MULTI-DIMENSIONAL HYDRAULIC ASSESSMENT OF HAZARD POSED TO MOAB URANIUM MILL TAILINGS BY FLOODING IN THE COLORADO RIVER, MOAB VALLEY, UTAH

The Moab uranium mill tailings are adjacent to the Colorado River in Moab Valley, Utah. More than 11.5 million tons of uranium tailings were generated by the Moab uranium mill which operated from 1956 to 1984. The Upper Colorado River drains more than 24,500 mi2 in Colorado and Utah before entering Moab Valley. The Colorado River downstream of Moab Valley provides drinking and irrigation water to more than 26 million people.

A multi-dimensional hydrodynamic model was applied to aid in the assessment of the potential hazard posed to the uranium mill tailings near Moab, Utah, by flooding in the Colorado River as it flows through Moab Valley. Discharge estimates for the 100- and 500-year recurrence intervals and for the Probable Maximum Flood (PMF) were evaluated with the model for the existing channel geometry. Water-surface elevation, velocity distribution, and shear-stress distribution were predicted for each simulation.

Predicted water-surface elevations indicated that the tailings pile would be inundated by about 4 feet by the 100-year discharge and 25 feet by the PMF discharge. A small area at the most streamward edge of the tailings pile was characterized by velocities of about 1 to 2 feet per second for the 100-year discharge. Predicted velocities at this location for the PMF discharge increased to between 2 and 4 feet per second over a somewhat larger area. The manner to which velocities progress from the 100-year discharge to the PMF discharge in the area of the tailings pile indicates that the tailings pile obstructs the over-bank flow of flood discharges. The predicted path of flow for all simulations along the existing Colorado River channel indicates that the current distribution of tamarisk or salt cedar in the over-bank regions affects how flood-flow velocities are spatially distributed. Shear-stress distributions were predicted throughout the study reach for each discharge and channel geometry examined. Material transport was evaluated by applying these shear-stress values to empirically determine critical shear-stress values for grain sizes ranging from very fine sands to very coarse gravels.