Delaware Bay is a large estuary with a deep, relatively narrow channel and wide, shallow banks, providing a clear example of a “channel‐shoal” estuary. This numerical modeling study addresses the exchange flow in this channel‐shoal estuary, specifically to examine how the lateral geometry affects the strength and mechanisms of exchange flow. We find that the exchange flow is exclusively confined to the channel region during spring tides, when stratification is weak, and it broadens laterally over the shoals during the more stratified neap tides but still occupies a small fraction of the total width of the estuary. Exchange flow is relatively weak during spring tides, resulting from oscillatory shear dispersion in the channel augmented by weak Eulerian exchange flow. During neap tides, stratification and shear increase markedly, resulting in a strong Eulerian residual shear flow driven mainly by the along‐estuary density gradient, with a net exchange flow roughly 5 times that of the spring tide. During both spring and neap tides, lateral salinity gradients generated by differential advection at the edge of the channel drive a tidally oscillating cross‐channel flow, which strongly influences the stratification, along‐estuary salt balance, and momentum balance. The lateral flow also causes the phase variation in salinity that results in oscillatory shear dispersion and is an advective momentum source contributing to the residual circulation. Whereas the shoals make a negligible direct contribution to the exchange flow, they have an indirect influence due to the salinity gradients between the channel and the shoal.
The Damariscotta River in midcoast Maine is a weakly stratified estuary characterized by several constrictions and channel bends that affect tidal asymmetry and material transport. Microstructure and current velocity measurements were collected at a cross‐channel transect in the northern reach of the estuary during spring and neap tidal conditions to study the effects of a channel bend immediately upstream of a constricted sill on intratidal dynamics. During the flood phase, a counterclockwise gyre is formed upstream of the headland, which enhances landward‐directed flow in the channel and is countered by seaward‐directed flow over the shallow shoal. Semidiurnal and quarter‐diurnal patterns of lateral advection and stress divergence emerge in the surface layer because of these secondary flows. Lateral advection effects dominate the dynamics in neap tide, and although they are stronger in spring tide, they are partially obscured by bottom friction forces that are proportional to the along‐channel velocity squared. A novel harmonic decomposition technique is introduced to determine the relative importance of advection and stress divergence on quarter‐diurnal velocity generation, and their implications to neap/spring variability in estuarine water quality are discussed.
more » « less- PAR ID:
- 10460660
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 2
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 955-980
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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