Sediment transport by liquid surficial flow: Application to Titan

Sediment transport by surficial flow likely occurs on Titan. Titan is thought to have a volatile cycle, such as on Earth and likely in the past on Mars, which would entail surficial liquid flow. And surficial flow is implied in interpretations of Cassini–Hyugens data as showing fluvial channels, which would require sediment transport by surficial flow to form the observable features. We present calculations from basic hydraulic formulae of sediment entrainment and transport by surficial flow. First, we describe the conditions for (non-cohesive) sediment entrainment by grain size through use of the Shieldsʹ threshold curve. We then calculate settling velocities by grain size to describe the type of sediment transport—washload, suspended load, or bedload—that would follow entrainment. These calculations allow derivation of required flow depths for sediment transport by grain size over a given slope. A technique to estimate required flow velocities and unit discharges is also presented. We show the results of these calculations for organic and water ice sediment movement by liquid methane flow under Titan gravity. For comparative purposes, plots for movement of quartz sediment by water on Earth and basalt sediment by water on Mars are also included. These results indicate that (non-cohesive) material would move more easily on Titan than on Earth or Mars. Terrestrial field observations suggest that coarse grain transport is enhanced by hyperconcentration of fine-grained sediment; and the apparent availability of organic (fine grained) sediment on Titan, in conjunction with the possibility of convection-driven rainstorms, may lead to hyperconcentrated flows. Thus, significant sediment transport may occur on Titan during individual overland flow events.