Filtration enhances suspended sediment deposition from surface water to granular permeable beds

We present results from an experimental study of suspended particle (4.5-36.5 μm silicon carbide powder) deposition from surface water to 'clean' equi-granular permeable beds in a small 12.5x12.5x15cm box and a re- circulating flume. Enhanced deposition rates of up to 5 times the accepted sediment deposition model (e.g., Einstein, 1968) are explained by filtration of particles in the bed. Compared to this model deposition increases with increasing surface fluid speed, decreasing suspended particle size and increasing bed particle size. These results can be explained by an increased ability of particles to penetrate into the bed with the pore water which increases the effective filter thickness. The predominant deposition mechanism within the bed pores appears to be settling. Enhanced deposition, evident in Einstein's (1968) experimental data, was previously attributed to flocculation but may be explained better by filtration. These enhanced deposition rates drop off to close to the accepted model predicted rate after a certain volume of sediment has entered the bed, which may be due to the slumping of deposits from the top of bed elements. This reduction in deposition rate occurs long before the bed is filled with fine sediment.