Understanding the Long-Term Evolution of the Coupled Natural-Human Coastal System The Future of the U.S. Gulf Coast (2018) / Chapter Skim
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2 Background of the Gulf Coast System
2 Background of the Gulf Coast System
Pages 17-60
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From page 17... ...
, including barrier islands and peninsulas, tidal inlets and tidal deltas, rivers and river deltas, cheniers, bays, marshes, cypress swamps, and mangrove forests, all of which respond differently to changing environmental and anthropogenic stressors. About two-thirds of the northern Gulf Coast consists of barrier islands or spits (e.g., Morton et al., 2004)
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Highest rate of relative sea level kilometers of rise. Extensive coastal wetlands and eroding beach.
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are drowned river valleys that flooded as sea level rose following the Last Glacial Maximum or are associated with inter-distributary bays in the Mississippi River Delta. The Gulf barrier islands, along with other coastal landforms such as most modern major river deltas, estuaries, and coastal wetlands, were formed approximately 5,000 to 6,000 years ago as sea level rise slowed during the late Holocene Epoch (Stutz and Pilkey, 2011; Anderson et al., 2014)
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. A growing literature suggests that these characteristics are associated with greater social vulnerability to the effects of sea level rise, hurricanes, and other hazards -- meaning that they are less able to prepare, respond, rebuild, or relocate before or after disasters (Cutter and Emrich, 2006; Cutter et al., 2006, 2008; Picou and Marshall, 2007)
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. Examples of physical-ecological interactions in the coastal zone are numerous and include loss or gain of coastal habitats in response to sea level rise (e.g., Day et al., 2008; Kirwan and Megonigal, 2013)
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T H E F U T U R E O F T H E U.S . G U L F C O A S T BOX 2.1 Interactions and Feedbacks This box draws on a wide range of sources from the coupled human-natural system literature (e.g., Turner et al., 2003; Kennedy, 2006; Liu et al., 2007; Werner and McNamara, 2007; Smith, 2008; Carpenter et al., 2009; Horan et al., 2011; Brashares et al., 2014; Smith, 2014; Withey et al., 2014)
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For example, continued sea level rise and changes in sediment supplies could eventually trigger a breach in a barrier island. While the initial breach area would be relatively small, given limited sediment supply and rising sea level the breach would very likely continue to widen, allowing more water in and out of the system, eventually leading to substantial land loss.
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Even without full-scale disappearance of barrier islands, physical changes in the coastal zone could lead to tipping points in the coupled natural-human system. For example, as sea level rise and wetland loss continue, many oil and gas pipelines continue to be maintained even though their vulnerability to wave action increases.
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. RSL rise is determined by global sea level rise plus change in land elevation 25
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Despite the slowdown in the rate of global sea level rise, which persisted until the early 20th century (Gehrels and Woodworth, 2013) , the Gulf Coast has seen continuous RSL rise due to subsidence associated with glacial isostatic adjustment.
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Future sea level rise predictions suggest a rise in global sea level of 0.3 to 2.5 meters by the year 2100, with an intermediate-high value of 1.5 meters (Sweet et al., 2017)
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of representative concentration pathway-based (RCP) global sea level rise projections (Sweet et al., 2017, and references therein)
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Important drivers include tectonic processes, faulting, sediment loading, glacial isostatic adjustment, compaction, and fluid withdrawal (see Figure 2.5)
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Glacial isostatic adjustment is related to the existence of the up to 4 kilometer thick Laurentide and Cordilleran ice sheets during the Last Glacial Maximum. The weight associated with these ice sheets led to a depression of the Earth's crust at their locations and an associated uplift in a large region surrounding the ice sheets.
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. Because of the Gulf Coast's relatively gently sloping terrain, hurricanes generate significant storm surge when making landfall, with wind setup (i.e., water level rise due to momentum transfer by wind to the water column)
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The exacerbation of hurricane impacts along the Gulf Coast by the combined influence of sea level rise and urban growth cannot be overlooked. RSL rise and substantial wetland TABLE 2.1 Mainland U.S.
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. Given future changes in storm climatology and mean sea level -- but ignoring sea level rise's influence on long-term coastal change -- several studies project a dramatic increase in hazard exposure.
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34 FIGURE 2.6 Changes in 100-year flood risk in the next 50 years in Louisiana with proposed master plan structural risk-reduction projects in place. Areas highlighted in yellow show induced flooding in front of proposed levees and floodwalls (pink lines)
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. Coastal Sediment Transport and Hydrodynamics Sediment sources within the Gulf of Mexico include offshore deposits from river discharge during periods of lower sea level and earlier periods in the Holocene Epoch, longshore drift from modern rivers, subsidence of coastal landforms, and biogenic production (Khalil et al., 2018)
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From page 36... ...
