The dynamic nature of the coastal environment and predominance of coastal development and infrastructure located along the Gulf Coast at low elevations—in association with estuaries, river channels, floodplains, wetlands, barrier islands, and inlets, all of which are responding to rapidly changing conditions—puts the natural (physical and ecological) and human components of the complex coastal system at odds in ways that significantly alter the future evolution of both the components and the system as a whole. For example, when the natural system begins to change in ways that are undesirable to people, decisions are often made to modify it to prevent change and protect valuable infrastructure. This further alters how the natural system evolves. In this way, the natural and human components are tightly coupled, influencing each other through time. Although the two systems can be considered in isolation, a true understanding of how each will evolve in the face of changing environmental conditions necessitates an understanding of the ways in which each system feeds back into the other. Understanding these feedbacks becomes especially challenging when environmental conditions change at unprecedented rates, as is projected to occur for the Gulf Coast (see Chapter 2 for additional discussion). When this occurs, there is the potential for both the natural and the human systems to evolve in ways that cannot be predicted by past empirical data, leading to the need for novel approaches to project and understand future changes in the Gulf Coast’s coupled coastal system.
Understanding long-term coastal zone dynamics under changing conditions within the coupled natural-human coastal system will require addressing research gaps related to (1) the physical and ecological components of the natural system (as well as interactions and feedbacks among these components), (2) the human system, and (3) the dynamics of interactions and feedbacks between the natural and the human systems. In this chapter, high-priority research gaps are presented that, if addressed, would transform the ability to understand the Gulf Coast coupled system and the capability to project its future evolution.
Given the interconnectedness of the coupled coastal system, all of the research gaps that were identified require some level of consideration of the entire system. As with Chapter 2, this chapter begins with the holistic view. Subsequently, research gaps that involve one-way interactions between various aspects of the system are grouped by considering the disciplinary lens through which they can best be addressed. Research gaps that involve the consideration of feedbacks among all system components are discussed separately in the last section. Each research gap presented is accompanied by a selected list of associated research questions that, if answered, would address the gap.
It is important to note that the research gaps are not provided in any type of ranking, nor are the associated research questions listed with them. The research gaps, and to some extent the number of associated research questions, do tend to reflect the fact that there is a much greater amount of scientific research that has already been conducted on the physical and ecological components; the research on the human system is less well-developed. This leads to varying levels of granularity in the questions related to different research gaps.
Physical drivers (e.g., sea level rise, episodic storms) can cause not only physical modifications such as coastal erosion and landform migration, but also ecological alterations such as wetland loss and displacement of biological communities. Such changes can trigger human responses that, in turn, lead to further adjustments in the physical and ecological systems. Human activities can also be drivers for change in the natural system. For instance, the impacts of coastal development on coastal ecosystem function or physical processes can generate feedbacks. Currently, understanding of these feedbacks for the coupled natural-human system in the Gulf Coast is limited.
Projecting the range of potential future behaviors of the coupled natural-human coastal system is challenging, particularly for longer timescales such as decades and centuries. Systems models can be applied to consider a range of scenarios representing a future without action or with a range of proposed coastal restoration and risk-reduction projects implemented, yielding future forecasts or projections of system behavior. Developing such projections depends not only on sufficient understanding of the physical, ecological, and human processes involved, but also on an understanding of the interactions and feedbacks among these components and accurate projections of external factors that could influence the system. Models that couple the natural and the human components of the system to explore its holistic evolution are under development, though such efforts are still in their infancy, and the encoded relationships are at times simplistic. Furthermore, such complex and nonlinear models can also give rise to emergent (even chaotic) behavior and may need to be constrained with observations to lead to usable results. Although the performance of simulation models over the historical long term can be evaluated using hindcast analysis,
verification of future projections is exceedingly difficult (and rarely undertaken, though potentially possible). Evaluation requires long-term iteration between making future projections and subsequently collecting targeted long-term datasets for comparison over long timescales.
Natural coastal and nearshore systems have been the focus of several white papers that lay out future research agendas (e.g., Elko et al., 2014). This section refines the information from those white papers, focusing on research gaps that are most pressing to further understanding of the Gulf Coast. The importance of studying the natural and the human systems in tandem, and their feedbacks and interactions, is also emphasized.
Physical processes that drive changes in the natural coastal system occur over varying timescales and range from episodic (e.g., hurricanes, local or distant rainfall events) to longer-term processes (e.g., rising sea level, subsidence). These processes also act across a range of spatial scales and cause short-term disturbances such as flooding, shoreline erosion, and changes in water quality, as well as longer-term changes in the landscape such as river channel migration, wetland loss, and the migration of barrier islands and tidal inlets.
