Limitations of Traditional Capacity Equations for Long Curb Inlets

Abstract

Recent work with full-scale experiments indicates that there are fundamental problems with standard curb inlet design equations when applied to depressed curb inlets of 10 ft (3 m) or longer. A full-scale laboratory experiment showed that the latter part of a long inlet does not have a simple linear water surface profile at 100% interception, which is assumed for the form of Izzard’s equation that is adopted for many common design approaches [including the Hydraulic Engineering Circular 22 (HEC-22) design equations recommended by FHWA]. For a long inlet, thin flow sheets were observed for a substantial portion of the inlet length, which is consistent with previous observations that have not been incorporated into standard design equations. Experimental results indicate that the HEC-22 design equations significantly overestimate the interception capacity of long, depressed curb inlets for an on-grade gutter. This issue has potential safety implications in that the gutter bypass and spread for a design storm will be larger than expected for such inlets. The present work is preliminary, so an L2 measure previously proposed by Izzard is recommended as a maximum inlet length pending the outcome of further studies.