Jon J. Major1, Richard M. Iverson1
(1) U.S. Geological Survey, Vancouver, WA
ABSTRACT: Sedimentation by Debris Flows -- Insights from Large-Scale Subaerial Flume Experiments
Reproducible experiments at the USGS debris-flow flume show that subaerial debris flows composed of ~ 10 m3 of water-saturated gravel, sand, and mud can deposit sediment incrementally, yet produce massively textured, matrix-supported deposits. Flows released down the steeply sloping (31°) 95-m long, 2-m wide flume deposited sediment beyond the flume mouth on a smooth, gently sloping (3°), unconfined concrete surface. In each experimental flow, surge waves developed spontaneously and deposited successive layers of sediment as they overran or partly displaced deposits of previous waves. The experimental debris-flow deposits exhibited many morphologic features typical of natural debris-flow deposits, such as lobate snouts, blunt margins, marginal levees, arcuate surface ridges, clusters and streaks of accumulated surface gravel, and particle long axes that preferentially aligned along wave perimeters. Internally, deposits were poorly sorted, massively textured, and contained "floating" clasts; locally they exhibited inverse grading of the largest clasts. Incremental accretion of massively textured sediment by a single debris flow contradicts the widely held view that the sedimentology of a debris-flow deposit results from sudden "freezing" of the flow. Simultaneous measurements of pore-fluid pressure and total normal stress at the bases of flows demonstrate that even during deceleration and deposition flows are nearly liquefied behind snouts having little pore pressure; elevated fluid pressure dissipates significantly only after deposition. Hence, deposition results from grain-contact friction and bed friction concentrated at flow margins.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado