One-dimensional flood inundation modeling and sediment transport characterization for a potential mining site in southern Virginia, USA

A one-dimensional flood inundation modeling effort and a qualitative sediment transport characterization that were conducted as part of a series of site-specific investigations for the Coles Hill uranium deposit in southern Virginia, USA are presented herein. A study watershed with an area of 1415 km(2) was defined with the objectives of: (1) generating flood inundation maps for the 10-, 50-, 100-, and 500-yr storms and the Probable Maximum Flood (PMF), and (2) studying the impact of various hydraulic characteristics on the overall sediment transport process. The employed methodology includes the construction of a river terrain model by combining data from publically available sources and field survey campaigns, a calibration procedure based on developed site-specific stage-discharge predictor curves, and unsteady flow simulations using the results from an event-based hydrologic model. The results highlight the severe conditions associated with the PMF in terms of predicted water depths (max. value of 23.5 m) and flood inundation spatial extents (inundated area of 28.8 km(2)), demonstrating the necessity of considering it as the worst-case flooding scenario. Moreover, it was found that the Banister Lake Dam and its reservoir, the Banister Lake, located nearly 54 km downstream of the deposit act as a control point in the river network. The results indicate that a significant reduction of the mean flow velocity (max. reduction of 57%) and the mean shear stress (max. reduction of 73%) as water enters the lake are caused by the presence of the dam and its reservoir, reducing the sediment transport capacity and hence facilitating sediment deposition. Field surveys downstream of the dam revealed the existence of an armor layer considerably coarser than the material located under it (median sizes of 51.0 mm and 0.9 mm, respectively), indicating that the dam and its reservoir do act as a barrier that disrupt the natural sediment transport process. The results presented herein aim to assist in the estimation of the potential consequences of a tailings (radioactive waste by-product) containment cell failure downstream of the Coles Hill mining site.