Sea level rise and subsidence

The formation and maintenance of shorelines, wetlands, barrier islands, estuaries and lagoons of the Gulf Coast are intimately linked with sea level. Geologic records indicate that global sea level has risen about 120 m since the Last Glacial Maximum approximately 20,000 years ago. Sea level rose rapidly (averaging almost 0.9 m per century) between 20,000 and 6000 years before present and slowed to about 0.02 m per century or less during the past 3000 years (IPCC, 2001). From the mid-19th century through much of the 20th century, the average rate of global sea level rise increased to between 0.17 m and 0.18 ±0.05 m per century, and satellite data for the last decade suggest that the contemporary rate is as much as 0.3 m per century (IPCC, 2007). Most atmosphere-ocean general circulation model simulations project that sea level rise will accelerate during the 21st century and beyond due to human-induced warming. If the rate of sea level rise increases as projected, the Gulf Coast and other low-lying coastal mainlands, barrier islands and wetlands that are not experiencing significant uplift or high rates of sediment accretion will be seriously impacted. Most of the Mississippi, Louisiana and Texas coastline has been classified by the US Geological Survey as 'highly vulnerable' to erosion due to sea level rise (Thieler and Hammar-Klose, 2000).

Relative sea level change at any coastal location is determined by the combination of eustasy (global sea level rise) and local processes that affect elevation of the land surface, such as tectonism, isostasy (glacial rebound) and subsidence (sinking of the land surface). Subsidence is the predominant direction of elevation change in the Gulf Coast region. Subsidence is highest in southeast Louisiana due to its geologic age and structure. Subsidence generally decreases westward and eastward of the Mississippi delta. The western Mississippi coastline is experiencing higher subsidence rates than to the east in Alabama and Florida, but subsidence has been observed in the marshes of Grand Bay, Mississippi (Schmid, 2001) and Mobile Bay, Alabama (Roach, undated).

Tide gauge records for the region indicate that relative sea level rise during the 20th century was greatest in the Mississippi River Deltaic Plain and in southeast Texas (NOAA, 2001). The linear mean relative sea level trend at the tide gauge at Grand Isle, Louisiana (established in 1947) indicates relative sea level rise of 9.85 mm/yr. The two tide gauges in the vicinity of Galveston, Texas (established in 1908 and 1957) indicate an average rate of relative sea level rise of 6.9 mm/yr. The contribution of global sea level rise at these gauges during the past century could be estimated at the global average rate of 1.7 mm/yr. However, the rate of sea level rise is not uniform spatially due to differences in ocean basin geometry, depth/heat uptake, circulation and other factors that influence ocean volume. Twentieth century tide gauge records and satellite altimetry data since 1993 indicate that the rate of sea level rise in the Gulf of Mexico is higher than many other parts of the world ocean (see Figure 8.4, Plate 9). Hence, assessments of sea level rise impacts that are based on the global average rate may underestimate impacts in the Gulf Coast region.

Deltas have long been recognized as highly sensitive to sea-level rise (Coleman et al, 1998; Woodroffe, 2003). Rates of relative sea level rise in many of the world's deltas are at least double the current global average rate of rise (Saito, 2001; Waltham, 2002) because of human activities and the fact that deltas are generally compacting under their own weight. The heavy sediment load deposited by the Mississippi River during the past several million years has caused high rates of natural subsidence that were offset to some degree by delta building processes during much of the Holocene. Sea level has an important influence on Mississippi River deltaic processes. During periods of falling sea level, sediment deposition shifts seaward and the deltaic land mass is built on the continental shelf; during times of rising sea level, the delta deposition shifts landward and, depending upon the rate of sea level rise, may cease to deposit sediments above mean sea level. Sediment inputs to shallow waters of the deltaic plain have been curtailed by the closing of distributary channels (for example, Bayou La Fourche in 1904) and the construction of levees, upstream dams, deep-water channels and other engineering works during the past century.

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fr mm/year

Figure 8.4 Geographic distribution of short-term linear trends in mean sea level for 1993-2003 based on TOPEX/Poseidon satellite altimetry (see Plate 9 for color version)

Source: IPCC, 2007, updated from Cazenave and Nerem, 2004.

Subsidence as a result of both hydrocarbon and groundwater extraction has been recognized as a problem in the Texas Gulf Coast since the mid-1920s. Subsidence caused by ground-water extraction has led to substantial and expensive problems in areas such as Houston, Baytown and Galveston. Subsidence in the vicinity of the Port Neches oil and gas field resulted in wetland losses that can be related to fault reactivation as a result of hydrocarbon production. State and local organizations (such as the Harris-Galveston Coastal Subsidence District) have implemented regulations that have reduced the rate of subsidence while helping to maintain ground-water supplies.

Sea level rise will generally increase marine transgression on natural estuarine shorelines (Pethick, 2001) and the frequency of barrier island overwash during storms, with effects most severe in habitats that are already stressed and deteriorating. Salt-water intrusion and increased mean water levels will lead to a change in plant and animal communities. In coastal Florida, for example, sea level rise has been identified as a causal factor in the die-off of cabbage palm (Sabal palmetto) (Williams et al, 1999). Salt-water intrusion has destroyed large tracts of coastal bald cypress (Taxodium disticum) forests in Louisiana (Krauss et al, 1998, 2000; Melillo et al, 2000). One effect of rising sea level in the Laguna Madre of

Texas will be greater water depths and increased tidal exchange, which will further reduce salinity in the lagoon (Nicholls et al, 2007).

Sea level rise does not necessarily lead to loss of saltmarshes because some marshes accrete sufficient sediments vertically to maintain their elevation with respect to sea level rise (Cahoon et al, 2006). The threshold at which coastal wetlands and other intertidal habitats are inundated by sea level rise varies widely depending upon local morphodynamic processes. Sediment inputs, even from frequent hurricanes, have not been able to compensate for subsidence and sea level rise effects in the rapidly deteriorating marshes of the Mississippi River delta (Rybczyk and Cahoon, 2002).

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