SEA LEVEL RISE
Future sea level rise along the Gulf Coast cannot be viewed in isolation from global sea level rise, even though there is an important role for regional considerations. Understanding the oceanic component of sea level rise along the Gulf Coast is foundational to projecting the future evolution of the coupled natural-human coastal system. Beyond global-scale influences, sea level rise will depend on regional changes in temperature and salinity, dynamic changes associated with ocean circulation, and subsidence (which is particularly important in the northern and western Gulf Coast and discussed in the next section). While there are numerous tide gauges along the entire Gulf Coast, the majority have produced time series that are incomplete or too short (NOAA, 2018), probably reflecting the fact that many of these instruments were never established to monitor relative sea level change. Consequently, there is only a sparse network of tide gauges that have operated long enough (i.e., more than 30 to 50 years; Pugh, 1987; Douglas, 1991; Shennan and Woodworth, 1992) to track rates of relative sea level rise in a meaningful way.
Perhaps the greatest challenge to monitoring sea level rise in coastal zones is that current space-based sensors are unable to produce meaningful signals of sea level change in the
near-coastal ocean from satellite altimetry (NASA, 2016). The Surface Water and Ocean Topography1 (SWOT) satellite mission, scheduled to be launched in April 2021, may offer a new means to monitor climate-driven sea level rise near the coastline.
Research Gap 1: Current datasets, monitoring systems, and approaches are insufficient to track and understand how the oceanic component of sea level (i.e., excluding subsidence) is changing along the Gulf Coast and to predict how it will change in the future.
- What are the spatial and temporal variations and impacts of steric sea level rise (i.e., due to changes in ocean temperature and salinity, including the effect of changes in freshwater discharge from large rivers)?
- What are the spatial and temporal variations and impacts of dynamic sea level rise due to factors such as shifts in the position of major ocean currents (such as the Loop Current) and shifts in winds that force water on and offshore?
- What are the spatial and temporal variations and impacts of changes to the Earth’s gravitational field, primarily due to melting ice sheets?
Subsidence rates are higher than the rate of climate-driven sea level rise along considerable portions of the Gulf Coast, notably its western half (e.g., Nienhuis et al., 2017). Although this will likely change in the future because global sea level rise rates are projected to accelerate, subsidence will remain a compounding factor for many regions. There are compelling scientific and practical reasons to reduce the uncertainty in the quantification of subsidence rates and their spatial patterns. While considerable progress has been made over the past decade in understanding subsidence mechanisms and rates, substantial gaps in such understanding still remain.
The rod surface-elevation table-marker horizon (RSET-MH) method (Webb et al., 2013) has led to major advances in understanding shallow subsidence rates in coastal Louisiana (e.g., Jankowski et al., 2017; Osland et al., 2017). This method could be expanded to encompass wetlands along the entire Gulf Coast, similar to the broad and comprehensive Coastwide Reference Monitoring System network in Louisiana.2 Although there is a considerable network of global positioning system (GPS) stations along the Gulf Coast (Karegar et al., 2015; Yu and Wang, 2016), the network has inadequate spatial resolution to resolve localized phenomena such as faulting and fluid extraction. It is critical that newly
installed GPS stations have well-documented anchor depths so that the interpretation of their records is straightforward. To date, this has not always been the case.
One of the obstacles toward progress in subsidence research has been the fact that this problem has traditionally been investigated in a monodisciplinary fashion. Recently, concerted efforts have been undertaken to integrate different measurement techniques in a systematic way through experiments known as “subsidence superstations” (Allison et al., 2016). Superstations can integrate measurements obtained with different techniques, such as GPS, optical fiber strainmeters, and RSET-MH, with stratigraphic and geotechnical analyses of continuous sediment cores. These new integrated data are expected to lead to a host of new insights.
There are also opportunities to introduce other new measurement techniques, including corner reflectors installed in wetland environments to be used by satellites for InSAR (Interferometric Satellite Aperture Radar) measurements, a space geodetic technique that can track changes in surface elevation. This method has been used successfully elsewhere (Strozzi et al., 2013), with emerging utilities around the Gulf Coast (e.g., Jones and Blom, 2014; Jones et al., 2016). However, while traditional InSAR has been applied successfully in urbanized settings (Dixon et al., 2006; Jones et al., 2016), extending it to wetlands has proven challenging due to the scarcity of hard surfaces suitable for signal reflection (though it has been done successfully; e.g., Jones and Blom, 2014).
Significant gaps in understanding remain with regard to the role of faulting and fluid extraction as contributors to subsidence along the Gulf Coast. While progress can be expected from recent studies of fault patterns based on industry-grade 3D seismic data, little is known about the contribution of hydrocarbon extraction to subsidence in areas with significant production (Morton and Bernier, 2010; Kolker et al., 2011; Chang et al., 2014).
There is a need for dedicated studies that link the production history of oil and gas fields to subsidence rates and the impacts of shallow fluid withdrawal, both to understand drivers of historic change and to inform future projections. In addition, because shallow subsidence due to compaction of Holocene sediments is a dominant process in many portions of the Gulf Coast (notably in coastal Louisiana; Törnqvist et al., 2008), there is a particularly great need for geotechnical models that include loading, dewatering, and other compaction processes in the prediction of subsidence rates. To date, there have been few studies of this kind, so there is significant scope for continued work, especially for studies that use newly collected data to calibrate and validate predictive geotechnical models.
