Coastal Hydrodynamics and Timescales in Meso-Macrotidal Estuaries in the Gulf of Maine: a Model Study

Sustainability solutions to water quality management problems, many generated by human activities, require knowledge and understanding of circulation patterns and transport rates that govern water quality conditions. This research demonstrates approaches to frame and implement estuary investigations with the purpose of quantifying hydrodynamics influencing estuary water pollution residence time and transport rates governed by freshwater flows and estuary tidal dynamics. The study area is a coastal location with connected estuaries in the Gulf of Maine, USA, near Acadia National Park where complex coastal morphometry controls on estuary hydrodynamics and pollution problems have been observed. A Lagrangian particle tracking study is implemented to examine particle transport patterns and timescales to identify the conditions and mechanisms that influence them. Conservative virtual particles are released and tracked in hydrodynamic simulations over month-long periods with a range of conditions specified by freshwater flow input, tide range, and varied density gradients. Results show that residual eddy size surpasses the tidal excursion length and transport behavior is controlled by the residual flow rather than the tide in wide and geometrically simple estuaries with a simple bed profile and few complications from localized geomorphological features. Estuaries with similar characteristics are pre-disposed to suppression of the estuary water density gradient, a primary driver of residual flow, producing an order of magnitude increase in residence time under conditions comparable to those simulated by our analysis. Narrow channels and the presence of complex geomorphic features such as islands and constrictions limit the length of residual eddies and increase the tidal excursion. Transport timescales in estuaries with these conditions are more sensitive to variations in the tidal range than to changes in residual flow. Explorations of the relations between residual eddy length estimated as the estuary aspect ratio and a normalized tidal excursion provide a practical way for managers to identify controls on particle transport and pollution dynamics in estuaries and determine appropriate management actions to monitor and respond to coastal water pollution problems.

Automated summary

This research investigates the influence of estuary hydrodynamics on the residence time and transport rates of estuarine water pollution. The study finds that complex coastal morphometry can affect these mechanisms. The results show that the residual flow has a stronger control in wide and simple estuaries, while narrow channels and complex geomorphic features increase the tidal excursion. These findings are significant for the management and monitoring of particle transport and pollution dynamics in estuaries.