Introduction

The major elements, inlet, ebb delta and flood delta, together with the adjacent coast constitute a sand sharing system (Dean, 1988); sand is transported among these elements by tide- and wave-generated currents. Because at tidal inlets direct measurements are difficult, much of what is known of sediment transport and sediment transport pathways has been inferred from migration and shape of bed forms and swash bars, dredging records, comparison of sequences of bathymetric maps and aerial photographs (Bruun and Gerritsen, 1959; Hanisch, 1981; Hine, 1975).

Sand is transported towards a tidal inlet by longshore currents. Longshore currents and the resulting longshore sand transport result from waves approaching the coast at an oblique angle (Kamphuis, 2006). Some of the longshore sand transport is carried into the inlet by the flood currents and is deposited in the back-barrier lagoon. Another part is jetted to the deeper parts of the ocean and some of it is transported over the ebb delta to the downdrift coast. The sand stored in the lagoon and the deeper parts of the ocean is lost to the littoral zone. As a result, the supply of sand to the downdrift coast is less than the longshore sand transport causing erosion of this part of the coast. The details of the transport of sand from the updrift to the downdrift coasts are discussed in Section 3.3.

An example of sand entering and leaving an inlet is presented in Fig. 3.1. Sand enters through the porous breakwater and is temporarily stored on the updrift side of the inlet in the form of a protruding sand bank. During ebb, sand is carried from the bank in an offshore direction. A similar process was observed in a small inlet in the Bay Islands, Honduras. In that case the clarity of the water and the size of the inlet (width 3 m, depth 0.3 m) made it possible to visually observe the deposition and formation of the sand bank on the updrift side of the inlet and the subsequent removal of some of the sand during ebb.