Lateral outflow from supercritical channels

Supercritical channels present a number of design issues as the flow is readily disturbed. Yet, the design of supercritical channels for storm water drainage is necessary in steep environments. Storm drains flowing into supercritical channels are typically aligned to minimize the angle between the drain and the channel. By doing so, standing waves are minimized. Channels are often designed with additional freeboard downstream to accommodate the standing wave. In contrast, flow can be removed from channels via storm drain pipes for the purpose of Best Management Practices consisting of storm water cleaning such as debris removal and/or natural filtration. Various lateral outflow structures have been used in Albuquerque, NM, USA. In some cases, low to moderate flows with lateral outflow result in undesirable hydraulics, while the effects of the lateral outflow are negligible at design flow rates. A physical model of a supercritical trapezoidal channel was built to assist in design guidance of lateral outflow. The angle and size of the outlet pipe were changed; various guiding vanes were employed; and a range of flow rates were tested. A numerical model was also developed to further the range of variables tested and to develop discharge coefficients for lateral outflow in supercritical channels. Preliminary results show that a 45 degree angled storm drain with a tapered vane can divert a reasonable amount of flow without adverse hydraulics for a range of flow rates.