Royal Navy port dredging in the 1920s

How deeply did the Royal Navy dredge port areas intended for destroyers and cruisers in the 1920s?

Usually it did little special dredging because existing ports were deep enough; larger later ships and fewer active bases created the greater infrastructure problem.

The rule of thumb offered is to choose water depth equal to the ship's draft plus another 50 percent for clearance. Most Royal Navy destroyers and cruisers in the 1920s could use existing ports without substantial dredging, and a broad base network gave the fleet alternatives. Dredging became more pressing as ships grew and the number of available bases declined. Later aircraft-carrier access at Portsmouth supplies the contrasting example, with channel work and the removal of obstructions associated with much larger ships.