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United States Department of Agriculture

Agricultural Research Service

Title: Prediction of Effects of Large Woody Debris Removal on Flow Resistance

Authors
item Shields F D Jr,
item Gippel C J, - UNIV OF MELBOURNE

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 10, 1993
Publication Date: N/A

Interpretive Summary: Stream channels are often obstructed by fallen trees and logs, which causes local flooding and channel erosion. Until recently, conventional practice was to clear obstructed streams of all woody debris to alleviate these problems. However, in the last 10 years, scientists have discovered more about the importance of woody debris to channel stability and aquatic ecosystems. Conservation interests have suggested (and in some cases, required) action agencies to practice selective removal of debris, but existing methods for estimating the hydraulic effects of debris removal were inexact. The amounts of debris and the effects of debris removal on the flow capacity of a river in Western Tennessee and another in Australia were measured. These measurements and results of some scale model experiments were used to develop and refine a procedure to predict effects of debris removal on flow capacity. Engineers can use these results to analyze benefits of selective debris removal proposals and weigh benefits against likely environmental impacts.

Technical Abstract: The relative importance of factors controlling the contribution of large woody debris (LWD) to stream channel roughness was studied using a series of flume experiments, and a simple technique for predicting the Darcy- Weisbach friction factor for river channels with varying amounts of LWD was developed. First, LWD density is determined based on measurement or visual estimation of cross-sectional areas of debris formations in the plane perpendicular to flow. The Darcy-Weisbach friction factor is then computed using debris density, channel geometry, mean flow velocity, and the LWD drag coefficient. The LWD drag coefficient may be computed from a power function with experimentally determined coefficients. For verification of the proposed procedure, debris density and friction factors were measured in river reaches in western Tennessee and southeastern New South Wales, Australia. Friction factors computed using the procedure were within 14% of measured values for straight, sand-bed reaches but within 38% of measured values for sinuous, gravel-bed reaches.

Last Modified: 12/19/2014
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