Interception Rate Calculation Method for Inverted Trapezoid Curb Inlets in Urban Roads

The curb inlet is a runoff interception structure connecting the road and bioretention cell. In the literature review, most of the equations for calculating the curb-inlet interception rate were only suitable for rectangular forms, while there was a knowledge gap regarding the hydraulic calculation methods and design parameters for inverted trapezoid curb inlets. In this work, a theoretical equation for calculating the interception rate of 45° inverted trapezoid curb inlets was derived, and comparing the interception rate under different road slopes and curb opening lengths by experiments, the results show that with an increase in the road cross slope and curb opening length, and a decrease in the longitudinal slope, the interception rate of the curb inlet will increase. The theoretical formula was then verified by experiments and computational fluid dynamics (CFD) numerical simulation results under different inflow rates. It was found that the error of theoretical formula at lower runoff flow rates (1–6 L/s) was ranged from -223.85% to -86.03%. Therefore, a modified theoretical formula was proposed based on experimental and CFD simulation results. The difference of the modified equation ranged from -7.80% to 8.50% under different runoff flow rates, which was more accurate than the theoretical formula. Comparing the interception rates obtained by the HEC-22 method of rectangular curb inlets and the revised inverted trapezoidal theoretical formula based on the rainfall data of Beijing, it was found that the interception rate of 45° inverted trapezoidal curb inlets is better than that of rectangular ones. So, select inverted trapezoidal curb inlets means more runoff can be intercepted into the bioretention cell.