Introduction

In the preceding chapters, the emphasis is on tidal inlets where tidal currents are dominant and river flow is of secondary importance. Sand is carried towards the inlets by longshore sand transport and cross-shore sand transport is small. Inlets are open at all times. The few that closed did so gradually through spit formation, thereby increasing the inlet length and decreasing the inlet velocity. Examples are Captain Sam's Inlet and Mason Inlet, both located in South Carolina and described, respectively, in Sections 4.4 and 4.5.

A different category of inlets is where river flow is dominant and the tide is of secondary importance in keeping the inlet open. Inlets in this category are found in Vietnam (Lam, 2009; Tung, 2011), South Africa (Cooper, 2001; Whitfield, 1992) and Australia (Baldock et al., 2008; Hinwood and McLean, 2015b; Hinwood et al., 2012; Morris and Turner, 2010; Ranasinghe and Pattiaratchi, 2003). Many of these inlets connect to small lagoons and have a small tidal range, resulting in a small tidal prism. In addition to longshore sand transport, cross-shore sand transport plays an important role in carrying the sand towards the entrance. The river flow shows strong inter-annual variations with periods of high alternating with periods of low river flow. The height and the period between peak flows have an important bearing on whether the inlets stay open or close. In this respect, a distinction is made between inlets that remain open at all times and inlets that are open only seasonally or intermittently.

Even though only open part of the year, many seasonally and intermittently closed inlets are used extensively as harbors for small fishing boats and as recreational areas for swimming and boating. Closure presents a three-fold problem. Firstly, ocean access for boats that use the back-barrier lagoon as a harbor is limited to when the inlet is open. Secondly, the water quality in the lagoon could deteriorate during the months of inlet closure. Thirdly, flooding of low-lying land may disrupt land use and access and lead to land siltation. Consequently, there is an interest in keeping the inlet permanently open.