Floc morphology and size distributions of cohesive sediment in steady-state flow

Fractal dimensions of particle populations of cohesive sediment were examined during deposition experiments in an annular flume at four conditions of steady-state flow (0.058, 0.123, 0.212 and 0.323 Pa). Light microscopy and an image analysis system were used to determine area, longest axis and perimeter of suspended solids. Four fractal dimensions ( D, D 1, D 2, D k ) were calculated from the slopes of regression lines of the relevant variables on double log plots. The fractal dimension D, which relates the projected area ( A) to the perimeter ( P) of the particle ( P∝ A D/2 ), increased from 1.25±0.005 at a shear stress of 0.058 Pa to a maximum of 1.36±0.003 at 0.121 Pa then decreased to 1.34±0.001 at 0.323 Pa. The change in D indicated that particle boundaries became more convoluted and the shape of larger particles was more irregular at higher levels of shear stress. At the highest shear stress, the observed decrease in D resulted from floc breakage due to increased particle collisions. The fractal dimension D 1, which relates the longest axis ( l) to the perimeter of the particle ( P∝ l D 1 ), increased from 1.00±0.006 at a shear stress of 0.058 Pa to a maximum of 1.25±0.003 at 0.325 Pa. The fractal dimension D 2, which relates the longest axis with the projected area of the particle ( A∝ l D 2 ), increased from 1.35±0.014 at a shear stress of 0.058 Pa to a maximum of 1.81±0.005 at 0.323 Pa. The observed increases in D 1 and D 2 indicate that particles became more elongated with increasing shear stress. Values of the fractal dimension D k , resulting from the Korcak's empirical law for particle population, decreased from 3.68±0.002 at a shear stress of 0.058 Pa to 1.33±0.001 at 0.323 Pa and indicate that the particle size distribution changed from a population of similar sized particles at low shear to larger flocculated particles at higher levels of shear. The results show that small particle clusters (micro-flocs) are the formational units of larger flocs in the water column and the stability of larger flocs is a function of the shear stress at steady state.