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

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.