For water rights / irrigation flows, freeboard is the distance between the water surface and the top of the canal.
Freeboard is necessary or the channel can overtop, with water spilling out and flood the surrounding lands. Without freeboard, channels that experience unanticipated surges in flow, wave action, or turbulence can overtop.
For Parshall flumes, the maximum flow depth is capped on the flow tables so that there is generally 6-inches [15.24 cm] of freeboard. For other flumes, freeboard is recommended to be 1/6th of the flume’s depth.
Keep in mind that overtopping is NOT flood irrigation. Overtopping is uncontrolled and is generally damaging and not beneficial to crops. Overtopping can also damage water distribution infrastructure.