Evaluating the Accuracy of Design Equations for Different Curb Inlet Designs on Capture Efficiency

Current equations used to predict the capture efficiency of curb inlets in green stormwater infrastructure (GSI) are mostly based on design equations created by the Federal Highway Administration (HEC-22), as this is the standard in roadway drainage design. These equations were determined using experiments and models based on curb opening sizes traditionally found on highways, which are usually much larger than curb openings found in urban infrastructure. The problem this presents is that urban street curb openings, particularly leading to GSI systems, are specified to be oversized when designing with these equations. There is limited guidance on how a municipality should set its guidance for the construction of curb openings for GSI. The present work set out to measure the capture efficiency of typical curb opening combinations (i.e., opening size and shape) using a physical model. The experimental observations were compared to capture efficiencies calculated using existing design equations. Findings show that the existing design equations can overestimate the expected flow capture and also identify the range of curb opening designs in which the design equations may be accurate.