|Figure 1. Stormtracks of Category 4 and category 5 hurricanes that have entered the Gulf of Mexico since 1900.
The coastal region of the northern Gulf of Mexico (NGOM) owes its current morphology to an array of tectonic forces and depositional processes that have collectively shaped its shelf, estuarine, wetland, and upland ecosystems. The region experiences a subtropical climate that is frequently punctuated by tropical cyclones.
Some of the most complex, dynamic and productive ecosystems in the nation are located in the north - central Gulf Basin, especially in southern Louisiana, which began to form in the early Holocene, about seven thousand years ago, when sea-level rise slowed following the most recent major glaciation. Since their formation, Mississippi River Delta (MRD) wetlands have experienced a net increase as the lower Mississippi River switched between distributaries and various delta lobes became active and prograded.
Prior to European settlement, numerous distributaries were active across the Mississippi River Delta, either constantly or during spring flooding, and the associated wetland landscape was sustained by hydrologic pulsing that occurred across a range of time and space scales. The resulting variability in sediment fluxes was driven by periodic events, primarily the shifting geographic centers of deltaic deposition, formation of crevasses, large floods, hurricanes, annual river floods, frontal passages, and tides.
Although several major drainages, including the Atchafalaya, Mississippi, Pearl, Pascagoula, Escatawpa, and Mobile Rivers, empty into the northern Gulf at present, the hydrology of the region is dominated by the Mississippi River watershed, an area of about 3 million square kilometers, or roughly 40 percent of the conterminous United States. The Mississippi River is one of the largest rivers in the world, supplying about 90 percent of all the freshwater flow received by the Gulf of Mexico from its drainage basin, the largest in North America. Presently, only two Mississippi River distributaries, the lower Mississippi River and the Atchafalaya River, are functional, with roughly one third of the total flow under engineering control discharged via the Atchafalaya River.
The northern Gulf of Mexico coast holds both renewable and nonrenewable resources that are of great importance to the Nation, including some of the world’s most diverse and productive ecosystems. For example, coast ecosystems support most of the Nation’s wintering waterfowl, and account for about one third of the fisheries production in the lower 48 states. Underscoring the ecological importance of the Mississippi River Delta to the health of the north - central Gulf, approximately 60 percent of the estuaries and marshes found along the Gulf of Mexico coast are located in southern Louisiana. Within a broad national context, the Louisiana coast, mainly the MRD and the Chenier Plain just to the west, contains one quarter of U.S. wetlands, an area that is equal to roughly one half of all U.S. coastal wetlands. In addition to playing a central role in the NGOM coast ecosystem by providing habitat for fish and wildlife, the Louisiana wetlands regulate chemical transformations, maintain water quality, store and release water, and buffer storm energy.
The NGOM coast, especially the coastal wetlands of the Mississippi River Delta, are economically significant to the United States, as this region supports resource - based activities that generate billions of dollars annually. Commerce in the region includes recreational and commercial fisheries, the harvesting of fur mammals and alligators, as well as ecotourism and recreational hunting. The lower Mississippi River in southern Louisiana is the site of the largest port activity by tonnage in the world. Further, refineries along the NGOM coast produce about $30 billion worth of petroleum products per year, and approximately 20 percent of U.S. crude oil and 33 percent of U.S. natural gas flow through the region.
Hurricanes are among nature’s most destructive natural forces. Driven by heat from the underlying warm tropical ocean, hurricanes produce both devastating winds and heavy rainfall that can extend outward from the eye of the storm for hundreds of kilometers. An average of 85 tropical cyclones annually form in the world’s oceans, but only a minor proportion that form in the North Atlantic result in threats to the U.S. mainland, and only a subset of those storms eventually make landfall. In the Western Hemisphere, Atlantic tropical cyclones result in more fatalities than any other natural disasters, and in this regard the NGOM coast is no exception.
Hurricanes are unquestionably the most visible and violent natural events that occur along the Gulf Coast. Economic losses due to hurricanes are roughly proportional to the fourth power of the wind speed, and storm surges driven by the low atmospheric pressure and high winds of hurricanes can destroy low elevation coastal communities and industries and claim many lives. Moreover, heavy rainfall from hurricanes as they move inland can cause massive coastal river and stream flooding that exacerbates the damage and death toll due to high winds and storm surge.
Rapid growth has only happened in the last 100 to 150 years. Growth of coastal communities throughout the Caribbean region and along the U.S. Atlantic seaboard and Gulf Coast has resulted in an ever - increasing human population and economic infrastructure that is vulnerable to catastrophic hurricane impacts.
In September 1900, a major tropical cyclone struck the Gulf Coast city of Galveston, Texas, resulting in 8,000 deaths, the second largest loss of life due to an Atlantic hurricane, and the largest ever in the U.S. Damaged heavily by the storm, Galveston’s preeminence as a Gulf Coast financial center ended, and after more than a century, the Galveston Hurricane death toll still totals about one third of all tabulated Atlantic tropical cyclone - related deaths in the nation. This devastating storm was followed by the Gulf Coast landfall of 49 category 4 or higher hurricanes, most recently the highly destructive Hurricanes Katrina and Rita in summer 2005 (Figure 1). This history illustrates the extreme vulnerability of the Gulf of Mexico coast to severe storms.