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

植被河道中的沉积物输送：评估平均床应力和湍流脉冲对初始运动的作用

• 批准号: 1414499
• 负责人: Heidi Nepf
• 金额: $ 36万
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-01 至 2018-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1414499&HistoricalAwards=false
• 关键词: Sediment; Transport; Vegetated; Channels; Evaluating

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.

许多沿海地区希望恢复沿海沼泽地，作为抵御风暴潮的防线和污染沿海沃茨的营养流失的缓冲区。此外，沿海沼泽地也是重要的碳汇。近几十年来，由于沿海开发，这些景观已经下降，并继续受到海平面上升和/或上游水坝造成的沉积物供应减少的威胁。沿海沼泽景观通过水流、植被和沉积物的相互作用而生长。为了更好地了解这一过程，该项目将利用实验室研究来研究植被如何影响沉积物的迁移。 此外，该项目还将建立新的模型，以预测植被地区内的沉积物迁移。该项目的成果将为恢复植被以保护海岸和改善水质提供信息。美国每年花费数十亿美元用于海岸和河流的恢复，因此，提高这些项目的成功率具有经济和环境的重要性。大多数以前的泥沙输运研究都集中在明渠条件下，湍流结构（包括冲动）与河床应力有关，这是河道湍流的主要来源。在这项研究中进行的植被实验提供了一个新的角度泥沙夹带，因为在植被系统的湍流和平均床面应力是断开的。这使得设计的实验中，床应力和湍流结构是独立变化的，允许一个明确的评价的作用，每个参数在泥沙夹带。 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，并将其与泥沙运动的阈值联系起来。两种模式的植被被认为是。第一种是交错排列的刚性圆柱，这是一个合理的替代芦苇和莎草，和植被流的基础研究的共同选择。第二个模型再现了在一些挺水植物中发现的聚集分布，使用芦苇，灯心草，作为几何模型。这个更复杂的模型将探索增加第二个长度尺度（团块直径）如何影响湍流，这反过来又可能影响颗粒夹带。这两种模式都是紧急的，延伸到整个水柱。 详细的流速测量将用于建立植被内床面应力的预测模型。此外，颗粒夹带的范围内的流动条件和植物的几何形状将量化使用数字成像的床，并通过测量散装泥沙输运率。这些观测结果将用于确定沉积物通量开始的临界流速。沉积物运动的阈值将与基于平均床应力和湍流结构的度量相关。相关性将揭示哪种度量（床应力或湍流冲量）更好地预测卷吸。