A computer model of sediment dynamics and radionuclide dispersion in a macrotidal estuary

Abstract

In macrotidal, partially mixed estuaries, tidal pumping can become a dominant mechanism that progressively accumulates sediment in the upper part of the estuary. This mechanism is also reinforced by different mixing behaviour during flood and ebb, due to significandy different stratification patterns. The flux and retention of particle reactive radionuclides within an estuary is complicated by their adsorption onto suspended silts and clays that may then accumulate in an estuary due to these tidal pumping processes. Predicting the accumulation of radionuclides in an estuary is further complicated by radioactive decay processes where daughter products may have distinctly different geochemical properties to those of the parent. The VERSE model has been recently developed in order to replicate the sediment transport and the advection-dispersion of dissolved and particle bound radionuclides with up to nine daughter products. The model is two-dimensional laterally averaged and uses finite difference schemes on a fixed grid. It contains four modules that compute the hydrodynamics, the cohesive sediment dynamics, and the advection- dispersion of conservative and non-conservative contaminants. VERSE has been developed to provide industry and regulatory authorities with a tool that can predict the transport of sediment and the dispersion of contaminants in estuaries, under various hydrodynamic conditions. This paper provides an overview of the model structure and results are compared to field data collected in a macrotidal estuary during several measurement campaigns.