This study was conducted to investigate the hydraulic efficiency of curb opening
Inlet structures adopted by the South Carolina Department of Transportation (SCDOT). The
structure consists of a rectangular Inlet placed in a depressed gutter. The effects of many
design parameters on the efficiency of the curb Inlet were experimentally investigated and
reported in other sources, but cited in the text of the paper. Based on the results of the
previous studies, an equation was developed for determining the efficiency of the Inlet
structure. The equation defines a relationship between three variables: 1) allowable flow
spread on the road, 2) road longitudinal slope, and 3) a coefficient K which was itself found
to be a function of structure length and road cross slope. Satisfactory agreement was obtained
between the calculated and experimental values of efficiency. The empirical equation
provides a practical method for the determination of curb Inlet structure efficiency and
spacing of the Inlet structures along the roadway. Application of the equation to geometries
not tested in this study can be done by experimentally determining the appropriate values of
the coefficient K. An example for the application of the equation for design of curb Inlets has
been given at the end of the paper.