Three potential responses that could occur along the mid-Atlantic coast in response to sea-level rise over the next century were identified.
- Bluff and upland erosion. Shorelines composed of older geologic units that form headland regions of the coast will retreat landward with rising sea level. As sea level rises over time, the uplands are eroded, and sandy materials are incorporated into the beach and dune systems along the shore and adjacent compartments. It is expected that bluff and upland erosion will persist for all four sea-level rise scenarios. A possible management reaction to bluff erosion is armoring of the shore. This may reduce bluff erosion in the short term, but will probably increase erosion of the beach in front of the armored bluff due to wave reflection as well as increased erosion of adjacent coastal segments by modifying the littoral sediment budget.
- Overwash, inlet processes, shoreline retreat, and barrier island narrowing. Five main processes were identified as agents of change as sea-level rise occurs. First, storm overwash will become more likely. In addition, recent studies suggest that hurricanes have become more intense over the last century (Emanuel, 2005; Webster and others, 2005). Some have argued that there is insufficient data to support this finding (Landsea and others, 2006), but recent work supports this trend for the North Atlantic (Kossin and others, 2007) and the contention that the increased storm activity is linked to 20th century climate and ocean warming (Holland and Webster, 2007).
Tidal inlet formation and migration will also be important components of future shoreline changes. Barrier islands are often subject to inlet formation by storms. If the storm surge produces channels that extend below sea level, an inlet may persist after the storm abates. These inlets can persist for some time until the inlet channels are filled with sediments accumulated from longshore transport, or they may remain open for months to decades. Geological investigations along the shores of the mid-Atlantic Bight have encountered numerous geomorphic features and deposits indicating former inlet positions (Fisher, 1962; Everts and others, 1983; Leatherman, 1985; McBride and Moslow, 1991; Moslow and Heron, 1994; Riggs and others, 1995; McBride, 1999). Historically, most inlets have opened by the storm surge associated with major hurricanes. In the 20th century four of the most important inlets in the mid-Atlantic Bight were formed by storm surges and breaches from the 1933 hurricane (Barden's Inlet, NC; Ocean City Inlet, MD; Indian River Inlet, DE; and Moriches Inlet, NY). Most recently, tidal inlets have formed in the North Carolina Outer Banks in response to Hurricane Isabel in 2003 and on Nauset beach, Cape Cod, MA in response to a spring 2007 storm.
The combined effect of rising sea level and stronger storms potentially acting at higher elevations on the barrier could be expected to accelerate shoreline retreat in many locations. Assessments of shoreline change on barrier islands indicate that barrier island narrowing has been observed on some islands over the last century (Leatherman, 1979; Jarrett, 1983; Everts and others, 1983; McBride and Byrnes, 1997; Penland and others, 2005). Actual barrier island migration is less widespread, but has been noted at Core Banks, NC (Riggs and Ames, 2007), the Virginia barriers (Byrnes and Gingerich, 1987; Byrnes and others, 1989), and the northern end of Assateague Island, MD (Leatherman, 1984).
- Threshold Behavior. Barrier islands are dynamic environments that are sensitive to a variety of driving forces. Some evidence suggests that changes in some or all of these processes can lead to conditions where a barrier system becomes less stable and crosses a geomorphic threshold. In this situation, the potential for rapid barrier-island migration or segmentation/disintegration is high. It is difficult to precisely define an unstable barrier but indications of instability can be:
- rapid landward recession of the ocean shoreline
- decrease in barrier width and height
- increased overwash during storms
- increased barrier breaching and inlet formation
- chronic loss of beach and dune sand volume.
Given the unstable state of some barrier islands under current rates of sea-level rise and climate trends, it is very likely that conditions will worsen under accelerated sea-level rise rates. The unfavorable conditions for barrier maintenance could result in barrier segmentation/disintegration as witnessed in coastal Louisiana (McBride and others, 1995; McBride and Byrnes, 1997; Penland and others, 2005; Day and others, 2007; Sallenger and others, 2007). This segmentation/disintegration may result from a combination of 1) limited sediment supply by longshore or cross-shore transport, 2) accelerated rates of sea-level rise, and 3) permanent removal of sand from the barrier system by storms. Changes in sea level coupled with changes in the hydrodynamic climate and sediment supply in the broader coastal environment contribute to the development of unstable behavior. The threshold behavior of unstable barriers could result in: a) landward migration/roll-over, barrier segmentation, or c) disintegration. If the barrier were to disintegrate, portions of the ocean shoreline could migrate or back-step toward and/or merge with the mainland.
During storms, large portions of low-elevation, narrow barriers can be inundated under high waves and storm surge. The parts of the mid-Atlantic coast most vulnerable to threshold behavior can be estimated based on their physical dimensions. Narrow, low-elevation barrier islands are most susceptible to storm overwash, which can lead to landward migration, and the formation of new tidal inlets. The northern portion of Assateague Island and segments of the North Carolina Outer Banks are examples of barrier islands that are extremely vulnerable to even modest storms because of their narrow width and low elevation (e.g., Leatherman, 1979; Riggs and Ames, 2003).
The future evolution of narrow, low-elevation barriers will likely depend in part on the ability of salt marshes in back-barrier lagoons and estuaries to keep pace with sea-level rise (FitzGerald and others, 2003 and 2006; Reed and others, 2007). It has been suggested that a reduction of salt marsh in back-barrier regions could change the hydraulics of back-barrier systems, altering local sediment budgets and leading to a reduction in sandy sediment available to sustain barrier systems (FitzGerald and others, 2003 and 2006). In these cases, even barrier systems that are relatively wide and exhibit well-developed dunes may evolve toward narrow, low-elevation barriers as local sand supplies are reduced.