**1. Introduction**

R.A. Bagnold [1] identified two distinct mechanisms of particle support: fluid suspension and intergranular contact, in his studies of desert sand in Egypt. It was Professor K.C. Wilson [2, 3] who proposed the application to particle-bearing liquids in pipes, i.e., an upper fluid suspension and a lower layer taking support from hindered settling and the pipe walls: the *Two-Layer Model*. This is a gross simplification in some eyes, but a great help in identifying the velocity and magnitude of the stratified burden. **Figure 1** shows a typical concentration map for a liquid with settling particles obtained by *electrical resistance tomography* (*ERT*) for low-velocity pipe flow (0.75 m/s). Interestingly, the supernatant phase (in pale blue) has not stacked itself in height order but has spread around the boundaries of the other pipe contents—actually emphasising the interface between two layers. The two-layer model was later modified for computer modelling by Professor C.A. Shook [4]. Shook proposed a *contact load* within an all-pervading suspension fraction.

**Figure 1.**

*Concentration map (5.5% v/v) for 2 mm beads (relative density 1.4) in water, obtained by ERT. High concentration red and low concentration blue.*
