Although barrier islands occur under a variety of coastal settings, coasts with an ample supply of sand-sized sediment, a wide and gentle continental shelf, and a broad coastal plain are more conducive to the formation of barrier islands. When the above three conditions, combined with a slow sea-level rise rate of 1–3 mm/year, occur, barrier islands can be the dominant coastal features, such as along the US Gulf of Mexico and Atlantic Ocean coasts. Although not as common, barrier islands also occur along rocky coasts and river-delta coasts. This chapter discusses different morphologic and sedimentologic characteristics under different geologic and oceanographic settings. The various sub-environments, also referred to as morphologic features, that compose a barrier–inlet system are described, along with a morphodynamic classification of barrier island under different coastal settings. These morphologic features and the morphodynamic classification are used in the following chapters discussing sediment transport processes, sediment exchange among the different features and the associated pathways, and management of the sediment resources among the various morphologic features.

The 2010 US Census indicates that 39% of the US population, or 123.3 million people, live in Coastal Shoreline Counties which comprise less than 10% of the US land area. This results in a population density that is 4.2-times the national average. Similar dense population occurs in coastal areas worldwide. Barrier islands constitute a major coastal environment. Barrier islands are separated by tidal inlets which serve as essential navigation channels connecting harbors within the estuaries and the open ocean. Barrier island beach and tidal inlet are parts of an inter-connected system and should be understood and managed as such. This book discusses beach–inlet interaction in terms of sediment exchange among various morphologic features and processes that drive the sediment exchange. Balancing valuable sediment resources in a beach–inlet system is essential to modern shore protection and coastal management.