Amphidromic point

Abstract: 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.

Abstract: The generation of tidal asymmetries is clarified via numerical integration of the one-dimensional equations for channel geometries characteristic of shallow estuaries. Channels without tidal flats develop a time asymmetry characterized by a longer falling than rising tide. This behavior is enhanced by strong friction and large channel cross-sectional area variability over a tidal cycle. Resulting tidal currents have a shorter, intense flood and a longer, weak ebb (flood-dominant). Addition of tidal flats to the channels can produce a longer rising tide and stronger ebb currents (ebb-dominant), if the area of tidal flats is large enough to overcome the effects of time-variable channel geometry. Weaker friction with flats can also produce this asymmetry. Despite the physical complexity of these systems, essential features of estuarine tidal response can be recovered from one-dimensional models. Shallow estuaries are shown to have a system response leading to stable, uniform senses of tidal asymmetry (either flood- or ebb-dominated, due to phase-locking of forced tidal constituents), with down-channel development in magnitude of asymmetry. These concepts are illustrated by modeling idealized representations of tidal channels at Nauset Inlet, MA, and Wachapreague Inlet, VA, which have flood- and ebb-dominance, respectively.

Abstract: Tides and their dynamic processes in the South China Sea (SCS) are studied by assimilating Topex/Poseidon altimetry data into a barotropic ocean tide model for the eight major constituents (M 2 S 2 K 1 O 1 N 2 K 2 P 1 Q 1 ) using a tidal data inversion scheme. High resolution (∼10 km) and large model domain are adopted to better resolve the physical processes involved and to minimize the uncertainty from the open boundary condition. The model results, which are optimized by an inversion scheme, compare well with tidal gauge measurements. The study reveals that the amplitude of the semi-diurnal tide, M 2 , decreases, while the amplitude of the diurnal tide, K 1 , increases similar to the Helmholtz resonance after the tidal waves propagate from the western Pacific into the SCS through the Luzon Strait (LS). Analyses of the energy studies show that the LS is a place where both M 2 and K 1 tidal energy dissipates the most, and strong M 2 tidal dissipation also occurs in the Taiwan Strait (TS). The work rate of the tidal generating force in the SCS basin is negative for M 2 and positive for K 1 . It is found that the responses of tides in the SCS are largely associated with the propagating directions of the tides in the Pacific, the tidal frequency, the wavelengths, the local geometry and bottom topography.