Abstract: The accurate calculation of drainage capacity from urban surface to underground pipe is essential for urban flood modelling.A full-scale physical model was built to study the surface-to-drainage flow of a grate inlet under different floodwater depths ranging in 3~50cm.Quantitative effects of water depth and flow velocity on the drainage capacity of a grate inlet were analyzed,based on 144 sets of experimental tests.The results show that the discharge capacity of grate inlet increases with a higher pre-grate water depth,and general drainage patterns of the grate can be classified into weir and nozzle flows.Meanwhile,the discharge capacity has a positive and negative correlation with the inflow velocity of surface runoff during the weir flow and nozzle flow stages,respectively.In addition,the discharge coefficients were calibrated for the weir and nozzle formulae,and the comprehensive discharge coefficients (C_w=0.0596,C_n=0.4413) were calibrated for the weir and nozzle formulae.Based on the method of dimensionless analysis,correlation function was derived between the relative velocity across the grate and the incoming Froude number,in order to calculate the discharge capacity,with the calibrated dimensionless parameters of a=0.281,and b=-0.73.Discrepancies between experimental data and national standards highlight the latter's overestimation of discharge capacity at large water depths,due to neglecting the limitations imposed by the drain box and connecting tube:the reference value of discharge is 0.270m³/s under the pre-grate water depth of 50cm,but the experimental value is only 0.084m³/s.