Study On The Coupling Relationship Between Urban Expansion And Mountain Hazards Risk In Sichuan And Yunnan Province

Abstract:With the advancement of economic globalization and market economization,the urbanization level has been rapidly improved.Urban expansion has become the most direct form of urbanization in space.The rapid expansion of cities has deepened the contradiction between urban development and disaster risk,especially in mountainous areas.Grasping the spatial and temporal coupling relationship between the urban expansion characteristics and the mountain disaster risk is conducive to identifying the problems existing in the process of urban expansion in mountainous areas and proposing the optimization strategy of urban expansion,which is of great significance for improving the urban resilience level and promoting the sustainable urban development.In this paper,278 mountainous counties in Sichuan and Yunnan province are regarded as research units,and the spatial-temporal characteristics indicators of urban expansion are used to quantitatively analysis spatial and temporal characteristics of urban expansion in the study area.The random forest model(RF)which belongs to a machine learning algorithm is used to the dynamic risk assessment of mountain hazard.Geographically and Temporally Weight Regression model(GTWR)is applied to reveal the dynamic coupling relationship between urban expansion and mountain hazards risk and its evolution path.Then county samples are selected to explored coupling model between urban expansion and mountain hazards risk at a micro-scale,and the targeted strategies for urban expansion are proposed.So as to improve the urban resilience level and promote the safe and sustainable development of cities in the Sichuan and Yunnan province.The main conclusions are as follows:(1)There are obvious spatial and temporal differences in the urban expansion in Sichuan and Yunnan province.Urban construction land shows a trend of continuous expansion,mainly occupying cultivated land,forest land,and grassland.The expansion rate of urban areas has shown a pattern of rapid rise and then slowdecline.The urban expansion intensity index(EI)also shows a continuous upward trend.Low-value areas dominate the study area,and there is a spatial distribution characteristic that expands from the center to the surrounding areas.The urban expansion in the study area is dominated by the marginal expansion,and the filling expansion city is the least.The fragmentation of urban landscape shows the change rule of decreasing first and then increasing,and the overall situation of east high and low west along the Hu Huanyong line becomes more and more obvious.(2)Mountain hazards risk is generally highly in the southeast and low in the northwest along the Hu Huanyong line for the study area,and high-value areas are roughly distributed along fault zones,river valleys,and low hills.The high and higher risk areas are mainly distributed in the Longmenshan fault and the Xianshuihe fault zone,along the Anning river valley,the eastern Sichuan hills and ridge-valleys,and the Nujiang valley area.The medium-risk,the high-risk,and the higher-risk areas are adjacent to each other.Medium-risk area are mainly concentrated in eastern and southern Sichuan hills,Daliang and Xiaoliang Mountain,northwestern Yunnan plateau,Nujiang valley,and some counties in the Anning valley.The lower risk areas are distributed in the lower hills,the pingba area of Yunnan-Guizhou plateau and the north plateau area of west Yunnan.The low risk areas are mainly distributed in the west Sichuan plateau in various districts and counties.(3)At the macro scale,there is a strong coupling relationship between mountain disaster risk,urban expansion intensity,and urban landscape fragmentation,which presents different spatial differentiation characteristics in different periods.The coupling relationship between the MESH change rate of the urban landscape and the disaster risk shows a strong spatial agglomeration characteristic,and the MESH change rate influence change trend is gradually from the negative to the positive.It indicates that the positive promoting effect of the urban landscape fragmentation on the disaster risk is weakened and the negative inhibiting effect is gradually strengthened.The impact of the EI value on disaster risk shows a gradual change from positive to negative,similar to landscape fragmentation,indicating that the EI has gradually weakened the disaster risk promotion effect of the study area,and the negative inhibition effect has gradually increased.The evolution path of the dominant coupling relationship types in the counties of the study area is dominated by"E-E-E"type,"E-E-M"type counties show a circular distribution in the Yunnan-Guizhou plateau.(4)At the micro scale,the spatial clustering characteristics of the dominant coupling relationship between urban expansion and mountain hazards risk in the case study area are obvious,which can be divided into four coupling types:"E+","E-","M+"and"M-",and the"E+"type has the largest proportion.The"E+"type urban expansion units are mainly distributed in Tongdu,Tangdan and Tuobuka township in Dongchuan county,as well as the mountainous and hilly area in the north of shifang city,with an urban expansion area of 11.12 km2.The"E-"type urban extension units are mainly distributed in Tangdan,Yinmin,Wulong,Awang Shekuai,and Hongtudi township in Dongchuan county,with the urban expansion area of 2.29 km2.The"M+"urban expansion units are mainly concentrated in Shifang county and distributed adjacent to the"E+"urban expansion units,with the urban expansion area up to 1.39km~2.The"M-"type urban extension units are scattered in Tongdu,Tangdan,Bingchuan and Jiandi township,with the urban expansion area of 0.35 km2.(5)Based on different coupling modes between urban expansion and mountain hazards risk,adopting differentiated urban expansion optimization strategies and combining the urban expansion optimization framework based on the concept of resilience can improve the overall level of urban resilience."E+"type cities should reasonably control their expansion intensity,strengthen their scientific planning,and actively promote the construction of disaster prevention projects."E-"type cities should appropriately enhance the intensity of urban expansion,promote the decentralized distribution of population and economy,and strengthen the construction of roads infrastructure and slope protection projects."M+"type towns should appropriately improve the fragmentation of urban landscape,reasonably reduce the compactness of city,and take relocation and migration measures when it is necessary to reduce the impact of regional mountain disaster risk."M-"type cities should pay attention to the reconstruction and optimization of the town interior,promote the group development of the town,and improve the compactness.In addition,Incorporating the concept of resilience throughout the entire process of urbanization development,and promoting the optimization of urban expansion units'layout and the resilience construction in the process of urbanization are conducive to improving the overall resilience level and promoting the sustainable development of cities.