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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 

D. Scour (Piers & Abutments)

 

Introduction

Problem:

Damage to bridge piers and abutments resulting from scouring of the streambed along and under their footings.

Mitigation Objective:

Reduce flood flow velocities along bridge piers and abutments, thereby eliminating scouring of the streambed along and under their footings.

D.1 Increase Footing Depth

The depth of pier and abutment footings should be extended below the expected depth of streambed scour or to bedrock. The expected depth of scour depends on the flood flow velocities along the footing and the nature of the streambed materials.

image053.png (38786 bytes)

Effectiveness:

  • Very effective, particularly when flood flow velocities are relatively high.
  • Consider flow deflectors, debris deflectors, or replacing multi-spans with a single span for maximum effectiveness.

Limitations:

  • The depth of pier and abutment footings may be limited by streambed characteristics.
  • Footings should be inspected periodically after floods for streambed erosion.

Considerations:  

D.2 Install Flow Deflectors

Install "V" shaped flow deflectors on or immediately upstream from the upstream sections of piers and abutments to reduce flow velocities and protect footings from scouring. Install a concrete collar on lower section of piers immediately above the footing. Also extend lower sections of abutments and the wingwalls, if present. This will assist in deflecting flood flows away from the piers and abutments, and will eliminate streambed scour along and under them.

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Effectiveness:

  • Flow deflectors are very effective, particularly for flood flows with high velocities.
  • Pier collars and abutment sub-walls are moderately effective
  • Pier collars and extended abutment and wingwalls may provide additional protection from impact of rocks and debris.

Limitations:

  • Flow deflectors should be inspected periodically after floods for impact damage and for streambed erosion.

Considerations:

D.3 Install Semicircular or Triangular Endnoses

Semicircular or triangular endnoses may be installed on the upstream ends of the piers to redirect flood flow velocities. Pier endnoses are a protection measure, such as sheet metal attached to the pier to redirect flow. Endnoses should also be designed to both prevent debris accumulation and to protect the piers and abutments from floating debris impact.

image059.png (32481 bytes)

Effectiveness:

  • Moderately effective where flood flow velocities are relatively high.
  • Less effective when flood flow velocities are relatively low.

Limitations:

  • Piers should be inspected periodically after floods for impact damage and for streambed erosion.
  • Bridge decks need to be high enough to pass floating debris.
  • Any debris that accumulates in the bridge opening needs to be removed during the flood or immediately after the flood peak has passed

Considerations:

D.4 Construct Bridge Wingwalls.

(see 3.A.1)

D.5 Realign Piers and Abutments.

(see 3.A.4)

D.6 Replace Multi-Spans With Single Span Bridge.

(see 3.B.3)

D.7 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