Characterizing the physical drivers of vegetation-forced bar evolution in gravel-bed streams

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Riparian vegetation patches rapidly colonize exposed bars in gravel-bed rivers, impacting the nearby flow hydraulics and subsequent erosional-depositional processes during flood events. This vegetation-forced erosion and deposition on gravel bars is important as it alters the size and composition of the bed sediments that affects aquatic and riparian habitat quality, drives changes in subsurface hydraulics and associated water quality, as well as impacts flow conveyance and community flood risk. This project seeks to understand how the hydraulics and sediment transport capacity change through vegetation patches to explain the spatial patterns of observed bed sediments and resultant shape of gravel bars. The magnitude of changes is expected to depend on the vegetation density within a patch. Results can help guide water agencies in deciding what vegetation planting densities are necessary during river restoration activities to achieve the desired evolutionary outcomes. Additionally, results are expected to help flood managers predict gravel bar dimensions from existing vegetation densities and determine if vegetation thinning or removal is necessary for public safety.Predicting vegetated bar evolution is difficult because the local hydraulics and sediment mobility (initiation of sediment motion) are spatially variable through a vegetation patch. Currently, no single parameter is available to predict coarse-sediment mobility in both open and vegetated channels. Here, a combination of fieldwork and large-scale flume experiments with vegetation simulants will be used to develop a mechanistic driver of coarse-sediment mobility that is applicable in both vegetated and unvegetated conditions. In the planned experiments, vegetation density will be varied to identify a density threshold that initiates grain-size changes, spatial sorting, and bar evolution in gravel-bed rivers. Further, a predictive relation between vegetation density and equilibrium bar dimensions will be developed.This project is jointly funded by Geomorphology and Land-use Dynamics (GLD) and by the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.