Grain-size sorting and slope failure in experimental subaqueous grain flows

Abstract

Grain-size sorting in subaqueous grain flows of a continuous range of grain sizes is studied experimentally with three mixtures. The observed pattern is a combination of stratification and gradual segregation. The stratification is caused by kinematic sieving in the grain flow. The segregation is caused by the down-slope overpassing of larger grains over smaller grains subsequent to kinematic sorting, and by the drag of the grain flows on the underlying grains causing erosion and additional down-slope transport of the larger grains. The sorting rate along the grain flows increases with the grain size standard deviation as expected. However, for the largest grain size range, a new slope failure mechanism emerged that increases the downslope sorting rate. Instead of drag and erosion of larger grains by the grain flows moving over them, about the lower half of the slope fails as a whole when the grain flow arrives at about the midpoint of the slope. This slope instability occurs due to stratification in the previous flow: the large grains lie on top of small grains which causes a smaller friction angle (static angle of repose), which is nearly equal to the topographic angle of the deposit.