Water Quality Models for Small Tidal Inlet Systems

A simplified mathematical modeling methodology is proposed for the assessment of water quality in small tidal inlet systems. Towards this objective, a quasi‐steady, tidal exchange model is developed for predicting the periodic tidal variations in the concentration of conservative and decaying constituents in small, completely mixed embayments subject to steady loading and steady freshwater inflow. This model provides an inexpensive alternative to conventional multidimensional schemes and provides insight into the physical nature of the water quality response of these systems to the external forcing effects of ocean tides, freshwater inflow, and constituent loading. It is demonstrated in both theoretical arguments and by sample applications of the model that the low‐pass, hydraulic filtering response of tidal inlet systems influences water quality. The model demonstrates that tidal exchange processes are less efficient in hydraulically constricted systems, which have relatively short Helmholtz periods and large inlet impedances, than in systems with long natural periods. Consequently, the model reveals a greater tendency for pollutant accumulation in systems with short Helmholtz periods. Thus, the model provides both a conceptual framework and a screening tool for assessing the susceptibility of small tidal systems to pollutant accumulation.