Geoscience Reference
In-Depth Information
settling velocity increases rapidly as salinity increases. However, when salinity exceeds
a certain value, its influence on floc settling becomes very slight.
Turbulence intensity
The turbulence of flow affects flocculation in two different ways (Owen, 1970;
McConnachie, 1991; Haralampides
et al
., 2003). For low shears, turbulence increases
the chance of collision among sediment particles and thus strengthens flocculation;
for high shears, strong turbulence may break apart the flocs and attentuate floc-
culation. Fig. 11.7 shows the relation between the residual turbidity and stirrer
speed observed by McConnachie (1991) in a Perspex reactor 0.1 m by 0.1 m in
plan view, filled to a depth of 0.1 m. Three types of stirrers were used. It was
found that as the stirrer speed increased, the residual turbidity decreased first and
then increased. Haralampides
et al
. (2003) performed experiments on the settling
of flocs in a rotating circular flume using sediment samples from the St. Clair
River near Sarnia, Ontario; they observed that the median size d
50
of flocs had
a maximum value at the bed shear stress of 0.17 N
m
−
2
and decreased for shear
·
m
−
2
. These experiments have proven that as
the turbulence intensity increases, flocculation will first be strengthened and then
attenuated.
stresses above and below 0.17 N
·
Figure 11.7
Residual turbidity in a Perspex reactor versus stirrer speed (McConnachie, 1991).
Temperature
Temperature affects the thermal motion of ions and, in turn, flocculation. According
to Huang's (1981) experiments on the settling of flocs using the Lianyun Harbor mud
at temperatures of 6.1, 21.5, and 32
◦
C, the settling velocity of the flocculated sediment
