Sediment Transport in Vegetated Channels: Evaluating the Roles of Mean Bed Stress and Turbulent Impulse on Incipient Motion

许多沿海地区希望恢复沿海沼泽，以此作为抵御风暴潮的防线，并缓冲污染沿海水域的营养物质流失。此外，沿海沼泽提供了一个重要的碳汇。近几十年来，由于沿海开发，这些地貌有所下降，并继续受到海平面上升和/或上游大坝造成的泥沙供应减少的威胁。滨海沼泽景观是通过水流、植被和泥沙堆积的相互作用而形成的。为了更好地了解这一过程，该项目将使用实验室研究来研究植被如何影响泥沙运输。此外，该项目将产生新的模型来预测植被覆盖地区的泥沙输送。该项目的成果将为恢复植被以保护海岸和改善水质提供参考。美国每年在海岸和河流修复上花费数十亿美元，因此提高这些项目的成功具有经济和环境方面的重要性。以前对泥沙输移的研究大多集中在明渠条件下，在明渠条件下，湍动结构(包括冲量)与河床应力有关，而河床应力是河道湍动的主要来源。本研究中进行的植被实验为泥沙卷吸提供了一个新的视角，因为在植被系统中，湍流和平均床面应力是分离的。这使得能够设计床面应力和湍流结构独立变化的实验，从而可以清楚地评估每个参数在泥沙卷吸中所起的作用。这项实验室研究将描述树干密度和直径对平均和湍流结构的影响，并将这些与泥沙运动的门槛联系起来。考虑了两种植被模型。第一种是交错排列的刚性圆柱体，这是芦苇和莎草的合理替代品，也是植物流基础研究的常见选择。第二个模型使用灯心草作为几何模型，再现了在一些新兴植物中发现的丛生分布。这个更复杂的模型将探索第二个长度尺度(束直径)的添加如何影响湍流，进而可能影响颗粒卷吸。这两种模式都是新兴的，都贯穿了整个水柱。将使用详细的速度测量来开发植被内床面应力的预测模型。此外，一系列水流条件和植物几何形状的颗粒夹带量将使用床的数字成像和测量总体泥沙输移率来量化。这些观测将被用来定义泥沙通量启动的临界速度。泥沙运动的门槛将与基于平均床面应力和湍流结构的量度相关联。相关关系将揭示哪一种度量(床面应力或湍流冲量)更好地预测卷吸。

Many coastal regions would like to restore coastal marshes as a line of protection against storm surge and a buffer for nutrient run-off that pollutes coastal waters. In addition, coastal marshes provide an important carbon sink. These landscapes have declined in recent decades due to coastal development and continue to be threatened by sea level rise and/or by diminished sediment supply due to upstream dams. Coastal marsh landscapes grow through the interplay of flow, vegetation, and sediment accretion. To better understand this process, this project will use laboratory studies to examine how vegetation impacts sediment transport. In addition, the project will produce new models to predict sediment transport within vegetated regions. The results of this project will inform the restoration of vegetation for coastal protection and water quality improvement. The US spends billions of dollars annually on coastal and river restoration, and improving the success of these projects is thus of economic, as well as environmental, importance.Most previous studies of sediment transport have focused on open channel conditions, for which turbulence structure (including impulse) is linked to bed stress, which is the primary source of channel turbulence. The vegetation experiments conducted in this study offer a new perspective on sediment entrainment, because in vegetated systems the turbulence and mean bed stress are disconnected. This enables the design of experiments in which bed stress and turbulence structure are varied independently, allowing a clear evaluation of the role each parameter plays in sediment entrainment. This laboratory study will describe the impact of stem density and diameter on mean and turbulent flow structure and relate these to the threshold of sediment motion. Two models of vegetation are considered. The first is a staggered array of rigid cylinders, which is a reasonable surrogate for reeds and sedges, and a common choice for fundamental studies of vegetated flow. The second model recreates the clumped distribution found in some emergent plants, using the bulrush, Juncus effuses, as a geometric model. This more complex model will explore how the addition of a second length-scale (clump diameter) impacts turbulence, which in turn may impact particle entrainment. Both models are emergent, extending through the full water column. Detailed velocity measurements will be used to develop a predictive model for bed-stress within vegetation. In addition, particle entrainment for a range of flow conditions and plant geometries will be quantified using digital imaging of the bed and by measuring the bulk sediment transport rate. These observations will be used to define a critical velocity at which sediment flux is initiated. The threshold of sediment motion will be correlated to metrics based on mean bed stress and turbulence structure. The correlations will reveal which metric (bed stress or turbulent impulse) better predicts entrainment.