Bayhead deltas, often found within coastal bays, are particularly sensitive to the balance between sea level rise and sediment input. When sediment input is sufficient, these deltas may grow despite rapid rates of RSL rise (e.g., Wellner et al., 2005; Anderson and Rodriguez, 2008)
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From page 37... ...
In this way, storms promote the maintenance of islands and sandy peninsulas by building them upward and moving them landward, thereby maintaining island elevation relative to sea level as sea level rises. Barriers constructed from muddy deltaic deposits require a greater amount of landward migration to liberate the same amount of sand, compared with locations where the substrate contains a higher proportion of sand (Moore et al., 2010)
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From page 38... ...
For example, sea level is currently rising far faster than before, and so erosion rates in the future may be far higher than what has occurred in the past several thousand years. Coastal evolution models, which simulate the evolution of the physical parts of the system, can be categorized by considering the domain across which a model operates and its underlying assumptions.
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These models are often used for short-term events, such as bar migration or barrier island evolution, over timescales of days to weeks. The synthesist models are based on reduced-complexity formulations developed by choosing macroscale variables to drive the model, such as sediment transport rates, which inherently consider all the sediment transport modes and physics, but use simplified relationships instead of explicitly calculating them as a function of the overlying hydrodynamics.
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; (C) Horn Island, a barrier island (photo by Caitlin Wessel)
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. Barrier islands support other important habitats along the Gulf Coast.
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. Seagrass beds, oyster reefs, wetlands, and barrier islands have the potential to reduce wave energy and help protect shorelines from erosion (Shepard et al., 2011)
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. Sea level rise will also affect barrier island ecosystems.
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noted that the effect of sea level rise on barrier island vegetation had not been studied. Over the nearly three decades since, and facilitated greatly by the development and increasing availability of the global positioning system (GPS)
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. THE HUMAN SYSTEM Although the scientific challenges of projecting physical and ecological changes, such as sea level rise or marsh loss, over the next 10 to 200 years are formidable, and sizable uncertainty is expected, the scientific basis to predict most significant human processes with any degree of confidence on these timescales is lacking.
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It may not be entirely absurd to model human processes with the expectation that fundamental ways of living and basic construction, transportation, and infrastructure technologies will continue to be similar to today. Many aspects of coastal storm damage or erosion reduction have remained unchanged over centuries.
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. An important Gulf Coast example is the Greater New Orleans Hurricane and Storm Damage Risk Reduction System, which is intended to prevent or reduce flooding from storm surge and waves from tropical cyclones in New Orleans.
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. A recent study found that among individuals impacted by Hurricane Sandy, those who reported greater social support also reported greater tolerance for flood risks and greater confidence in community adaptation measures, suggesting an important but complex role of personal connections to collective resilience―both keeping people in place and helping them survive there (Wong-Parodi et al., 2017)
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(2018) examines the predisposition to action and the impact of a sea level rise decision aid that emphasizes the risk, the adaptation measures, both, or neither on the adoption of adaptation measures among a representative sample of 1,201 at-risk coastal residents (living within a 100-year floodplain)
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ecological restoration and land-creation projects that are funded with the explicit intent of reducing storm surge impacts and restoring coastal habitat (LACPRA, 2017)
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. Based on the historic trends of land loss and future projections of sea level rise, as well as damage from storms, the island will most likely not be habitable in a few decades.
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grades the current state of national energy infrastructure as "D+" due to a combination of aging and increased vulnerability to storm impacts (ASCE, 2017)
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The four storage facilities are at risk from storm surge caused by sea level rise and hurricanes (USACE, 2013a; Bradbury et al., 2015)
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Other critical infrastructure includes numerous ports; the Mississippi River, which is among the most important shipping pathways in the nation; and the Gulf Intracoastal Waterway, which connects inland waterways across the Gulf. The region has an extensive road and railway network, with some roads located at or near sea level.
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This strategy posits that a combination of natural features (e.g., marshes, barrier islands) , built infrastructure (e.g., levees, floodwalls)
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. Another example of feedbacks related to coastal engineering approaches comes from Dauphin Island, the only barrier island located offshore of coastal Alabama.
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. With continuing chronic erosion, an average elevation of only 7.2 feet, and a width of barely a mile at its widest point, Dauphin Island is highly vulnerable to changing conditions, such as accelerating sea level rise and predictions for increased frequency of intense hurricanes.
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West Coast, where sea level rise, increased storminess, and recent development have left communities more susceptible to flooding and erosion. The work involves sustained engagement with the Tillamook County Coastal Knowledge-to-Action Networka that comprises researchers, students, outreach specialists, and coastal community members in Tillamook County, Oregon.
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Background of the Gulf Coast System tem of models, however, does not yet consider dynamic feedbacks between the human and natural systems, except for the effects of specific restoration or risk-reduction projects considered for the master plan. Coupled models can be perceived as oversimplifications of human agency and the human decision environment.
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