Experimental studies of deposition by debris flows: process, characteristics of deposits, and effects of pore-fluid pressure

This study examines deposition by experimental debris flows (to 15 m3 ) composed ofmixtures of gravel (to 32 mm), sand, and loam using the 95-m-long, 2-m-wide, 31° U.S.Geological Survey debris-flow flume. It examines the depositional process, relations betweenflow kinematics and deposit character, and fluid pressure in debris at, and following,deposition. These data permit evaluation of competing hypotheses regarding debris-flowdeposition.
Experimental debris flows invariably developed surges; deposits developed abruptly on a3° runout slope as sediment transported by shallow ( The depositional process is recorded primarily by deposit morphology and surface textureand is not faithfully registered by interior sedimentary texture. Homogeneous internaltextures can be interpreted as the result of deposition by a single surge. Individual debris
flows as well may leave little distinctive signature in the sedimentary record. Superposeddeposits from similar yet separate flows could not be distinguished without the aid of anartificial marker horizon. These results show that methods of estimating flow properties fromdeposit thickness or from relations between particle size and bed thickness are in error.Relations between sediment composition and deposit thickness are incompatible withdeposition by a simple viscoplastic material.
Experimental debris flows deposited sediment despite measured basal fluid pressures thatwere lithostatic to nearly lithostatic. These data refute hypotheses that propose uniform fluid-pressure dissipation as a control on deposition. Modeling and laboratory analyses of gravity-driven consolidation reveal that characteristic pressure-dissipation times in quasistatic debrisexceed surge periods and durations typical of debris flows. Numerical simulations oftransient changes in fluid-pressure and effective-stress fields in 2-dimensional quasistaticdomains reveal that excess fluid pressures remain elevated, and effective stresses depressed,everywhere except adjacent to margins in wide thin bodies for time scales that exceeddurations typical of debris flows. Observed deposition, measured fluid pressures, andmodeling results suggest that debris-flow deposition is controlled by a balance between adiminishing driving stress and locally increasing resisting stresses along flow margins ratherthan by a uniform bodywide increase of effective stress