Hydrographs of a Martian flood from a breached crater lake, with insights about flow calculations, channel erosion rates, and chasma growth

The Elaver Vallis channels provide a unique opportunity to develop calibrated hydrographs for a Martian flood. Groundwater filled Morella Crater until the enclosed lake overtopped the rim, confirming that the groundwater potentiometric surface exceeded 1771 m. The overtopping flow rapidly breached the crater wall, catastrophically drained the lake, and eroded a broad scabland and two main channels. Laser altimeter data were used to calculate a preflood lake volume of 2.216 x 10(12)m(3). Dam breach methods were used to develop hydrographs of discharge and lake stage over time. The peak discharge was in the range of 1.94 x 10(7) to 3.51 x 10(7) m(3) s(-1), depending on the underlying breach erosion scenario. Approximately 95% of the drainable lake volume discharged in 6.4 to 7.5 days. The 240 m deep southern channel entirely formed in 1 to 1.7 days, indicating a mean erosion rate of 0.10 to 0.17 m min(-1) . The volume of material eroded from the channels was 0.45 of the flood volume, but the actual eroded fraction was probably greater because distant channel reaches were obliterated by the postflood growth of Ganges Chasma. For comparison, flow calculations were performed using open-channel methods. Channel energy slopes had to be corrected for a postflood tectonic tilt in the regional surface. Comparing the hydrographs to the open-channel calculations reveals that the northern Elaver Vallis channel as seen today never flowed bank full or even at half its maximum depth because the implied flows would have exceeded the peak discharges. Citation: Coleman, N. M. (2013), Hydrographs of a Martian flood from a breached crater lake, with insights about flow calculations, channel erosion rates, and chasma growth, J. Geophys. Res. Planets, 118, 263-277, doi: 10.1029/2012JE004193.