Tidally-modulated stratification in a channel-shoal estuary

Abstract

This study integrates field observations and numerical simulations in the Delaware Estuary. The geometry of the Delaware Estuary is similar to that of South San Francisco Bay, the German Wadden Sea, and the mouth of the Changjiang Estuary. Both the observations and model results demonstrate that the water column was stratified (Δs > 15 psu), in contrast to the prior designations of the Delaware Estuary as a “weakly-stratified” estuary (Garvine et al., 1992). This study implements the salinity variance method to qualify the contributions of different mechanisms to initiating estuarine stratification. The analysis indicates that the tendency for stratification is initialized by the interplay between tidal straining and mixing from the end of spring tides. Although the longitudinal straining decreases during flood tides, the lateral straining contributes to a positive tendency for estuarine stratification. The lateral salinity gradient is not significant under the annual mean discharge condition. The strong vertical shear of flow and deviation from the mean salinity are favorable for the generation of lateral tidal straining. In contrast, a prior study showed that strong lateral salinity gradients enhance vertical stratification during the flood. The lateral straining at the end of the flood provides initial stratification, resulting in enhanced the longitudinal tidal straining during the early ebb due to the inhibition of turbulence and thereby enhanced tidal shear.