When fine sediment concentrations (by volume) rise above about 30%, (Rickenmann, 1992) the viscosity of the mixture increases enough that the viscosity of water is no longer an appropriate approximation. The Debris library models Hyperconcentrated flows with a Bingham Plastic model.
The Bingham Plastic model has a linear stress-strain relationship like the Newtonian model, but it diverges from Newtonian assumptions in two ways. First, the Bingham model includes a yield stress. The yield stress (τy) introduces a non-zero intercept in the stress-strain relationship. In other words, there is a range of stress that does not deform the fluid (a range of stresses that do not induce strain). Second, while the Bingham stress-strain relationship is linear it does not have the same slope as the Newtonian fluid. The viscosity of the mixture (μm, which is higher than the viscosity of the fluid alone) dictates the slope of the Hyperconcentrated stress-strain relationship.
