Evaluating Sediment Stability at Sites with Historic Contamination

The stability of cohesive sediment deposits during a rare storm is a critical component in the evaluation of remedial options at a contaminated sediment site. Estimating scour depths during a rare storm, and the resulting contaminant concentrations in the surficial layer of the bed, is necessary for comparing the efficacy of various remedial alternatives. Evaluation of sediment stability is accomplished using sediment transport analyses that employ quantitative procedures. Qualitative analyses or conceptual models can be useful for developing and validating quantitative analysis tools; however, qualitative techniques alone generally are insufficient for conducting defensible remedial alternative evaluations. The level of analysis used for a specific site depends on data availability, required level of accuracy, and time and budget constraints. A tier 1 analysis involves the use of approximate equations to produce order-of-magnitude estimates of scour depths during a rare storm. The second tier of this analysis scheme employs the development and application of a sediment transport model to evaluate bed stability. State-of-the-science sediment transport models have been effectively used as management tools for evaluating remedial options at several contaminated sediment sites. It should not be presumed that rare storm events cause catastrophic impacts at the site under review. Two case studies demonstrate that a rare storm is not necessarily catastrophic; significant increases in surficial bed concentrations caused by reexposure of elevated concentrations buried at depth in the bed will not necessarily occur during a rare storm. However, it is important to note that sediment stability is site-specific.