Levee-channel Deposits in Dry or Wet Debris Flows: a Tool to Understand Gullies Formation

Introduction: Recent gullies on Mars are observed on the wallslopes of the mid-latitude regions. They might sign the presence of fluid flows, likely involving liquid water, in a recent past [1]. However, authors have shown that dry flows might be an alternative to the formation of Martian gullies [e.g.2]. Levees are frequently present together Martian gullies independently of their location (dunes, crater wallslopes, isolated hillslopes) [1,3,4]. Levees morphometry is different in a dry or a wet flow therefore enabling us to measure Martian levees and to compare which case fits best. The high resolution images HiRISE allow us to look in detail to levees characteristics and measure their size using photoclinometry. Despite levees are not ubiquitously observed together gullies, many leveed channels have been identified on several images of gullies with HiRISE (Fig. 1, Fig. 2). Thus, the aim of this work is to identify critical parameters that discriminate the processes of the levees formation and test them on Mars from levees observations on HiRISE images. Physical basis for levees morphometry: Terrestrial wet debris flows are often modeled using the Bingham fluid properties [5]. A Bingham plastic material has a linear relation between the strain rate and the stress but with a finite yield strength [4, 5]. This means that it does not deform until a critical shear stress is reached, after which it deforms as a Newtonian fluid (linear relation). The occurrence of the critical shear stress is the main reason of the presence of lateral deposits: As the flow gets thinner near the lateral shoulders of the channel, the gravity force which is proportional to the thickness of the flow gets smaller. As a result, the driving forces are not strong enough for the material to exceed the critical shear stress, thus leading to the formation of lateral levees. For granular flows, laboratory experiments show that self-channeling lobes and levee-channel deposits can be obtained when the slope higher than 20° [e.g. 6]. The appropriate flow law to describe dissipation in dry granular flows is still under debate. An empirical parameterization of the friction law has been suggested in the recent years [7]. This theory shows that the friction coefficient involved in the classical Coulomb friction law increases with decreasing thickness and increasing velocity. The thickness h stop left on an inclined plane by a steady granular flows when the supply is cut is an empirical parameter of the …