Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis

The importance of asymmetric tidal cycles in the transport and accumulation of sediment in shallow well-mixed estuaries is well established. Along the U.S. Atlantic Coast, tidal amplitude, bottom friction, and system geometry determine tidal distortion as documented at 54 tide gauges in 26 tidally dominated estuaries of varying geometry having negligible freshwater inflow. Analyses of sea-surface heights are compared to the results of one-dimensional numerical modelling to clarify the physics of tidal response in well-mixed estuaries. Concise measurements of estuarine geometry and ocean tidal range are used to predict consistently the nature of tidal sea-surface distortion. Numerical modelling then is utilized to extend theoretical and observational relationships between geometry and sea-height to predict trends in velocity distortion and near-bed sediment transport. Non-linear tidal distortion is a composite of two principal effects: (1) frictional interaction between the tide and channel bottoms (reflected in a h = tidal amplitude channel depth ) causes relatively shorter floods; (2) intertidal storage (measured by V s V c = volume of intertidal storage volume of channels at mean sea level) causes relatively shorter ebbs. Variations in V s V c and a h trigger consistent and predictable changes in tidal distortion as measured through the first harmonic of the principal tidal constitutent.