Chapter 8: Culverts

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Section 1: Introduction

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Definition and Purpose

A culvert conveys surface water through a roadway embankment or away from the highway right-of-way (ROW) or into a channel along the ROW. In addition to the hydraulic function, the culvert must also support construction and highway traffic and earth loads; therefore, culvert design involves both hydraulic and structural design. The hydraulic and structural designs must be such that minimal risks to traffic, property damage, and failure from floods prove the results of good engineering practice and economics. Culverts are considered minor structures, but they are of great importance to adequate drainage and the integrity of the facility. This chapter describes the hydraulic aspects of culvert design, construction and operation of culverts, and makes references to structural aspects only as they are related to the hydraulic design.

Culverts, as distinguished from bridges, are usually covered with embankment and are composed of structural material around the entire perimeter, although some are supported on spread footings with the streambed or concrete riprap channel serving as the bottom of the culvert. For economy and hydraulic efficiency, engineers should design culverts to operate with the inlet submerged during flood flows, if conditions permit. Bridges, on the other hand, are not covered with embankment or designed to take advantage of submergence to increase hydraulic capacity, even though some are designed to be inundated under flood conditions. Any culvert with a clear opening of more than 20-feet, measured along the center of the roadway between inside of end walls, is considered a bridge by FHWA, and is designated a bridge class culvert. (See Chapter 9, Section 1). This chapter addresses structures designed hydraulically as culverts, regardless of length.

At many locations, either a bridge or a culvert fulfills both the structural and hydraulic requirements for the stream crossing. The appropriate structure should be chosen based on the following criteria:

  • construction and maintenance costs
  • risk of failure
  • risk of property damage
  • traffic safety
  • environmental and aesthetic considerations
  • construction expedience.

Although the cost of individual culverts is usually relatively small, the total cost of culvert construction constitutes a substantial share of the total cost of highway construction. Similarly, culvert maintenance may account for a large share of the total cost of maintaining highway hydraulic features. Improved traffic service and reduced cost can be achieved by judicious choice of design criteria and careful attention to the hydraulic design of each culvert.

Before starting culvert design, the site and roadway data, design parameters (including shape, material, and orientation), hydrology (flood magnitude versus frequency relation), and channel analysis (stage versus discharge relation) must be considered.

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Construction

Culverts are constructed from a variety of materials and are available in many different shapes and configurations. When selecting a culvert, the following should be considered:

  • roadway profiles
  • channel characteristics
  • flood damage evaluations
  • construction and maintenance costs
  • estimates of service life.

Numerous cross-sectional shapes are available. The most commonly used shapes are circular, pipe-arch and elliptical, box (rectangular), modified box, and arch. Shape selection should be based on the cost of construction, limitation on upstream water surface elevation, roadway embankment height, and hydraulic performance. Commonly used culvert materials include concrete (reinforced and non-reinforced), steel (smooth and corrugated), aluminum (smooth and corrugated), and plastic (smooth and corrugated).

The selection of material for a culvert depends on several factors that can vary considerably according to location. The following groups of variables should be considered:

  • structure strength, considering fill height, loading condition, and foundation condition
  • hydraulic efficiency, considering Manning’s roughness, cross section area, and shape
  • installation, local construction practices, availability of pipe embedment material, and joint tightness requirements
  • durability, considering water and soil environment (pH and resistivity), corrosion (metallic coating selection), and abrasion
  • cost, considering availability of materials.

The most economical culvert is the one that has the lowest total annual cost over the design life of the project. Culvert material selection should not be based solely on the initial cost. Replacement costs and traffic delay are usually the primary factors in selecting a material that has a long service life. If two or more culvert materials are equally acceptable for use at a site, including hydraulic performance and annual costs for a given life expectancy, bidding the materials as alternates should be considered, allowing the contractor to make the most economical material selection.

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Inlets

Several inlet configurations are utilized on culvert barrels. These include both prefabricated and constructed-in-place installations. Commonly used inlet configurations include the following:

  • projecting culvert barrels
  • cast-in-place concrete headwalls
  • pre-cast or prefabricated end sections
  • culvert ends mitered to conform to the fill slope.

When selecting various inlet configurations, structural stability, aesthetics, erosion control, and fill retention should be considered.

Culvert hydraulic capacity may be improved by selecting appropriate inlets. Because the natural channel is usually wider than the culvert barrel, the culvert inlet edge represents a flow contraction and may be the primary flow control. A more gradual flow transition lessens the energy loss and thus creates a more hydraulically efficient inlet condition. Beveled inlet edges are more efficient than square edges. Side-tapered inlets and slope-tapered inlets, commonly referred to as improved inlets, further reduce head loss due to flow contraction. Depressed inlets, such as slope-tapered inlets, increase the effective head on the flow control section, thereby further increasing the culvert efficiency.

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