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首页> 外文期刊>Estuarine Coastal and Shelf Science >Numerical modelling of hydraulics and sediment dynamics around mangrove seedlings: Implications for mangrove establishment and reforestation
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Numerical modelling of hydraulics and sediment dynamics around mangrove seedlings: Implications for mangrove establishment and reforestation

机译：红树林幼苗周围水力学和泥沙动力学的数值模拟：对建立红树林和重新造林的意义

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Mangroves grow in the coastal and intertidal zones at tropical and subtropical latitudes. It is widely accepted that the establishment, growth and survival of mangrove seedling depend on the environmental conditions such as temperature, tidal regime and hydrodynamics. To date we know that, in cohesionless sediment, the higher the flow velocity the greater the eroded volume and, thus, the stronger the scour around the mangrove seedling which can lead to its uprooting or death. However, how a mangrove seedling (cm-scale) alters the flow pattern and the sediment transport is, to date, still poorly understood. Four suits of numerical experiments were setup with the same mangrove seedling and subjected to flow speeds of 5 cm.s(-1), 10 cm.s(-1) and 15 cm.s(-1) as well as a rarely occurring extreme scenario 50 cm.s(-1). The hydro- and sediment dynamics were simulated using a coupled sediment-hydrodynamic continuum approach: the Finite Volume Method utilizing the software package OpenFOAM. The two phases, silty sediment and water, constitute a mixture. The distribution of the sediment phase in the water phase was estimated with the drift-flux approximation. A natural mangrove seedling (Rhizophora rnucronata) was digitized employing photogrammetry and discretized into the model domain. This numerical model was validated against experimental observations and is able to capture the main features of the flow and sediment transport around a mangrove seedling. The numerical simulations showed that a downward flow associated to a horseshoe vortex enhances scour in front of the mangrove seedling and a vortex shedding keeps the sediment in suspension or re-suspends the sediment in the rear of the mangrove seedling. Thus, a mangrove seedling has a significant influence on the flow pattern and sediment transport: the higher the flow speed, the less stable the sediment bed. Additionally, these findings could help to better understand how the settlement of mangrove seedlings and sediment dynamics affect mangrove establishment and why the colonization or restoration of tidal flats is successful or not. This paper is an initial study focusing on a single mangrove seedling on a cm-scale. It is anticipated to extend the interactions between several seedlings as well as mangrove roots. With this successfully utilized approach, it will be possible in the future to investigate the impact of several mangrove seedlings forming a matrix on a m-scale on sediment dynamics. So, this work creates the basis to study more complex sediment transport processes such as erosion and sediment trapping in mangrove forests.
机译：红树林生长在热带和亚热带纬度的沿海和潮间带。人们普遍认为，红树林幼苗的建立，生长和存活取决于环境条件，例如温度，潮汐状态和流体动力学。迄今为止，我们知道，在无粘性的沉积物中，流速越高，侵蚀量越大，因此，红树林幼苗周围的冲刷力越强，可能导致其生根或死亡。然而，迄今为止，对红树林幼苗（厘米级）如何改变水流模式和沉积物迁移的了解仍然很少。使用相同的红树林幼苗设置了四套数值实验，其流速分别为5 cm.s（-1），10 cm.s（-1）和15 cm.s（-1），并且很少发生极端情况50 cm.s（-1）。使用耦合的沉积物-水动力连续体方法：利用OpenFOAM软件包的有限体积法模拟了水和沉积物动力学。粉质沉积物和水这两个阶段混合在一起。用漂移通量近似法估算水相中的泥沙相分布。使用摄影测量法将天然红树林幼苗（Rhizophora rnucronata）数字化，并离散到模型域中。该数值模型已针对实验观察进行了验证，并且能够捕获红树林幼苗周围的水流和泥沙输送的主要特征。数值模拟表明，与马蹄形涡旋相关的向下流动增强了红树林幼苗前的冲刷，而涡旋脱落使沉积物保持悬浮状态或使红树林幼苗后部的沉积物重新悬浮。因此，红树林幼苗对流态和沉积物的传输有重大影响：流速越高，沉积物床的稳定性就越差。此外，这些发现可能有助于更好地理解红树林幼苗的沉积和沉积物动力学如何影响红树林的建立以及为什么定居或恢复潮滩成功与否。本文是一项初步研究，着重于厘米级的单个红树林幼苗。预计将扩展几种幼苗以及红树林根之间的相互作用。通过这种成功利用的方法，将来有可能研究形成m尺度基质的几种红树林幼苗对沉积物动力学的影响。因此，这项工作为研究更复杂的泥沙输送过程（例如红树林中的侵蚀和泥沙捕获）奠定了基础。

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