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, ). 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:

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.