Ocean and Mechanical Engineering, Florida Atlantic University, Boca Raton, 33431, USA.
School of Mechanical Engineering, Purdue University, West Lafayette, 47907, USA.
Sci Rep. 2021 May 11;11(1):9969. doi: 10.1038/s41598-021-88119-5.
Mangrove swamps are extremely productive ecosystems providing many ecological services in coastal regions. The hydrodynamic interactions of mangrove roots and water flow have been proposed as a key element to mitigate erosion. Several studies reveal that precise prediction of the morphological evolution of coastal areas, in the face of global warming and the consequent sea-level rise, requires an understanding of interactions between root porosity (the fraction of the volume of void space over the total volume), water flows, and sediment transport. Water flows around the mangrove prop roots create a complex energetic process that mixes up sediments and generates a depositional region posterior to the roots. In this work, we investigated the boundary layer behind permeable arrays of cylinders (patch) that represent the mangrove roots to explore the impact of patch porosity on the onset of sediment transport. The flow measurements were performed in a vertical plane along the water depth downstream of the mangrove root models. A high-resolution Particle Image Velocimetry (PIV) was used in a flume to observe the impact of porosity on the mean flow, velocity derivatives, skin friction coefficient, and production of turbulent kinetic energy for Reynolds number of 2500 (based on patch diameter length-scale). Here, we proposed a predictive model for critical velocity for incipient motion that takes into account the mangrove roots porosity and the near-bed turbulence effect. It is found that the patch with the [Formula: see text] porosity, has the maximum critical velocity over which the sediment transport initiates. We found the optimum porosity has the minimum sediment erosion and creates negative vorticity sources near the bed that increases the critical velocity. This signifies an optimum porosity for the onset of sediment transport consistent with the porosity of mangroves in nature. The phenomenological model is elucidated based on an analysis of the vorticity evolution equation for viscous incompressible flows. For the optimum porous patch, a sink of vorticity was formed which yielded to lower the near-bed turbulence and vorticity. The minimum velocity fluctuations were sufficient to initiate the boundary layer transition, however, the viscous dissipation dominated the turbulence production to obstruct the sediment transport. This work identified the pivotal role of mangrove root porosity in sediment transport in terms of velocity and its derivatives in wall-bounded flows. Our work also provides insight into the sediment transport and erosion processes that govern the evolution of the shapes of shorelines.
红树林沼泽是非常高产的生态系统，在沿海地区提供许多生态服务。红树林根系与水流的水动力相互作用被认为是减轻侵蚀的关键因素。多项研究表明，要精确预测在全球变暖及随之而来的海平面上升的情况下沿海地区的形态演变，就需要了解根孔隙率（空隙空间体积与总体积之比）、水流和泥沙输移之间的相互作用。红树林支柱根周围的水流会产生复杂的能量过程，将泥沙混合，并在根部后方形成沉积区。在这项工作中，我们研究了代表红树林根系的多孔圆柱阵列（斑块）后面的边界层，以探讨斑块孔隙率对泥沙输移开始的影响。在红树林根系模型下游的水深处，在垂直平面上进行了流动测量。在水槽中使用高分辨率粒子图像测速（PIV）观察孔隙率对平均流、速度导数、壁面摩擦系数和湍流动能耗散率的影响，雷诺数为 2500（基于斑块直径长度尺度）。在这里，我们提出了一个考虑红树林根系孔隙率和近床湍流效应的临界起沙速度预测模型。结果表明，孔隙率为 [Formula: see text] 的斑块具有最大的临界速度，超过该速度就会开始输沙。我们发现，最优孔隙率具有最小的泥沙侵蚀作用，并在近床处产生负涡度源，从而提高临界速度。这表明，对于泥沙输移的开始，存在一个最优的孔隙率，与自然界中红树林的孔隙率一致。基于粘性不可压缩流的涡度演化方程的分析，阐明了这个现象学模型。对于最优的多孔斑块，形成了一个涡旋汇，从而降低了近床层的湍流和涡度。最小的速度波动足以引发边界层转捩，然而，粘性耗散主导了湍流的产生，阻碍了泥沙输移。这项工作确定了红树林根系孔隙率在壁面边界层流中速度及其导数方面对泥沙输移的关键作用。我们的工作还深入了解了控制海岸线形状演变的泥沙输移和侵蚀过程。
