波致层移条件下粘性细颗粒泥沙高含沙层底部动力机制

Sediment transport is one of the key problems in the development and utilization of estuarine and coastal areas. Under strong flow dynamics conditions (especially waves), ripples disappear and the bed form becomes moving flat. In this case, sheet flow happens in bottom and high suspended load concentration exists in upper layer. Sheet flow layer is the key area of sediment suspension and transport, which is the source of turbulence, sediment entrainment and induces the high suspended load concentration. At present, the study of sheet flow was mainly focused on sand. There are many silty-muddy coasts in China, and the sediments belong to cohesive fine sediments. The high concentration phenomenon under sheet flow has been found in some laboratory experiments for fine cohesive sediments; however, their sheet flow patterns and the flow-sediment mechanisms were poorly understood. The contents of this study are as following. First, wave flume experiments will be carried out for cohesive fine sediments under wave conditions and the sediment transport data from the unmoved bed to water surface will be measured by high precision instruments. Combined with the measured data and the definition of sheet flow, whether the sediment movement near bottom belongs to sheet flow or not will be identified, and the "sheet flow" patterns will be studied, including the critical conditions, sheet thickness and vertical distribution of sediment concentration etc. Then, combined with turbulence diffusion theory and wave boundary layer theory, the bottom flow-sediment mechanisms of suspended load high concentration layer are expected to be studied, which could be further used in numerical simulation or project analysis. This study is of great significance for both the sediment fundamental study and engineering practices.

河口海岸的泥沙问题是人类开发利用近海资源的关键问题之一。在较强动力（尤其是波浪）条件下，底部沙纹消失出现动平整床面，此时底部的层移质运动对上部悬沙高含沙层发展起到重要作用。目前对层移质运动的研究主要集中在沙质泥沙。我国分布有大量的粉沙淤泥质海岸，属于粘性细颗粒泥沙的范畴。近年波浪水槽试验已观测到粘性细颗粒泥沙在疑似层移条件下的高含沙运动现象，但对其特征和动力机制分析较少。本项目拟通过波浪水槽细颗粒泥沙试验和理论分析相结合的方法，通过精细测量得到从不动床面到悬沙层的水沙数据，引入层移概念明析近底床面泥沙运动特征，解析其“层移”发生的临界条件、“层移层”厚度及含沙量分布等；然后结合波浪边界层理论和紊动扩散机理揭示高含沙层的底部动力机制，为进一步开展悬沙高含沙层运动研究奠定基础，指导工程实践。对于促进河口海岸泥沙运动研究具有重大的理论意义和工程意义。

河口海岸的泥沙问题是人类开发利用各种资源和生态保护的关键问题。相对较强动力条件下的泥沙运动尤其是层移质运动长期以来一直受到大量学者和工程技术人员的关注。目前对粉沙淤泥质泥沙运动的研究大多基于宏观特性统计，对底部泥沙运动机制仍需要进一步研究。本项目针对波致层移条件下的细颗粒泥沙运动，采用水槽实验、波流边界层水沙数学模型与理论分析相结合的方法，观测了含沙量剖面特征，研究了底部高含沙层的动力机制，推导了参数化表达式。取得的主要成果包括：.解析了底部高含沙层特征，提出底部高含沙层与边界层密切相关，其厚度为波浪边界层厚度的2倍。定量分析了制约沉速、层化效应、层移运动动床阻力等对底部高含沙层的影响机制。制约沉速效应使得泥沙沉速减小，从而将增大含沙量。层化效应明显降低了含沙量和紊动粘滞系数。对细颗粒泥沙而言，层移运动产生的动床阻力在粗糙度中起主导作用。.明确了床面形态对细颗粒悬沙运动具有重要影响。在层移床面，近底最大含沙量基本发生在水动力最大时刻，这主要是由紊动剪切造成的；而沙波床面则相反，近底最大含沙量基本发生在转流时刻，主要是由漩涡造成的。床面形态决定近底含沙量剖面形状，水动力影响了含沙量大小。在模拟方法上需采用不同的处理方式，仅通过床面粗糙高度不足以反映泥沙的运动机理。.基于底部高含沙层敏感性分析，提出层移床面波浪相关的时均泥沙扩散系数分布，推导了基于边界层过程的波浪作用下时均含沙量剖面表达式。提出的层移床面波浪作用下泥沙扩散系数为“趾”形分布。推导的波浪作用下时均含沙量剖面表达式中考虑了底部高含沙层主要影响因子，包括床面形态影响、层化效应、制约沉速等。实验资料验证表明，公式计算与实测基本吻合较好。.本研究揭示泥沙运动的基本问题，对促进泥沙学科的发展具有理论意义，对正确模拟泥沙运动，应用于工程实践起到指导作用，具有重要的实际价值。

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