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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

B. Plugging

Introduction

Problem:

Damage or failure of a culvert caused by overtopping and erosion of the embankment due to plugging of the culvert with debris and/or silt. Debris deposition across the culvert entrance, or debris caught or wedged in the culvert, restricts the water flow. A culvert can then be washed out or damaged due to increased water surface elevations upstream.

Mitigation Objective:

To prevent future damage to a culvert by preventing it from becoming plugged.

B.1.  Install an Entrance Debris Barrier

Install an entrance debris barrier (debris deflector or debris crib) to prevent blockage of the culvert or debris fins designed to orient the floating debris for easy passage through the culvert. Install a "V" shaped or semi-circular rack at the culvert entrance or a straight rack at the end of wingwalls to allow for overtopping of rack by the flow when debris accumulates around the rack. Install debris deflector or debris fins at upstream entrance to the culvert and install a debris crib over the entrance with a drop inlet.

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Effectiveness:

Limitations:

  • Adequate stream channel storage for debris accumulation must be available.

Considerations: 

B.2.  Install a Sediment Catch Basin

Install a sediment catch basin upstream of the culvert. The basin should be located far enough upstream and the openings should be sized to allow the suspended sediment sufficient time to settle out prior to entering the culvert.

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Effectiveness:

  • Generally very effective.
  • Particularly effective in areas where heavy silt and/or sand loading occurs during flood events and velocity levels do not provide for scouring of the streambed and culvert.

Limitations:

  • Requires adequate storage in basin.
  • Must be easily accessible for sediment clean out.

Considerations:

B.3.  Install a Relief Culvert

Relief culvert(s) should be located at the crossing site and in the embankment above the flow line of the primary culvert. This configuration provides an alternate route for the flow, if the main culvert gets plugged, and prevents sedimentation through the high flow scouring action.

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Effectiveness:

Limitations:

  • Embankment geometry may limit ability to locate culvert(s) above flow line of primary culvert.

Considerations:

B.4.  Install a Perforated Standpipe

Install a perforated standpipe in lieu of the traditional culvert entrance. The standpipe allows debris to float up with the rising floodwaters without impacting flow into the culvert. The standpipe should be armored at its base by constructing a cone of free draining gravel around it. The area upstream of the entrance should be suitable for storing floodwaters.

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Effectiveness:

  • Generally very effective.

Limitations:

  • Standpipe may be vulnerable to damage from high velocity flows.

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