Impacts Of Urban Complex Conditions On Detailed Hydrological Processes

The increasing urban population and urban size determine the importance of the urban hydrology,while the strong heterogeneities of rainfall,underlying surface,and infrastructures determine its complexity.This study aims to systematically evaluate how the time resolution of the rainfall,the spatial resolution of the underlying surface,and the infrastructures will affect the urban hydrological processes.With Tsinghua Campus as a typical urban area,we prepared multivariate high-resolution data,carried out fine hydro-meteorological measurements,and then developed a routing-enhanced detailed urban stormwater model as the tool.The study contributes to understanding the complex urban conditions with their hydrological impacts,and supporting the Sponge City Programme(SCP)in China.To account for the complex urban flow paths,and facilitate the application of high-resolution data,the study developed a routing-enhanced detailed urban stormwater model(REDUS),which has a four-layer structure including surface,road network,pipe network,and river network.For the case of Tsinghua Campus,the model identified the topological relationship between the complex underlying surface and the infrastructures with the help of multivariate high-resolution data,and achieved detailed simulation of the urban stormwater processes.Simulations were carried out for different events of multiple time resolutions,and the results showed that the decrease in resolution will lead to the reduction of the regional runoff peak,and the reduction extent is related to the hyetograph variation,the area size,and the area imperviousness.We used rainfall accuracy index,and proposed a method to describe the impacts of rainfall time resolutions with characteristics of both the rainfall and the underlying surface.In order to limit the variation rate of the runoff peak,at least 5-minute rainfall or 15-minute rainfall is required for an urban area smaller or larger than 1 km~2,respectively.Simulations were carried out for different spatial resolutions of the underlying surface and the results indicated that the underlying spatial resolution will impact the urban hydrological processes by changing both the storage in surface sinks and the infiltration amounts.On one hand,the underlying spatial resolution affects the description of urban micro-topography and the connection between surface sinks and road network,resulting in a decrease of the storages in the case of coarse resolutions.On the other hand,the underlying spatial resolution affects the flow length and the complexity of flow paths,inducing a change in infiltration which is also related to the rainfall patterns.Therefore we proposed a method to adjust the model parameters by taking into account the characteristics of the flow paths,which is able to accomodate the infiltration change introduced by the resolution.To test the role of infrastructures in draining storm water,simulations were carried out with different modelling schemes of the roads,drainage pipe networks and gully distributions.The simulation results showed that urban roads can play an important role when facing a severely excessive rainfall,and road drainage effectiveness has a good correlation with maximum 30-minute rainfall volume,while the impact of the longitudinal slope on road drainage effectiveness works closely with the underground pipes.The drainage pipe network contributes to the rapid and directional discharge of surface runoff,and lower complexity of the drainage pipe network can seriously underestimate the flood volume and peak.The simulation results also showed that for a certain total drainage capacity,the distribution of gullies has little effect on the discharge of the major outlet,while affects significantly the local flood processes.