ABSTRACT: The new Texas Department of Transportation curb inlet uses 6 inch flush slab supports for the top slab of a curb inlet. HEC-22, which provides design equations used by TxDOT, states flush slab supports can reduce an on-grade inlet’s interception capacity by as much as 50%, yet does not provide any guidance on quantifying these effects. Full-scale physical modeling of the TxDOT curb inlet on-grade was performed to investigate the effects of flush slab supports on hydraulic performance. In addition, the modeled curb inlet is compared with HEC-22 and other curb inlet design equations. No measurable difference in interception capacity or ponded width was found between curb inlets with flush slab supports and without. For the 5 ft modeled curb inlet a combination of Guo and MacKenzie (2012) design equation and HEC-22 align best, yet neither align with every tested slope combination. HEC-22 design equations were found to over-predict the 15 ft modeled curb inlet by an average factor of 2.3:1. No other design equations were found to accurately predict hydraulic performance for the 15 ft modeled curb inlet.