Tidal distortion in shallow estuaries

Abstract

The offshore tide becomes distorted as it propagates into shallow inlet/estuarine systems. Time asymmetries develop in the rise and fall of sea surface with consequent time and magnitude asymmetries in tidal currents. Flood-dominant estuaries are characterized by longer falling tides and stronger flood currents while ebb-dominant estuaries have longer rising tides and stronger ebb currents. The asymmetries are reflected in the non-linear growth of harmonics and compound tides of the principal equilibrium tidal constituents. This dissertation consists of three papers which examine the development of tidal asymmetries in shallow estuarine systems: a study of the recent migration history of Nauset Inlet (MA), a shallow estuarine system located on Cape Cod; an analysis of the results of a series of field experiments conducted at Nauset; a numerical model study of the types of estuarine characteristics controlling tidal asymmetry. The analysis of field results focuses on sea surface measurements. Non-linear distortion of the tide at Nauset is characterized by the strong growth of harmonics and compound constituents particularly in the quarter-diurnal band. Phase relationships between the forced constituents and their parent produce a flood-dominant estuary. Numerical modeling of M2 tidal propagation in shallow estuarine channels utilizes the one-dimensional equations of motion. Shallow, frictionally dominated channels with moderate tidal flat area develop a flood-dominant asymmetry while deeper channels with extensive tidal flats develop an ebb-dominant asymmetry. Model results are supported by observations of tidal asymmetry in natural estuaries. Implications of non-linear tidal distortion on bedload and suspended material transport are profound. Flood-dominant estuaries tend to import sediment if the supply is adequate whereas ebb-dominant estuaries can flush entering sediment effectively. Over long time periods, flood-dominant estuaries may eventually fill. Ebb-dominant estuaries may represent more stable long-term configurations.