| Under the dual background of climate change and urbanization,the frequency of extreme rainfall events has increased,and land use/cover changes have been obvious.In China,especially in some cities with severe rainstorms and waterlogging,the phenomenon of"urban watching the sea"occurs frequently,leading to increased losses and impacts.The Yangtze River Delta Eco-Green Integrated Development Demonstration Zone is located in the subtropical monsoon area.The overall terrain is low-lying,which is prone to rainstorms and waterlogging,and related waterlogging disasters have been reported.This area is the core area for implementing the national strategy of integrated development of the Yangtze River Delta.However,there are not many studies on the land use/cover change and the effects of heavy rain and waterlogging in the area,which cannot support the strategic development of the area.Based on the 1990?2010 land use/cover data and the 2020 Landsat 8 remote sensing data interpretation of the land use/cover data,this paper makes spatio-temporal changes of the land use/cover in the Yangtze River Delta Eco-Green Integration Demonstration Zone from 1990 to 2020.And transfer matrix analysis.Then use the SCS model to study the response of surface runoff depth to land use/cover changes by taking the design of the rainstorm return period of 100 years and the rainstorm duration of 1h as an example.Then construct the study area's return period of 2,5,10,20,50,100 years of heavy rain scenarios and revise the study area's elevation.The submerged water depth is calculated by the SCS model and the equal volume method,and the spatial mapping expression and statistical analysis are carried out.Finally,taking residential buildings in the study area as an example,the storm waterlogging risk assessment is carried out,and the surpassing probability-loss curve of the residential buildings in the study area is constructed,and the average annual expected loss of the residential buildings in the study area is the risk value.Research can provide references for urban planning,sponge city construction,and disaster prevention and mitigation,and is of practical significance.The main conclusions are as follows:(1)Analyze land use/cover changes in the study area from 1990 to 2020.The land use/cover changes in this area have been more obvious in the past 30 years.The main manifestations are the decrease of arable land(the proportion of which has dropped from 69.2%to 29.4%),and the increase of construction land(the proportion of which has increased from 10.4%to 42.9%).There are relatively few changes in woodland,grassland,and waters in this area.From 1990 to 2020,the types of construction land used in Qingpu District,Wujiang District,and Jiashan County have mostly increased.The increased spatial locations are mainly distributed in the northern area of Qingpu District,the northern and southeastern areas of Wujiang District,and the central and southern areas of Jiashan County.The study also found that the increase in forest land in the southern part of Wujiang District is more concentrated.Regarding the conversion of cultivated land types to construction land types in the Yangtze River Delta Eco-Green Integrated Development Demonstration Zone,from 1990 to 2020,cultivated land has been continuously converted to construction land,and the number of converted areas has been increasing every ten years.That is,from 1990 to 2000,from 2000 to 2010,from 2010 to 2020,from 1990 to 2020,the cultivated land type in the study area was converted to the construction land type,and the area conversion values were 44 km~2,292.9 km~2,485.5 km~2,and 767 km~2 respectively.Taking the study area from 1990 to2020,the area converted from cultivated land type to construction land in the latter ten years(2010 to 2020)is 11 times that of the area converted from the former ten years(1990 to 2000).The contribution of the area of cultivated land type converted to construction land type to the total area of construction land type converted from 1990to 2000,2000 to 2010,2010 to 2020,and 1990 to 2020 in the study area.Calculated,the contribution rates are 98.0%,93.2%,83.9%,and 90.1%,respectively.This result shows that the contribution of the conversion of cultivated land types to the total conversion of construction land types has been declining at intervals of ten years,but its value is still as high as 80%or more,and the conversion of cultivated land types is still the leading factor in the transfer of construction land.This result also means that the contribution of other types of land to construction land is increasing.(2)Explore the response of surface runoff to land use/cover changes.Taking a design rainstorm return period of 100 years and a rainstorm duration of 1h as an example,the response of surface runoff to land use/cover changes is studied.Through analysis,the average surface runoff depth in the study area has been increasing from1990 to 2020.With a ten-year interval,2000 compared to 1990,2010 compared to 2000,and 2020 compared to 2010,the average surface runoff increased by 0.8 mm,4.5 mm,and 4.4 mm,respectively.In 2020,compared with 1990,the average surface runoff depth in the study area increased from 67.1 mm to 76.8mm,with a total increase of 9.7mm.(3)Analyze the change of runoff depth under the scenario of the return period of heavy rain and waterlogging in the study area in 2020.The runoff depth in the study area increases with the increase of the return period.When the return period is increased from 1/2-year to 1/5-year,the average runoff depth increases by 10.8 mm;when the return period is from 1/5-year to 1/10-year,the average runoff depth increases by 8.5mm;the return period is increased from 1/10-year to 1/20-year,the average runoff depth increases by 8.6 mm;when the return period increases from 1/20-year to 1/50-year,the average runoff depth increases by 11.6 mm;the return period increases from 1/50-year to 1/100-year,the average runoff depth increases by 8.9 mm.When the return period increased from 1/2-year to 1/100-year,the average surface runoff depth increased from28.4 mm to 76.8 mm,with a total increase of 48.4 mm.(4)Analyze the changes of submerged area and submerged water depth under the scenario of the recurrence period of heavy rain and waterlogging in the study area in2020.The submerged area and submerged water depth of the study area increased with the increase of the rainstorm return period.When the return period increases from once in two years to once in five years,the submerged area increases by 63.8 km~2.When the return period is from once in five years to once in ten years,the submerged area increases by 47.7 km~2.When the return period increases from once in ten years to once in twenty years,the submerged area increases by 46.5 km~2.When the return period increases from once in 20 years to once in 50 years,the submerged area increases by60.1 km~2.When the return period increases from once in 50 years to once in 100 years,the submerged area increases by 44.5 km~2.When the return period increased from once in two years to once in a hundred years,the submerged area increased from 186.9km~2to 449.5km~2,a total increase of 262.6 km~2.The maximum submerged water depth is1.56 m.(5)Analyze the exposure and losses of residential buildings under the rainstorm waterlogging return period scenario in the study area,and evaluate their risks.Residential building exposure and losses are mainly distributed in Xujing Town in the east of Qingpu District,Jiangling Street and Songling Street in the north of Wujiang District,and Luoxing Street in the south of Jiashan County.With the increase of the rainstorm recurrence period,the exposure and loss of residential buildings gradually spread to the adjacent townships,and the exposure and loss value of residential buildings in the study area increased accordingly.Residential building losses increase as the return period of the storm increases.When the return period increases from 1/2-year to 1/5-year,residential building losses increase by 350 million CNY;when the return period increases from 1/5-year to 1/10-year,residential building losses increase by 340 million CNY;when the return period increases from 1/10-year to 1/20-year,residential building losses increase by 360 million CNY;when the return period increases from 1/20-year to 1/50-year,residential building losses increase by 540million CNY;when the return period increases from 1/50-year to 1/100-year,residential building losses increase by 400 million CNY.When the return period increases from1/2-year event to 1/100-year event,residential building losses increase from 1.59 billion CNY to 3.58 billion CNY,a total increase of 1.99 billion CNY.By constructing a probability-loss curve for flooded residential buildings in the study area,the value of risk for flooded residential buildings in the study area was calculated by integration to be 1.758 billion CNY. |
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