Change Patterns Of Flood Disaster And Their Disaster-causing Mechanism In Highly Urbanized Areas

With the rapid development of global climate change and,urbanization,extreme weather events have increased and enhanced,and the problem of flooding in highly urbanized areas has become increasingly prominent,becoming an important factor limiting the economic and social development of China.Heavy precipitation is a direct cause of flooding.The impact of urbanization on storm water is inevitable in the context of global climate change.Therefore,an in-depth analysis of the characteristics of the spatial and temporal distribution and evolution patterns of heavy rainfall and flooding in highly urbanized areas,and an analysis of the causes and mechanisms of heavy rainfall and flooding in highly urbanized areas is an important scientific problem focusing on the foundation that needs to be solved urgently.Taking the Yangtze River Delta region as an example,linear regression method,MannKendall trend test method and extreme variation method were used to analyze the change pattern of flooding in highly urbanized areas.The changing patterns of extreme precipitation and socioeconomic factors were analyzed.Using hyperbolic tangent function to reveal the relationship between causative factors and disaster losses.Revealing disaster-causing mechanisms with disaster loss data from four cities:Shanghai,Hangzhou,Huzhou and Jiaxing.The following conclusions were mainly obtained:(1)Use of publicly available provincial and municipal flood and drought disaster bulletins.The rate of change of flooding was analyzed.The impact of urbanization rate on flooding was revealed.The proportion of the affected population to the resident population and the proportion of direct economic loss to the GDP of the year both show a decreasing trend.Among them,the rate of decline of the affected population is 0,2565%/year,and the rate of decline of the direct economic loss is 0.0177%/year.The spatial distribution of the affected population and direct economic losses is similar,showing a distribution with more in the north and south and less in the center.With the increase of urbanization rate,the loss rate of affected population and direct economic loss shows an exponential decrease.As the percentage of affected population increases,the percentage of direct economic loss also shows an increasing trend,i.e.,there is a strong positive correlation between them,the correlation of determination of the 0.7573.For every 1%increase in population,the direct economic loss increases by 0.0535%.(2)The change pattern of extreme precipitation was analyzed by using precipitation data from 25 meteorological stations in the study area,and the change pattern of socio-economic factors was analyzed by using piblic socio-economic factors by linear regression method and land use transfer matrix.The trend of annual maximum continuous precipitation days(CWD)is not obvious,and the trend of increasing extreme precipitation indicators(MAXI and MAX3)is obvious.The annual maximum daily precipitation(MAXI)passed the significance test whenα=0.05.Population density shows an upward trend.Between 1990 and 2010,the growth rate was 15.361 persons/km2/a,and after 2010,the growth rate gradually slowed to half of the original rate,at 7.8621 persons/km2/a.The natural population growth rate has shown an overall downward trend,and it fell below 1‰ again in 2019.Economic density shows an upward trend,and the growth rate of economic density after 2000 has increased rapidly to 10 times of that after 1990,and the economy is gathering in Suzhou,Wuxi,Shanghai,Hangzhou,etc.Before 2000,the area of cropland,forest land and water body accounted for a relatively large proportion,which could basically reach about 80%of the land in the whole watershed,and there was relatively little transformation among different land types.After 2000,the development of the city has the characteristics of being along the river,around the lake and along the coast.At the same time the transformation of different land types becomes dramatic,the ratio of cropland area decreases at a similar rate as the ratio of building land increases,while the ratio of other land types remains basically unchanged.Operating speed of different road network on working days and nonworking days have different patterns.During a weekday day,the speed variation shows a clear"tidal" phenomenon,with a significant decrease in speed during peak hours compared to other period of the day.On non-working days,only traffic road network speeds on daytime are shown to be lower than nighttime speeds.(3)The development of flood losses in highly urbanized areas exhibits three stages of initiation,development and conclusion.The beginning and end phases are gradual,and the development phase is a phase of dramatic increase in losses.The shape of the function is that the disaster development process is "S" shaped with the increase of the disaster driver.The bestfitting rainfall index was selected as a representative value of the regional drivers of disaster.The representative values of the causative drivers for Shanghai,Hangzhou,Huzhou and Jiaxing are annual maximum 24h rainfall,annual maximum 3d rainfall,annual maximum 3d rainfall and annual maximum 3d rainfall,respectively.The simulated values of the flood loss model of the four cities are in good agreement with the measured data.Using the constructed flood loss model,the disaster losses generated by the disaster-causing factors in Shanghai,Hangzhou,Huzhou and Jiaxing were calculated for the 50-year,100-year and 500-year events.The minimum values of the intensity of the causative factor when the upper limit of loss was reached were calculated using the model as 350.0 mm,420.0 mm,480.0 mm,and 495.0 mm.