Hydrodynamics and Associated Morphological Variations on an Estuarine Intertidal Sand Flat

Abstract

Hydrodynamics, sediment suspension, and morphological response on an estuarine intertidal sand flat in Tokyo Bay, Japan, are examined through a field experiment performed for 16 days in winter 2000 and using a bathymetry data set based on a 6-year series of surveys. Topography of the sandy flat was found to fluctuate by approximately 8 cm during the deployment, while the long-term accumulation rate is estimated with the surveyed bathymetries to be only 3.8 cm/y. Cross-spectral analysis of the measured data indicates that on the sandy tidal flat, semidiurnal or shorter-period fluctuations in current velocity are mostly attributed to semidiurnal tides and waves, whereas wind above the sea generally drives diurnal or longer-period fluctuations. The field data also confirm that suspended sediment concentrations were highly correlated with bed shear stress, which is generated by combination of tidal current, wind-induced current, and wind waves. Episodic erosion is observed on the sandy flat with high turbidities on ebbing phases. Erosion evidently occurs at phases between the high slack and the mean tidal levels during ebb flows when combined tidal, wind-driven, and wave-driven currents are maximized.