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WEB-BASED HANDBOOK (Legacy Edition). CLICK HERE to submit comments.
Because of staff changes and Randolph Langenbach's retirement from FEMA, these handbooks were never published on the FEMA website.  These are the only copies available.


Chapter 3: BRIDGES 

B. Insufficient Flow Capacity (Decks)


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

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  • Most effective mitigation for passing flood flows.


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


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.

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  • 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


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


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.

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


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.

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  • 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.


  • Crossing and stream channel geometry may preclude this option.


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,")

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  • 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.


  • Geometry of drainage area may preclude this option.


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)



NOTE:  None of the mitigation measures in these Handbooks should be considered ‘pre-approved’ or otherwise automatically eligible for FEMA funding. Only FEMA staff can determine eligibility, once they have determined that an applicant is eligible and they have reviewed a project proposal.

FEMA HAZARD MITIGATION HANDBOOKS                                                                        Updated: June 13, 2002