Integrated coastal subsidence modeling is currently still in its infancy (Allison et al., 2016), and existing models typically focus on just one of the many processes previously discussed. Some modeling efforts, such as glacial isostatic adjustment modeling, have seen considerable progress (Love et al., 2016). An example of an underexplored yet critical area is geotechnical modeling of shallow, highly unconsolidated coastal deposits. Ultimately,
subsidence models developed by one specific community will need to become fully accessible to other communities (e.g., geophysics versus geotechnical engineering).
Research Gap 2: The causes, rates, and patterns of subsidence along the Gulf Coast are not sufficiently well understood to allow for accurate prediction at the local to regional scale.
- How can next-generation subsidence modeling approaches that integrate models from different fields improve the capability to accurately predict subsidence?
- What are the contributions of faulting and fluid extraction to subsidence, as revealed through studies of oil and gas field production histories and denser networks of instruments (e.g., GPS) that monitor deeper crustal processes?
- How can geotechnical models contribute to a better understanding and prediction of shallow subsidence processes?
EPISODIC COASTAL AND RIVERINE FLOODING
Flooding events along the Gulf Coast are related to the effects of major landfalling hurricanes and other storms, but are also affected by precipitation and related changes in river discharge. Storms can cause major devastation and their flood damage potential will increase in the future due to concomitant sea level rise. Studies have suggested that sea level rise, more than changes in storm climatology, will be a dominant driver of future hurricane-induced flooding (e.g., Woodruff et al., 2013). However, successfully predicting how sea level rise will affect storm-induced flooding along the Gulf Coast will require improved understanding of the processes involved, particularly in areas farther inland where precipitation and related changes in river discharge are also important.
Climate change stressors (e.g., changes in precipitation intensity and patterns, sea level rise, potential backwater effects) are likely to change the hydrology of watersheds draining into the Gulf Coast. Humans also alter the hydrology of the system by diverting freshwater for uses such as drinking water or flood control, modifying land use patterns (e.g., development, agriculture, industry, deforestation, channelization), constructing and operating dams, and developing river diversions that direct water away from large rivers and into coastal embayments for coastal land creation. Changes in watershed hydrology alter sediment availability, salinity, and nutrient flow (including those from upstream runoff) into the coastal system, which contribute to both physical and ecological alterations (e.g., shoreline erosion, subsidence, wetlands accretion, changes in river channel morphodynamics, barrier island formation, oyster reef formation, and estuarine and marine eutrophication). Bio-physical changes, in turn, influence important ecosystem services such as storm damage reduction,
flood control, waterfowl, and fisheries. Many major changes to the coastal system are directly or indirectly linked to inland water resources (NRC, 1994).
The effects of human activities on the coast, including urbanization, building of infrastructure, and loss of natural systems such as wetlands, may exacerbate the impacts of storm-driven flooding. However, these effects are not well understood. An emerging area of concern is how the development of levees, floodwalls, storm gates, and other infrastructure designed to reduce flooding impacts in designated areas might impact flooding in other neighboring areas. For example, building storm surge levees may have the effect of redirecting surge and waves from the design area toward neighboring areas, which may then experience adverse effects. Proposals to build a storm surge barrier across the mouth of Lake Pontchartrain show that the barrier could redirect floodwaters toward communities outside the lake, including St. Bernard Parish in Louisiana and Hancock County in Mississippi (Fischbach et al., 2017). In addition, in several locations in Louisiana, efforts to control storm surge through the use of storm surge barriers or gates could exacerbate vulnerability to river flooding—closing the gates to keep coastal flooding out could lead to flooding from the river instead, as the freshwater piles up and has nowhere to go.
Research Gap 3: The combined effects of freshwater input from Gulf Coast watersheds, storm surge, sea level rise, and development on coastal flood hazards are not well understood, thereby limiting the capacity to include and model these effects in predictions of Gulf Coast dynamics.
- How is storm surge affected as the coastal landscape (e.g., landforms, ecosystems, land use) changes in response to sea level rise and coastal development?
- What controls the relative importance of storm climatology and sea level rise in driving the storm flood hazard, including consideration of flooding due to the combined effects of precipitation (and associated river discharge) and storm surge?
- What processes (e.g., astronomical tides, cold fronts, meteotsunamis, coastal evolution, landcover changes associated with ecosystem change) most influence currently observed and future increases in nuisance flooding as sea level rises?
- How do human modifications of the coastline, including urbanization, built infrastructure, gray and green coastal engineering approaches, and land use, affect storm surge and nuisance flood hazards?
