Table of Contents
FREE OVERFALL OR STRAIGHT DROP SPILLWAY
- 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.