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Some physical and biological factors influencing the fate of fine clastic particles in flowing water Salant, Nira Liat

Abstract

An experimental flume study was conducted to assess the influence of several physical and biological factors on the movement and deposition of fine particles (< 125 µm) in flowing water. Mechanisms of particle movement were elucidated from measurements of flow hydraulics, particle concentrations, surface deposition, and subsurface infiltration for varying flow rates, bed sand fractions, particle densities, initial concentrations, and periphyton structures. Results showed that low flows slowed total deposition, an unexpected result given the lower near-bed Reynolds stresses and velocities of this condition. Similarly, a bed with a high sand fraction also slowed total deposition despite having lower near-bed Reynolds stresses. A higher amount of surface deposition to the high sand bed was offset by limited subsurface deposition, likely due to the clogging of pore spaces by fine sand and reduced advective transport. Particle density also significantly altered deposition rate but had no effect on particle infiltration or flow hydraulics. Along a gradient of low to high initial concentrations, deposition rate and infiltration increased, due to greater particle availability and an increase in particle interactions. A comparison of theoretical and measured concentration profiles showed that for fine particles the Rouse equation, using a depth-integrated particle size, performed as well as or better than more complex models. All models under-predicted concentrations of low-density plastic particles, over-predicted at low concentrations, and performed better with a high sand bed. Periphyton had a significant effect on hydraulics and deposition for a range of structures, densities and spatial scales. High density, closed periphyton patches compacted under high flows resulted in higher velocities and lower near-bed Reynolds stresses by constricting the flow depth and smoothing the bed surface. Lower density patches increased bed roughness, reducing near-bed velocities and transferring turbulent shear upward. Mucilaginous diatoms at low to moderate biomasses increased deposition rate and surface deposition by reducing near-bed Reynolds stress and enhancing particle adhesion. However, at high biomasses, diatom assemblages clogged interstitial spaces and reduced the amount of subsurface deposition thus slowing total deposition. In contrast, deposition occurred more slowly for most growth stages of filamentous algae, possibly due to partial clogging of the bed and a lack of surface adhesion. However, later algal growth stages increased Reynolds stress and advective transport, in turn increasing the amount of subsurface deposition and thus total deposition rate.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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