RIVERINE SEDIMENT TRANSPORT
Many low-elevation coastal zones worldwide experience reduced sediment input from their drainage basins, commonly due to damming upstream (Syvitski et al., 2005). This is the case in several Gulf Coast rivers (e.g., Rio Grande River) that have seen dramatic
reductions in their sediment delivery to the coast (Milliken et al., 2017). Reduced sediment fluxes have been proposed as a major cause of coastal degradation along the Gulf Coast (e.g., Blum and Roberts, 2009) and may be compounded by the embankment of major rivers for flood-reduction purposes. Levees prevent the natural dispersion of sediment across the coastal landscape, resulting in elevation deficits and, in many cases, eventual wetland loss. This problem is particularly acute in the Mississippi River Delta, where planning is currently well under way for mitigation measures via managed sediment diversions at strategically selected locations (LACPRA, 2017). Decreased sediment delivery by rivers also affects smaller bayhead deltas that occupy estuaries (e.g., in Texas) and barrier islands (Anderson, 2007). However, this is less of an issue for Florida’s Gulf Coast, which has historically had lower sediment inputs due to relatively small, shallow gradient watersheds (Kumpf et al., 1999).
One poorly resolved issue is whether the present-day sediment flux of the heavily managed Mississippi River, by far the largest sediment source in the Gulf of Mexico, is significantly smaller than in its natural state without human modifications. Many studies suggest this is the case, based on instrumental records that show a distinct reduction of sediment flux during the 1950s when a series of major dams were constructed on the Missouri River, the largest sediment source within the Mississippi River drainage basin (Meade and Moody, 2010). It is possible, however, that historic sediment fluxes were significantly higher than prehistoric fluxes, due to clearing of the drainage basin for agriculture and the associated increase in erosion rates (Keown et al., 1986; Tweel and Turner, 2012b). Resolving this uncertainty would contribute to a better understanding of sediment management within this large region, because it would help clarify the degree to which current landform change is a response to historic changes in sediment supply, current changes in hydrodynamics, or relative sea level rise.
Another poorly resolved issue is the amount of sediment that is available for ecosystem creation and land-building projects across the Gulf Coast, whether “new” sediment recently transported to the Gulf of Mexico via rivers or “existing” sediment from river beds, the seafloor, submarine shoals and tidal deltas, and other coastal sedimentary features. While there has been some progress to date to quantify these resources, much remains unknown about the total volume of sediment that exists for ecosystem creation projects, its quality (e.g., particle size, contaminant potential), and the location of sedimentary resources in relation to coastal restoration areas where sediment is needed.
Along the eastern Gulf Coast, questions also remain about the availability of sediment necessary to sustain coastal systems in the face of sea level rise. These stem from a need to better understand alongshore sediment transport pathways and the impacts of structures (e.g., groins, jetties) on sediment transport pathways, as well as the capacity for biogenic sediment production to supply meaningful amounts of sediment.
Research Gap 4: The relative contributions of naturally occurring and artificially managed riverine sediment delivery (e.g., availability and fluxes), diversion and management activities, and how they impact the evolution of coastal landforms (e.g., river deltas, barrier islands) and ecosystems (e.g., wetlands) is poorly understood.
- How does the present-day sediment flux of the Mississippi River and other Gulf Coast rivers compare with historical and prehistorical fluxes?
- What sources of sediment are available for restoration efforts and in what quantities?
- How will the availability of sediment to wetlands and barrier islands change in the future, and what will be the likely effect of these changes on coastal evolution?
COASTAL SEDIMENT TRANSPORT AND HYDRODYNAMICS
The fate, delivery, and transport of sediments exert major control on the evolution of the coastal landscape. Significant progress has been made in understanding the dynamics of sediment transport on sand and cobble beaches and barrier islands along unprotected coasts. However, understanding of sediment transport remains limited in areas where cohesive sediments or mixed grain sizes are dominant, and in the presence of coastal development and hazard mitigation infrastructure. Formulations of sediment transport are an important component of many coastal evolution models. Currently, sediment transport formulations in wide use within coastal evolution models are empirically based and do not fully represent all governing physical processes. Though there have been significant recent advances in understanding transport at the sand grain scale (e.g., Simeonov and Calantoni, 2011), there is currently an inability to upscale these grain-scale processes to the spatial and temporal scales required for predicting long-term coastal change while retaining the fundamental physics of sediment transport. Recent advances use statistical approaches as a way to move toward addressing this issue (Palmsten et al., 2017) and have the potential to provide an important additional means for predicting patterns of coastal behavior at large spatial and temporal scales under changing conditions. It is also important to better understand the critical role of biota (e.g., microphytobenthos, dune grasses, mangroves) in influencing sediment transport and to incorporate improved understanding of this role into sediment transport formulations.
Developing reliable projections of long-term coastal evolution will also require improvements in the ability to project future hydrodynamic conditions. The recent development of a novel statistical approach that uses climate-model-derived wind field output to predict future wave climate (e.g., Camus et al., 2014a,b) represents an opportunity to improve the representation of such processes in future projections (Antolínez et al., 2016). It is critical to develop more such methods for making reasonable projections of hydro-
dynamic conditions. In addition to wind and wave forcing, regional-scale hydrodynamics such as the Loop Current have the potential to influence large-scale evolution of coastal features, yet the understanding of hydrodynamics at this scale and the relationship among such processes is insufficient to include their effects in modeling efforts.
Research Gap 5: Limited understanding of sediment transport processes and uncertainties in predicting future hydrodynamic conditions hampers the ability to project long-term coastal evolution.
