粉砂质底床波致层化堆积及再起动机制研究

Silt-dominated sediment is defined as a mixture of clay (with limited content), silt and very fine sand. Bed liquefaction is commonly observed when strong wind waves exert on silt-dominated bed, causing re-packing of sediment grains and forming a layered bed network structure. Such phenomena can further influence re-initiation of motion process of bed material. Due to high dynamics of silt-dominated bed, this project aims to gain more in-depth insights into characteristics of wave-induced layered bed network structure and its influence on sediment re-initiation of motion. This project focuses on the following three aspects: (a) by carrying out a series of flume experiments on silt-dominated bed with different silt contents, we will investigate the evolution processes of bed forms under waves and the effect of silt content on the evolution processes; (b) by conducting geotechnical tests on the sediment beds developed under still water and wave conditions, respectively, we will explore characteristics of the wave-induced network structure and its response to different silt contents; (c) based on the knowledge gained from the above two exploratory researches, together with the help of several tests on initial of motion over sediment beds with different types of bed network structures, we will study the influence of network structure on the initiation of motion of silt-dominated bed. Outcome of this research will deepen understanding on the initiation of motion over silt-dominated bed and provide a fundamental basis for improving performance of sediment transport models.

粉砂质泥沙一般指由粘土（少量）、粉砂和极细砂组成的混合沙。大风浪驱动下粉砂质底床易发生液化，致使底床内部泥沙颗粒重新排列，常形成层化的堆积结构，进而影响泥沙再起动，因此粉砂质底床波致层化堆积及再起动问题是泥沙研究的前沿问题，具有重要科学意义。项目针对粉砂质底床动力堆积结构复杂多变的问题，以粉砂含量为切入点，结合水槽实验及土工试验，研究粉砂质底床波致层化堆积机制及其对泥沙再起动的影响。设计多种粉砂含量底床的水槽系列实验，探究波浪作用下底床发育模式及其对粉砂含量的响应；对比分析静水沉积与波浪作用后底床垂向各层的土工指标，探究波致底床的层化堆积机制；进而开展波致层化堆积底床的再起动实验，并结合前两个层次对底床堆积结构的探索性研究，阐明不同底床堆积结构下泥沙的再起动机制，完善泥沙起动理论。本项目可从底床堆积结构角度加深对泥沙起动的认知，为进一步提高河口海岸泥沙运动的模拟精度等提供基础理论支持。

粉砂组分作为处于粘性淤泥质泥沙与非粘性砂质泥沙过渡区域的泥沙，其在波浪、潮流作用下的动力特性较为复杂，将其归类为粘性沙还是非粘性沙存在争议，对粉砂运动机理层面的研究相对较少。本项目针对波浪、水流作用下的粉砂质泥沙冲刷、悬扬问题，以粉砂含量为切入点，结合水槽实验、数值模拟、理论分析等研究手段，开展的主要研究内容包括，粉砂含量对粉砂质泥沙冲刷悬扬特性的影响、波致粉砂质底床响应过程以及混合泥沙数值模块研发和应用等。本项目主要取得的重要结果有：（1）粉砂含量超过30%时，底床堆积结构发生转变，引起底床抗冲刷性增强，表现为临界起动切应力提高；（2）对于未有波浪作用历史的底床，随粉砂含量的增加，波致床面液化深度增加，同时沙纹形成的时间尺度逐步延长；有波浪作用历史的底床，底床液化不显著；（3）研发了粘土-粉砂-细砂混合泥沙运动计算模块并在江苏沿海进行了应用，发现更新粉砂计算方法后，模型可连续模拟从潮周期到年际间的泥沙输运过程，并保持稳健；（4）江苏沿海悬沙在辐射沙脊群中部和废黄河三角洲存在浓度高值区，悬沙运动主要受潮汐控制，而风主要影响悬沙浓度的季节性变化。项目研究成果对完善河口海岸泥沙运动力学学科具有推动作用，提出的数值计算方法可进一步在其他河口海岸进行应用，为河口海岸保护开发提供科学依据。