Hyperconcentrated sediment flows such as mudflows and mud floods are fluids to distinguish them from granular flows or landslides. Mudflows and mud floods are unsteady, nonuniform, viscous fluid phenomena. The ubiquitous term ‘debris flow' is discouraged because it provides little insight about the flow physical processes. If a water and sediment mixture deforms continuously when subjected to shear stress, it is fluid and is governed by the physics of fluid motion. Fluid motion is classified as Newtonian if the shear stress is a linear function of the shear rate or as nonNewtonian when the viscous fluid behavior is more complex. Sediment particle motion that exhibits pore water pressure or significant dispersive stress is either as a landslide or granular flow. Savage (1979) defined ‘bulk solids motion' as an assemblage of discrete particles dispersed in a fluid such that the particles contact their neighbors. In bulk solid flows, the concentration exceeds 50% by volume and motion is governed by particle cohesion, friction and collision. While the transition from solid particle motion (rolling, tumbling and sliding) to fluid motion may be gradual, the existence of a pore water pressure is an indication of solids motion. Pore water pressure in granular material means that a portion of the overburden load is carried by both the fluid phase and the solids phase. It also means that the fluid cannot escape through the interstices or that the upward motion of fluid through the solid particles is relatively minor. The distinction between a fluid flow and a granular flow is important when formulating the equations of motion, and the dominance of the fluid turbulence, viscous stress, dispersive stress and particle friction terms.

This discussion addresses fluid motion of hyperconcentrated sediment flows referred to as mudflows or mud floods. In these flows, course clastic material such as boulders, vegetation and garbage are only along for the ride and do not significantly affect the fluid behavior. Simulation of the fluid motion of hyperconcentrated sediment flows can be accomplished with the FLO-2D model which considers viscous, turbulence and dispersive shear stress terms in the equations of motion. A number of FLO-2D results from mudflows in the Rockies are presented including several model simulations.