Submerged and unsubmerged minor bridge

A minor bridge has a total length of about 6 to 60 m (IRC). Its hydraulic behaviour depends on the flood level relative to the bridge deck.

Un-submerged (free-surface) condition

The High Flood Level (HFL) stays below the underside (soffit) of the deck, with a free board (vertical clearance). As per IRC:5, the clearance increases with the discharge: from about 150 mm for very small discharges, to 600 mm at moderate discharges, and 1.2 to 1.5 m for large discharges, with more allowed for floating debris. The flow under the bridge is open-channel flow with a free surface, and the road is not overtopped. This is the normal design condition.

Design: the afflux (rise in water level upstream due to the contraction), the velocity and the scour depth are computed (scour by Lacey's regime theory), and the foundation is taken below the scour level.

Submerged condition

The water level rises above the deck soffit or even over the road. The flow then becomes pressure (orifice) flow under the deck, or flow over the deck when it is overtopped. The structure is then subject to hydrostatic and drag forces, buoyancy and uplift, debris impact, and heavy scour. The road may be damaged and traffic is interrupted.

A submerged bridge (a "submersible bridge" or a vented causeway) is accepted only on low-traffic roads where floods are rare and of short duration, as it is much cheaper than a high-level bridge. It is designed with:

• strongly anchored decks (to resist uplift),
• a smooth soffit and rounded edges,
• no parapet (or collapsible railings),
• heavy bed protection and downstream aprons.

 Un-submerged                Submerged
 ____deck____ soffit          ~~~~~~~~~~~ HFL
 ~~~~~ HFL ~~~~ clearance     ____deck____