Submarine debris flows are often accompanied by isolated blocks located some distance beyond the rest of the failed mass. These so-called outrunner blocks have the ability to travel over long distances on very gentle slopes. Glide tracks of various depths are observed in some cases, while in others no traces of significant erosion can be detected, which indicates that outrunner blocks are able to travel completely separated from the bed. Similar phenomena occur in laboratory experiments, where chunks detach from the front of a small-scale debris flow and move out ahead of the rest of the flow. We present a two-dimensional, small-scale model of a rigid block subjected to gravity combined with the complete dynamical interaction with the surrounding liquid. Our simulations indicate that the block is able to hydroplane completely separated from the bed and attain long runout distances. The maximum velocity of the block is close correlated with the thickness of the block. For the simple shape assumed in our model we find that the densimetric Froude number is ?0.8 . Depending on the geometrical shape of the block, size and the slope angle, we observe oscillatory motion, where the front of the block is lifted periodically and the rear part tends to scrape the bed. The pressure distribution around the block indicates that the block is likely to deform at the rear end as well as the front.
Small scale simulations of outrunner blocks