Hydrodynamics and sediment processes at beach–inlet systems are dynamic and complicated. Beach–inlet systems are characteristic of mostly shallow and complicated bathymetry. As ocean waves propagate into shallow water with complex bathymetry, wave refraction, diffraction, shoaling, and breaking occur. The shallow water associated with the ebb tidal delta has significant influence on the pattern of wave propagation. Eventually in the vicinity of the shoreline and over the very shallow portions of the ebb tidal delta, waves break and generate intense turbulence that induce active sediment transport. Both wave and tide play significant roles in transporting sediments and causing morphology change at barrier–inlet systems. In general, wave-induced sediment transport dominates in areas far from the inlet where tidal currents tend to be weak. Within the deeper parts of inlet channels, tide-driven current constitutes the main mechanism for moving sediments. Over the ebb and flood tidal deltas, sediment movements are driven by both wave and current. This chapter reviews the above topics and commonly used equations on the hydrodynamics and sediment transport relevant to the beach–inlet system.

Building upon regional scale discussions in Chapter 2, this chapter discusses the interactions between barrier beaches and tidal inlets at a local scale from the perspective of inlet stability, and mechanisms and pathways of sediment bypassing across tidal inlets. Inlet stability can be significantly influenced by local geologic conditions such as outcropping of erosion resistant bedrocks or stiff mud, in addition to hydraulic conditions as controlled by tidal prism. Local geology such as shoreline exposure of bedrocks and stiff mud can also influence beach morphodynamics by controlling its planform. However, tidal inlets and their ebb tidal deltas impose the major interruption to the continuity of longshore sand transport. How sand moving along the beach can reach from one barrier island to another constitutes the main issue for beach–inlet interaction, and subsequently the erosion or accretion of barrier beaches. Several conceptual models have been developed and are discussed in this chapter. The rapidly improving numerical modeling provides a promising tool to quantify sediment bypassing and is introduced here.