[1] This paper summarizes state-of-the-art models for water flow and sediment transport and suggests implications for the sediment grain size distribution, transport process, and delta formation. The flow velocity in Martian outflow channels is commonly calculated from the Manning equation, which is dimensionally incorrect and masks the large uncertainty of the reconstructed flow velocity. More modern friction predictors based on surface grain size distribution are tested on 190 rivers on Earth including moderately catastrophic events. The uncertainty for the flow velocity is a factor of 3–4. The sediment transport is commonly assumed to amount to 40% of the water flux (hyperconcentration), but this is only true for special conditions. A debris-flow origin of the channels is unlikely. Application of modern sediment transport models to typical Martian conditions indicates orders of magnitude smaller sediment fluxes dominated by bed load transport, resulting in much larger timescales for sediment emplacement in crater lake deltas and in the potential northern ocean. This is in part caused by the unexpected grain size distribution of the sediment derived from observations of landers and of delta morphologies. The implied duration of hydrological activity and channel and delta formation is of the order of 103–106 years, which is still very short on the geological timescale of Mars.

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Flow discharge and sediment transport models for estimating a minimum timescale of hydrological activity and channel and delta formation on Mars

[1] The large outflow channels of Chryse Planitia, Mars, are thought to have been carved by floodwaters discharged from an aquifer beneath a confining cryosphere. However, conventional models of groundwater flow require optimistically high permeabilities to produce, in a single flooding event, the discharge rates and volumes inferred from channel morphology. Additionally, discharge likely ceased upon refreezing of chaotic terrain fractures carrying it to the surface, further limiting the volume of water produced in a single flood. It is thus probable that multiple surface discharge events were required, and we quantify this hypothesis with regional groundwater simulations. Each discharge event is triggered by cryosphere disruption due to superlithostatic hydraulic head at the channel source region and is terminated by cryosphere fracture refreezing. Before the next event, head is allowed to recover with the aid of distal aquifer recharge constrained by previous global groundwater models. Our baseline model, which emulates the source region of Kasei Valles, yields a minimum of 2900 flooding events even for high depth-averaged aquifer permeabilities corresponding to near-surface values of 10−9 m2. Although fewer events are required for other circum-Chryse channels, formation by a single flood remains improbable. We suggest that unrealistically high numbers of floods may be circumvented by an alternative model involving the local ponding of surface discharge to form standing bodies of water. Episodic failure of these bodies produces flood outbursts which erode the channels.

/pdf/multiple-flooding-events-in-martian-outflow-channels-dytfmuje7q.pdf

Multiple flooding events in Martian outflow channels

[1] The large martian outflow channels terminating in Chryse Planitia are remnants of flooding events involving the localized discharge of millions of cubic kilometers of subsurface water, more than can be regionally stored in the crust. We use a dynamic groundwater model to demonstrate that snowpack or glaciers on the Tharsis rise, formed during periods of high obliquity when ice was stable at low latitudes, may have provided an efficient source of recharge and hydraulic head for the circum-Chryse outflow channels. Water is discharged through the outflow channels several times faster in Tharsis recharge models than in comparable South Pole recharge models.

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Tharsis recharge: A source of groundwater for Martian outflow channels

http://www.geo.brown.edu/geocourses/geo292/papers/Coleman_Dinwiddie_Casteel_2007.pdf

High outflow channels on Mars indicate Hesperian recharge at low latitudes and the presence of Canyon Lakes

Design of Stable Channels with Gravel Beds

Introduction: The question of survival times of water on the surface of Mars has implications for the composition, thickness, and surface temperature of the martian atmosphere. Whether water was present for extremely brief periods or for geologically significant time spans may be determined in part by examination of the times necessary for sustained flow to empty reservoirs or to excavate channels. Previous estimates of the duration of the Maja Valles [1] outflow system were poorly constrained by lack of accur ate channel geometry. The acquisition of the MOLA data has provided the needed data to constrain the channel geometries sufficiently to determine the channel slope and cross-sectional geometry for several transects across the Maumee and Vedra trunk channels. Method: Equilibrium Sediment Transport Theory [2, 3, 4] postulates that an equilibrium stream achieves the minimum ratio of stream power to sediment load, i.e., that the greatest possible proportion of stream energy is employed in the transport of available sediment load and that the maximum sediment transport rate is achieved for the flow rate. In achieving this equilibrium, the stream adjusts its channel geometry until the channel hydraulics achieve the ideal balance between stream power and bed mobility to maximize the transport of the available sediment. Achievement of equilibrium conditions is indicated by the development of a parabolic channel cross-section [5]. The theory [4, 6] indicates that there are three independent variables that control the equilibrium conditions (i.e., sediment discharge vs. stream power) for any natural channel: discharge, grain-size, and channel slope. If any two of these can be constrained, the sediment transport rate can be determined by analysis of the balance between sediment discharge and the unknown variable to maximize the mobile sediment concentration within the stream. Because the limits on the parabolic cross-sectional geometry define the upper limits of the equilibrium channel, the equilibrium discharge was able to be determined by synthesis of the limiting discharges determined for the various transects [7]; the equilibrium discharge for the Maumee channel is approximately 1.93 x 10 6 m

/pdf/excavation-time-for-the-vedra-and-maumee-channels-mars-by-4yj4ccgap2.pdf

Excavation time for the vedra and maumee channels (mars) by application of

Excavation Time for the Vedra and Maumee Channels (Mars) by Application of Equilibrium Sediment Transport Theory

Introduction: Catastrophic outflows are one agent responsible for the redistribution of martian surface material The late Hesperian, Maja Valles outflow stretches 1200 km from Juvenate Chasma into western Chryse Planitia [1] The initial outflow ponded on Lunae Planum before spilling across Xanthe Terra highlands to debouche into the Chryse basin The material moved by the outflow is dependent upon the sediment transport rate and the flow rate This paper examines the probable grain-size moved in channels across Xanthe Terra Method: Equilibrium Sediment Transport Theory [2-4] postulates that an equilibrium stream achieves the minimum ratio of stream power to sediment load, ie, that the greatest possible proportion of stream energy is employed in the transport of available sediment load and that the maximum sediment transport rate is achieved for the flow rate In achieving this equilibrium, the stream adjusts its channel geometry until the channel hydraulics achieve the ideal balance between stream power and bed mobility to maximize the transport of the available sediment One of the most critical factors in this balance is the grain-size of the bed material Although this theory was developed for channel design in engineered systems [4], it can be applied to the analysis of natural channels created by high paleoflows assuming that the channel hydraulics can be sufficiently constrained The acquisition of the MOLA data has provided the needed data to constrain the channel hydraulics for the Maumee and Vedra channels Using equations for sediment transport [5, 6], White et al [4] showed that sediment concentration, X, could be expressed as

/pdf/grain-size-analysis-of-maumee-and-vedra-channel-sediments-3eor9p5gg3.pdf

C. J. Thibodeaux

Papers

Excavation time for the vedra and maumee channels (mars) by application of

Excavation Time for the Vedra and Maumee Channels (Mars) by Application of Equilibrium Sediment Transport Theory

Grain-Size Analysis of Maumee and Vedra Channel Sediments (Mars) Using Equilibrium Sediment Transport Theory