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.
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.
