BARRIER ISLANDS IN LOW ENERGY COASTAL SETTINGS: CHESAPEAKE AND DELAWARE BAYS

Barrier islands are well known from open ocean shorelines. In this paper the geology, environmental setting and evolution of barrier islands in low energy environments are described, taking Chesapeake and Delaware Bays as examples. These low-energy barrier islands number in the thousands worldwide and are much more abundant than their open ocean equivalents (approximately 2500 in number).

There are approximately 200 low energy barrier islands in Chesapeake Bay and 100 in Delaware Bay. They are typically 100s of metres long, a few metres wide and a few metres high. They consist of a shallow shoreface (sometimes resting on eroded marsh surfaces), overwash aprons and occasionally are topped by dunes.

We identify three main active barrier types on the basis of morphology that are developed along a wave-energy and sediment abundance gradient. In higher energy settings with abundant sediment are linear islands separated by broad inlets. These are backed by marshes and have well-developed shorefaces including multiple nearshore bars. In medium energy, and moderate sediment abundance environments, semi-continuous irregular islands exist in whose morphology a strong geological control is evident. The ultimate expression of geological control is wrap-around islands that enclose small areas of marsh. In the lowest energy and low sediment supply environments, islands comprise discontinuous rims of coastal plain sand thrown up onto the margins of marshes, separated by areas of marsh rather than inlets.

In addition, several inactive islands in Chesapeake Bay were identified that comprise semi-circular ridges of sand entirely surrounded by marsh. These islands are interpreted as the margins of former Carolina Bays.

From an evolutionary perspective we identify several processes including shoreface progradation, island erosion, migration and overwashing, spit elongation, inlet formation and inlet closure.