ON STREAM PERIPHYTON-TURBULENCE INTERACTIONS

A set of experiments was carried out to determine what effects periphy ton communities could have on near-bed hydraulic fields. We analysed v elocity distribution, skewness and kurtosis coefficients, Reynolds str esses, relative turbulence intensity, coefficient of eddy diffusivity, Velocity spectra, and turbulence scales at two flows with, and withou t, diatom-dominated periphyton on the bed. We found that turbulence wa s affected by the periphyton mat selectively. The largest influence oc curred for the velocity distribution, Reynolds stress, coefficient of eddy diffusivity, and velocity cross-spectra. Changes as a result of t he periphyton mat were revealed in the large-scale turbulence structur e. The periphyton affected not only the region near the bottom but als o the entire logarithmic layer. The periphyton mat increased roughness length by a factor of approximate to 5 whereas the integral resistanc e to flow (reciprocal Chezy coefficient and Manning's roughness coeffi cient) was increased by 20-25%. The microscale turbulence structure wa s investigated using values of Kolmogorov's microscale eta(K). These a ppeared to be commensurate with the characteristic scales of periphyto n filaments. This allowed us to hypothesise that the mechanism of the ''periphyton-turbulence'' interaction is connected, to a certain degre e, with viscous effects.