freeboard: one construct, many systems

In naval architecture, freeboard is the vertical distance from the waterline up to the freeboard deck at the side of a hull, the measure of how much hull stands clear of the sea. It represents reserve buoyancy and the margin against waves coming aboard, which is why minimum freeboard is set by load line (Plimsoll) rules that mark the deepest a ship may legally be loaded in given conditions. Rawson and Tupper's Basic ship theory treats freeboard and the load line. The construct is a safety height margin between a water surface and the top of a structure that must not be overtopped, the same idea that reappears in the design of dams and channels.[1]

Unit

Vertical distance from the waterline to the freeboard deck

Recovery means

Not applicable

Measured by

Set against assigned load-line marks and reserve-buoyancy requirements for the service condition

In hydraulic and dam engineering, freeboard is the vertical distance between the maximum design or still-water level and the crest of a dam, levee, or channel, the margin kept to stop waves, surges, and run-up from overtopping the structure. It is sized from wind-driven wave height, run-up, and a safety allowance, because overtopping of an embankment can erode and breach it. Novak and colleagues' Hydraulic structures sets out freeboard allowances for dams and channels. This is the same construct as a ship's freeboard, a reserve height between a water surface and a crest that must stay dry, transposed from a hull to a fixed water-retaining structure.[2]

Unit

Vertical distance between design water level and the crest of a dam, levee, or channel

Recovery means

Not applicable

Measured by

Computed from design wave height, wind set-up and run-up, and a prescribed safety allowance above maximum water level