Field and laboratory investigations of the fluid in fully developed debris flows

Fluid (composed of fines and water) and its interaction with solid (coarser grains) are thought to greatly affect debris-flow behaviors. However, the high variability of the proposed cutoff diameter between the fines in fluid and coarser grains would seem to indicate that additional fluid is nonexistent in debris flows. The presence or absence of fluid in fully developed debris flows is here investigated. Results show that in debris-flow slurries fluid independent of coarser grains is nonexistent or negligible. Field evidence, including close observations of 124 moving debris-flow surges and comprehensive inspection of 64 surge deposits, indicates the en masse propagation and deposition of solid particles and water, i.e., the absence of additional fluid in debris-flow slurries. The lack of fluid in the fronts of 80 experimental debris-flow deposits, Solidity values of the deposits approaching 1, the massive structure of 32 sediments evolving from the experimental slurries, and the Bingham behavior exhibited by 80 experimental slurries consistently demonstrate that fluid is not present in debris flows, and that debris flows themselves can be regarded as continuum single-phase fluid. The degrees of saturation of > 108% of 80 slurries, determined using soil mechanics methods, indicate that debris flows are water-supported, and that the sediments in debris flows do not constitute porous media. The sustained high excess water pressure and very low cumulative percentage of decant water suggest that two mechanisms are simultaneously at work in debris flows: water supporting grains and granular assembly holding water.