Hydraulic Efficiency of Grate and Curb Inlets for Urban Storm Drainage

A curb inlet and two combination inlets (with a grate and curb opening) used for storm-water drainage were tested under varying road conditions, and equations were developed for their application. The need for this study arose from a general uncertainty in matching the inlets tested to previously published methods. This uncertainty relates to sizing inlets and in determining the flow captured. A one-third Froude-scale model of a two-lane road section was designed and built for testing. A total of 120 tests were performed for longitudinal street slopes from 0.5 to 4%, cross slopes from 1 to 2%, gutter prototype flow depths from 0.1 to 0.15 m, and prototype inlet lengths from 1 to 4.6 m. Collected test data on inlet efficiency were compared to previously published methods for the inlets and were found to deviate by as much as 45% and an average of 13% at typical design depths of 0.1–0.15 m. Existing methods for the curb inlet were modified, and empirical design equations were developed for the combination inlets. Dimensionless parameter groups developed for empirical equations are based largely upon flow characteristics and may be applicable to other inlets. New methods were found to deviate by as much as 19% and by an average of 5% from the observed test data for typical design depths of 0.1–0.15 m.