scope:

INTRODUCTION: Bridges serve a variety of highway purposes including the elimination of conflicts with traffic and other modes of transportation, such as rail, marine, air, and pedestrian. Bridges enable watercourses to maintain the natural function of flow conveyance and sustain aquatic life. Bridges are also important and expensive highway-hydraulic structures and are vulnerable to failure from flood-related causes. To minimize the risk of failure, the hydraulic requirements of stream crossings must be recognized and considered in all phases of highway development, construction, and maintenance.

Features that are important to the hydraulic performance of a bridge include the approach fill alignment, skew, and profile; bridge location, skew, and length; span lengths; bent and pier location and design; and foundation and superstructure configuration and elevations. These features of a highway-stream crossing are usually the responsibility of location, design, and bridge engineers; however, the integrity and safety of the facility are often as dependent upon competent hydraulic design as on competent structural and geometric design.

In this chapter, the hydraulic engineering aspects of bridged stream crossings are discussed, proceeding from hydraulic considerations in planning and location through studies necessary for design and construction to hydraulic considerations in maintenance and operation. Tidal areas, such as bays and estuaries, are not discussed in detail although much of the discussion is applicable to these areas. Another chapter will discuss tidal hydraulics.

Structures measuring more than 6.1 m (20 ft) along the roadway centerline are conventionally classified as bridges. However, structures designed hydraulically as bridges are treated in this chapter, regardless of length. Discussion of the hydraulics of bridges will include consideration of the total crossing, including approach embankments and structures on the floodplains.