertically and thus lot of energy is generat due to impact.
  • This energy can be dissipat by adopting any of the following measures.
  • Provide concrete apron for some length on the D/S side.
  • Develop a small pool of water, below the falling jet of water, by constructing a small secondary dam at some distance on the D/S side.
  • Impact blocks of small heights are construct on the D/S side.
  • The energy is dissipated by means of turbulences caused by the impingement of the flow against the impact blocks.
  • By developing hydraulic jump. This type of spillway is recommend for large heads.

Fig 145.3. Free overall or straght drop spillway.


Fig. 14.4. Straight drop spillway with over-hanging lip.

  • The difference between ogee type and free overfill spillway can now be easily understood.
  • In case of free overfall spillway, the water jet falls clearly away from the D/S face and space between D/S face and underside of the jet is ventilat.
  • In ogee spillway nappe of falling water remains in touch with the D/S surface of the spillway.

Fig. 14.5. Side channel spillway.