In an open channel, freeboard is the head measured from the maximum water level to the uppermost watertight portion of the surrounding channel.
Freeboard is an important consideration when evaluating the impact of the installation of a flume or weir. Weirs in particular require considerable head to ensure that the nappe can freely spring from the weir crest.
For steep channels, it is preferable to have the height of the freeboard equal to the flow dpeth to account for high varitions of swift flow induced by waves, surges, and splashes.
Freeboard is a hedge against overtopping the channel and its always advisable to ensure there is freeboard in an installation. In general, the recommended amount of freeboard is 1/6 of the channel depth - this typically allows provides enough room so that wave action or flow surges don't overtop the channel.
Sources: Civil Engineering, FAO
