Freeboard in Dams

 Freeboard

Free Board is the vertical distance between the top of the dam and the still water level. Freeboard is computed from the following two considerations:

Wave height considerations

It is equal to wind set up plus 1 1/3 times the wave height above FRL or above MWL (corresponding to design flood) whichever gives higher dam top level. A minimum freeboard of 1m above MWL corresponding to design flood shall be available. If design flood is not equal to PMF then the top of dam should be at least equal to MWL corresponding to PMF. At least 1m high solid parapet is to be provided, not withstanding the above requirements.

Wind velocity generally assumed as below in absence of meteorological data:

For FRL condition - 120 km/hr

For MWL condition - 80 km/hr

T. Saville’s method as given in IS:6512-1984 is used for calculating the wave height/freeboard.

Operation considerations

IS:11223 specifies the following:

The freeboard as specified in IS: 6512 shall be available at FRL and MWL corresponding to all bays operative condition. For gated spillways a contingency of 10% of gates (min. one gate) being inoperative is considered as an emergency. A reduced freeboard may be acceptable under the emergency condition. The dam shall not be allowed to overtop in any case.