- What improvements can be made in methodologies to predict sediment transport in the coastal zone?
- How do biota influence sediment transport and what modifications to sediment transport formulations are necessary to include these effects?
- What are the main drivers and sensitivities connecting regional-scale hydrodynamics to large-scale coastal evolution?
- How will hydrodynamic conditions (e.g., waves, tides, inundation, currents) change in the future?
- How might shelf-scale oceanographic processes control coastal evolution (e.g., effects of the Loop Current)?
EVOLUTION OF COASTAL LANDFORMS AND EMBAYMENTS
Coastal landforms and embayments in the Gulf (e.g., mainland shores, deltaic plains, chenier plains, river deltas, barrier islands, peninsulas, tidal inlets, bays, and lagoons) change and evolve over a range of timescales in response to waves, tides, and currents that drive sediment transport (both in fair weather and during storms), as well as sea level rise, subsidence, ecological processes, and human processes. As sea level rise accelerates and environmental conditions such as storminess, precipitation, and temperature shift in the future, the dynamic coastal landforms and embayments of the Gulf Coast will respond by evolving even more rapidly than they have in the recent past.
The ability to simulate and forecast the long-term evolution of coastal landforms and embayments is critical to understanding the coupled natural-human system and would draw directly from progress toward addressing Research Gaps 1 to 5 (described in previous sections). Models that forecast evolution of the physical coastal system have progressed over the past decade, though the capability to make regional-scale, long-term predictions of coastal change across landscapes that include a range of different types of coastal environments is still far away. Development and testing of these models also hinges on the availability of recent, synoptic-scale bathymetry and nearshore topography for the Gulf Coast.
Contributions of sediment from rivers, alongshore sediment transport, and shoreface erosion, along with human activities that modify them, determine the rates at which sediment is supplied to or lost from the coast across a range of timescales. Assessing these contributions is essential for modeling and projecting coastal evolution; however, these contributions are not well understood (see Research Gaps 4 and 5). The amount of sediment supplied to wetlands and barrier islands is especially critical in determining whether they can accrete and maintain their position and function as sea level rises. Sediment reduction or blocking can have long-term consequences (e.g., Rogers et al., 2015). Climate change–induced shifts in dominant vegetation type (e.g., Zarnetske et al., 2012; Armitage, 2015) or characteristics (e.g., Emery et al., 2015) on barrier islands and mainland, or estuarine shores, can alter patterns of sediment transport and deposition (e.g., Hacker et al., 2012; Durán and Moore, 2013). This can lead to landscape alterations that affect ecosystem function and coastal vulnerability. Furthermore, although it is well known that vegetation can act as a strong stabilizing agent against erosion in a variety of ecosystems (e.g., forest, riparian, agricultural), much remains to be understood regarding the specific interaction of barrier island vegetation and substrate accretion and stability (e.g., Maun, 1998; Tsoar, 2005). Wave climate changes can also alter sediment transport, affecting patterns of shoreline erosion and accretion and leading to changes in coastline shape (e.g., Slott et al., 2006; Moore et al., 2013; Thomas et al., 2016), and current and future coastal vulnerability (e.g., Wahl and Plant, 2015). In addition, understanding the role of preexisting geologic conditions (e.g., Rodriguez et al., 2004; Mallinson et al., 2010) and feedbacks between physical processes and ecological processes (e.g., Short and Hesp, 1982; Psuty, 2008; Houser and Hamilton, 2009) along the Gulf Coast is still evolving. These all lead to a need to identify coastal tipping points, such as the loss of wetlands and barrier islands, in light of anticipated rapid changes in environmental conditions in the future.
As discussed in Chapter 2, there have recently been important advances in the development of coastal area models. These predictive models require significant computational resources (e.g., Delft3D3 [Lesser et al., 2004], XBeach [Roelvink et al., 2009]) and typically are only capable of simulating short-term coastal change. Various schemes, however, have been proposed to reduce computational time (e.g., time-averaging the equations over timescales of tides or longer to allow for longer model time steps or morphological factors that accelerate bathymetric change) to arrive at models that can be executed rapidly for very long periods of time (Roelvink et al., 2006). Further reductions in the computational needs of predictive coastal area models are also possible (e.g., new computational methods and future computational power) and would enhance their utility by allowing simulation of coastal evolution across longer time and spatial scales.
In addition to developments in predictive coastal area models, there have been advances in reduced-complexity modeling approaches (e.g., French et al., 2016) that include explicit treatment of key processes (e.g., Coastline Evolution Model [CEM; Ashton and Murray, 2006]; COastal Vector Evolution Model [COVE; Hurst et al., 2015]; ShorelineS [Roelvink, personal communication4]). These models are computationally efficient and lend themselves to being coupled with other system models to make predictions over large spatial and temporal scales. These reduced-complexity models, however, can involve significant simplifying assumptions and may need extensive, site-specific calibration. Further development of these types of models is needed, for example through data assimilation approaches (e.g., Vitousek et al., 2017), to expand the temporal and spatial scales across which they can make realistic predictions.
