20 40

20 40

FIGURE 12.4 Floc growth measured by Hogg et al.10 — the various symbols refer to various test conditions.

flocculation conditions were made extremely favorable through the addition of the flocculant. Yet, the flocculation time was still about 1 h. Also McAnally and Mehta11 found flocculation times of many hours, depending on the turbulence intensity and sediment concentration.

The evolution of flocs in the natural environment is governed by a large number of physical, and often also chemical and biological processes. At present it is impossible to quantify all these processes, as many basic questions have not yet been answered. An analysis of these processes is, therefore, only possible when making severe simplifications. Here we schematize the floc size distribution with one mean floc size only. We first treat a number of processes in isolation to establish their relevance and scaling rules, upon which these processes are integrated to understand the effect of nonlinear interactions as occurring in the real world. It is emphasized that this chapter yields qualitative conclusions only, as model schematizations are crude and the data scarce.

In Section 12.2, we present a simple relation between floc size and settling velocity and a Lagrangian model for flocculation of cohesive sediment under turbulent conditions; here we also present our formal definition of the flocculation time. Note that from studies by McCave,12 Wacholder and Sather,13 and Stolzenbach and Elimelech,14 it can be concluded that Brownian motion and differential settling play a small role in the high-dynamic coastal and estuarine environments. In Section 12.3, we elaborate on the vertical floc size distribution, discussing the role of residence times on maximal floc size. In Section 12.4, we integrate the various physical processes and study the evolution of floc size over the tidal cycle, elaborating on the processes behind rapid settling and sediment-induced stratification. Whereas Sections 12.3 and 12.4 deal with low concentrated suspensions, Section 12.5 treats the effects of flocculation on the sediment concentration in fluid mud layers. In Section 12.6, we present some conclusions, and contemplate on the role of flocculation time on floc size and settling velocity, and how to measure these.

0 0

Post a comment