Hydrodynamics of a Large and Shallow Back-Barrier Estuarine System and Responses to Long-Term Changes in Geomorphology

Abstract

Coupled hydrodynamic and wave models were used to simulate water levels, currents, waves and salinity transport in the Albemarle-Pamlico Estuarine System (APES), a large shallow back-barrier basin in eastern North Carolina, over a one-month long period. Simulations were run for four different bathymetric grids pertaining to distinct time slices during the evolution of the APES corresponding to the present day, and 500, 1000, and 4000 calibrated years before present to determine responses to long-term changes in geomorphology. The present day simulation was used to evaluate a spatially varying wind field developed from the North American Regional Reanalysis (NARR) dataset in comparison to spatially uniform winds from observations at different sites across the region. Simulations using winds observed offshore result in statistically better hydrodynamic simulations of water levels (R=0.88) in the estuaries than the NARR dataset (R=0.48). The removal of a long shoal from present day bathymetry resulted in a decrease in water level setup by 3% at the estuarine shoreline, a decrease in current magnitude up to 40% and an increase in significant wave height up to 25%, indicating the importance of this shallow feature as a major control on the present day hydrodynamic response of Pamlico Sound. Paleobathymetric grids for three time slices over the late Holocene were developed from sediment core and seismic observations described by Zaremba (2014). Model results were compared to assess the impacts of: 1) varying degrees of barrier island segmentation; 2) long-term changes in basin geomorphology; and 3) sea-level rise on the hydrodynamic response in Pamlico Sound. All three of these influenced the hydrodynamics (e.g., up to 3 times higher tidal range and current velocity) when compared to present day observations and model results. Salinity in the present day simulation was validated using observations, and salinity distributions predicted for each time slice were compared to salinity ranges from foraminiferal assemblages at sediment cores. The results indicate that salinity in Pamlico Sound is strongly influenced by wind and wave-driven mixing and transport, the hydraulic connectivity between Albemarle and Pamlico Sounds, and the quantity and size of tidal inlets through the barrier island system.