Finally, opportunities exist to combine the strengths of coastal area, coastal evolution (planform), and landform evolution models (e.g., of barrier islands, wetlands, estuaries) by coupling them in appropriate ways. For example, importing relevant information from high-resolution hydrodynamics-based coastal area models into the input conditions of other types of models can more effectively address long-term evolution of the coastal system (e.g., van Maanen et al., 2016). In addition to coupling different types of coastal models, it is important to include feedbacks between physical processes and ecological processes in models of coastal evolution. There has been some progress toward this end, such as the inclusion of connected barrier-marsh and bay processes in landform evolution models (e.g., Walters et al., 2014) and inclusion of the effects of vegetation on bed friction in XBeach (Passeri et al., 2018).
Ensemble approaches, including stochastic forecasting such as Monte Carlo methods, have been used effectively in other branches of geoscience and engineering, as well as in meteorology, to characterize uncertainty associated with variable or unknown processes, or to consider the range of predictions derived from the use of different modeling approaches. Many studies of long-term coastal change already adopt an ensemble approach to address a lack of constraints on the exact sequencing and spatial distribution of physical, ecological, and human drivers (e.g., Barkwith et al., 2014). Extending the ensemble approach to include consideration of the range of future behavior predicted for the same future conditions by different long-term models and modeling approaches, such as those so commonly used in climate modeling and hurricane forecasting, would provide a more comprehensive means for developing and assessing the reliability and range of future predictions (e.g., Kirwan et al., 2010). Pairing this approach with long-term monitoring of landscape change would allow comparisons with and among model results over time, facilitating model development and improvements in capability.
4 Communication during committee’s open session in New Orleans, Louisiana, on September 18, 2017, with Dano Roelvink, IHE Delft Institute for Water Education.
Data assimilative approaches have also been explored in related geoscience fields to constrain and guide model solutions or determine poorly known boundary conditions or parameter values (e.g., surf zone bathymetry estimation [Wilson et al., 2014], sediment distribution estimation [Wiberg et al., 2015]). Such approaches can lead to model configurations that are better suited to accurate predictions. They can also result in an understanding of crucial observations and can therefore inform the design of long-term observational programs.
Research Gap 6: There is a critical need to understand and project the future response of coastal landforms and embayments to changing climate and the conditions under which they will no longer be able to keep pace with relative sea level rise.
- What are the critical tipping points beyond which barrier islands, river deltas, and wetlands will be unable to keep up with relative sea level rise, and what indicators can be used to reveal when the system is approaching them?
- How will sediment budgets, transport rates, and patterns change in the future and what will the impacts be on the evolution of barrier islands, mainland shores, and wetlands?
- How do feedbacks between biota and sediment dynamics, under current and future conditions, affect the structure and evolution of coastal landforms along the Gulf Coast?
- How has the Gulf coastline responded to changes in wave climate, rates of relative sea level rise, and sediment supply in the past, and what effects might future changes in these variables have on coastal evolution?
- How can future evolution of the physical coastal system (e.g., open-ocean coastline, estuaries, barrier islands, river deltas) be reliably modeled and projected over long timescales and large spatial scales?
- What monitoring and observational studies can be developed to capture and understand changes as they begin to happen even more rapidly in the future?
- How can existing approaches to modeling the physical system be improved or new approaches developed (e.g., by incorporating data assimilation approaches and the expanded use of ensemble modeling) to encompass a full range of future landform and embayment projections?
- What improvements in the reliability of modeling future landform and embayment projections can be realized through iteratively incorporating new understanding of the physical system?
- What are the best means for testing and further improving the reliability of modeled future projections of landforms and embayments?
Gulf Coast ecosystems continue to evolve over a decadal to centennial scale. These changes are inherently linked to changes in the physical system and are often human induced, arising from factors such as changes in the built environment, industrial practices along the coast, and climate change. Placing the ecological component within the coupled natural-human Gulf Coast system first entails understanding how ecosystems function under natural conditions, and then how human alterations affect ecosystem function.
EFFECTS OF CURRENT AND FUTURE PHYSICAL FORCING AND ENVIRONMENTAL CONDITIONS ON COASTAL ECOSYSTEMS
The Gulf Coast is characterized by sharp longitudinal contrasts in terms of sediment, nutrient and freshwater inputs, and resulting environmental conditions for its biota. Although these natural environmental gradients strongly affect the composition, structure, and species richness of coastal ecosystems, the impacts of these gradients on ecosystem function and resilience are not well understood. For instance, understanding the functional changes from the phytoplankton-dominated western Gulf to the macrophyte-dominated eastern Gulf (Anton et al., 2011; McDonald et al., 2016b) is far from complete. While the knowledge of compositional and structural shifts in Gulf Coast ecosystems is detailed, it is not well known how these shifts translate into functional changes.
