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Boundary Shear Stress Along Vegetated Streambanks
EPA Grant Number: FP916953Title: Boundary Shear Stress Along Vegetated Streambanks
Investigators: Clark, Leslie
Institution: Virginia Polytechnic Institute and State University
EPA Project Officer: Thompson, Delores
Project Period: September 1, 2008 through August 31, 2009
RFA: STAR Graduate Fellowships (2008)
Research Category: Academic Fellowships
Description:
Objective:Sediment, a leading cause of water quality impairment, damages aquatic ecosystems and interferes with recreational uses and water treatment processes. Streambank retreat can contribute as much as 85% of watershed sediment yield. Vegetation is an important component of stream restoration designs used to control streambank retreat, but vegetation effects on near bank flows and boundary shear stress (BSS) need to be quantified.
The overall goal of this experiment is to predict boundary shear stress (BSS), a parameter associated with streambank erosion, for vegetated streambanks. This goal will be met by determining a method for measuring BSS in the field along hydraulically rough streambanks, evaluating the effects of streambank vegetation on BSS, and developing predictive methods for estimating BSS based on easily measured vegetative properties.
Approach:A flume experiment will be conducted, comparing three distinct streambank vegetation types: herbaceous, shrubs, and woody. A second order prototype stream (Tom’s Creek in Blacksburg, VA), with individual reaches dominated by the three vegetation types was modeled using a fixed-bed Froude-scale modeling technique. Initial field measurements (topography, vegetation, and grain roughness) were made to characterize the stream reaches. One model streambank of the prototype stream will be constructed for each vegetation type, resulting in three model streambanks. Simulated vegetation (e.g. wooden dowels and a woven grass mat) will be attached in the locations identified in the field survey
A Dantec MiniCTA system with a flush mounted hot-film probe (55R46) will be used to measure shear stress at the streambank wall. For this study, measurements made by the hot-film anemometer will be considered representative of the actual boundary shear stress. To determine whether there are differences in BSS as a function of vegetation type, the hot-film anemometer measurements will be examined using a randomized design approach. Velocity profiles perpendicular to the flume model streambank will also be evaluated. The velocity profiles will be compared among vegetation types to determine if profiles are similar along the bank face. Predictive methods for estimating boundary shear stress based on common vegetation measures will be developed in the flume study.
Expected Results:This research is intended to improve our understanding of the role of riparian vegetation in stream morphology by evaluating the effects of vegetation on boundary shear stress, providing insight to the type and density of vegetation required for streambank stability. The results will also aide in quantifying sediment inputs from streambanks, providing quantitative information for stream restoration projects and watershed management planning.
Supplemental Keywords:Streambank erosion, boundary shear stress, streambank vegetation, turbulence, vegetated flow,