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Potential for Shoreline Changes Due to Sea-Level Rise Along the U.S. Mid-Atlantic Region

H-Coastal Landforms in the Mid-Atlantic bight

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Figure 1. Map of the mid-Atlantic coast of the United States showing the seventeen coastal compartments and their coastal geomorphic type.
Figure 1.
Map of the mid-Atlantic coast of the United States showing the seventeen coastal compartments and their coastal geomorphic type.

A consensus was reached by the committee that coastal landforms along the shores of the mid-Atlantic Bight can best be classified by merging the schemes developed by Fisher (1982), Hayes (1979), and Davis and Hayes (1984). Four distinct geomorphic settings were identified (fig. 1).

Spits

The accumulation of sand from longshore transport has built large spits that extend from adjacent headlands into the mouths of large coastal embayments. Outstanding examples of these occur at the entrances of Raritan (Sandy Hook, NJ) and Delaware Bays (Cape Henlopen, DE). The evolution and existence of these spits result from the interaction between alongshore transport driven by incoming waves and the tidal flow through the large embayments. Morphologically these areas can extend and evolve rapidly. Since 1842, Cape Henlopen (fig. 1, compartment 9) has extended over 1.5 km to the north into the mouth of Delaware Bay as the northern Delaware shoreline has retreated and sediment has been transported north by longshore currents (Kraft 1971; Ramsey and others, 2001).

Headlands

Figure 2. Paleogeography of the Delaware Bay region inferred from geological investigations of shallow marine sediments.
Figure 2.
Paleogeography of the Delaware Bay region inferred from geological investigations of shallow marine sediments. Compiled from Fletcher and others (1990 and 1992).

In the mid-Atlantic Bight, coastal headlands typically front drainage divides that separate creeks and rivers from one another in the older landscape (fig. 2). These regions provide a source of sediment that is eroded and incorporated into the longshore transport system that maintains adjacent beaches and barriers. Coastal headlands are present on Long Island, NY (See fig. 1), from Southhampton to Montauk (compartment 1), in northern New Jersey from Monmouth to Point Pleasant (compartment 5; Oertel and Kraft, 1994), in southern New Jersey at Cape May (compartment 8), on Delaware north and south of Indian River and Rehoboth Bays (compartments 10 and 12; Kraft, 1971; Oertel and Kraft, 1994; Ramsey and others, 2001), and on the Virginia coast, from Cape Henry to Sandbridge (compartment 16).

Wave-Dominated Barrier Islands

Wave-dominated barrier islands occur as relatively long and thin stretches of sand fronting shallow estuaries, lagoons, or embayments and are bisected by widely-spaced tidal inlets. These barriers are present in regions where wave energy is large relative to tidal energy (Hayes, 1979; Davis and Hayes, 1984). Limited tidal ranges result in flow through tidal inlets that is marginally sufficient to flush the sediments that accumulate from longshore sediment transport. In some cases this causes the inlet to migrate over time in response to a changing balance between tidal flow through the inlet and wave driven alongshore transport. Inlets on wave-dominated coasts often exhibit large flood-tidal deltas and small ebb-tidal deltas as tidal currents are often stronger during the flooding stage of the tide. In addition, inlets on wave-dominated barriers are often ephemeral. They open intermittently in response to storm-generated overwash and migrate laterally in the direction of net littoral drift. In many cases these inlets are prone to filling with sands from alongshore transport (e.g., Riggs and others, 1995; McBride, 1999).

Overwash produced by storms is common on wave dominated barriers (e.g., Morton and Sallenger, 2003; Riggs and Ames, 2007). Overwash cuts through low-lying dunes into the island interior. Sediment deposition from overwash adds to the island's elevation. Washover fans that extend into the back-barrier waterways form substrates for back-barrier marshes and submerged aquatic vegetation. Overwash is critical for the evolution of these barriers over time. The process of overwash is an important mechanism by which some types of barriers migrate landward and upward over time. This process of landward migration has been referred to as "roll-over" (Dillon, 1970; Godfrey and Godfrey, 1976; Fisher, 1982; Riggs and Ames, 2007). Over decades to centuries, the intermittent processes of overwash and inlet formation enable the barrier to migrate over and erode into back-barrier environments such as marshes as relative sea-level rise occurs over time. As this occurs, back-barrier environments such as marshes are eroded and buried by barrier beach and dune sands. Wave-dominated barrier islands and spits are found along Long Island, NY (fig. 1, compartments 2 and 3), the New Jersey coast north of Little Egg inlet (fig. 1, compartment 6), the Delaware and Maryland coasts (fig. 1, compartments 10 and 13) , and much of the North Carolina coast that are considered in this assessment (fig. 1, compartment 17).

Mixed-Energy Barrier Islands

The other barrier island type present along the U.S. Atlantic coast, mixed-energy barrier islands, are shorter and wider than their wave-dominated counterparts (Hayes, 1979). They are punctuated by well-developed tidal inlets. The large sediment transport capacity of the tidal currents within the inlets of these systems maintains large ebb-tidal deltas seaward of the inlet mouth. The shoals that comprise ebb-tidal deltas cause incoming waves to refract around the large sand body that forms the delta so that local reversals of alongshore currents and sediment transport occur downdrift of the inlet. As a result, portions of the barrier downdrift of inlets become localized sediment sinks that are manifest as recurved sand ridges, giving the barrier islands a 'drumstick'-like shape (Hayes, 1979; Davis, 1994). Mixed-energy barriers are present along the Virginia coast (fig. 1, compartment 14), and the New Jersey coast between Little Egg Inlet and Cape May (Oertel and Kraft, 1994). Some authors have referred to the mixed-energy barriers as tide-dominated barriers along the Delmarva shoreline (e.g., Oertel and Kraft, 1994).

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