Chapter 3: BRIDGES 

B. Insufficient Flow Capacity (Decks)

Introduction

Problem: Damage to bridge decks and associated superstructures (railings and truss) as a result of overtopping due to insufficient capacity for flow through the bridge opening.

Mitigation Objective: To prevent damage to bridge decks and associated superstructures by increasing the design capacity for the bridge opening, and/or modifying the bridge deck design to allow for controlled overflow.

B.1 Elevate the Bridge Deck

The bridge deck and associated superstructure should be elevated to a level sufficient to pass anticipated flood flows. Approach sections to the bridge may likewise need to be raised

Effectiveness:

• Most effective mitigation for passing flood flows.

Limitations:

• The pier and abutment supports may need to be redesigned to accommodate and support the elevated bridge deck and associated superstructure

Considerations: 

B.2 Replace a Steel Truss Bridge With an Open Deck Bridge

Replacing a steel truss bridge with an open deck bridge will reduce the backwater conditions upstream, and eliminate the accumulation of debris should the bridge become over-topped during flood events.

Effectiveness:

• Generally very effective
• An open deck bridge does not trap floating debris to the same extent that a steel truss bridge will when overtopped
• effectiveness is increased if open deck bridge is elevated

Limitations:

• Bridge piers and abutments may require extensive redesign to accommodate open deck bridge

Considerations: 

B.3 Replace Multi-Spans With a Single Span Bridge

Replace the multiple spans of a bridge with a single clear-span to eliminate the need for piers. This will increase the flow through the bridge and reduce upstream backwater conditions.

Effectiveness:

• Very effective
• Increases the effective size of the bridge opening and flow capacity.
• Reduces debris accumulation.
• Decreases the backwater conditions upstream from the bridge and the effects of drawdown through it.
• Consider relief openings, wingwalls, realignment of piers and abutments, embankment slope protection, and abutment debris deflectors for maximum effectiveness.

Limitations:

• Length of span may be limited by strength of materials.

Considerations:

B.4 Increase Bridge Opening Size

Increase the size of the bridge opening(s) by lengthening the opening or raising the bridge deck. Increasing bridge opening size will decrease any backwater conditions upstream from the bridge and reduce the effects of drawdown through the bridge.

Effectiveness:

• Very effective.  Particularly effective where damage was caused by overtopping of the bridge due to excessively high water surface elevations upstream or by excessively high water velocities eroding the pier and abutment foundations.
• Degree of effectiveness varies with the difference of the water surfaces upstream and downstream from the bridge, and with the water velocities through the bridge
• Consider relief openings, wingwalls, realignment of piers and abutments, embankment slope protection, and debris deflectors for maximum effectiveness.

Limitations:

• Crossing and stream channel geometry may preclude this option.

Considerations: 

B.5 Construct a Relief Opening

Construct one or more relief openings through the road prism at a location that will carry excess floodwaters. The relief opening may be a culvert or bridge, or multiple culverts or bridges. The openings should be located at natural side channels and in line with heavy flow areas located on the stream overbanks. (See "Culverts-Plugging,")

Effectiveness:

• Generally very effective, particularly if combined with appropriate culvert and/or bridge entrance and outlet treatments.
• Consider wingwalls, embankment slope protection, and debris deflectors for maximum effectiveness.

Limitations:

• Geometry of drainage area may preclude this option.

Considerations:  

B.6 Construct Bridge Wingwalls

(SEE 3.A.1)

B.7 Install A High Water Overflow Crossing

(SEE 2.A.8)

B.8 Realign Piers & Abutments

(SEE 3.A.4)

B.9 Install Additional Bridge Openings or Spans

(SEE 3.A.3)