Stage-Discharge Models for Concrete Orifices: Impact on Estimating Detention Basin Drawdown Time

Abstract

Thick-walled concrete orifices are widely used to control outflow rates from stormwater detention basins. There are limited data on stage-discharge relationships for such orifices, particularly for low head and partially full conditions that are relevant to extended detention applications for water quality control. This paper presents laboratory and field testing of stage-discharge relationships for concrete orifices under such conditions. Steady-state and transient drainage curve experiments were conducted in a horizontal flume, and a detention basin with a multistage concrete outlet structure was instrumented to collect drawdown data in the field. For fully submerged conditions, the orifice equation with single-valued discharge coefficient (