Physical And Biological Factors Influencing The Fate Of Fine Particles In Shear Flows

An experimental flume study tests the influence of several geomorphic, hydraulic and biological factors on the deposition rate and infiltration of fine particles to a coarse gravel-sand bed. Factors include flow rate, particle density, initial particle concentration, and the density, growth stage, and type of streambed periphyton. Our experimental design independently compares the influence of 1) two flow rates; 2) high- and low-density particles; 3) high and low particle concentrations; and 4) two forms of periphyton assemblages, one dominated by diatoms and the other by algae, at a range of biomass densities and growth stages. Over an eight hour period, measured suspended particle concentrations show a roughly exponential decline, from which deposition rates are derived. Stratified bed samples are sieved for fine particle content. Among physical factors, particle density and initial concentration have the greatest effects on deposition rate, although the magnitude and direction of effects differ over time and among particle size classes. Particle density has no effect on particle infiltration or flow hydraulics. Along a gradient of low to high initial concentrations, deposition rate and infiltration amounts increase and near-bed Reynolds stresses decrease. Low flow rates have lower velocities, Reynolds stresses, and deposition rates but similar infiltration amounts to higher flows. Among periphyton types and densities, diatoms generally increase deposition rates relative to non-periphyton surfaces by reducing Reynolds stress and enhancing surface deposition via adhesion. However, increases in diatom biomass reduce particle infiltration by clogging interstitial spaces, in turn lowering deposition rates. In contrast, all algal growth stages have slower or similar deposition rates to non- periphyton conditions, possibly due to partial clogging by high biomass and a lack of adhesion at the bed surface. Clogging effects are counteracted at later algal growth stages by the increase in Reynolds stress and downward advection, in turn increasing infiltration amounts and deposition rates.