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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 2:  CULVERTS

D. Misalignment

Introduction

Problem:

Damage to a culvert caused by its horizontal and/or vertical misalignment with the stream channel and subsequent erosion of the embankment. The misalignment may be a result of original design miscalculations and/or subsequent stream migration.

Mitigation Objective:

To prevent future damage to a culvert by aligning the culvert to the axis of the stream; to prevent future migration of the stream away from the culvert; and to design supplementary drainage structures to accommodate future migration of the stream channel.

D.1.  Install Additional Culverts

Locate additional culverts at previous and/or potential stream alignments at road crossing site. The additional culverts should be located some distance from the mainstream culvert.

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Effectiveness:

Limitations:

  • Potential for sedimentation of culverts that are not carrying higher floodwater velocities.
  • Consider varying the culverts' elevations.

Considerations:

D.2.  Realign Culvert

Align centerline of the culvert (either vertically or horizontally) to the centerline of the stream to eliminate erosion along the embankment and subsequent damage to the culvert. Alignment may require relocating culvert to present location of stream channel.

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Effectiveness:

Considerations:

D.3.  Install Approach Berms

Install approach berms on the stream overbanks. Berms should be aligned so that flow is directed into and at the same angle as the culvert and away from the embankment.

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Effectiveness:

Limitations:

  • Overbank geometry may preclude this option.
  • Berms need to be placed on the stream overbanks near or at the edge of the floodway so that water-surface elevations are not significantly increased at the culvert or upstream.

Considerations:

D.4.  Install Flow Diverters

Install flow diverters (barbs) in the stream. Design barbs to redirect the flow away from the embankment and into the culvert.

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Effectiveness:

  • Particularly effective during relatively low flows.
  • May not be effective at high flows.
  • May help prevent channel migration at all flow levels.
  • Consider culvert endwalls, wingwalls, energy dissipaters, and debris barriers for maximum effectiveness.
  • Consider using natural materials such as root balls or anchored logs.

Limitations:

  • Requires additional stream embankment slope protection for high flows.
  • Design should include measures, such as sheet steel piling, to eliminate wash-out of landward end of barbs.
  • Height of barbs should not cause significant increase in water-surface elevations for high flows at or upstream of the culvert.

Considerations:    

D.5.  Realign the Stream Channel

Channel flow should be directed into and at same angle as the culvert and away from the embankment to reduce erosion along the embankment and consequent damage to the culvert.

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Effectiveness:

Limitations:

  • Stream may reclaim original channel over time due to natural meandering.

Considerations:

D.6  Install Appropriate Culvert End Sections

(SEE 2.C.3)

D.7  Place Riprap       

(SEE 1.B.5)

 

 

 

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