Chapter 9: Bridges

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Section 1: Introduction and Definitions

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Hydraulically Designed Bridges

A bridge is defined in the TxDOT Standard Specifications For Construction and Maintenance of Highways, Streets, and Bridges 2004 as a structure, including supports, erected over a depression or an obstruction (e.g., water, a highway, or a railway) having a roadway or track for carrying traffic or other moving loads, and having an opening measured along the center of the roadway of more than 20 feet between faces of abutments, spring lines of arches, or extreme ends of the openings for multiple box culverts. Bridges, as opposed to culverts, 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.

Bridges enable streams to maintain flow conveyance and to sustain aquatic life. They are important and expensive highway hydraulic structures vulnerable to failure from flood related causes. In order to minimize the risk of failure, the hydraulic requirements of a stream crossing during the development, construction, and maintenance highway phases must be recognized and addressed.

This chapter addresses hydraulic engineering aspects of bridge stream crossings, including approach embankments and structures on floodplains. It does not provide detailed information on tidal areas such as bays and estuaries. Risk is discussed in Chapter 3, Section 3 Evaluation of Risk.


One-Dimensional Analysis – A steady state or standard step model, meaning that there is no direct modeling of the hydraulic effect of cross section shape changes, bends, and other two- and three-dimensional aspects of flow. HEC-1, WSPRO, and HEC-RAS are examples of a one-dimensional analysis models.

Two-Dimensional Analysis – A spatially distributed hydraulic model which models dynamic unsteady flow and is therefore capable of delivering results far more accurately and closer to real life than a steady state model. Dynamic models allow the effects of storage and backwater in conduits and floodplains and the timing of the hydrographs to yield a true representation of the HGL at any point in space and time. Two-dimensional analysis models require such a high level of expertise and time to run effectively that they are used for unusual situations.

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