Many experimental studies on the cross-sectional distribution of particles in vertical risers have revealed marked segregation of particles. These flows are inherently unsteady with large fluctuations in suspension density. In this article, we have analyzed the time-smoothed equations for the motion of dense suspensions to demonstrate the role of these fluctuations on the occurrence of segregation. It is shown that the particles will congregate in regions where the kinetic energy of fluctuations associated with the particles is small. In the context of transport of small particles such as FCC, a simplified model based on the mixture velocity can be constructed. A speculative K-epsilon model based on this velocity is analyzed to illustrate the extent of segregation of particles afforded by unsteady fluctuations.