Abrupt Transition from a Circular Pipe to a Rectangular Open Channel

The development of criteria and a generalized procedure for the design of an abrupt transition from a circular pipe to a rectangular open channel are presented. The rectangular channel must be 1.0 pipe diameters wide. Wider channels cause high waves which reflect from the channel sidewalls, may overtop the sidewalls, and produce severe disturbances in the channel. To permit the pipe to expand, the channel may be widened for a distance not exceeding 0.5 pipe diameters downstream from the pipe exit, and the floor of the channel may be lowered. The equations developed describe the locations of the water surface elements to within an average of 0.11 pipe diameters of their correct locations. The maximum anticipated location error is +/- 1.4 pipe diameters. The equations for the envelope curves covering the crests of the sidewall waves, which determine the channel sidewall height, provide an average freeboard of 0.08 pipe diameters and a maximum freeboard of 0.31 pipe diameters. When the envelope equations are used only 2 percent of the wall waves will overtop the sidewalls, the maximum overtopping being 0.04 pipe diameters. The average depth of flow-the depth at the wave nodes-is predicted by the equations to within a maximum deviation of +0.13 and -0.06 pipe diameters of the observed depths. The average depth at the nodes is predicted by the equations within 0.01 pipe diameters of the observed average depth.