SEAWALL SUBSIDENCE IN SHANGHAI: CHARACTERISTICS AND DRIVING MECHANISMS
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摘要: 上海是地面沉降较为严重的地区,持续的地面沉降使上海面临着风暴潮的潜在威胁。利用2009-2015年的海堤高程测量成果,对上海海堤沉降时空变化特征以及驱动因素进行分析,结果显示,上海海堤沉降呈现明显的时空分异性,7年来上海海堤沉降总体呈现了加剧-减缓-加剧的波动性,沉降岸段占比为87%,70%以上的岸段沉降与抬升的变化范围在20 mm/a以内;沉降较为严重的地区发生在浦东机场外侧、南汇嘴以及奉贤部分岸段,宝山、金山部分岸段以及崇明南侧缓慢沉降;崇明西侧以及长兴岛地区发生一定量的抬升;上海海堤沉降驱动因素复杂,下伏地层的岩性控制了海堤沉降的空间格局,地下水位变化与海岸带开发活动则是导致沉降速率时间上波动的主因,海堤沉降空间差异性与海堤建设时间相关性不强。鉴于工程建设对周边海堤的显著影响,建议在工程建设前后对海堤沉降进行长期监测,对发生沉降大的区域及时采取加高措施,降低风暴潮风险。Abstract: Shanghai has long been suffered from land subsidence since 1920s. Accumulated subsidence imposes a huge threat to the coastland, which could get high risk of inundation by storm surges under the combined impact of rapid sea level rise. Using high precision-leveling of 2009 to 2015 along the seawall, together with the historical GPS data and leveling covering the entire city since 1980s, we attempt to describe the vertical displacement of the seawall, and try to interpret the natural and anthropogenic mechanisms that induce the pattern of the displacement. Results displayed subsidence rates of the seawall exhibit significant spatial variability.87% of seawalls have experienced overall subsidence in the past 7 years. After analyzing the possible correlation between sediment rates and groundwater level, Quaternary sediments, thickness of clayey and compressible Holocene deposit as well as anthropogenic activities, we come to the conclusion that, consolidation of late Holocene deposit and the external loading are the primary factors responsible for the observed displacement variability, and groundwater level and land reclamation on the tidal flat also have significant impact on the rapid subsidence. Conversely, tectonic subsidence and differential post-construction settlement could not be the triggering factors that made the settlement in the coastal zone. Due to the significant impact of external loading from engineering construction on the seawall, continuous monitoring is urgently needed during the process of the projection construction.
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