The Effect of Tidal Exchange on Residence Time

Abstract:

As the conduit between the ocean and an inland body of water such as a lagoon, estuary or harbor, tidal inlets serve an important role in the hydrodynamics of the nearshore environment. As the global population grows in a world where 13% of the coastline is composed of barrier islands, an understanding of the physical processes that influence the transport of inland waters offshore is increasingly paramount. Water renewal, or the replacement of old lagoon water with new seawater has been well studied and is controlled by the tides through the process of tidal exchange or ‘tidal pumping’. The magnitude of tidal exchange is influenced by the inlet hydraulics and geometry of the inlet and various physical processes driven by the tide, wind, and waves. To examine the correlation between tidal exchange and the renewal of lagoon water with seawater, a new method to quantify lagoon residence time is explored.

Modeling experiments in Delft3D of idealized inlet systems are used to quantify the effect that tidal exchange has on residence time. Tidal exchange is decomposed into two fractions, an ocean exchange fraction and a lagoon exchange fraction. A simple transport model that utilizes both these fractions is compared to both a tidal prism model that neglects the lagoon exchange fraction and Delft3D. The tidal prism model over predicts the seaward transport of lagoon water as compared to Delft3D for all inlet channel geometries studied. The transport model shows good agreement with Delft3D for narrower inlets that support high ocean exchange fractions, but less agreement for wider inlets that support low ocean exchange fractions. Residence time distributions for each geometry are calculated using a new virtual tracer method in Delft3D. The spatially averaged residence time in the lagoon is found to be inversely proportional to the product of the two exchange fractions of each inlet system.

Funded by ONR and NDSEG