Along the Gulf Coast, physical forcing varies over diverse timescales (e.g., daily, weekly, monthly, seasonal, inter-annual). Episodic events, such as hurricanes and cold fronts, can also induce substantial temporal variability. However, the impacts of physical temporal variability on the dynamics and function of coastal ecosystems are not fully understood. For instance, more research is needed on how Gulf Coast ecosystems respond to rising water levels driven by global sea level rise and subsidence, which takes place over years to decades; changes in wind-driven setup, which take place over hours to days (but also have a multi-decadal component); and river floods, which take place over days to months; and the interactions among these.
Coastal shorelines in the Gulf of Mexico will continue to experience impacts from coastal development such as residential, commercial, and energy infrastructure; legacy impacts associated with this infrastructure; hydrological alterations; and exploitation of natural resources. All of these impacts can cause deterioration and compromise the resiliency and sustainability of coastal ecosystems. There have been meaningful advances in understanding the impact of coastal development on Gulf Coast ecosystems, but much remains to be learned. A warming climate and rising sea level will certainly cause further coastal ecosystem change in the decades ahead. Changes in watershed hydrology will also affect sediment flux, resulting in wetland accretion or erosion and affecting services provided by wetlands
and other coastal ecosystems. There is substantial evidence (Osland et al., 2013; Weston, 2014) that anthropogenic impacts on Gulf Coast ecosystems will likely become more pronounced in the future, but how this may alter their dynamics and function is less well known. In particular, the compounding effects of coastal development and climate change on ecosystem dynamics and function are poorly understood, yet are crucial to understand and co-manage ecosystems and human populations along the Gulf Coast in future years. It is also important to put the interactive effects of development and climate change on coastal ecosystems in the context of associated changes in the physical system (e.g., open-ocean coastline, estuaries, barrier islands, river deltas) to acquire a holistic understanding of the future evolution of the natural-human coastal system in the Gulf Coast. This can be achieved through the use of integrated models that synthesize the response by physical and ecological systems to climate change and coastal development.
Research Gap 7: There is limited understanding of the individual and combined effects of current environmental gradients, physical forcing, climate change, and coastal development (including energy-related infrastructure) on Gulf Coast ecosystems.
- What are the effects of current and predicted future (e.g., climate change, coastal development) gradients in physical and environmental conditions (e.g., sea level rise, sediment loading, water clarity, tropicalization, ocean acidification, changes in storm frequency and intensity, watershed hydrology) on the distribution and abundance of key ecosystems along the Gulf Coast, including oyster reefs, salt marshes, submerged grass beds, and barrier islands?
- How are ecosystem services (e.g., fisheries productivity, shoreline stabilization through wave attenuation, filtration and removal of pollutants, mitigation of climate change through carbon sequestration) affected by these current and predicted future gradients?
- How prominent will the effects of relative sea level rise be in relation to other stressors such as coastal development, and what are the interactions among these processes and the consequences for ecosystem survivability?
- What are the most important interactions between and among physical forcing, coastal development, and climate change driving changes in environmental conditions and ecosystem response, both in terms of their structure and function?
EFFECTS OF STRATEGIC NATURAL RESOURCE CONSERVATION AND RESTORATION ON COASTAL ECOSYSTEMS
There is evidence to suggest that strategic natural resource conservation (e.g., functional hotspots) can help attain sustainable and resilient coastal systems (McDonald et al.,
2016a). However, knowledge on how to identify functional hot spots, and their viability and effectiveness to maintain ecosystem function under current and future climate regimes, is still in its infancy. Restoration efforts (e.g., creating land through river diversions) can fill a similar role. These efforts come in many forms and can vary widely in design, cost, execution, evaluation, and performance. Unsuccessful efforts are often grounded in a lack of understanding and consensus about desired recovery targets, as well as limited understanding of the response of the resource to existing environmental conditions (Sparks et al., 2013). Research on the response of restored resources to environmental conditions on the developed coast under current and future climates, as well as criteria for the establishment of restoration benchmarks sought relative to current or past conditions, is needed for more effective outcomes and higher success in restoration efforts.
Research Gap 8: The understanding of strategic natural resource conservation and restoration activities for effective coastal management is limited.
- Which functional hot spots best contribute to the preservation of ecosystem resources, function, and services in developed coasts under current and future climate conditions?
- How do restored resources respond to stressors from human development and climate change?
- What are reasonable, realistic benchmarks for gauging restoration success in developed coasts under current and future climate conditions?
Understanding the evolution of the coupled natural-human coastal system necessitates an understanding of the components of the human system that interact with and feed back on the natural (physical and ecological) system. The examples of human dynamics and decision making discussed in Chapter 2 underscore the lack of scientific basis to predict significant human processes with any degree of confidence over decadal to centennial timescales. These types of projections will be particularly challenged if thresholds and tipping points are exceeded, leading to new equilibrium states for any of the coupled natural or human system components. An argument can be made that many of the most important human changes in the 20th century are unique to that time, and such significant changes are not likely to occur again. It may be reasonable to model human processes with the expectation that fundamental ways of living over the next 10 to 200 years will be similar to today. However, many environmental changes in the coastal zone will be unprecedented and may lead to entirely new ways of living in the coastal zone and new patterns of economic activity—these will need to be considered in modeling efforts. The essential features of the human system
that models will need to consider can be grouped into three broad areas: decision making and adaptation, the built environment, and human migration.
