TABLE OF CONTENTS Open channels are designed to carry a design discharge in a safe and economical way. For flood control channels the design discharge represents the peak discharge expected to result from a flood event of a specified return period. Normally, the design discharge is obtained from the hydrologic study of upstream watersheds. For water distribution channels, however, such as those used in irrigation and water supply projects, the design discharge is determined on the basis of total delivery requirements. Open channels are usually designed for uniform or normal flow conditions.
Designing an open channel involves the selection of channel alignment, size and shape of the channel, longitudinal slope, and the type of lining material. Normally, we consider several hydraulically feasible alternatives, and compare them to determine the most cost-effective alternative. This chapter will emphasize the hydraulic considerations involved in channel design rather than economic analyses of different alternatives.
Selection of channel alignment is the first step in designing an open channel. Generally, the topography of the area, available width of right-of-way, and existing and planned adjacent structures and transportation facilities control the channel alignment. The topography also controls the invert elevations and bottom slope of the channel.
Most manmade surface channels are trapezoidal in cross-section, although triangular, parabolic and rectangular channels are also used. The primary concern in selecting a cross-sectional shape and size is the section s hydraulic capacity to accommodate the design discharge. There are, however, other factors to be considered. For instance, the...
