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Title: Turbulence measurements over immobile gravel with additions of sand from supply limited to capacity transport conditions

item Wren, Daniel
item Kuhnle, Roger
item Langendoen, Eddy
item Ursic, Michael - Mick

Submitted to: International Conference on Fluvial Hydraulics
Publication Type: Proceedings
Publication Acceptance Date: 7/13/2016
Publication Date: 7/13/2016
Citation: Wren, D.G., Kuhnle, R.A., Langendoen, E.J., Ursic, M.E. 2016. Turbulence measurements over immobile gravel with additions of sand from supply limited to capacity transport conditions. International Association of Hydraulic Research 2016 River Flow Conference in St. Louis, MO ( July 12-15, 2016. PP. 361-362.

Interpretive Summary: The availability of commercially produced acoustic Doppler instruments has made it much easier to obtain the turbulence quantities necessary for calculating TKE, making it an attractive method for estimating bottom shear stress in stream restoration evaluation and research. For instance, in the case of stream management and restoration practices that aim to enhance physical habitat heterogeneity or increase it through morphological improvements of the stream bed. 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 gravel bed that was covered by sand, the ratio of TKE to bed shear stress was similar to the value suggested by previous researchers, but, for the clean gravel bed, the ratio was increased by about 30%. The main conclusion for this conference proceedings paper is that the value of the ratio of TKE to bed shear stress depends on bed roughness, and the approach should be used with caution.

Technical Abstract: Measurement of the turbulence that drives sand transport over and through immobile gravels is relevant to efforts to model sediment movement downstream of dams, where fine sediments are eroded from coarse substrates and are not replaced due to the presence of the upstream dam. The relative elevations of the sand and gravel control the near-bed turbulence and sand transport capacity. A profiling acoustic Doppler velocimeter was used to make turbulence measurements in a recirculating laboratory flume with a gravel bed (D50=35 mm). Sand (D50=0.3 mm) was added to the gravel bed in increments until the bed was completely covered by sand. Changes in Reynolds stress, TKE, and the ration between bed shear stress and TKE were investigated for each sand elevation. Both Reynolds stress and TKE decreased with increasing sand elevation and the effect was greater for higher flow strengths. For the lowest three sand fills in the gravel bed, 0/TKE=0.25. Additions of sand above 81% fill, which began to introduce bed forms, reduced 0/TKE to 0.20.