| Climate change leads to extreme weather and climate events show a trend of frequent,widespread,strong and concurrent.In the process of rapid urbanization,the urban underlying surfaces have changed,leading to frequent urban waterlogging disasters,which seriously endanger the life and property safety of urban residents.The mode of urban development has changed from incremental expansion to stock renewal.In the face of frequent waterlogging disasters,urban downtown are crowded with people,outdated facilities,and limited space.It is urgent to find an effective "micro-regeneration" mode to improve the ability of cities to cope with waterlogging disasters.Taking Xi’an downtown(within the Third Ring Road)as the research object,this study formulated the regional waterlogging control and urban regeneration countermeasures under the guidance of urban micro-regeneration and employing sponge city construction based on the waterlogging risk assessment and hazard-formative environment research of Xi’an downtown.The main results are as follows:(1)The waterlogging high-risk areas in Xi’an are mainly concentrated in the moat drainage zone,Xiaozhai area,and Tuanjie Village area in Weiyang District.Since a large number of urban residents and most kinds of social-economic infrastructures,including life,business,science and education,and entertainment,are highly concentrated in the urban center within the Second Ring Road of Xi’an,High-Tec Development Zone and Qujiang New Area,the urban waterlogging vulnerability index of built-up area in Xi’an shows a gradually decreasing trend,from the city center to the surrounding,from south to north,from east to west.(2)The comprehensive waterlogging risk in the built-up area of Xi’an shows an attenuation trend from the city center to the surrounding,forming a high-risk area with one center(moat drainage zone)-two sub-centers(Xiaozhai area and Beiguan area)and a ring-shaped sub-high-risk area.There are four types of situations in high-risk areas and sub-high-risk areas: high risk-high vulnerability,high risk-medium vulnerability,high risk-high vulnerability,and medium risk-high vulnerability.Different countermeasures should be taken according to the type of the situations of urban waterlogging disasters.(3)The factors with the highest explanatory power for the waterlogging risk in Xi’an built-up area are the landscape aggregation index and the maximum patch area index,and their explanatory powers for the spatial differentiation of waterlogging are 12.08% and8.91%,respectively;geomorphology,slope,green patch area,and land-use factors have low explanatory power on spatial differentiation of urban waterlogging.Therefore,the spatial pattern of urban green space plays a vital role in the formation of waterlogging disasters in the built-up area of Xi’an.(4)The spatial distribution of slope,maximum patch area of green space,and landscape aggregation index have a significant correlation with the risk of urban waterlogging: as long as the slope is larger,the maximum patch area index is smaller,and the landscape aggregation index is smaller,the urban waterlogging risk is higher.The spatial differentiation of urban waterlogging hazard is not the result of a single factor but multiple factors.Among them,the synergy of elevation factors and other factors has more significant impact on the risk of urban waterlogging,followed by the synergy of landform factors and other factors.(5)LID measures can sufficiently reduce the total runoff,reduce the runoff peak,and delay the runoff generation time and runoff peak.However,with the increase in the return period,the capacity of LID measures to reduce runoff is significantly weakened after reaching saturation.The regulating pond mainly plays a role in the process of rainwater transportation.The regulating effect of the regulating pond on delaying the time of outflow and the peak time of outflow is better than that of LID measures.Especially in regulating the peak time of outflow,the regulating pond can better delay the peak time with the increase of the return period.(6)Under the six return periods of the two-level regulation combined measures of "LID measures + regulating pond,” the outflow time was delayed by 34 min,33min,33 min,32min,32 min,and 31 min,and the total outflow reduction rates were 79.1%,58.7%,41.0%,33.4%,29.2%,and 27.1%,respectively;With the increase of the return period,the peak time of outflow was postponed by 23 min,17min,15 min,15min,17 min and 20 min,and the peak value of outflow was reduced by 87.6%,71.2%,52.9%,44.1%,and 33.9% respectively.It can be seen that under different return periods,the combination of LID measures and the regulating pond can not only better reduce the peak value of discharge but also further delay the peak time of discharge,thus relieving the overall pressure of the urban pipe network.(7)The moat drainage area is densely populated,with old houses,many cultural relics and historic sites,and backward infrastructure,making it difficult to regenerate the city.The leading causes of waterlogging disasters in this region include: the remaining problems in the process of urbanization;Low-lying terrain;Combined sewer,construction of low standard pipe network,and jacking of moat water level during rainstorms;The green space rate is low,and the green space structure is unreasonable.Coping with the above problems,the strategies that should be followed in the urban regeneration of moat drainage zone include: adhering to the overall planning,focusing on the large and starting from the small in the regeneration process;Optimize the urban pattern,and improving the urban green space system;The sponge city transformation should combine source control with strengthening and regulation of rainfall runoff,to improve the overall disaster resistance of the city;Strengthen the popular science propaganda to enhance public participation in the process of urban regeneration. |
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