Impact Of Urbanization On River Networks System And Its Storage And Flood Control Capacity In The Typical Areas Of Yangtze River Delta

River network system,regarded as "the pulse of the city",is reflective to economic development,social civilization and human settlements environment of a city.With the rapid urbanization,the natural process of river network systems has been disturbed across the Yangtze River Delta(YRD).On the one hand,the amount of rivers and lakes decreased dramatically,which cause a series of problems,such as the deterioration of the riverine habitat and the increasing in flood risk.On the other hand,the temporal variability in the river network,water level,flood control and storage capacity caused by rapid urbanization have led directly to the flood disaster,which may restrict the healthy development of urban and river nerwork system.Therefore,how to properly deal with the man-land relationship and the urbanization-river network system during the urbanization process is the key factor to guarantee the healthy development of urbanization.This study focuses on the flooding caused by river network systems decay in the rapid urbanization process and the contradiction between urbanization and river network protection.We used the integrated remote sensing and geographic information spatial analysis technology to analyze the land-river progress.The multi-disciplinary method including river geomorphology,hydrology,econometrics,et al.were also adopted in analysis process.The study took the following frame,"the evolution of river water system pattern-hydrology factors under urbanization-the development tendency and threshold range of urbanization and river network system-impact and changes of storage and flood control capacity under the urbanization".The study are to reveal the characteristics of variation of climate change and human activity,the impacts of both changes on the water level,and the correlation between precipitation and water level;to illustrate the evolution characteristics of river network system in typical areas of Yangtze river Delta under high urbanization;based on the healthy development of urbanization and river network system,to analyze the development tendency and threshold of between urbanization and river network system;to identify the changes of storage and flood control capacity,and reflect of flood process of river network systems under urbanization.The main content and conclusions are as follows:(1)Impact and evolution of the river network to urbanizationBased on the topographic map(1:50000),satellite image,water conservancy survey,the river network was extracted and river hierarchies were made.Several field investigations were also carried out to verify accuracy of the data above.The evolution characteristics of river network systems from 1960s to 2010s in Suzhou city were emphatically studied;the mutual feedback mechanism and trend between urbanization and river network systems were carried out;and the response threshold of Suzhou river network systems to the urbanization was revealed.The results indicated that:?The shape and structure of river network systems became simplified in Suzhou in the last 50 years.The drainage density,water surface ratio,fractal dimension,evolution coefficient of branch and river sinuosity were decreased by 5.6%,19%,1.2%,3%and 3.4%,respectively.Meanwhile,the river area length ratio increased by 1.5%.?The precipitation change and urbanization rate are important factors to alter drainage density,water surface ratio,coefficient of fractal dimension and tributary development.This displays a common sense within the relationship between river network systems with urbanization or precipitation.The response of river network systems change in Suzhou is more sensitive to urbanization,and about 67%of the change of river system is caused by urbanization.?The relationship between river network systems and the urbanization showed an "inverted U" curve,and presented a characteristics of " loose at the beginning,tight in the end",and lies in " natural development and antagonism" stage.(2)Impact and changes of the hydrological processes to urbanizationBased on daily precipitation and water level,the trend and discrete of precipitation and water level in flood season and non-flood season over the past half century were analyzed.The characteristics of flood level under different rain conditions and the hydrological alteration analysis were studied in Suzhou.The long-term relationships between precipitation and water level in flood season and non-flood season in last 55 years were revealed.And the contribution rate of urbanization and precipitation change to the water level alteration was assessed.Finally,the changes of water production under the influence of urbanization in the study area were analyzed.The results indicated that:?the precipitation shows increase trend,but the discreteness was on decrease trend;the overall water level,return period and flooding risk rise,while the discrete degree is decreasing.? high water level is more sensitive to precipitation,while the low water level tends to be influenced stronger by urbanization.The contribution of precipitation to water level changes is about 67%,and rest were contributed by urbanization.?the water production is relatively more uniform under the lower urbanization.With the rapidly urbanization rate,the regional spatial heterogeneity of water production of was getting larger.(3)Impact of urbanization on regional flood inundation and storage/regulation ca parity of river networksThe spatial and temporal characteristics of storage and flood control capacity of the study area were analyzed in this part.The spatial and temporal characteristics of water conservancy facilities and drainage capacity in highly urbanized areas were studied,and then the analysis of the inundation area and volume of the flood with different return periods under the artificial water conservancy facilities condition were conducted.The results showed that:?The overall storage and flood control capacity and the regulation storage capacity of the study area are improved.?The increase of water production in Suzhou and Kunshan were greater than other regions.The improvement of the maximum storage capacity was less than the increase of water production.The total water production is seven times of the maximum storage capacity in the study area.?Using urbanization rate as constraint conditions,we found that the threshold range of water surface ratio and drainage density are 12.5%and 3.1 in the study area.When the urbanization rate is 80%,the storage and regulation capacity of the river network is greater than 46346 m3/km2.Consequently,the current level has yet to be improved.Theoretically,Suzhou city can withstand the rain off type of 2013 year with the current storage and regulation capacity.The 1.05 billion m3 regulation capacity of the lake area nearby would be utilized,when the rain off type of 1991 year occurred in Suzhou.According to the inundation map,25%of Suzhou city area would be submerged under the rain off type of 1991 year.Due to the construction of the urban levees,the risk of urban floods transfers to the surrounding area.This study focuses on the flood disaster caused by river network systems decay,as well as the contradiction between urbanization and river network under highly developed urbanization.The multidisciplinary methodology was adopted in the analysis of the morphological and structural variation of river network systems in in Suzhou.The research results can support river network systems protection and flood risk control in the YRD.