Let us assume that by 2100 sea level will rise by amounts similar to the upper bounds of the 2007 IPCC estimates, roughly 60 cm. At the same time, let's assume that subsidence rates on the New Orleans region continue at current rates between 2 and 28 mm/year. The result would be between 0.8 and 3.1m of sea level rise relative to the current land surface.
Even if sea level rise does not inundate low-lying coastal regions, it will make them more prone to flooding during storms and prolonged periods of heavy rainfall. In fact, this is the predominant fear in places like Bangladesh and the Mississippi Delta region near New Orleans. Let's consider the area around New Orleans where hurricanes are a constant threat. The extensive damage caused by Hurricane Katrina did not arise from wind or rain, rather the massive storm surge, the giant wall of water that was pushed up onto the land during the hurricane. This storm surge was over 9 meters to the northeast of the city and peaked at about 5 meters within the city limits. This water either overtopped the levees and flood walls that were built to protect low-lying areas of the city, or, more frequently, combined with the wave action to topple levees from the base upwards. In the aftermath of Katrina, the Army Corps of Engineers has rebuilt the levee and floodwall system in New Orleans, and upgraded pump stations, to defend the city from a similar storm surge in the future. The new system offers multiple lines of defense beginning outside of the perimeter of the city. The massive Inner Harbor Navigation Canal Lake Borgne Surge Barrier is the largest flooding structure in the US.
The following videos describe how subsidence is leading to sea level rise in the Mississippi Delta region and how engineering is being used to combat it.
Video: Sea-Level Rise, Subsidence, and Wetland Loss (9:44)
Video: New Orleans Levees (2:07)
The design of all of the New Orleans flood protection is based on the elevation of a flood that occurs every 100 years. In other words, there is a 1% chance that the system will fail each year. You might ask why this risk is being taken and the structures have not been built higher. The answer is that it costs a large amount of money to build of the structures to withstand higher water levels.
Although New Orleans has received much attention after Katrina, and rightly so, many other areas are also at great risk from hurricane winds, waves and storm surge.
Outer Banks, North Carolina
One of the most popular holiday spots on the East Coast, the Outer Banks (OBX) of North Carolina, is also highly susceptible to sea level rise. In fact, recent research suggests that the OBX may be experiencing some of the fastest sea level rise on the planet at least as a result of very rapid subsidence of the land. Sea levels in OBX have climbed as much as 3.7 centimeters (1.5 inches) per decade since 1980, while globally they've risen up to 1.0 cm (0.4 inches). Models suggest that sea level may rise by up to 1.6 meters (5 ft) by 2100! The Outer Banks are part of a chain of barrier islands that stretch from Florida to Massachusetts along the Atlantic seaboard. Barrier Islands are delicate sand bodies that are generally moving towards the adjacent continent as sea level rises; this process occurs largely because storms erode sand from the seaward side of the island and deposit it on the landward side. The OBX is moving at a rate that is quite alarming from a development point of view, a point illustrated best by the famous Cape Hatteras lighthouse. The lighthouse was built in 1870 some 1,500 feet from the ocean. By 1970, the lighthouse was just 120 feet from the ocean, and its fate was not very uncertain. Fortunately, the lighthouse was moved some 2900 feet inland. At the rate the OBX are shifting, all development is threatened. However, development modifies the response of the coastal environment to erosion from storms often increasing erosion rates along the undeveloped parts of the coastline.
The OBX are extremely vulnerable to storms as a result of their exposed position in the open Atlantic Ocean, which has led to a greater number of hurricanes. As we have seen, experts predict there will be fewer more powerful storms in the future. The impact of these storms will be amplified by the continuing sea level rise and a powerful hurricane could have an extremely destructive impact on the fragile barrier islands.