Inlet Flood Tidal Delta Development through Sediment Transport Processes

Abstract

Structurally controlled inlets along microtidal coasts that open to large lagoons develop extensive flood tidal deltas while ebb tidal shoals are small or absent. Tidal hydrodynamics indicate a sediment transport dominance in the ebb direction. To understand the processes of such a system, and this apparent conflict between theory and observation, aerial photographs, sedimentological data, and sediment transport simulations are examined.

Sebastian Inlet is used as an example to define a class of “lagoonal-type” inlets. This inlet is located on Florida’s east coast connecting the Indian River Lagoon to the Atlantic Ocean. The inlet is on a coastline with moderate to high wave energy and a high net southerly drift of littoral material. Construction of two jetties and extensive dredging have been required to maintain a navigable channel. An updrift offset has developed in response to jetty construction.

This inlet has only a small ebb tidal shoal configuration, but the extensive flood tidal delta has grown to be the most dominant feature. Growth of this delta and changes in channel orientation through time have been documented with a detailed series of aerial photographs beginning in 1943. As a result of dredging and natural processes, the main channel has displayed a dynamic history, changing orientation and splitting into two subchannels as the flood tidal delta has grown. An ebb-type shield has developed and grown with the creation of a dredge spoil area in the center of the delta. The delta has continued to grow into the lagoon by the process of flood spillover lobe formation with little evidence of ebb sand transport.

Cores taken in the area of the flood delta have documented the lagoonward growth of distinctive inlet-derived sands deposited over the lagoonal shelly fine grained sediment. Grain size distributions vertically and laterally show the transport and sorting occurring under the influence of the tidally driven sand transport processes as this delta grows.

A set of predictive equations has been developed and tested to investigate tidally driven sediment transport. The results suggest that there is an ebb dominance of sediment transport in the inlet throat and flood dominance of sediment transport in the region of delta growth. The hydrodynamic and sand transport simulations result in qualitative agreement with the descriptive and sedimentological investigations. This provides a clear understanding of the phenomena occurring in this class of inlets.