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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

E. Debris Impact (Piers & Abutments) 

 

Introduction

Problem:

Damage to bridge piers and abutments resulting from the impact and accumulation of debris.

Mitigation Objective:

To prevent future debris damage to bridge piers and abutments by directing debris around and away from them, by providing clear passage of debris through the bridge opening, and by minimizing amount of debris catching on the structural elements of the bridge

E.1 Install Debris Deflectors

Debris deflectors or debris fins should be installed on the upstream ends of piers and abutments and angled so as to direct floating debris into areas of high flood flow velocities. The debris deflectors and fins should be "V" shaped and extend upstream a sufficient distance to orient the floating debris for easy passage through the bridge. Debris deflectors or fins should be designed to both prevent debris accumulation and to protect the piers and abutments from floating debris impact.

image063.png (29047 bytes)

Effectiveness:

  • Very effective in areas that have significant debris loading in the upstream drainage and flood flow velocities are high.
  • Less effective when flood flow velocities are low.

Limitations:

  • Bridge decks need to be high enough to pass floating debris.

Considerations:

E.2 Install Batters

Install batters (steel plates) on the upstream ends of concrete piers with semicircular or "V" shaped endnoses, or on wingwall ends and wingwall/abutment junctions to protect them from the impact of floating debris.

image067.png (28905 bytes)

Effectiveness:

  • Very effective in protecting piers from debris impact damage

Considerations:

E.3 Replace Wood Pile Bent Pier Structure With

Solid Concrete Column Pier

Replace a pier constructed with wooden piling with a solid concrete column pier. This measure will prevent debris from becoming caught and accumulating in the pile bent pier configuration, and will protect the pier from debris impact.

image070.png (20260 bytes)

Effectiveness:

  • Very effective in areas that have significant debris loading in the upstream drainage
  • effectiveness increases with debris deflectors or debris fins, semicircular or "V" shaped endnoses, and/or batters.

Considerations:

E.4 Construct Debris Catchments

Debris catchment structures, such as debris barriers (trash racks) or low height dams, may be constructed on small tributary streams upstream from the bridge. The catchment structures should be designed to trap debris while passing the stream flow. If a debris catchment dam is constructed, it must include an emergency spillway.

image073.png (25089 bytes)

Effectiveness:

  • effective where the source of debris is from highly vegetated drainage areas upstream from the bridge and where there are adequate storage areas upstream from the catchment structures.
  • Less effective on larger tributary streams.

Limitations:

  • Any debris that accumulates upstream from the catchment structures needs to be removed during the flood or immediately after the flood peak has passed

Considerations:  

 

E.5 Install Semicircular or Triangular Endnoses.

(SEE 3.D.3)

E.6 Construct Bridge Wingwalls.

(SEE 3.A.1)

E.7 Realign Piers and Abutments.

(SEE 3.A.4)

E.8 Replace Multi-Spans With Single Span Bridge.

(SEE 3.B.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