Influence Of Urbanization On River System And Hydrological Process In Taihu Plain

Rapid urbanization in China has severely disturbed the underlying surface and river systems,causing a series of flood disaster.Taking Suzhou-Wuxi-Changzhou area,a typical plain river network in Yangtze River Delta,as an example,based on the remote sensing images of the 1970s,the 1990s and the 2010s and the river system data of the 1960s,the 1980s and the 2010s,using RS/GIS technology and spatial analyzing method,the streams structure parameters were adopted to reveal temporal and spatial variations of river structure in plain river network areas during urbanization.Combined with the law of urban expansion in highly urbanized areas,we mainly discussed the disturbance of different urban expansion modes on impervious area and different levels of rivers.Meanwhile,the evolution of relations between city spatial expansion and river system structure was analyzed.On this basis,this paper mainly discussed the influence of urbanization on the structural and hydrological characteristics of the river network by applying statistical methods and simulation analysis,and the variation law of static regulation and storage function of river network was explored.At the same time,combined with the flow calculation of different urbanized areas,the influencing pattern of different urbanization processes on river system structure and hydrological processes was detected.This study can provide reference and theoretical support for river system protection,urban planning and flood prevention and mitigation in urbanized areas.The results show that:(1)The process of urban sprawl follows different stages and has different trends,oriented to urban planning policies,and is influenced and restricted by both the natural environment and the social economy.The process of Suzhou-Wuxi-Changzhou urbanization has developed rapidly and can be divided into two stages.From the 1970s to the 1990s,urbanization developed slowly,with expansion rate at about 1 km2/a,but rapid development occurred during the 1990s-2010s.As the urbanization process continued to develop,urban space continued to expand at a rate of 4 km2/a.The city gradually merged into pieces,primarily at the expense of farmland and water surface area.(2)At different stages of urbanization,the spatial urban expansion and the natural river characteristics influence the evolution of the river network.Taking Suzhou as an example,the decline in surface water ratio and river density is more intense during rapid urbanization.The decrease of surface water ratio and river density in rapidly urbanized areas was the greatest,followed by that in area urbanized in the 1980s,with few changes in the old town.Under high urbanization,river system indicators is less intense and tend to stabilize.In the past 50 years,the attenuation trends of river density in these three districts are-21.06%,-16.23%and 1.88%,respectively.The development of river network structure is significantly affected by the demand for urbanization development in different periods.The structure of the river network in the natural state is relatively stable,and after entering the urbanization stage,the stability of river structure is reduced.(3)The hydrological characteristics of the river are deeply affected by the urbanization process.Compared to the stage before urbanization,the inter-annual water level,maximum and minimum water levels increase significantly in the stages after urbanization.Urbanization also has a profound impact on the regulation and storage function of the river network.In the past 50 years,the storage capacity and adjustable storage capacity in Suzhou central district have been significantly attenuated by 10.13%and 29.70%respectively.The tributary storage capacity is degraded more seriously.At the same time,the change of water yield has a strong response to the change of land use.The higher the urbanization,the greater the water yield will be.The water yield per unit area increased from 0.81 to 0.95 m3/m2 in the whole study area(1991-2015).Floods caused by the same rainfall intensity have a higher flood level and greater risk of flooding under rapid urbanization.In the period of high urbanization,the water yield changes in a reverse direction.Due to flood control pressure,measures are taken to increase urban green spaces and widen the main river channels.As a result,impervious surface and water yield will decrease.This study focused on the Suzhou-Wuxi-Changzhou area in the Taihu Plain,and the "urbanization?river system?hydrological process" in the highly urbanized areas was taken as the main line.A multi-disciplinary and integrated approach was applied to combine the combination of physical geography and human geography,statistical analysis and model simulation to explore the influence of different urbanization processes on river system structure and hydrological characteristics.The research results can provide reference for urban planning and river system protection in the Yangtze River Delta.And they also have guiding significance for exploring China's new urbanization road.