Fisher (1983) emphasized the concept of flow transformations, which involve the change from laminar to turbulent - or turbulent to laminar - behavior in sedment gravity flows. I thought that the distinctions between his four different types were a little hard to keep track of, and were not as important as simply the fact that transformations occur and leave a signal in the stratigraphic record. Also important, Fisher notes that flow transformations result primarily from dynamic changes in particle concentration (which determines density), flow thickness, and flow velocity. With a few examples.

The paper by Postma et al. (1988) was motivated by a desire to explain floating out-sized clasts in coarse grained turbidites. To me, this was (and still is) a relatively minor aspect of the paper. I think the main contribution of this paper is the recognition that a subaqueous debris flow can tranform down slope into a “bipartite” flow that includes two co-evolving parts: (1) a lower gravel-rich grain flow that we call a “traction carpet” because its motioin is significantly aided by shear stress at the base of; (2) an overflowing, faster moving low-density turbulent suspension of fine-grained sediment and water. I believe this is a widespread process in subaqueous sediment gravity flows, and/but it has generated a fair amount of discussion and controversy since 1988.

Back to Sediment Gravity Flows