On stream periphyton-turbulence interactions

A set of experiments was carried out to determine what effects periphyton communities could have on near-bed hydraulic fields. We analysed velocity distribution, skewness and kurtosis coefficients, Reynolds stresses, relative turbulence intensity, coefficient of eddy diffusivity, velocity spectra, and turbulence scales at two flows with, and without, diatom-dominated periphyton on the bed. We found that turbulence was affected by the periphyton mat selectively. The largest influence occurred for the velocity distribution, Reynolds stress, coefficient of eddy diffusivity, and velocity cross-spectra. Changes as a result of the periphyton mat were revealed in the large-scale turbulence structure. The periphyton affected not only the region near the bottom but also the entire logarithmic layer. The periphyton mat increased roughness length by a factor of =5 whereas the integral resistance to flow (reciprocal Chezy coefficient and Manning's roughness coefficient) was increased by 20-25%. The microscale turbulence structure was investigated using values of Kolmogorov's microscale 1)^These appeared to be commensurate with the characteristic scales of periphyton filaments. This allowed us to hypothesise that the mechanism of the "periphytonturbulence" interaction is connected, to a certain degree, with viscous effects.