Urban Rainstorm Waterlogging Scenario Modelling And Mitigation Study

The drainage pipe network has low design standards and surface impervious area is large of urban area in shanghai.Under the background of global warming and extreme rainfall events increasing,the waterlogging caused by rainstorm has become more serious in the central area of Shanghai,which has a major impact on the city's social and economic security and residents' daily life.The population and buildings are concentrated in the urban area of downtown in Shanghai and the surface imperviousness is very high,thus it is prone to cause waterlogging during heavy rain.Due to the early development,the old drainage facilities,and the scarce green space,the original Jing'an District that is the old city of Shanghai is more likely to accumulate water above the ground during heavy rain.The participation of human activities makes the urban surface water accumulation process more complicated.However,the research on the hydrodynamic process of surface water accumulation in urban environment is still not deep enough.The research on the impact of rain-type structure on urban torrential rain is also involved not much.As a new type of green infrastructure,Low Impact Development has a significant effect in mitigating urban waterlogging.It has been theoretically studied and applied in many countries and regions,and LID performance analysis based on scenario simulation is the most common methods.However,there are few studies on the impact of rain patterns on the effectiveness of LID measures.The impact of LID measures on the degree of mitigation of rainstorms and the effect of rain patterns on mitigation needs further exploration.The practice of LID measures in Shanghai's old city is also lacking.In this study,the urban two-dimensional hydrodynamic model was established to carry out the urban surface water accumulation process based on the urban drainage network,urban measured rainfall data,urban DEM data,and urban land use data,by collecting data related to the urban stormwater intrusion model.The temporal and spatial distribution patterns of simulated and surface water were analyzed,and thegreen roof was used as the entry point to study the effectiveness of LID measures to reduce urban waterlogging.The empirical study was carried out in Shanghai Jingjingan District.The research content and main results are as follows:1.A two-dimensional hydrodynamic model for urban environments was established.Due to the intervention of human activities,urban drainage methods include urban drainage pipes distributed underground,in addition to rivers and other means.Therefore,the urban hydrodynamic process involves three stages: surface runoff and confluence,underground pipe network flow,and surface flood.The relevant basic data required for modeling was preprocessed by Arc GIS.Based on MIKE hydrology/hydraulic software,two-dimensional hydrodynamic models of urban were constructed including three mutually independent sub-model: surface runoff,underground pipe network flow and surface flood which can also work coupled.At the same time,a model database was established.The selection of model parameters mainly refers to field experiment observation,parameter migration under similar watershed and underlying surface conditions,default value of MIKE user manual,Arc GIS,related literature and so on.By inputting precipitation data,the surface water accumulation process was simulated in the study area under different rainfall scenarios.The results showed that the surface runoff occured during heavy rain,the runoff peak time slightly laged behind the rainfall peak time,and the peak time of underground pipe network laged slightly behind the surface runoff peak time.When the pipe network was saturated,the surface flood was generated,basically in line with the hydrodynamic process in urban environments.2.The effects of different rain peak locations on the surface water accumulation process in urban storms are discussed.The single-peak rain type has a greater impact on the waterlogging in Shanghai.In the single-peak rainfall,the poisson distribution rain type--the rain peak in the front--has the highest frequency,while the rain peak in the back is less.However,the urban rainstorm waterlogging process has been studied mainly under the normal distribution rain type,of which the rain peak in the middle,and rarely involves the influence of different rain patterns on urban rainstorm waterlogging.Based on the two-dimensional hydrological model constructed,this study simulated the urban surface water accumulation process with 70 rainfall events under Poisson rain type and normal rain type and analyzed the peak surface runoff and the peak of underground pipe network flow and the difference in the time of occurrence of surface water accumulation under two rain types.The results showedthat the different of the rainfall peak time affect the surface sedimentation process and the underground pipe network flow process,which leads to the difference in the development process of surface water accumulation.Under the same rainfall return period and rainfall duration,when the rain peak appears earlier,the peak of surface runoff and the peak of the pipe network flow appear earlier,the peak of the runoff and the peak of the pipe network flow coincided with the position of the corresponding rain-type rain peak.The pipe network had a large drainage pressure and the surface water appeared earlier,so that the waterlogging are more serious under the Poisson rain type than under the Normal rain type.3.The temporal and spatial distribution rule of urban rainstorms waterlogging is studied under the current land use.The overall terrain of the study area is low and the total area of green land which is mostly high with the sidewalk or convex terrain and water area is so low(only 3.99%),that the surface water permeability is poor,and the main roads in the area are low.Based on the hydrological model,the temporal and spatial variation characteristics of surface seeper were analyzed under 70 rainfall scenarios in the study area.The results show that under the same rainfall return period of the poisson rain type,with the increase of rainfall duration,the depth of water accumulation increases and the area of water accumulation increases.In space,it starts from the low-lying area in the middle of the study area,and the seeper area gradually spreads from the central part to the low-lying main road around it,as rainfall duration increases.Under the same rainfall duration,as the rainfall return period increases,the accumulated water depth increases,and the areas with serious water accumulation are mostly on both sides or intersections of the main road in urban areas.Under the normal rain type,the depth and area of the surface seeper change with time and the spatial distribution is similar to that under the poisson rain type in the study area.4.In this study,it is quantitatively analyzed that the difference of 4 diverse green roof schemes to reduce the risk of urban rainstorms waterlogging,and select the best option.Combined with the natural conditions of the study area,the implementation status of the LID measures and the feasibility of the measures,the Green Roof was selected to study its palliation effect on waterlogging during rainstorm and four schemes were designed.Taking the rain events of two rain types and rainfall duration of 60 minutes as an example,the surface water accumulation process with roof greening measures is simulated.Compared with the surface water accumulationprocess without LID measures,the four LID schemes were evaluated to alleviate the efficiency of urban rainstorms waterlogging in various rainfall scenarios.The results show that the four roof greening schemes can effectively reduce the risk of urban rainstorms waterlogging and generally perform better when the rainfall intensity is small.With the increase of rainfall intensity,the reduction of surface runoff,the relief of drainage pressure of the pipe network,and the reduction of the total area of maximum accumulated water depth showed a significant downward trend in each LID scheme,and the mitigation effect was relatively better under the poisson rain type than under the normal rain type.The slowdown of the total area of water accumulated in each water depth increases with the increase of water depth,and decreases with the increase of rainfall return period.Among the four schemes,the LID2 and LID3 schemes have a large slowdown in urban rainstorms waterlogging and are relatively stable under various rainfall scenarios.Therefore,the LID scheme design should be carried out for the specific waterlogging characteristics of the study area.