Open-channel ¢ow is distinguished from closed-conduit ¢ow by the presence of a free surface, or interface, between two different ¢uids of different densities. The two most common ¢uids involved are water and air. The presence of a free surface makes the subject of open-channel ¢ow more complex, and more dif’cult to compute commonly needed information about the ¢ow, than closedconduit ¢ow, or pipe ¢ow. In pipe-¢ow problems, the cross-sectional area of the ¢ow is known to equal the area of the pipe. In open-channel ¢ow, the area depends upon the depth of ¢ow, which is generally unknown, and must be determined as part of the solution processes. Coupling this added complexity with the fact that there are more open-channel ¢ows around us than there are pipe ¢ows, emphasizes the need for engineers, who plan work in water-related ’elds, to acquire pro’ciency in open-channel hydraulics. The wide use of computers in engineering practice reduces the need for graphical, table lookup, and other techniques learned by engineers who received their training a decade ago.