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Chapter 2:   CULVERTS

A. Insufficient Capacity and/or Inefficient End Sections

Introduction

Problem:

Damage or failure of a culvert resulting from overtopping and/or erosion of embankments due to insufficient culvert capacity and/or inefficient end sections. The inadequate capacity may be a result of inappropriate hydrologic analysis of flood peaks and volumes, and/or application of inappropriate culvert design criteria.

Mitigation Objective:

To prevent future damage to a pipe culvert by increasing the design capacity and adding effective end sections; redesigning the culvert installation; replacing the culvert with an alternate drainage structure(s); and/or adding an overflow channel.

A.1. Replace With Larger Pipe Culvert

A larger culvert allows for the passage of a greater volume of water.

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Effectiveness:

  • Very effective if, at the time of failure the culvert was flowing full. (See "Assessing Culvert Damage," pp. 17-19)

Limitations:

Considerations:

A.2.  Increase Efficiency of Entrance &/or Outlet Design

Culvert entrance rounding, entrance bevel rings, wingwalls, flared end sections, paving the culvert entrance bottom, and/or "U" shaped endwalls may increase the efficiency of a pipe culvert.

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Effectiveness:

Limitations:

  • The least effective of these treatments are entrance wingwalls and flared entrance end sections.
  • Effectiveness is less for endwalls and wingwalls when centerline of culvert is at an angle to that of the stream.

Considerations: 

A.3.  Change Culvert Alignment

Change culvert horizontal and vertical alignment to match centerline and slope of stream. Direct entrance and exit alignment to maximize culvert efficiency. (See also "Culverts-Misalignment," pp. 40-45)

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Effectiveness:

Limitations:

  • A culvert bottom slope less than that of the stream may cause backwater conditions with decreased efficiency and potential siltation.
  • A culvert bottom slope greater than that of the stream may cause scour of the streambed at the outlet and erosion downstream.

Considerations: 

A.4.  Add Multiple Pipe Culverts

Multiple culverts may be installed with an existing culvert at a single crossing site at either the same or at differing elevations. The culverts should be placed at different elevations in the embankment and should be separated by more than one-tenth the diameter of the individual culverts to minimize the potential for sedimentation build-up.

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Effectiveness:

  • Generally very effective.
  • Particularly effective when combined with culvert entrance and outlet treatments and embankment slope protection.
  • May be more suitable than a single, large diameter culvert pipe for low fill areas.
  • Sedimentation can be minimized by use of the scour effect from higher velocity flows.

Limitations:

  • Smaller pipe culverts will restrict debris passage.

Considerations:

A.5.  Replace With a Box or Arch Culvert

A box or arch culvert provides additional capacity in low fill situations. Can be designed for very minimal fill height.

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Effectiveness:

Considerations:

A.6.  Replace With a Bridge

Replace culvert with a bridge.

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Effectiveness:

  • Very effective in increasing flow capacity through embankment.

Limitations:

  • May require engineering analysis and design.

Considerations:  

A.7.  Replace With or Add a Low Water Crossing

Replace culvert with a depression in the roadway that will accommodate the anticipated flows (low water crossing), or add a roadway depression over a culvert.

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Effectiveness:

  • Generally very effective in seasonal intermittent drainages and as an emergency spillway.

Limitations:

  • Not appropriate if roadway provides access to a critical facility.
  • Road impassable during flooding events.
  • Adequate signage and barricades are necessary when water depth exceeds a safe level for vehicles.
  • Roadway and embankments should be designed and constructed to withstand anticipated flows.
  • The profile of the crossing should match the shape of the stream crossing as close as possible.

Considerations:  

A.8.  Install a High Water Overflow Crossing

Install overflow section (high water overflow crossing) in the roadway that will accommodate flows from the overbank areas of the stream. This effectively reduces and provides an emergency spillway. High water overflow crossings should be located at natural side channels and/or in line with heavy flow areas located on the stream overbanks.

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Effectiveness:

Generally very effective, and particularly when the road must remain passable during normal flows and topography makes a low water crossing infeasible.

Limitations:

  • Not appropriate if roadway provides access to a critical facility.
  • Road impassable during flooding events.
  • Adequate signage and barricades are necessary when water depth exceeds a safe level for vehicles.
  • Roadway and embankments should be designed and constructed to withstand anticipated 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