  Quasi-2D experiments of a submerged sediment layer creeping downward were
performed, varying the channel tilt and a porous flow under the respective
thresholds for yielding. Logarithmic decay rates of the deformation are
observed, with the rate increasing with both control parameters. A new
dimensionless parameter, $P^*$, accounting for both mean porous flow and
gravity force effects on particle motion, allows a collapse of all the
deformation results on a single curve. Two distinct creep regimes are
identified, and correspond to a systematic change of the void size distribution
as $P^*$ increases.
