Observations of Tidal and Subtidal Currents Around Inlets
Ocean currents on the continental shelf vary widely both spatially and temporally, are driven by winds, pressure gradients, and density variations, and at depth or in shallow water can be strongly influenced by local topography. Although away from shore ocean currents are important for transport of, for example, nutrients and heat, they are generally not a hazard to mariners. However, in shallower water closer to shore, ocean currents can become important to navigation and sediment transport, and constitute a coastal hazard that is complicated by the geometry of the coastline, the nature of the coastal geology, and mobile seabed sediments. Nowhere are ocean currents more important to mariners, coastal engineers, coastal managers, and homeland security than within inlets and estuaries where they undergo strong interactions with topography and mix with river discharge creating a complex system of flow that varies over the water column. As a consequence of shallow nonlinear forcing, and despite our robust knowledge of tidal forcing, currents in inlets and estuaries are often poorly predicted, a problem exacerbated by limited observations of the flow field that spans the water column. In this work, we present field observations of the vertical structure of mean (hourly averaged) currents obtained at Hampton/Seabrook Inlet in 2005 and 2011, the Piscataqua Estuarine system in 2007, and New River Inlet, NC, in 2012. Currents were measured with ADCP’s deployed on the shallow shelf just outside the inlet mouth, within the throat of the inlet, and upstream within the estuary. Currents are separated into tidal and subtidal components and compared to observed winds and atmospheric pressure obtained from nearby meteorological stations. Results show that tidal flows have strong vertical structure, particularly on the inner shelf, with current magnitudes attenuating and changing direction with depth in response to wind stress, bottom drag, and mixing. Results also show that subtidal flows outside the inlethave similar complex vertical structure, and within the inlet and estuary are strongly influenced by large scale weather patterns over the inner shelf.
Acknowledgements: Research at Hampton (2011) and New River (2012)was supported by ONR and NOAA, in Hampton (2005) by USACE, and (2007) by NOAA NOS. Observations were obtained in Hampton (2005) by Jim Irish and Larry Ward, in the Piscataqua (2007)by Carl Kammerer, and in Hampton (2011) and New River (2012) by Jim Irish and Jon Hunt.
Tom Lippmann is an oceanographer in the Department of Earth Sciences and Center for Coastal and Ocean Mapping. He is the Director of the Oceanography Graduate Program, and affiliated with the Ocean Engineering Graduate Program and School of Marine Science and Ocean Engineering. Dr. Lippmann’s research is focused on shallow water oceanography, hydrography, and bathymetric evolution in coastal waters spanning inner continental shelf, surf zone, and inlet environments. Research questions are collaboratively addressed with a combination of experimental, theoretical, and numerical approaches. He has participated in 24 oceanographic field experiments and spent over 3 years in the field.