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 mu m 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 th e accepted sediment deposition model (e.g., Einstein, 1968) are explai ned by filtration of particles in the bed. Compared to this model depo sition increases with increasing surface fluid speed, decreasing suspe nded particle size and increasing bed particle size. These results can be explained by an increased ability of particles to penetrate into t he bed with the pore water which increases the effective filter thickn ess. The predominant deposition mechanism within the bed pores appears to be settling. Enhanced deposition evident in Einstein's (1968) expe rimental data, was previously attributed to flocculation but may be ex plained 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 d eposits from the top of bed elements. This reduction in deposition rat e occurs long before the bed is filled with fine sediment.