| Stability is an important foundation for the health of urban human settlement environment.The development and utilization of surface space have seriously damaged the stability of ground environment,and more and more cities have happened land subsidence.However,there are still uncertainties and spatial heterogeneity in the range,size,spatial characteristics and causes of land subsidence.How to effectively,quickly and accurately monitor land subsidence has attracted widespread attention.In this study,Nansha district of Guangzhou was taken as a example.Firstly,the land subsidence in Nansha district was monitored by 34 Sentinel1-A SAR images based on SBAS-In SAR technology from June 2015 to April 2018,and the spatial and temporal characteristics were analyzed to identify and locate the high risk hot spots of land subsidence.Secondly,the validity of land subsidence is verified by four methods: the rationality of subsidence rate,the different datasources with same method,leveling data and field investigation.Then,based on the results of subsidence monitoring,the stability of large linear infrastructure,shoreline and other linear features and their surrounding areas are further evaluated by using buffer zone and profile analysis methods.Finally,combined with the basic geographic information data such as terrain,social and economic factors such as population density,building density and floor area ratio were obtained.Under the grid size of 500m*500m,356 effective grid data and 7 indicators used to represent the potential influencing factors of land subsidence were selected.Multi-scale geography-weighted regression(MGWR)was used to explore the potential impacts of different types of natural factors and human activities on the spatial and temporal pattern of land subsidence in Nansha district in 2016.The main conclusions are as follows:(1)SBAS-In SAR technique can better monitor the spatial distribution of land subsidence in Nansha district.The land subsidence in Nansha district is stable on the whole,and the southern part is higher than the northern part.There are four key subsidence areas with continuous subsidence,and the average subsidence rate in the whole area is-1.81mm/a.With the settlement rate within 5mm/a as the stability standard,the land subsidence of Wanqingsha Town and Longxue Street under the jurisdiction is the most serious,the land subsidence of Dongyong Town,Hengli Town,Dagang Town and Zhujiang Street is relatively stable,Huangge Town,Lanhe Town and Nansha Street has some uplift phenomenon,but it is still within the controllable range.Seasonal differences of subsidence were obvious in 2016.In2016,the cumulative subsidence reached the maximum of-56.4mm,and subsidence rebounded in some areas in summer with uplift of about 6mm,and the cumulative subsidence reached the maximum in winter.(2)Various accuracy verification methods prove that SBAS-In SAR is effective and reliable in extracting land subsidence information in Nansha district.Firstly,the frequency histogram of the monitoring results of land subsidence is to the left of zero,which is in line with the prior judgment of the overall stability of land subsidence in Nansha district.Secondly,the monitoring results are cross-verified with the data of different polarization modes in the same method,and the average monitoring results of VV polarization and VH polarization are 2.09 mm and 1.01 mm respectively.The root mean square errors were 1.12 mm and 2.65 mm,respectively,which showed a good consistency.Then,through historical leveling data and field visits,the corresponding subsidence trend was found in the monitoring results.(3)Land subsidence characteristics and analysis results of linear ground features show that within the 500 m buffer zone along the line,the subway stations of Guangzhou Metro Line 4(Nansha Part)are more stable than those along the line.The main subsidence lines are concentrated between Jiaomen Station and Jinzhou Station.The subsidence rate of Jinzhou Station is the highest,and the maximum subsidence rate of Jinzhou Station is about-5mm/a.But less than the maximum sedimentation rate of-10.47 mm/a along the line.However,except for slight uplift of S4-5km and S43-45 km in Nansha Port Expressway,land subsidence has occurred in other sections,and the S43-45 km section intersecting with the key serious subsidence area needs to be monitored in time.In addition,the shoreline subsidence of Nansha is closely related to the seashore exploitation activities,and the wetland and water body subsidence rate is the largest,but the accuracy is the lowest.The settlement rate of impervious water surface and cultivated land is more serious,while the settlement rate of forest is the least.The maximum settlement of Nansha Port wharf on the shore segment appeared on January 6,2018,with a cumulative settlement of about 26 mm.(4)Potential influencing factors of land subsidence show that the spatial unit size of 500m*500m is the optimal scale for analysis of influencing factors in Nansha district.In 2016,the accumulated land subsidence in Nansha district is most strongly correlated with groundwater and fault zone.Elevation,building density,floor area ratio and distance from fault zone are positively correlated with the accumulated land subsidence,while the distance from river is negatively correlated with the land subsidence.Therefore,the main characteristics of high cumulative subsidence are distance from the river,close to the fault zone and low altitude.And the scale of different land subsidence factors is very different.Floor area ratio and building density are much higher than other variables,while the distance from the fault zone is the second.Due to the spatial heterogeneity and scale distance of the influence degree of the potential influencing factors on land subsidence,different natural and human activities either independently or jointly affect the spatial distribution of land subsidence in Nansha district.Therefore,the spatial difference and scale effect of different influencing factors can be considered when making the policy of land subsidence disaster prevention and control,so as to make the decision more targeted. |
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