The use of Lagrangian particle methods to investigate ocean-estuary exchange in well-mixed estuaries

A Lagrangian particle method which has been parallelized and embedded within a 2-D finite element code is used to study the transport and fate of contaminant plumes and ocean-estuary exchange processes in a well-mixed Gulf of Maine estuary. The particle method has been extended to include a random walk model in the horizontal that simulates sub-grid scale turbulent transport processes. This module has been formulated to allow for spatial variability in the diffusivity. The 2-D finite element model includes a porous medium transport module to treat the effects of wetting and drying of estuarine tidal flats. Due to the highly-complex, spatially dependent nature of tidal mixing and shear dispersion, contaminant transport is most naturally addressed through the Lagrangian methodology. Our approach involves instantaneous, massive particle releases that enable the quantification of ocean-estuary and inter-bay exchange along with the associated residence time. The results show that estuary-ocean exchange is significantly enhanced, and hence residence times reduced, by the presence of turbulent mixing, which combines with the effects of the sheared tidal currents to drive strong interbay exchange, and/or river input, which drives a mean throughflow. The particle approach helps to uncover the strong spatial dependent nature of the residence time within the estuary which has important ramifications for local water quality. The interbay exchanges are considered as a Markov process as discussed by Thompson et al. [11] and this framework is found to be useful.