GEOMORPHOLOGICAL MODELING OF THE LONG-TERM REDISTRIBUTION OF TEPHRA DEPOSITS

A process-level model is being developed to evaluate the long-term remobilization and redistribution of tephra following a scoria-cone (violent strombolian) eruption. Initial modeling efforts focused on Sunset Crater, Arizona, and Parícutin, Mexico, and incorporated a simplified mass-balance approach to examine sediment erosion and transport processes in ephemeral drainage systems. Rates of erosion are often poorly constrained; however, a suitable range of values can be entered into a sediment budget to demonstrate the quantitative or mass flux relationship between such components as annual sediment transport rates (primarily the tephra), discharge from the drainage system, sediment mixing (dilution of tephra deposits), balance of remaining tephra, and associated changes in sediment storage. For this first-order model, all hillslope and erosion processes (including mass wasting and sheet wash) are combined into an annual measure of sediment production. Using this sediment budget technique with a sediment yield of 100 m³km^-2yr^-1 and a tephra-fall deposit with a reported volume of 1.3 km³, 5.3 percent of the Parícutin deposit was eroded and redistributed in the 62 years from the start of the eruption. Applying the same erosion rate, 14 percent of the Sunset Crater tephra-fall deposit (0.75 km³) was eroded and redistributed in the approximately 1,000 years since the onset of that eruption. This modeling approach has additional applications, including remobilization of contaminated tephra from a possible volcanic eruption at the potential high-level waste repository at Yucca Mountain, Nevada. Appropriate model parameters are being substantiated for expected dryland conditions in the Fortymile Wash drainage system following a modeled eruption at Yucca Mountain.  [This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the U.S. Nuclear Regulatory Commission.]