Gravity-driven consolidation of granular slurries--implications for debris-flow deposition and deposit characteristics

Fresh debris-flow deposits consolidate under their own weight. How quickly they consolidate (dissipate excess pore-fluid pressure and compact) affects their resistance to remobilization as well as their sedimentologic and stratigraphic characteristics. Here, analysis of small-volume (-0.05 m3) noncohesive debris-flow slurries and larger ( -10 m3) experimental debris-flow deposits reveals the nature, rate, and magnitude of consolidation of typical debris-flow deposits.
A simple, linear, one-dimensional model describing the diffusion of excess pore-fluid pressure satisfactorily approximates the overall timing and magnitude of consolidation of noncohesive debris-flow deposits. The model and measurements of pore-fluid pressure demonstrate that changes in fluid pressure and effective stress evolve upward from the base of a deposit, and show that hydraulic diffusivities of muddy slurries containing about 5 to 50 wt% mud are remarkably similar, about 10-6-10-7 m2/s. By comparison, sandy-gravel debris-flow deposits containing Low hydraulic diffusivities promote high and persistent pore-fluid pressure in debris flows, key factors enhancing mobilization. Elsewhere, pore-fluid pressures nearly sufficient to liquefy debris have been shown to persist through transit and deposition. Here, I show that significant dissipation of such fluid pressure is restricted to post depositional consolidation. Therefore, neither uniform decay of excess pore-fluid pressure nor intrinsic viscoplastic yield strength explain debris-flow deposition. Instead. debris-flow deposition results from friction concentrated along flow margins where high pore-fluid pressures are absent. Sustained high pore-fluid pressure following deposition fosters deposit remobilization, which can mute or obliterate stratigraphic evidence for multiple events. A thick deposit of homogeneous, poorly sorted debris can result from mingling of soft deposits and recurrent surges rather than from a single flow wave if deposit consolidation time greatly exceeds typical sediment emplacement times.