Herschel-Bulkley generalized modeling of real debris flow: set-up of rheological parameters accounting for sediment concentration

Natural flows as mud, debris and hyper-concentrated flows are fast-flow phenomena typically characterized by the rapid motion of a large amount of soil mixed with water, and the study of these events is particularly important with reference to their catastrophic and destructive capability worldwide. In this perspective, a complete and appropriate knowledge of the rheological properties of these kinds of granular flows could be an effective support to the assessment of invasion’s areas, travel distance and flow velocity (Schippa & Pavan, 2011). For such materials, the bulk flow properties (related to failure, propagation and spreading phase) can be extrapolated from the study of the involved soil-liquid mixture paying particular attention to the influence of solid volumetric concentration and grain size distribution (Coussot et al., 1998; Pellegrino & Schippa, 2013; Pellegrino, Scotto et al. 2015). We refer to the Herschel-Bulkely rheological model, which is widely used in hydrodynamic modeling of mud and debris-flows (Laigle & Coussot, 1997; Fraccarollo & Papa, 2000; Schippa & Pavan, 2011). Generally speaking, the flow-like behaviour of slurries appears dominated by the rheological properties of the interstitial fluid phase composed by water and finer sediment fraction, whereas yield stress are greatly affected also by coarser fraction (Major & Pierson, 1992). The experimental activity herein presented focuses on the former of these aspects, and puts in evidence the effect associated with the granular bulk concentration of the mixture in terms of rheological properties of the mixture. It was carried out using a standard rheometer equipped with vane rotor system on natural soils collected from the source area of two real events of debris flow occurred in Campania region (Italy).