The Effect of Lateral Contractions on Super-critical Flow in Open Channels

It is frequently desirable, if not absolutely necessary, to change from a wide, shallow channel to a relatively narrow, deep channel, and in so doing, some form of transition section must be employed to "ease" the water into the narrower channel. Such a transition section would, for example, be required to ease the water from an irrigation ditch into a wooden flume used to carry the water over a gulch or dry wash. Another case where a transition section would be required is in the spillway of a dam. In many instances the crest of the spillway is located at one end of the dam or at a low spot in the surrounding hills. In either case, a structure of considerable length must be built to conduct the water from the crest to the tailrace. It is desirable to have the crest of the spillway as wide as possible in order to keep the depth of over-flow a minimum. The water flowing over the crest will be moving at the critical depth and velocity, but below the crest the velocity of the water will increase owing to the drop in elevation, and thus the depth of water must decrease. This low-depth, high-velocity flow is to be contracted into a narrower and deeper channel. The contraction from spillway, or crest width to a narrower channel width will cause standing waves which may be two or three times the average depth of flow, particularly if the transition section is not well designed. These waves will extend downstream until they are either dampened out by friction or are suppressed by a second set of waves which will cancel out the original ones by interference. Because these waves are set up, the channel must, therefore, either be dug deeper or else the sides of the channel must be raised in order to prevent these waves from over-topping the structure. If the spillway, as is frequently the case, is cut in solid rock, additional cut would be quite expensive, and any method of hydraulic design which will give a better transition section and thus reduce this cost would obviously be highly desirable. Because of the variety of ways in which transition sections may be built, and the importance of securing satisfactory operation, these studies have been undertaken to eventually determine the best design. This paper deals with studies made with a high-velocity flow on a particular contraction in an open channel and the waves created by this contraction.