AASHTO HDG CHAPTER 6

Abstract

INTRODUCTION: An open channel is a conveyance in which water flows with a free surface and may be natural or constructed. Natural streams usually consist of a normal or low-flow channel and adjacent floodplains. For purposes of this chapter, the term "open channel" will include the total conveyance facility, floodplain, and stream channel. <br>Open channel hydraulics is of particular importance to highway design because of the interrelationship of channels to all highway hydraulic structures. In the hydraulic analysis and design of bridges and culverts, open-channel hydraulic principles are utilized to evaluate the effects of proposed structures on water surface profiles, flow, and velocity distributions, lateral and vertical stability of the channel, stream regime, flood risk, and the potential reaction of the stream to changes in variables (e.g., structure type, shape, location, scour control measures). <br>The hydraulic design process for open channels consists of establishing criteria, developing and evaluating alternatives, and selecting the alternative that best satisfies the established criteria. Capital investment and probable future costs, including maintenance and flood damages to properties, traffic service requirements, and the stream and floodplain environment must be considered in the design process. The detail in which risks are considered should be commensurate with the flood hazard at the site, economics, and current engineering practices. <br>Highway encroachments on streams and floodplains should be avoided where practicable; however, encroachments are necessary in some locations. The short- and long-term effects of changes to natural streams should be evaluated during both the planning and design phase of project development. Highway-related channel work is generally local in nature and should not be associated with extensive modifications generally referred to as channelization projects. <br>In this chapter, the hydraulic engineering aspects of open channels are discussed, proceeding from hydraulic considerations in planning and location through studies necessary for design and construction to hydraulic considerations as related to maintenance. Channels along, across, approaching, and leaving the highway are included. <br>The chapter is not all inclusive, but salient considerations are discussed, suggested procedures are presented and references to sources of more detailed information are cited.