Freeboard

Total depth of gutter minus the designed water depth.
File:File:Freeboard-Part1.jpeg
File:File:Freeboard-Part2.jpeg

In accordance to BS EN 12056-3, (Clause 5.2 Valley and parapet gutters Table 5 Minimum freeboard for valley and parapet gutters)
File:Freeboard-forValleyandParapetGutters.jpeg

In accordance to (SG Gov.) PUB Code of Practice on Surface water drainage, Freeboard refers to the depth from the top of the drain (cope/ bank) to the top of the water surface in the drain at design flow condition. Sufficient freeboard shall be provided to prevent waves or fluctuation of the water surface from overflowing the cope/ bank. Generally, a depth of freeboard equivalent to 15% of the depth of the drain is required.

References

1. BS EN 12056-3:2000
2. Code of Practice on Surface Water Drainage (p.43)