bascule bridge height

[cal40john]

NOAA chart 18447 shows the Montlake Bridge height as 32 feet, 48 at center. The sign on the bridge says 46 feet.

The University bridge has 30 feet, 45 at center on the chart. The sign on the bridge says 42.5 feet.

Chart 18447

These bridges are on a lock/dam controlled waterway. Coast Pilot states bridge clearance is based on mean water level, at 21 feet above MLLW, and depths are based on low water of 20 feet above MLLW. So there is a 2 foot range of water level.

Any guesses? or links to how bridge clearances are determined, like how much of the center span is used?

John