Location: Watershed Physical Processes ResearchTitle: Examining the generality of Tau_0/TKE for gravel and cobble beds with sand fill
Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2016
Publication Date: 4/1/2017
Publication URL: http://handle.nal.usda.gov/10113/5661728
Citation: Wren, D.G., Ursic, M.E., Langendoen, E.J., Kuhnle, R.A. 2017. Examining the generality of Tau_0/TKE for gravel and cobble beds with sand fill. Journal of Hydraulic Engineering. 143(4):06016028. doi:10.1061/(ASCE)HY.1943-7900.0001265.
Interpretive Summary: The availability of commercially produced acoustic Doppler instruments that can measure turbulence in flowing water has made it much easier to obtain the data necessary for calculating Turbulent Kinetic Energy (TKE), which is a measure of how energetic the turbulence is. TKE can be converted into bed shear stress, yielding a measure of the force of the flowing water acting on the bottom of a stream. The method has been used for estimating bottom shear stress in stream restoration evaluation and research. For example, TKE has been used in ecological studies to aid in investigating the effectiveness of stream restoration techniques for enabling the recovery of stream invertebrate populations, where elevated levels of TKE were associated with higher heterogeneity, defined as variability in particle size, in riffle sections that were manipulated during the study. The conversion of TKE into bed shear stress was investigated for gravel and cobble beds with varying levels of infilling sand. It was found that, for the clean gravel bed, the ratio of TKE to bed shear stress was similar to the value suggested by previous researchers, but, for the cobble bed, it was higher and for cases with sand filling >87% of the gravel bed, the value was reduced. For cobble beds with sand, the value was again similar to the value found in previous research. The conclusion to this study is that the value of the ratio of TKE to bed shear stress depends on bed configuration, and the approach should be used with caution.
Technical Abstract: Turbulence measurements over rough beds are used for a variety of purposes, including studies of habitat diversity for aquatic organisms, stream restoration efforts, and assessment of drag induced by vegetation. Turbulent Kinetic Energy (TKE) has been used to determine bed shear stress by use of a proportionality constant previously estimated to be between 0.19 and 0.21. Use of TKE to determine bed shear stress may have an advantage over Reynolds stress measurements since misalignment errors with the primary flow direction have less effect on TKE measurements than Reynolds stress measurements. In the work reported here, TKE was compared to bed shear stresses for gravel (D50=35 mm) and cobble (D50=156 mm) beds with varying discharge and levels of sand (D50=0.3 mm) fill. The ratio of bed shear stress (0) with TKE was not strongly affected by flow strength for either the gravel or cobble beds but was affected by the roughness height and presence of dunes. The mean of the median 0/TKE ratio was 0.25 for gravel with no sand in the bed, while for mean sand elevations in the range of 87-139% of the gravel bed height, 0/TKE was reduced to 0.20. For mean sand elevations of 0-38% of the cobble bed height, 0/TKE =0.48, and, for sand elevations of 55-74% of the cobble bed height, 0/TKE=0.40 for the cobble bed. These results show that the roughness of the bed can strongly affect the ratio of bed shear stress to TKE.