The Dynamic Expansion Of Cross-river Cities In The Middle Reaches Of The Yellow River And Its Influence On River Hydrology

In the context of ecological protection and high-quality development in the Yellow River Basin,the coordination of ecological land and construction land has become an important research topic.As a typical urban spatial form in the middle reaches of the Yellow River Basin,river-crossing cities are facing serious spatial constraints.It is of great significance to carry out the research on the spatial expansion evolution and river change of river-crossing cities in the middle reaches of the Yellow River Basin and put forward relevant optimization strategies for guiding the reasonable and orderly expansion of river-crossing cities.This paper takes the middle reaches of the Yellow River Basin as the research area,takes Suide,a typical river-crossing city,as the research object,uses Landsat series remote sensing images as the main data source,supplemented by hydrological and socio-economic data,and uses mathematical statistics,spatial autocorrelation,standard deviation ellipse and other methods to study the spatial and temporal characteristics of urbanization and the spatial distribution pattern of river-crossing cities in the middle reaches of the Yellow River from a macro perspective.Based on this,the supervised classification algorithm is used to extract the construction land,and the qualitative and quantitative analysis of typical cross-river urban expansion and encroachment on riverbank beaches are carried out from the perspective of timing and space.Finally,combined with the evolution of urban spatial morphology,the changes of urban river morphology and hydrological situation in Suide County are studied,and the problems and optimization strategies in the process of cross-river urban expansion are put forward.The main conclusions are as follows:(1)The urban population and built-up area in the middle reaches of the Yellow River have increased greatly,showing that land urbanization is faster than population urbanization.River-crossing cities show unique quantitative and spatial distribution characteristics in the middle reaches of the Yellow River.According to the quantitative characteristics,the provincial capitals Xi’an and Taiyuan only develop large-scale first-level tributaries of the Yellow River.The first-level tributaries of river-crossing cities in the prefecture level develop the most,followed by the second-level tributaries,accounting for 60%and 33%respectively,while the second-level tributaries of river-crossing cities in the county level develop the most,followed by the first-level tributaries,accounting for 47%and 33%respectively.In terms of spatial distribution characteristics,the distribution of county-level cross-river cities with different levels of tributaries is spatial aggregation.Among them,the second branch county-level river-crossing city distribution range is the widest,the fourth branch county-level river-crossing city spatial distribution is the most concentrated;with the increase of river level,the direction of cross-river city distribution is more obvious.(2)In recent years,the urban built-up area of Suide County,a typical county-level cross-river city,has been increasing.The expansion intensity has increased first and then decreased.The expansion form has gradually changed from "extension" to "filling".The indicators such as compactness,fractal dimension and radiation number show that the urban spatial structure of Suide County is loose,and the urban boundary is extremely tortuous and complex.The overall trend of the center of gravity transfer in the built-up area is in the southeast direction.The hot spots of the expansion are mainly around the Wuding River and Dali River.The closer to the urban river,the more intense the urban expansion is.The farther away from the river,the weaker the urban expansion is.In recent years,the inward extrusion of rivers and the occupation of river bank beaches have become the main form of expansion.(3)From the morphological evolution of urban rivers,the water surface area and river width of the Wuding River in the urban section of Suide City from 1986 to 2020 showed a serious attenuation trend.During the study period,the river channel shape fluctuated greatly,and the scattered flow was mostly irregular.The river bending coefficient fluctuated and the degree of shoreline fragmentation on both sides increased significantly.From the change of hydrological situation of the river,after the river in the urban section of Suide was occupied by the bank beach,the flow capacity of the river was weakened.The average peak time decreased from 11.63 h in 1986-1990 to 10.53 h in 2013-2020,and the average peak time decreased to some extent.(4)Based on the study of the spatial evolution process and river change of the typical cross-river city of Suide,this paper summarizes and analyzes the problems faced in the expansion of cross-river cities:inefficient urban sprawl,loose spatial form,unbalanced functional structure,unreasonable layout,excessive axial development,insufficient transportation and facilities service ability,blind occupation of construction land,and increasingly shrinking riverbank beaches.In view of the above problems,the corresponding optimization strategy suggestions are put forward:rational planning of urban space,revitalizing the existing construction land,intensive and economical use of land,strictly adhering to the urban ecological blue line,and realizing the sustainable and intensive development of cross-river cities.