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In real two-phase (debris) mass flows there exists a strong coupling between the [[solid]] and the [[fluid]] [[momentum transfer]], where the solid's normal [[stress (physics)|stress]] is reduced by [[buoyancy]], which in turn diminishes the [[friction]]al resistance, enhances the [[pressure gradient]], and reduces the [[drag]] on the solid component. Buoyancy is an important aspect of two-phase debris flow, because it enhances flow mobility (longer travel distances) by reducing the frictional resistance in the [[mixture]]. Buoyancy is present as long as there is fluid in the mixture.<ref>{{cite journal |author= E. B., Pitman and L. Le |title= A two-fluid model for avalanche and debris flows |journal= Philosophical Transactions of the Royal Society A |volume= 363 |year =2005|pages =1573–1602}}</ref> It reduces the solid normal stress, solid lateral normal stresses, and the basal [[shear stress]] (thus, frictional resistance) by a factor (<math> 1-\gamma </math>), where <math>\gamma </math> is the density [[ratio]] between the fluid and the solid phases. The effect is substantial when the density ratio (<math>\gamma </math>) is large (e.g., in the natural debris flow).

If the flow is neutrally buoyant, i.e., <math>\gamma = 1</math>, (see, e.g., Bagnold,<ref>{{cite journal |author= R. A. Bagnold |title= Experiments on a gravity-free dispersion of large solid spheres in a Newtonian fluid under shear |journal= [[Proceedings of the Royal Society A]] |volume= 225| year = 1954|pages = 49–63}}</ref> 1954) the debris mass is fluidized and moves longer travel distances. This can happen in highly [[viscosity|viscous]] natural debris flows.<ref>{{cite journal |author= B. W., McArdell, P. Bartelt, and J. Kowalski |title= Field observations of basal forces and fluid pore pressure in a debris flow |journal= Geophys. Res. Lett., |volume= 34 |year =2007 |doi=10.1029/2006GL029183}}</ref> For neutrally buoyant flows, [[Coulomb friction]] disappears, the lateral solid pressure gradient vanishes, the [[drag coefficient]] is zero, and the basal slope effect on the solid phase also vanishes. In this [[limiting case]], the only remaining solid force is due to [[gravity]], and thus the force associated with buoyancy.
Under these conditions of [[hydrodynamics|hydrodynamic]] support of the [[particle]]s by the fluid, the debris mass is fully fluidized (or [[lubrication|lubricated]]) and moves very economically, promoting long travel distances. Compared to buoyant flow, the neutrally buoyant flow shows completely different behaviour. For the latter case, the solid and fluid phases move together, the debris bulk mass is fluidized, the front moves substantially farther, the tail lags behind, and the overall flow height is also reduced. When <math>\gamma = 0</math>, the flow does not experience any buoyancy effect. Then the effective frictional shear stress for the solid phase is that of pure [[granular]] flow. In this case the force due to the pressure gradient is altered, the drag is high and the effect of the virtual mass disappears in the solid momentum. All this leads to slowing down the [[motion]].

==References==
{{Reflist}}

[[Category:Fluid dynamics]]
[[Category:Buoyancy]]

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en>ChrisGualtieri: /* References */Remove stub template(s). Page is start class or higher. Also check for and do General Fixes + Checkwiki fixes using AWB