Understanding Bridge Clearance: Free Board

When designing bridges that cross rivers or other water bodies, it's crucial to ensure there is enough vertical space between the water level and the underside of the bridge structure. This space is important for navigation (if applicable) and to prevent the bridge from being damaged by floodwaters carrying debris or exerting pressure on the superstructure.

The term for this critical vertical distance between a specific high water level and the bottom of the bridge superstructure is known as the free board.

Let's look at the key components:

• High Flood Level (H.F.L): This represents the highest level the water is expected to reach during a flood event, based on historical data and hydrological studies.
• Bottom of the Bridge Superstructure: This is the lowest point of the bridge deck or girders that span the water body.

The distance measured vertically from the High Flood Level (H.F.L) up to the bottom of the bridge superstructure is the free board. It acts as a safety margin above the anticipated maximum water level.

Why Other Options Are Not Correct

• Sluice level: This term relates to the water level controlled by a sluice gate, typically in irrigation systems or channels, not the clearance under a bridge.
• Full tank level: This refers to the maximum water level a reservoir or tank is designed to hold. It is not related to bridge clearance over a river.
• Top bund level: This refers to the height of the top of an embankment or dyke, usually built to contain water or protect an area from flooding. It is not the clearance under a bridge.

Therefore, the correct term for the distance between the bottom of the bridge superstructure and the High Flood Level (H.F.L) is free board.