Air draft
Image:Allanburg Bridge.jpg|thumb|The deck of the Allanburg Bridge on Canada's Welland Canal typically rests only a few metres above the water level. When a ship approaches, the deck is raised to provide sufficient air draft for the vessel to pass through. This bridge was involved in a collision with a lake freighter in 2001 as a result of lowering the span before the ship fully cleared the bridge.
Air draft is the vertical distance from the surface of the water to the highest point on a vessel. This is similar to the deep draft of a vessel which is measured from the surface of the water to the deepest part of the hull below the surface. However, air draft is expressed as a height, while deep draft is expressed as a depth.
Clearance below
The vessel's clearance is the distance in excess of the air draft which allows a vessel to pass safely under a bridge or obstacle such as power lines, etc. A bridge's "clearance below" is most often noted on charts as measured from the surface of the water to the underside of the bridge at the chart datum Mean High Water, a less restrictive clearance than Mean Higher High Water.In 2014, the United States Coast Guard reported that 1.2% of the collisions that it investigated in the recent past were caused by vessels attempting to pass under structures with insufficient clearance resulting in bridge strikes.
Examples
The Bridge of the Americas in Panama limits which ships can traverse the Panama Canal due to its height at above the water. The world's largest cruise ships,, and the will fit within the canal's new widened locks, but they are too tall to pass under the bridge, even at low tide. New vessels are rarely built not clearing, a height which accommodates all but the largest cruise and container ships.The Suez Canal Bridge has a clearance over the canal.
The Bayonne Bridge, an arch bridge connecting New Jersey with New York City, undertook a $1.7 billion modification to raise its roadbed to.