Study Of Outdoor Artificially Simulated Rainfall Runoff And Pollutant Transport Characteristics

With the continuous promotion of urbanization,some permeable substrates such as lawns and raw soil substrates are being replaced by weakly permeable substrates（e.g.,concrete,asphalt）.Weakly permeable sub-bedding surfaces have low retention rate of initial pollutants in rainfall runoff,and their runoff characteristics and pollutant transport characteristics have a greater impact on urban stormwater use.The study of rainfall runoff production and sink characteristics and pollutant transport characteristics on sub-bedding surfaces can help managers to grasp first-hand information on urban sub-bedding surface runoff water quality and quantity,and provide detailed data support for urban initial stormwater pollution control and stormwater reuse,which can guide model construction,law determination and stormwater reuse methods,and is of great significance.It is proposed to study the runoff characteristics and pollutant transport characteristics of the weakly permeable sub-bedding surface by artificially simulating rainfall in a runoff cell.The test site was selected at the runoff plot of the Institute of Soil and Water Conservation,Lanzhou City,Gansu Province.Four types of rainfall were simulated:heavy rain（1.71mm/min）,heavy rain（1.28mm/min）,medium rain（0.89mm/min）,and light rain（0.51mm/min）,and three types of substrates were selected:seasonal green substrate,impermeable brick surface,and natural soil,with two slopes（10°and 15°）.The parameters such as water content,infiltration rate,flow production intensity,flow production rate and runoff coefficient were analyzed.Six pollutant indicators,including sand content,turbidity,SS,TP,TN and COD_Mn,were selected to reflect the transport characteristics of pollutants in simulated rainfall,and the correlation between sand content and other five pollutants was analyzed.The experimental contents and main conclusions are as follows:（1）The test results of water content and infiltration rate for the sub-bedding surface of Four Seasons Green and Natural Raw Soil showed that the infiltration rate was ranked from largest to smallest:the slope of the sub-bedding surface of Four Seasons Green was 10°>the slope of the sub-bedding surface of Four Seasons Green was 15°>the slope of the sub-bedding surface of Natural Raw Soil was 10°>the slope of the sub-bedding surface of Natural Raw Soil was 15°,and the changes of soil infiltration rate and water content were linear,and the soil infiltration rate was positively correlated with the reduction rate of pollutants.（2）When the rain intensity is the same,the flow production intensity of weakly permeable sub-bedding surface（impermeable brick sub-bedding surface）increases the fastest,followed by natural soil sub-bedding surface,and the flow production intensity of Four Seasons Green sub-bedding surface increases the slowest;under the same conditions of rain intensity and slope,the relationship between the size of runoff coefficient is:impermeable brick sub-bedding surface>natural soil sub-bedding surface>Four Seasons Green sub-bedding surface,and the runoff coefficient is logarithmically related to the rain intensity.The stronger the permeability,the smaller the increase in runoff rate.（3）The pollutant transport patterns on the two types of permeable sub-bedding surfaces（Four Seasons Green sub-bedding surface and natural soil sub-bedding surface）are generally consistent,and the pollutant concentration and runoff ephemeris are in accordance with the intercept function relationship.The peak of pollutant concentration on the weakly permeable sub-bedding surface appears at the early stage of flow production.（4）The sand content and COD_Mn,TN,SS,turbidity and TP showed a linear relationship,and the study found good correlation between sand content and the other five pollutants.（5）Based on the results of simulated rainfall runoff tests on three types of sub-bedding surfaces,a rainfall runoff pollutant scour model was established,which reflected an e-exponential relationship between pollutant concentration and effective water depth.