New Method for Modeling Thin-Walled Orifice Flow under Partially Submerged Conditions

Orifices are often used in stormwater detention facilities for reducing runoff peak flow and achieving extended detention times to improve water quality. Orifices often flow partially full; therefore, accurate and easily implemented methods of modeling basin outflow under such conditions are needed. In this paper we describe two models of partially submerged flow in thin-walled orifices and conduct full-scale testing of these models. The best model (termed the top-width weir approach) treats the orifice as an equivalently sized rectangular weir, where the equivalent weir length equals the top width of flow in the orifice until half-full and the diameter of the orifice when greater than half-full. A single fitting coefficient Cw is required, which is directly proportional to the discharge coefficient for submerged orifice flow. This method provides results that, except for low submergence conditions, are within 2 to 5% of experimental data for steady flow, a significant improvement over the existing circular weir model in which errors are >10%. The model is also shown to closely match transient drainage data, demonstrating its utility for predicting drawdown times for stormwater detention facilities.