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In cities, the existence of impervious structures, such as road pavements, parking lots, footpaths, sidewalks, and roofs, can limit the magnitude of rainfall infiltration greatly. Stormwater run-off in storm sewer often dominates, which can cause problems of urban flooding easily during heave rainfalls. In rain and sewage diversion, perforated storm sewer is proposed to enable the occurrence of stormwater infiltration into the ground, which can change the prolonged drought-like condition. Model-scale laboratory tests are conducted to assess the infiltration process of water through drainage holes around the sewer circumference. The experimental data are also used to calibrate a numerical model, after which numerical parametric analysis is carried out. It is found that the feature of drainage holes only influences the wetting front in the initial stage of infiltration. In the end, the egg-shaped contour of wetting front with greater influencing zone below the sewer is obtained. The cumulative infiltration with time can be described by an infiltration model, which is positively correlated with the diameter and the number of drainage holes. The optimal opening ratio is recommended as 0.25%, which can provide the most effective drainage capacity. [ABSTRACT FROM AUTHOR]

• Perforation in storm sewer can ease the burden of rainfall runoff and help the groundwater recharge. • An alternating direction implicit algorithm is proposed to simulate the infiltration of stormwater. • The finite difference method is found to be effective by comparing against field measurements. • An opening ratio of 0.25% can optimize the strength and the hydraulic performance of perorated sewer. Due to urbanization, the ground in cities is often covered with impervious structures. Rainfall infiltration is then limited to cause the prolonged draught-like condition. Combined sanitary and storm sewers need to satisfy the requirements of rainfall runoff, such that urban flooding can occur easily due to insufficient discharge capacity of sewers during heavy rainfalls. Sanitary and storm sewers should be constructed separately, and perforation in storm sewer can enable the occurrence of partial rainfall infiltration to: (1) ease the burden of rainfall runoff, and (2) help the groundwater recharge. In this investigation, the alternating direction implicit method (ADI) is adopted to simulate the infiltration process of stormwater in the ground. Nonlinear second order differential equations for governing equation, initial and boundary conditions are established, and converted into tridiagonal systems of equations. The Thomas algorithm is employed to solve the flow conditions at all discretized nodes. The efficacy of the proposed finite difference method is assessed by comparing against field measurements of cumulative infiltration. Upon the successful calibration of the model, parametric analyses are conducted for design optimization of drainage holes. The opening ratio is suggested at 0.25%, such that the drainage performance of perforated sewer can be optimized. [ABSTRACT FROM AUTHOR]