Mapping exchange and residence time in a model of Willapa Bay, Washington, a branching, macrotidal estuary

The numerical model GETM is used to examine transport pathways and residence time in Willapa Bay, Washington, a macrotidal estuary with a complex channel geometry. When the model is run with realistic forcing, it reproduces both tidal velocities and the decrease of the salt intrusion length with increasing river flow with errors of 5-20%. Furthermore, a more stringent test, when the model is run with tidal forcing only, it reproduces the along-channel profile of the effective horizontal diffusivity K, a direct measure of the strength of subtidal dispersion, which is known from previous empirical estimates. A Lagrangian, particle-tracking method is used to map subtidal transport pathways at the resolution of the model grid. This analysis reveals an interweaving of coherent lateral exchange flows with discontinuous, small-scale dispersion as well as tidal residual currents that in some locations, sharpen rather than smooth gradients between water masses. Comparison between these Lagrangian results and an Eulerian salt flux decomposition suggests that along-channel complexity (channel junctions and channel curvature) is at least as important as cross-sectional depth variation in shaping the subtidal circulation. Finally, a nonconservative tracer method is used to produce high-resolution, three-dimensional maps of residence time. This analysis shows that consistent with previous observational work in Willapa, at all except the highest winter-storm-level river flows, river- and ocean-density-driven exchanges are discernable but secondary to tidal stirring. In all seasons, despite the fact that half the volume of the bay enters and leaves with every tide, average retention times in the upper third of the estuary are 3-5 weeks.