Sediment transport processes in vegetated canopies under full-scale wave conditions

与气候变化相关的海平面上升和暴风雨增加给沿海地区带来了风险，如洪水增加以及对娱乐和生态重要地点(例如动物栖息地)的破坏。传统的“硬”海岸保护方案，如海堤、丁坝和堤坝等，由于无法适应不断变化的条件和持续维护的费用，不能提供有效的长期解决方案。涉及沿海和水生植被的“较软”生态系统沿海防御解决方案因其适应气候变化的能力而被认为是一种更有前途的替代办法。以前的研究表明，海岸植被能够通过消散波浪能量和减少泥沙侵蚀来保护近岸遗址。虽然已有相当多的研究集中于海岸植被对波浪能量的消散，但对水动力学的研究要少得多，对沉积物动力学的研究更是少之又少，特别是在全尺度波浪条件下。一般认为，植被可通过以下方式影响泥沙的流动性：i)由于植被冠层上方流速的降低而使悬浮泥沙“滞留”；ii)由于冠层内流速、近床面湍流和床面剪切力的降低而减少沉积物的再悬浮；以及iii)由于植被的根部结构而使海底基质稳定。然而，这些过程的相对重要性尚不清楚，但开发沿海泥沙输送模型至关重要。在这项研究中，我将研究在香港仔振荡流隧道(AOFT)中，在全尺度波浪诱导的振荡流条件下，植被对泥沙输送过程的水动力效应。主要目的是量化生物力学植被特征，如几何形状、植被茎密度和灵活性对近床水动力、床面剪应力和泥沙输送过程的影响。从这些实验中获得的知识将被用于将植被的影响纳入到实际的泥沙输送模型中。这些泥沙输运模型是用于海岸工程的大型地貌动力学模拟系统的重要组成部分，用于预测气候变化引起的人为活动和自然过程造成的海岸长期演变。

Sea level rise and increased storminess associated with climate change presents risks for coastal areas such as increased flooding and damage to recreational and ecologically important (e.g. animal habitats) sites. Conventional "hard" coastal protection solutions such as seawalls, groynes, and dykes etc. do not offer effective long term solutions due to an inability to adapt to changing conditions and the expense of continued maintenance. "Softer" eco-system based coastal defence solutions, involving coastal and aquatic vegetation, are considered to be a more promising alternative because of their ability to adapt to climate change . Previous studies have shown the ability of coastal vegetation to protect nearshore sites by dissipating wave energy and reducing sediment erosion. Although a considerable amount of research has focussed on the dissipation of wave energy by coastal vegetation, far less studies have focused on the hydrodynamics and even fewer on the sediment dynamics, particularly under full-scale wave conditions. It is generally argued that vegetation can affect sediment mobility by: i) the "trapping" of suspended sediments due to reduced flow velocities above the vegetation canopy; ii) reduced sediment resuspension due to in-canopy reductions in flow velocities, near-bed turbulence, and bed shear stress and iii) the stabilisation of the seabed substrate due to the vegetation root structure. However, the relative importance these processes remains unknown, yet it is crucially important to develop coastal sediment transport models. In this study I will investigate the hydrodynamic effects of vegetation on sediment transport processes under full-scale wave-induced oscillatory flow conditions in the Aberdeen Oscillatory Flow Tunnel (AOFT). The main objectives are to quantify the effect of bio-mechanical vegetation characteristics such as geometry, vegetation stem density, and flexibility on the near-bed hydrodynamics, bed shear stress and the sediment transport processes. Knowledge gained from these experiments will be used to incorporate the effects of vegetation into practical sediment transport models. These sediment transport models are a crucial part of large-scale morphodynamic modelling systems used in coastal engineering to predict long-term coastal evolution as a result of anthropogenic activities and natural processes resulting from climate change.