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The acceleration of urbanization has brought significant changes to the urban underlying surface. As a result, the flood disaster caused by stormwater runoff has become increasingly prominent. The infiltration function of the permeable area can lead to flood disasters, but the extent and depth of the effect are still unclear. Therefore, based on the storm water management model (SWMM) and Green-Ampt infiltration model, this paper discussed the effect of improving soil saturated hydraulic conductivity (SSHC) and soil capillary suction head (SCSH) on the stormwater runoff process. The results show that the increase in SSHC and SCSH can significantly reduce runoff and increase infiltration. However, the improvement of SSHC can more effectively alleviate flood disasters compared with the improvement of SCSH. The change of SSHC has a significant effect on the stormwater runoff with a critical SSHC value while the effect can be ignored. In addition, there is a cross value; when the value of SSHC and SCSH is larger than the cross value, the difference between SSHC and SCSH in reducing runoff duration no longer exists. The critical value and cross value are not constant but change with the change of rainfall intensity. HIGHLIGHTS The effect of an urban underlying surface change on stormwater runoff is notionally studied.; Improving SSHC can reduce total runoff and increase total infiltration more effectively than improving SCSH.; There is an effect of rainfall intensity and urban IR in SSHC and SCSH on soil infiltration.; There exists a critical SSHC value, below which the change of SSHC has a significant effect on the stormwater runoff.

Due to extensive pollution and the relatively weak flow replacement in urban rivers, determining how to fully utilize the self-purification abilities of water bodies for water quality protection has been a complex and popular topic of research and social concern. Organic pollution is an important type of urban river pollution, and COD (chemical oxygen demand) is one of the key pollution factors. Currently, there is a lack of research on the relationship between COD degradation and the flow characteristics of urban rivers. In this paper, COD degradation experiments were conducted in an annular flume with Jinjiang River water at controlled flow velocities and the COD degradation coefficients under different hydraulic conditions were analyzed. A good correlation was observed between the degradation coefficient and hydraulic conditions. According to dimensional analysis, the relationship between the COD degradation coefficient and hydraulic conditions such as the flow velocity, water depth, Reynolds number (Re), and Froude number (Fr) was established as K COD = 86400 u h F r 0.8415 R e &minus, 1.2719 + 0.258 . The COD degradation coefficients of the Chishui River in Guizhou Province ranged from 0.175&ndash, 0.373 1/d based on this formula, and the field-measured values varied from 0.234&ndash, 0.463 1/d. The error in the formula ranged from 5.4&ndash, 25.3%. This study provides a scientific basis for the prediction of the COD degradation coefficients of urban rivers.