强涌潮作用下的河口底边界层湍流混合

Turbulent mixing in the bottom boundary layer (BBL) has been receiving increasing attention from physical oceanographers, sediment dynamic researchers, end coastal engineers, as it plays key role in energy distribution and sediment dynamics in estuaries. The Qiantang River Estuary breeds the world’s largest tidal bore, which induces intense large-scale vortices and massive sediment resuspensions modifying the BBL turbulent mixing. However, current research and knowledge of BBL dynamics with impact of tidal bore are limited by challenges in in-situ observation. To gain insight into the combining impact of tidal bore and suspended sediment on turbulent mixing in BBL, we propose to conduct 15-day in-situ observations covering a spring-neap tidal cycle at two sites with and without tidal bore in Qiantang estuary, on basis of our previous studies. Flow and suspended sediment concentration data in BBL will be collected synchronously with high spatiotemporal resolution (i.e., sampling frequency of 8-32 Hz and vertical resolution of several centimeters), using well-designed instrumented tripods. Subsequently, key BBL parameters such as bed shear stress, roughness length, turbulent kinetic energy (TKE), shear production and dissipation rate of TKE and turbulent flux of suspended sediment will be estimated to understand the influence of tidal bore and sediment stratification on turbulent mixing in BBL. Expected findings will broaden our understanding of the classical theories in BBL dynamics and provide theoretical basis for sediment transport research and engineering constructions in estuaries with large tidal bores.

底边界层湍流混合过程对河口能量分配和沉积动力起关键性作用，一直是物理海洋学、沉积动力学和海岸工程学等的研究热点。钱塘江河口发育世界上最强的涌潮，释放巨大的能量，造成强紊流和大量泥沙再悬浮，对底边界层湍流混合产生深刻影响。但因现场观测难度大，目前对涌潮作用下底边界层动力过程的研究和认识十分有限。本项目拟以前期工作为基础，在钱塘江河口最强涌潮河段及强潮流河段同步开展持续15天的座底三脚架观测，辅以船基观测，以获得底边界层内高时空分辨率的流场和悬沙浓度数据（8-32Hz，垂向分层数厘米）。基于精细流场结构，计算底边界层湍流混合参数，如底切应力、底粗糙度、湍动能剪切生成及耗散率、悬沙垂向紊动通量等。对比涌潮与强潮流作用下的底边界层的异同，探讨涌潮和高悬沙浓度对底边界层湍流混合的影响。该研究将拓展对传统底边界层的理论认识，同时为涌潮河口的泥沙输运研究和工程建设提供理论依据。

基于观测数据的深入分析，发现强涌潮作用下湍流来源除了通常认为的由潮流经底摩擦产生的，破碎潮头和次生波浪均能产生湍流，该湍流至表面向下衰减。这些过程能够加强涌潮水体中悬浮泥沙的垂向混合。悬沙浓度的升高直接导致了河口沉积物搬运的急剧上升，进而改变了潮控型河口的沉积动力地貌。结果显示湍流的强度随潮流和涌潮所伴生的次生波的增强而增强。在涌潮的初期阶段，潮头及次生波所产生的对流型湍流增强湍流的长度尺度，主导了悬浮沉积物的垂向混合。湍流的动量和垂向泥沙通量是由间歇性的、大强度的湍流事件所主导。上述结果揭示了不同于传统河口沉积动力学的过程，即涌潮通过在表层注入湍流而改变悬沙的分布和搬运。该结果已在海洋地质领域权威期刊Marine Geology（Tu et al., 2021）发表。.层化湍流中的混合效率（Mixing Efficiency，亦称通量理查森数Rf）对海洋环流和物质输移具有重要的作用。流体中的湍流动能主要通过两个过程来消耗：大部分能量通过摩擦消散，另一部分能量则用于输运重要的流动成分。后者也称为克服重力做功的动能。本项目在高浊度河口中对强湍流、悬沙层化的流动进行了现场观测；结合已发表的观测数据和模拟结果，证实了混合效率可从较易观测的湍流强度指标（即湍流弗劳德数）进行估算。该结果发表于国际知名海洋学期刊Geophysical Research Letters（Tu et al., 2022b）上。.高浓度细颗粒悬浮泥沙引起的密度分层和水流剪切之间的相互作用产生流动不稳定性。本项目通过船载回声探测仪对剪切不稳定结构进行精细观测，利用座底三脚架中的ADP-HR对近底层湍动能耗散率进行观测，结合流速和悬沙浓度剖面数据等，揭示了浮泥层中湍流剪切层生消规律和湍流混合特征。该结果发表于国际知名海洋学期刊Journal of Geophysical Research: Oceans（Tu et al., 2020）和Journal of Physical Oceanography（Tu et al., 2022a）上。