Mechanisms of exchange flow in an estuary with a narrow, deep channel and wide, shallow shoals

Abstract

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, 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 during both spring and neap tides and is a significant advective momentum source driving the residual circulation. Thus, although the shoals make a negligible direct contribution to the exchange flow, the salinity gradients between the channel and the shoal are critical to the stratification and exchange flow within the estuarine channel.