Mathematics – Logic
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27..265c&link_type=abstract
Lunar and Planetary Science, volume 27, page 265
Mathematics
Logic
2
Channels: Outflow, Mars Pathfinder, Velocities: Channel Flow
Scientific paper
To know what we're getting out of the Mars Pathfinder "grab bag" landing site it is imperative that the detailed geology and hydraulic history of the circum-Chryse outflow channel complex be understood ahead of time. Crude estimates of the maximum channel flow velocities can be made simply by knowing the depth and slopes of the outflow channels themselves. Although these characteristics have been derived in part by stereophotogrammetry, they are subject to a considerable amount of error. Fortunately some Earth-based radar data exist which are both reasonably accurate and provide the spatial coverage necessary for determining the slopes of some of the channels. Using these data, the bed shear stress of a flow, or the retarding stress at the base of a flow, Tb, can be estimated from the depth-slope formula ~b = pghS where p is the density of the fluid, g is gravitational acceleration, h is the flow (or channel) depth, and S is the slope of the channel. This is equal to the bottom stress created by a flow, tau, where tau = pCf-u2 and Cf is a dimensionless drag coefficient and u is the mean flow velocity. Thus, the mean flow velocity for a channel can be calculated from -u = (ghS/Cf) 1/2. The dimensionless drag coefficient can be adjustment for gravity by the expression Cf = g(n21hll3) where n is the Manning roughness coefficient (units of s/ml/2), which has been derived empirically from terrestrial observations. Application of an appropriate Manning roughness coefficient, n, to Martian outflow channels is uncertain, so a range of reasonable values (0.015 to 0.035) is used. Estimates of the mean flow velocities were calculated from this method, however, at best these represent maximum values. Large-scale geologic mapping indicates that most channels were subjected to multiple episodes of flooding, which suggests that the channels may not have been completely full of water at any one time (i.e., bankfull discharge). This method is also not directly applicable to Simud and Tiu Valles because the Earth-based radar data indicate a positive downslope gradient, which mav be due to modification (e.g.slack-water deposition or tectonism) post-dating channel formation.
Craddock Robert A.
Tanaka Ken L.
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