基于超大型波浪水槽的沙滩动力地貌响应及横向输沙机制研究

Sandy beaches are important coastal and tourism resources. A comprehensive investigation of the beach morphodynamics and the cross-shore sediment transport mechanism is mandatory for both coastline erosion prevention and the construction technique of artificial beach and beach nourishment. Nearshore beaches situate in the surfzone, the hydrodynamics and morphology evolution processes are extremely complex due to the mutual interaction among waves, undertow and movable bed. In historical works, the methodology of small-scaled wave flume experiments is mostly used. However, the small-scaled wave flume experiments can not repeat the realistic processes e.g. the in-situ wave intensity, surface roller turbulence, and wave boundary layer structure, which limits its theoretical application. In this study, the world’s largest wave flume is applied to directly generate hydro-sedimentological environment equivalent to the actual site condition in the laboratory. Firstly, through a series of fixed and movable bed experiments, the hydrodynamic structure e.g. the waves, undertow, suspended sediment concentration within the surfzone, as well as the beach profile evolution and sandbar migration characteristics are precisely measured to evaluate the mutual coupling process among wave propagation, distribution of the undertow structure and morphology evolution. Secondly, base on the measurement datasets, a process-based three-dimensional numerical model is further developed. Combining the conclusions of both experimental study and numerical modeling results, the nature of surfzone hydrodynamics and beach cross-shore sediment transport process under realistic nearshore condition is finally obtained. The results achieved in this study could promote the development of the coastal dynamics theory.

沙滩是重要的海岸国土和旅游资源。厘清沙滩的动力地貌响应及横向输沙机制，无论从防止海岸侵蚀，还是发展人工沙滩和养滩技术均有重要实际意义。沙滩位于破波带内，波浪、波生流和地貌演变三相互制，水沙运动机制极为复杂。在以往研究中，多基于小型波浪水槽实验手段，但其无法真实复演现场当量的水沙强度，导致破波水滚、波浪紊动及边界层结构与现场条件存在较大差异，限制了所得理论的拓展性。本项目基于国际最大尺度的波浪水槽，直接在实验室内复演接近原型的水沙环境，并设计具有代表性的定床、动床实验，精确测量破波带内外波浪、波生流、含沙量平面和垂向结构，以及剖面变化、沙坝发育和迁移等地貌过程，据此分析破波带水动力结构与沙滩剖面地貌演变的耦合响应关系；基于实验数据，同步开发完善基于动力地貌过程的三维数值模式，并开展数值试验。综合实验与模型成果，阐明天然现场条件下沙滩横向输沙内在物理机制，所得成果可推进海岸动力学理论的发展。

厘清沙滩的动力地貌响应及横向输沙机制，无论从抑制海岸侵蚀，还是发展人工沙滩和养滩技术均有重要实际意义。对沙滩动力地貌的历史研究多基于小型波浪水槽手段，但其无法真实复演现场当量的水沙强度，限制了成果的拓展性和通用性。本项目基于国际最大尺度的波浪水槽，直接于室内复演了原型沙滩地貌及水沙环境，精确测量了现场当量波浪作用下的底部边界层内部流系、破波带内外波生流垂向结构，以及沙坝发育、迁移等输沙与地貌过程，分析了破波带水动力与沙滩地貌演变的耦合响应关系；同时，进一步完善了自主产权的沙滩动力地貌三维数值模式，并开展大量数值试验论证。项目得到以下重要结论：（1）于国内外首次发现了强波致雷诺应力作用下波浪边界层的薄化效应和垂向紊动抑制效应，为未见报道的崭新科学现象。（2）发现破波点附近存在波生时均反向环流系统，泥沙运动存在离岸、向岸的交汇点，是水下沙坝发育的成因；（3）发现沙坝在局部地貌—波浪—波生流的三相耦合下，其发育具有猝发性和不稳定性，严格意义上的平衡剖面较难实现。（4）指出沙坝在风暴浪下的离岸迁移控制于底部离岸流，沙坝在弱浪条件下的向岸迁移受时均反向环流和净漂流共同控制。（5）指出风暴作用下底部离岸流和沿岸流同时存在，破波带内形成螺旋状时均三维环流系统，发现高强度的沿岸流可阻断横向泥沙供应、抑制沙坝发育，即弱浪下的典型恢复剖面并非一定能够形成。本项目成果明晰了现场波浪环境下的紊流边界层内部结构，阐明了沙滩横向输沙内在物理机制，推导形成了高精度、适用于现场的底部剪切力计算表达式；厘清了沙滩横向输沙及沙坝形成、迁移机制，发展了通用、高精度的沙滩动力地貌响应三维数值计算模式。所得成果推进了海岸动力学的学科理论进步，具有重要的科学意义，同时，所得到的自主产权公式、软件等产品成果具有显著的实际应用价值，得到了大量工程实践，取得了显著的经济社会和生态环境效益。