Estuarine flow is highly dependent on a number of factors, including freshwater discharge, wind, and tides. In multi-inlet bar-built systems, the flow can further be driven by mutual forcing between adjacent inlets. In the North Inlet–Winyah Bay National Estuarine Research Reserve, two distinct estuaries are connected by a subtidal channel. Within the channel, simultaneous tidal forcing from both ends produces a nodal point. To determine the implications of forcing from both ends on net exchange, we placed an RDI acoustic Doppler current profiler (ADCP) and a SonTek acoustic current profiler (ACP) on either side of the nodal region for 35 days beginning in March 2005. Asymmetries in tidal forcing on either side of the node lead to significant shifts in the timing of flow reversals. The longer ebb associated with the more distorted Winyah Bay tide causes flow reversals to occur at midebb in Winyah Bay. In addition to tidal asymmetries, subtidal flow patterns during periods of strong wind forcing varied greatly, yielding persistent net volume exchange between the two estuaries for periods of up to several days. Application of simple momentum-balance arguments indicates that the observed subtidal exchange is consistent with local setup and setdown in the larger exposed Winyah Bay, in which pressure gradients generated at the fringe of North Inlet drive the observed flow in the channels. However, local forcing alone does not explain the exchange during periods of persistent upwelling conditions, in which the implied setup at the fringe fails to maintain the direction and magnitude of the current. Rather, the low passed filtered exchange in the channel initially vanishes and then reverses direction around the period of maximum wind stress.
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