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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 2:  CULVERTS

C. Embankment Erosion

Introduction

Problem:

Damage or failure of a culvert caused by erosion of the embankment at its entrance and/or outlet, or around the outside of the culvert. The embankment erosion and subsequent culvert damage or failure may be a result of inadequate culvert end sections.

Mitigation Objective:

To prevent future damage to a pipe culvert by adding appropriate end sections.

C.1.  Shape Culvert Entrance

Shape culvert entrance (bevel/skew) to match the embankment slope or stream alignment. Culvert efficiency will be increased, and turbulence at the entrance and through the culvert will be decreased, reducing erosion of the bank.

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Effectiveness:

Limitations:

  • Cutting a culvert to bevel or skew its entrance may weaken a large diameter culvert's ability to resist ring compression; flanges may be required to stiffen it.

Considerations:

C.2.  Construct a Cutoff Wall

Construct sheet steel, low height, cutoff wall to prevent undermining of culvert entrance. The sheeting should extend below the culvert entrance to at least one half times the culvert diameter and above the culvert entrance to a point where it meets the junction of the embankment with other end treatments.

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Effectiveness:

Limitations:

  • Streambed geology may prohibit installation of cutoff wall.

Considerations:

C.3.  Install Appropriate Culvert End Sections

Construct an endwalls, wingwalls, and/or flared end section to direct flow into and out of the culvert and protect embankment slopes. Straight or "U" shaped endwalls or flared wingwalls may be used when the centerline of the stream is aligned with the culvert. A "U" shaped endwall or wingwall may be used when an abrupt change in the flow direction is necessary. An "L" shaped endwall may be constructed to redirect the flow to the angle of the culvert.

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Effectiveness:

Limitations:

  • Straight endwalls may decrease culvert capacities, but rounding of the entrance corners may offset it.
  • If stream velocities are high, lateral scour of embankments may result from eddies at the culvert end sections.
  • Attaching fabricated flared end sections to culvert entrances and outlets may cause separation of culvert joints if the culvert cannot support additional weight.

Considerations:

C.4.  Construct an Energy Dissipater

Construct an energy dissipater to minimize scouring at the culvert outlet. Energy dissipater designs may include concrete or rock sloping aprons, "bucket" outlets that throw the jet downstream, stilling basins, or other more elaborate structures.

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Effectiveness:

  • Very effective. Especially effective when there is a significant drop from the bottom of the outflow end of the culvert to the drainage flow line, or when the gradient of the culvert is steep.
  • If stream velocities are high, may be needed to eliminate lateral scour.

Limitations:

  • Stream channel geometry may dictate the design of energy dissipaters.
  • Depending on existing conditions, stream velocity, and depth of pool, measure may have considerable impacts on fish.

Considerations:

C.5.  Extend Culvert Inlet or Outlet

Extend culvert entrance and/or outlet past the embankment face.

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Effectiveness:

  • Moderately effective in reducing turbulence at culvert entrance.
  • Moderately effective in reducing erosion of the downstream embankment, but can cause erosion to upstream embankment due to eddies.
  • Consider energy dissipaters, embankment slope protection, and debris barriers to maximize effectiveness.

Limitations:

  • Extensions can cause scour of the streambed downstream.
  • Extensions are vulnerable to flow and debris impacts.

Considerations:

 

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