The Risk Assessment System And Risk Management Of Land Subsidence In Shanghai

Relied on a lot of long-term monitoring data of land subsidence, and according to themechanism of land subsidence in Shanghai area, this study has considered the serious impactof land subsidence disasters on social and economic sustainable development, andsummarized the main condition which can decide the risk level of land subsidence inShanghai. This study has carried out the research work of risk assessment on land subsidencein Shanghai, including the determination of the risk assessment elements, the establishment ofrisk assessment system and building risk assessment model. In addition, this study also hasanalysized the control effects of land subsidence risk by artificial recharge of groundwater. Atlast, this study has raised the partition management countermeasures on the risk of groundsubsidence in Shanghai. The main conclusions are as follows:1. Based on the geological background analysis of topography, bedrock geology,Quaternary geology, humanities geological conditions, and engineering geological conditionsin Shanghai, and combied with the history of land subsidence in recent years, this study hasanalyzed the causes of land subsidence in Shanghai, including: the Shanghai region formedtwo depressions and three ridges, in the Quaternary loose piled is thick in depressions band,which constitute the material basis of ground subsidence; the groundwater compareconcentrated in Shanghai, and it leads to serious land subsidence and sedimentation funneldevelopment as well as a number of ground fissure zones; the development and utilization ofgroundwater caused the changes of groundwater flow field, which caused a great impact onthe relative size and distribution of land subsidence. In addition, construction activities havean impact on land subsidence. Land subsidence has caused the damage of buildings, relativeuplift of wellhead, reduce of clearance dong, destroy of municipal facilities, failure of port orbank, intrusion of river water, flooding, salinization of farmland.2. According to the characteristics and mechanism of land subsidence in Shanghai, therisk factors of land subsidence in Shanghai include two elements, hazard and vulnerability.Combined with the actual situation in Shanghai, the increased hazard factors of landsubsidence can be summarized as prone extent of ground subsidence, historical disasterintensity, forecast sedimentation rate and terrain elevation; the increased vulnerability factors of land subsidence can be summarized as urban population density, GDP per unit area, theproportion of land for construction, as well as municipal facilities, including the length offlood wall, the density of water pipe, the density of elevated and rail traffic. The impactfactors of reduced vulnerability are the inputs of disaster protection, including the inputs ofdisaster protection and recharge per unit area.3. The main contents of risk assessment on land subsidence in Shanghai are as follows:the establishment of the model on hazard, vulnerability and risk assessment.The hazardassessment model was established by improved fuzzy AHP. The vulnerability assessmentmodel was established by discriminant fuzzy comprehensive evaluation method. The riskassessment model was established by statistical analysis method, and based on Mapgis.4. The conclusions of hazard assessment on land subsidence in Shanghai are as follows:Shanghai is mainly in the middle hazard areas of ground subsidence. The high-risk areas aremainly distributed in the central city and Jiading, Jinshan areas which are at the junction ofJiangsu and Zhejiang provinces. The low risk areas are mainly distributed in thewell-developed geological environment and the low prediction rate of land subsidence, suchas Qingpu, Fengxian areas. In addition, Sandao region are in the middle hazard areas of landsubsidence.5. The conclusions of vulnerability assessment on land subsidence in Shanghai are asfollows: the vulnerability of land subsidence in downtown is most serious. The landsubsidence is more serious in Pudong areas, leading to the regional subsidence funnel inGaoqiao, Zhangjiang, Tangqiao, which are classified as high vulnerability zone. Baoshan andMinhang are classified as moderate vulnerability zone. Songjiang, Nanhui, and JiadingDistricts are classified as the lower vulnerability zone. Fengxian, Chongming, Jinshan, andQingpu Districts are classified as the lowest vulnerability zone.6. The conclusions of risk assessment on land subsidence in Shanghai are as follows: thethree level of land subsidence system formed with the central city as the center. Thedowntown is in the highest risk areas of land subsidence, peri-urban areas are in the higherrisk areas of land subsidence to the middle risk areas, and the rural is in the low risk areas ofland subsidence.7. With the control of artificial groundwater recharge increased, the level of fourth confined aquifer groundwater was generally upward, and the center of the city, as well asBaoshan District, rise more than10m. Artificial groundwater recharge causes theconductivity of recharge water mixed with the groundwater decreased, redox potential rised,dissolved oxygen content increased, pH value decreased, the concentrations of chloride ionand bicarbonate ions decreased, the concentrations of sulfate and nitrate ions increased, theconcentrations of sodium, potassium calcium and magnesium ions decreased. In addition,with groundwater recharging, the total iron and manganese contents was decreased, which canmeet the drinking water standards. But with the recharge going for25-35days, the iron andmanganese mineral components was re-dissoluted, leading to the concentrations of heavymetals cannot meet the drinking water standards. The concentrations of TOC and COD weregradually increased, but did not exceed the back concentration of recharge source, which didnot cause any pollution. In addition, the water quality after artificial groundwater rechargemay contain hazardous chemical substances and pathogenic microorganisms, which arehazard on human health and ecosystems. Therefore, the groundwater quality after artificialrecharge has some effects to groundwater environment.8. With the risk assessment results of land subsidence in Shanghai, this study furtherproposed the risk management countermeasures of land subsidence in Shanghai. According tothe spatial distribution characteristics of land subsidence in Shanghai, this study raised thecountermeasures about three corresponding system of risk management divisions, includingthe focus area of prevention and management of land subsidence (Ⅰ area), the sub-focusarea of prevention and management of land subsidence (Ⅱ area), as well as the general areaof prevention and management of land subsidence (Ⅲ area). The detailed range of eachmanagement area was divided, and the risk management priorities in each managementdistrict was proposed targeted from the engineering measures, planning control, monitoring,forecasting and early warning, avoidance, acceptance and education of disaster prevention andmitigation.