Fine particle deposition to porous beds

Abstract

We summarize the results of flume experiments examining the transport behavior of dilute suspensions of silt‐sized particles carried in an open channel flow over three separate bed types including porous open mesh, glass beads, and small cobbles. In these experiments the time‐varying concentration of particles in suspension transport was measured as a function of suspended‐particle size and bed shear stress, and analyzed using a 1‐D model incorporating simultaneous deposition and entrainment. Rates of fine‐particle deposition to the bed are found to approach Stoke's settling velocity in slow flows, but diminish systematically with increasing bed shear stress and mean flow speed. No discernible re‐entrainment from the porous beds was observed, indicating that such beds act as an effective sink for fine particles. When our new results are compared to those of related, previously reported experiments examining fine‐particle transport over smooth impermeable beds, silt‐sized particles display similar behavior with regard to systematic reduction in deposition velocity independent of suspended‐silt‐particle size or bed porosity. This behavior is tentatively interpreted to reflect the effects of lift in a linear shear flow in excess of submerged weight of individual particles. Our findings compare favorably with values of effective settling velocity of fine particulate organic matter in natural channel flows reported elsewhere.