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

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 down-slope 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.