Task 4: 21st Century Northern Gulf Coast Landscape and Human Community Structure and Vulnerability

Task Leader: John Brock - USGS St. Petersburg Coastal and Marine Science Center, St. Petersburg, Florida

Task 4 Goals

The coastal plain across the northern Gulf of Mexico (NGOM) coast region landscape is dynamic, evolving on a range of time scales due to intertwined climatic, geological, hydrological, biological, and meteorological processes superimposed on this complex system is human development, commercial activities, and resource extraction. The Mississippi River deposited a huge volume of sediment at its mouth over the last several thousand years, as sea level has risen, building land out onto the continental shelf, and supporting the development of wetlands nourished with sediment-laden floodwaters and nutrients. During the 20th century, massive alteration of the Mississippi River Delta occurred due to the construction of river levees, construction of navigation channels, and oil and gas production. Consequently, the balance has shifted from net land-building deltaic processes to land loss due to altered hydrology, subsidence, and erosion. For example, during the decade from 1990 to 2000, southern Louisiana lost 60 square kilometers per year due to natural causes and human activities.

Global sea-level rise, occurring at a rate of about 1-2 mm per year during the last century, is a primary driver of NGOM coast landscape change, and is predicted to increase to 2 – 5 times this rate during the next few decades. In the NGOM coastal region, eustatic sea-level rise combines with shallow and deep subsidence, coastal geomorphology, variable sediment supply, and the frequency of major storms to cause regional submergence of coastal marshes and mangrove forests. Due to local subsidence, the rate of relative sea-level rise (RSLR) in the Mississippi River delta region is now up to 10 times the rate of global mean sea-level rise, creating vast potential for the continued submergence of coastal wetlands.

This extremely high rate of RSLR stems from the superposition of deep, or geologic, and shallow subsidence, that differ in cause, process, and geographic distribution. The basic geologic drivers of deep subsidence are compaction, faulting, and isostasy. Past movement of salt diapers established a framework of faults, but at present salt movement is largely dormant, and seismic evidence for tectonic activity in the NGOM coast is negligible. Natural compaction of deltaic plain sediments may be a significant cause of deep-seated subsidence, as previous work suggests that subsidence rates are higher in regions of thicker Holocene sediments. Moreover, extensive areas of wetland loss in the NGOM coast region coincide with large-volume fluid production from mature petroleum fields, an anthropogenic cause of geologic subsidence.

Hurricanes are a major agent of catastrophic disturbance throughout the Caribbean and along the Gulf Coast, where the hurricane return time is generally shorter than the life span of a canopy tree or a forest ecosystem. In the NGOM coast, hurricanes have for thousands of years played a role in driving plant population dynamics, soil development, and nutrient cycling. Although hurricanes are a recurrent meteorological phenomena along the Gulf Coast with typical return times of 5 to 20 years, the geomorphic and ecological change associated with severe storm disturbance has not been well documented. Most studies of hurricane impacts on coastal wetlands have been done at localized scales and therefore do not allow broad inferences, but Penland and others (1989) concluded that the major permanent effects of Gulf Coast hurricanes are mainly geomorphic. Recent hurricanes that have traversed the Gulf Coast have damaged coastal wetlands by trapping salt water in impoundments, scouring shallow ponds and lakes, depositing sediment and debris on marsh surfaces, and through the uprooting and tearing of marsh plants and substrate.

In contrast to such immediate and deleterious effects, storm-induced wetland sediment deposition, that may forestall wetland loss, was observed following the landfall of Hurricane Andrew. More generally, winter storms and hurricanes have been implicated as major sources of sediment influx on NGOM coast coastal marshes, and drive marsh accretion, especially where sediment input is low and subsidence is high. Moreover, hurricanes are a major factor in the structuring of coastal ecosystems in the NGOM because their combined landfalls create a “disturbance mosaic” that results in a heterogeneous landscape. Plant communities that develop within the resulting patches of the mosaic are dynamic and respond more readily to changing environmental conditions. The subsequent heterogeneity of the coastal landscape partially controls the magnitude and distribution of impacts due to later severe storms.

Task 4 Objectives

The goal of Task 4 is to provide a regional synthesis of present day NGOM coastal ecosystem and human community structure and vulnerability, and forecast the evolution of this landscape over the next century given ongoing natural processes, anthropogenic effects, and alternate redevelopment policies.

The specific objectives of Task 4 are to:

• Create up-to-date post Hurricanes Katrina and Wilma maps that describe the terrestrial landscape of the entire Northern Gulf Coast region at the highest practical spatial resolution
• Conduct EAARL lidar acquisition surveys over desired locations within the NGOM project study area, and process the resulting data sets to create GIS-ready, fully documented DEMs for bald-Earth, canopy-top, and submerged topography
• Model coastal wetland vulnerability and predict the spatial structure of the NGOM coast in the Year 2100
• Investigate the historic increase in coastal vulnerability using a storm surge model, and evaluate present and future vulnerability as a function of the evolution of the continental shelf, barrier island chains, coastlines and urbanization,
• Investigate the use of scientific information in the Gulf Coast recovery from the 2005 Hurricane season.