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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

Damage to culverts is caused primarily by floodwaters eroding culvert entrances or outlets and road embankments, and usually results in a full or partial washout or misalignment of the culvert. These damages may be due to insufficient design capacity or end treatments, inadequate slope protection, or inadequate protection from floating debris. Careful determination of the cause for the damage is necessary, as different causes require different mitigation.

Assessing the Causes of Culvert Damage

Selection of appropriate mitigation depends upon the culvert flow conditions at the time of damage. Surveys of high water marks located upstream and downstream from the culvert, and surveys of the inlet and outlet elevations are necessary to identify the flow conditions.

Flow through a culvert may be controlled by its entrance or outlet conditions, or by downstream channel features. A culvert will flow full if its outlet is submerged or if the depth of water above the top of its entrance is greater than 1.5 times its diameter. Damage to a culvert flowing full usually occurs when the road embankment is overtopped and is fully or partially washed out. Appropriate mitigation measures include:

  • Increasing culvert size;
  • Increasing efficiency of the entrance; and/or
  • Raising the culvert.

If raising the culvert causes it to flow partially full, adjust the slope to return it to full flow conditions.

A culvert may flow partially full when the water depth is above the top of the culvert's entrance and below the top of the outlet, or when the water surface is below the top of the culvert at both entrance and outlet. Damage to a culvert flowing partially full usually occurs when embankment erosion has occurred. Appropriate mitigation for these conditions include:

  • Increasing culvert entrance efficiency, and/or
  • Decreasing the slope of the culvert.

If the culvert was flowing partially full and the flow at the outlet was in a subcritical or tranquil state, the damage will likely be confined to its entrance. In this case, appropriate mitigation includes:

  • Increasing the efficiency of the entrance, and
  • Armoring the entrance embankment.

If the culvert was flowing partially full and the flow at the outlet was in a supercritical or turbulent state, damage may have occurred to either the entrance or outlet or both. Appropriate mitigation would then include:

  • Increasing efficiency of entrance and outlet conditions, and
  • Armoring the entrance and outlet embankments.

If, as a result of these conditions, erosion of the streambed with subsequent head cutting and embankment erosion occur, appropriate mitigation would be:

  • Installation of a stilling basin, and/or
  • Armoring of the stream channel and road embankment.

To identify the flow conditions that caused the damage, determine:

  • Water surface elevations upstream and downstream from the culvert at the time of damage;
  • Elevations of the culvert entrances and outlets;
  • Whether downstream channel erosion and head cutting occurred; and
  • Evidence of road embankment erosion.

Floodwaters frequently carry debris both as the flows rise or recede. Debris carried by rising flood flows may become caught or wedged in culverts, plugging the flow. Culverts can then be washed out or damaged due to increased surface flow elevations. Mitigation measures should be designed to protect against debris impact and accumulation, and to assist in passing debris through the structure openings. Debris carried by receding flood flows will generally be deposited on the stream overbanks and draped over culvert entrances. Damage to culverts will most likely occur due to factors other than floating debris, and mitigation should be developed based upon the most probable cause of damage.

A. Insufficient Capacity and/or Inefficient End Sections

B. Plugging

C. Embankment Erosion

D. Misalignment

 

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