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United States Department of Agriculture

Agricultural Research Service

Research Project: INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS Title: Calculation of Near-Bank Velocity and Boundary Shear Stress

Authors
item Kean, Jason - U.S. GEOLOGICAL SURVEY
item Kuhnle, Roger
item Smith, J - U.S. GEOLOGICAL SURVEY
item Alonso, Carlos
item Langendoen, Eddy

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 25, 2009
Publication Date: July 1, 2009
Citation: Kean, J.W., Kuhnle, R.A., Smith, J.D., Alonso, C.V., Langendoen, E.J. 2009. Calculation of Near-Bank Velocity and Boundary Shear Stress. Journal of Hydraulic Engineering. Vol. 135 Issue 6 pp. 588-601.

Interpretive Summary: Accurate determinations of the rate of sediment eroded from channel boundaries in streams and rivers are necessary because erosion of the channel boundary can destroy valuable crop or other lands, and can threaten transportation and other components of the infrastructure that are adjacent to stream and river channels. Water quality may also be adversely affected by sources of sediment originating from eroding channel banks. Accurate knowledge about the forces of moving water on channel boundaries is required to predict instability of channel boundaries. Once regions of instability are identified, stabilization measures can be targeted and applied. In this study a test of a fully predictive method for determining the force of the flowing water on an irregular channel boundary was made. Detailed measurements of the force of the moving water were made in a model channel in the laboratory at the National Sedimentation Laboratory. The calculated forces compared favorably with the measured forces on the boundary of the laboratory channel. This study has provided an independent test of this technique and has indicated its suitability for use in the prediction of sites of potential channel boundary erosion in natural streams. This type of information is necessary for watershed managers and land owners to effectively manage lands adjacent to streams and rivers in agricultural and other watersheds.

Technical Abstract: Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a test of a relatively simple, fully predictive, numerical method for determining the distribution of streamwise velocity and boundary shear stress over the entire cross-section of a straight channel. The method is based on the approach of Kean and Smith [2004], which is extended here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with high-resolution measurements made in a laboratory flume with a complex cross-section that includes a simulated, cobble-roughened floodplain. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of boundary shear stress across the wetted perimeter.

Last Modified: 10/21/2014
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