Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p41a1350p&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P41A-1350
Physics
5415 Erosion And Weathering, 5419 Hydrology And Fluvial Processes
Scientific paper
Using a stereo pair of HiRISE images of a crater slope incised by fourteen gullies at -37.86 N, 217.92 E we calculate relative elevation changes between pairs of hand-selected points. Using the method of Kreslavsky [1]. The background slope on which the gullies are located has a slope of 22 degrees. Out of the five gullies we analyzed, all show a steadily decreasing slope from an average of 30 ± 4 degrees at the alcove to 16 ± 2 degrees at the apron. These measurements are in agreement with previous gully slope measurements done at MOLA resolution in a different region [2]. The slope beyond the base of the gully aprons is 4±1 degrees. The depth of alcove incision in nine of the gullies is 17±8.5~m. We take advantage of this slope and incision data to determine the evolution of a one-dimensional gully profile over time with a 1D sediment transport model [3]. The shear stress applied to the channel bed by flowing water is τ = ρ g h sinθ where h is the channel depth, g is gravity, and h is the channel depth. The rate of transport is non-linearly related to τ/τrg where the reference stress for a gravel bed is τrg = 0.035 ( (s-1)ρ g Dg ) where s is the ratio of sediment to water density, ρ is 1000~m3, and Dg is the sediment grainsize. The two significant unknowns in applying the theory to Martian gullies are the sediment grainsize and channel depth. We ran simulations for various channel depths and grainsizes to get a range of water discharges and simulation times that result in alcoves 25~m deep. Erosion is rapid due to the high slopes; incision rates decrease with decreasing channel depth and increasing grainsize. For grains 20~cm in diameter and a conservatively low channel depth of 20~cm, alcove incision occurs over a 5~h period, discharging a volume of 8500 m3 of water. These discharges assume a 1~m wide channel and a constant, bank-full discharge over the duration of the simulation. Gullies are spaced about every 500~m along the slope. If liquid water is sourced from melting ice within a semi-circular disk with a radius (r) equal to half the gully spacing, the discharge volume V=0.5π r2 h φ, where h is the disk thickness, and φ is the ice volume fraction. If φ=0.1 and r=250~m, then the thickness of ice-rich material is 1~m (per meter width of channel). [1] Kreslavsky, M.A., Mars Gully Conf. #8034, 2007 [2] Dickson, J.L. et al., Icarus 188, 315323, 2007 [3] Wilcock, P.R. and Kenworthy, T., Water Resources Res., 38, 1194, 2002
Kreslavsky Mikhail
Nimmo Francis
Parsons Aaron R.
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