长江河口湿地演变对流域来沙减少的响应机理研究

The decrease in river sediment discharge into the sea has become a global concern in recent decades, since it has led to the erosion of many deltas. For Yangtze River, the decrease in sediment discharge began in the late 1960s. As a result, the recession of the Yangtze estuarine wetland at the Yangtze delta front has been observed since 2003. Most of previous studies have focused on the subaqueous Delta, the study of the estuarine wetland or nearshore area was documented poorly. It is necessary to conduct further research on the estuarine wetland since it can provide complementary evidence for the evolution of the whole delta system, under the background of a decreasing riverine sediment supply and sea level rise. The study focus on mechanism of the estuarine wetland evolution in response to decline in sediment supply from the Yangtze River, based on the remote sensing images, bathymetric data and in-situ observation data of more than 40 years since 1970s. It mainly covers three aspects. Firstly, the evolution process of the estuarine wetland, responding to the reduced sediment load, will be discussed. On this basis, to evaluate quantitatively the contribution of the riverine sediment load among multi-faceted influencing factors to the wetland evolution in the Yangtze estuary. Secondly, to assess the influence of the decrease in sediment discharge on the spatial distribution of the estuarine wetland, by exploring the relationship between the changes of the spatial distribution of the estuarine wetland objects, including wetland vegetation, wetland topography, bare tidal flat and submerged shallow areas, and the decreased sediment discharge. Lastly, based on the study of evolution process and spatial distribution of the estuarine wetland, the vegetation succession, shoreline change, longshore sediment transport and accretion and erosion of topography will be focused on, and their interaction and the effect of the decreased sediment discharge on them will be investigated, based on which a response model of the estuarine wetland evolution to reduced riverine sediment load will be developed. Expected results may be helpful in maintaining sustainable development of wetland and potentially provide a more overall picture of the evolution of the entire Yangtze River delta, combined with previous findings on subaqueous delta.

长江河口湿地是长江三角洲地区的宝贵资源和生态安全屏障。近年来受流域来沙减少的影响已呈现退化迹象，对区域社会经济的可持续发展构成了极大的威胁。本项目以长江河口湿地作为研究对象，综合1970年代以来40余年的遥感、海图和实地调查观测数据，开展河口湿地演变对流域来沙减少的响应机理研究：1）分析流域来沙减少背景下河口湿地的演变过程及响应，定量研究流域来沙减少对河口湿地演变的贡献；2）通过考察河口湿地植被、地形地貌、光滩、潮下带浅水域等湿地对象的分布格局及其与流域来沙减少之间的关系，辨析流域来沙减少对河口湿地分布格局的影响规律，掌握其影响范围、程度及影响途径；3）综合湿地演变过程和分布格局变化规律，重点分析湿地植被演替、岸线变化、沿岸输沙、地形冲淤等方面的交互影响及受流域来沙减少的影响规律，构建河口湿地演变对流域来沙减少的响应模型，揭示其响应机理。该研究将为全球变化背景下的河口湿地演变奠定理论基础。

河口湿地是地球上最重要的生态系统类型之一，近年来随着全球入海泥沙通量的降低和海平面的上升，河口湿地面临着严峻的威胁。但由于受到潮汐影响河口湿地的很大一部分会被潮水周期性地淹没，特别是像长江河口这样的世界级大河河口，很难实施大范围、长周期的全面观测，长期以来对长江河口湿地的研究存在着一定的空白。本项目利用1975年以来的遥感影像数据、历史海图数据和水动力数据，重点研究了长江河口湿地的时空分异模式，对流域来沙减少、围垦工程、深水航道工程、生态工程、海平面上升等对河口湿地演变的影响分时段、分区域进行了详细和深入的分析，并对崇明东滩鸟类国家级自然保护区和九段沙湿地国家级自然保护区两个重点湿地单独进行了研究。同时，研究了基于光学遥感影像数据的波浪特征参数的获取方法和基于GIS的淤泥质海岸侵蚀风险评估方法。重要结果包括：（1）1970年代以来长江河口湿地总体上呈降低趋势但存在区域差异，1990年代中期是长江河口湿地面积变化的拐点。在1990年代中期之前湿地面积基本保持稳定，之后降低趋势明显，湿地面积在1990年代中期达到最大值，约为1495.2 km2。当前长江河口湿地面积已降低到不足1000 km2，降低了约1/3。（2）研究区湿地主要集中在苏北沿岸、崇明岛周边、九段沙和南汇边滩，苏北沿岸湿地面积最大，南汇边滩湿地受围垦的影响减少最为明显。（3）长江河口湿地面积的波动与流域来沙的变化趋势基本保持一致，但长江入海泥沙的持续降低始于1980年代，而河口湿地面积的显著减少始于1990年代中期，说明河口湿地对入海泥沙的变化存在一定的延迟；同时，长江河口大面积的围垦工程也开始于1990年代中期，这一时间点与湿地面积变化的拐点一致。据此我们判断，到目前为止围垦工程可能是长江河口湿地面积减少的最重要因素，流域来沙也是其演变的主要控制因素之一。此外，深水航道工程、生态工程、海平面上升都会对长江河口湿地的演变产生一定影响。本研究的成果为全球变化背景下的河口湿地演变理论提供了技术资料。

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