潮流脊影响下开敞型潮滩的地貌动力过程及演化模式

The high spatial and temporal resolution of suspended sediment concentration and seabed elevation changes will be measured to estimate the sediment transport and tidal flat profile evolution by using the advanced instruments (e.g. MIDAS400, ADV, OBS and GPS-RTK) along the cross-sections in the tidal flat-sand ridge system in the southern Yellow Sea. On the basis of the sediment dynamics and numerical model, the sediment transport rate and net transport pathways can be determined in quantity. Then the feedback between the tidal flat geomorphology and associated hydrodynamics can be investigated, in terms of their response to the human activities. In addition, the parameters, including the tidal flat accommodation, sediment availability (or capacity), are calculated by detailed theoretical analysis for the tidal flat-sand ridge system of the study area. Further, the nonlinear residual currents in the extreme shallow water associated with sediment transport can be set forth. The sediment source and hydrodynamic controlls on the tidal flat evolution scales (in spatial and temporal) will then be evaluated. Finally, a scientific hypothesis could be tested: the sand ridge moves landward and form a "recombination tidal flat" with multimodal profile in terms of elevation along the onshore-offshore cross-sections; the offshore sand ridge could grow upward and form another type of tidal flat named "sand-ridge tidal flat". However, the highest elevation and extending of such tidal flats are limited by the accommodation space. This study could put forward a novel theory of tidal flat evolution and make contributions on the coastal exploitation, environment protection, oceanic disaster forecasting and alarm.

以南黄海江苏近岸海域潮滩-潮流脊复合系统中的开敞型潮滩为研究对象，应用现代化仪器MIDAS400、ADV、OBS、GPS-TRK等沿典型断面进行水动力和地形地貌的现场观测，获取高时空分辨率的沉积物输运及潮滩演化信息；采取沉积动力学与数学模型相结合的方法，重点分析两种类型潮滩（复合型及离岸型）体系中沉积物输运趋势、地貌-动力反馈机制，辨析潮滩演化对人类活动的响应。结合理论分析，估算潮滩的生长空间和沉积物可获得率，分析极浅水非线性余流的形成机理及其输沙效应，进而探讨物源供应和水动力条件对潮滩演化时间尺度的影响机制，验证两种新的潮滩演化模式的工作假说：潮流脊横向迁移、并陆形成复合型潮滩，及岸外潮流脊垂向淤长至大潮低潮位之上形成沙脊型潮滩；其滩顶高程和滩面展布受相应理论生长极限控制。该项研究成果将为构建新的潮滩演化模式理论奠定基础，并为海岸带资源开发、环境保护和海洋防灾减灾提供科学依据。

潮滩海岸是典型的“关键带（Critical Zone）”，地形地貌的变化也相对迅速，但现场观测、数值模拟较困难，是传统研究领域的薄弱区域。在本项目资助下，课题组进行了9个航次的现场观测，建立数值模型。获得了以下新认识。（1）江苏中部海岸淤涨型潮滩剖面均衡态一般表现为双凸型，即在物源供应充足的情况下，涨落潮流在平均高、低潮位线附近的流速突变，造成沉积物落淤，从而形成上、下凸点。双凸剖面的形成与潮滩动力特征、沉积物供应、地形等均有较大关系。（2）定量研究了极浅水环境的水动力和地貌演化过程，发现在水深 < 0.2米的极浅水阶段，其持续时间仅占整个潮周期的11%，但在极浅水阶段产生的侵蚀淤积厚度占整个潮周期地貌变化的35%，表明传统观测中经常被忽略的极浅水效应在沉积地貌演化中是非常关键的；在河口海岸环境的海底冲淤演化模拟中，只有考虑极浅水阶段的冲淤时，计算结果才能跟跟实测值相吻合；极浅水阶段的沉积动力过程与微地貌（如沙纹）之间存在相互反馈关系。这些认识有助于加深微地貌形成和消亡机制的理解。因此，在潮汐海岸环境中，考虑极浅水效应可提高海岸地貌演化的预测精度。（3）发现了潮滩海域平流为主的输沙模式，这些沉积物来自于以涨潮占优、低盐度的辐射沙脊群或沿岸河口区域，可解释江苏中部潮滩处于淤长状态和演化机制。（4）沙脊区的顺时针余环流格局，导致沙脊两侧的输运率变化，产生沿沙脊主轴方向的梯度，造成沙脊的横向迁移；在辐射状的水道—沙脊格局中，离岸沙脊的沉积物输运绕脊环流与近岸沙脊的输运环流，在离岸沙脊的近岸端相遇、交汇，形成“汇流”，沉积物在此处淤积，附近沙脊的高程变高。在数十年的时间尺度上，水下沙脊最初堆积的粗颗粒沉积物，逐渐淤高至平均海平面附近时，沉积层中的细颗粒组分含量开始增加，发育成为离岸加积型潮滩。这些新成果发表于JGR-Oceans, Marine Geology等主流刊物，为构建新的潮滩演化模式奠定理论基础，为海岸带资源开发、环境保护和海洋防灾减灾提供科学依据。