In Chapter 1, it was noted that nonuniform ¢ows in open channels are subdivided into gradually varied and rapidly varied on the basis of whether normal accelerations of ¢uid moving along the curved streamlines can be ignored or not. Situations, such as the ¢ow directly underneath a gate, and over the crest of a weir or a dam are examples of rapidly varied ¢ows. For such situations, the stream lines curve rapidly and normal accelerations have a signi’cant in¢uence on the ¢ow pattern, i.e., the ¢ow needs to be handled as a two-or three-dimensional problem. When the water depth and the velocity change over long distances as water impounds behind a gate, or the depth gradually decreases in a mild channel upstream from a break in grade to a steeper channel, then normal accelerations are insigni’cant, and the ¢ow can be considered one-dimensional. In this chapter, only gradually varied ¢ows will be dealt with. Furthermore, only steady-state ¢ows are considered in this chapter.