Introduction

Depending on their origin, tidal inlets are identified as primary or secondary inlets. Regardless of the origin, the morphology is characterized by three major elements: the inlet, the ebb delta and the flood delta. The morphology of each element is determined by tide and waves. In particular, the tidal prism (the volume of water entering on the flood and leaving on the ebb) and the wave-induced longshore sand transport play an important role in determining the cross-sectional area of the inlet and the size and shape of the ebb delta.

Origin of Tidal Inlets

Following Ehlers (1988), in tracing the origin of tidal inlets a distinction is made between primary and secondary inlets. Primary inlets are those where pre-existing relief, characterized by troughs and adjacent highs, plays a decisive role in the formation of the inlet. During the Holocene, starting some 10,000 BP, onshore sand transport associated with the rapid rise in sea level caused these existing troughs to fill while barrier islands formed on the adjacent highs (Jelgersma, 1983). Examples of primary inlets are the Ameland Inlet (Fig. 4.7) and the Frisian Inlet (Fig. 12.4) along the Dutch Wadden Sea, both relics of drowned river valleys (Beets and van der Spek, 2000).

Apart from these primary tidal inlets, secondary inlets can be identified. Secondary inlets originate from flooding of narrow and shallow parts of barrier islands during a storm. Once the fore-dune ridge is dismantled as a result of storm erosion, a shallow washover channel develops. As the storm passes and winds change direction, return flow forces water against the landward side of the barrier. Often the return flow is funneled across the low portion of the barrier island through the washover channel. Depending on the tidal prism and the longshore sand transport, the washover channel closes or remains open. When remaining open, this channel is then gradually enlarged by the ensuing tidal currents. Sand from the channel is deposited both offshore and in the basin, forming the onset to, respectively, the ebb delta and the flood delta. All these secondary or washover inlets are located in a sand-rich environment. As discussed in more detail in the following chapters, the ultimate shape and size of newly opened inlets depends on tide and waves.