Decision Making and Adaptation
Individual households and governments sometimes invest in durable goods to adapt to climate change. Households might purchase high-clearance cars or trucks to transit areas of minor flooding, replace carpet with tile to deal with nuisance flooding, or raise homes on pilings to avoid damages from storm surge. Governments might build a seawall or fund a beach nourishment project to mitigate against storm risk or erosion associated with sea level rise. Future investments in durable goods may come in the form of new innovations or adaptation technologies based on ideas that are not yet known or cannot yet be feasibly implemented. Which durable goods individuals and governments choose, and when they choose them, ultimately influences the long-term evolution of the coupled natural-human system.
ADAPTIVE DECISION MAKING
Most efforts to model human adaptation measures do not incorporate insights on how humans make decisions. Models of defensive adaptation expenditures, such as sea walls, levees, beach nourishment, and other gray, green, or hybrid interventions, typically impose decisions on the physical system. Similarly, to the extent that there is modeling of zoning, setback rules, or building codes, the focus is on modeling the effects on the physical, ecological, or human system. Less understood in the long-term evolution of the coupled natural-human system is how these decisions are determined endogenously (i.e., within the system in response to feedbacks, rather than external to the system). Imposing a particular decision or defensive expenditure on the coupled system without a model of how future decisions will adapt to future conditions will likely not produce complete or useful scenarios to analyze long-term changes in the coastal zone. A handful of studies have coupled human adaptive decisions with physical coastline change models and have focused on beach nourishment along sandy coastlines, including property value and beliefs about climate change as socioeconomic drivers (McNamara and Keeler, 2013; Williams et al., 2013; McNamara et al., 2015), or both policy and climate change scenarios on a range of stakeholder-defined metrics, such as number of affected buildings or considerations of beach access (Lipiec et al., 2018).
Modeling long-term coastal zone change requires understanding a broad set of mechanisms that drive a range of defensive expenditures and development restrictions, as well as an understanding of the feedbacks that these decisions create on the physical and hu-
man systems, particularly how individual decisions evolve with associated human-imposed physical change. More research is needed to apply insights to coastline types and adaptation settings beyond those included in previous studies. More data are needed regarding state, federal, and community-level defensive expenditures and decisions to restrict development. While there are some datasets that track federal payouts on disaster relief (FEMA, 2018), flood insurance, and expenditures on projects such as beach nourishment (PSDS, 2018), datasets that compile federal, state, and local decisions about defensive capital expenditures and local development restrictions are sparse and not uniformly tracked over time and space.
Research Gap 9: There is a need to understand how decisions about the built environment will be affected by coastal change and how these decisions create feedbacks between the natural and human systems.
- How do flooding events influence decisions about future infrastructure siting? How does existing infrastructure influence current and future flood hazard?
- How do decisions about coastal development and abandonment alter the natural evolution of coastal landforms, and what are the feedbacks on subsequent behaviors and decisions?
- How will existing policies or policy changes that provide incentives or disincentives to develop or redevelop (e.g., flood insurance, zoning, buyouts) influence future decision making?
- How willing are coastal residents to pay higher taxes to support defensive capital expenditures or other adaptation interventions to support existing coastal communities?
- At the household level, what decision-making process and what circumstances lead to investments in adaptation measures such as structure elevation or flood proofing?
The Built Environment
The built environment refers to the places and spaces created by people. These are the areas where they live, work, and recreate on a day-to-day basis, such as buildings, infrastructure, and parks. The built environment can be affected by coastal change as seen, for example, in the damage to energy and port infrastructure, destroyed homes and commercial buildings, and damaged levees and other structural defenses during and in the aftermath of Hurricane Katrina and other major Gulf Coast hurricanes. Subsequent decisions about the built environment can also affect coastal environmental change.
EFFECTIVENESS OF GREEN OR HYBRID INFRASTRUCTURE
One important issue that exemplifies the need for a better understanding of natural-human system feedbacks is the use of gray-green (or hybrid) infrastructure. Hybrid approaches use nature-based approaches (e.g., wetland creation) to enhance the effectiveness of built infrastructure (Sharma et al., 2016a). Besides helping traditional, human-made (gray) features become more efficient, nature-based (green) approaches can also help preserve ecosystem function and services. However, like gray infrastructure, nature-based methods also affect the long-term evolution of the coupled coastal system. However, there are critical gaps in understanding how green infrastructure performs from an engineering perspective (e.g., Cunnif and Schwartz, 2015), as well as how these systems evolve over time, particularly in the face of rapid sea level rise, and how this evolution impacts their performance. An understanding of how to best combine both green and gray infrastructure in cost-effective ways is limited, as is reconciling variations in performance between the two (green infrastructure is inherently variable), overcoming differences in gray and green infrastructure useable life span, and mitigating adverse effects that may stem from a combination of approaches.
Even when engineered with spatial uniformity, within a single season green infrastructure can undergo significant life-cycle changes (e.g., marsh plants senescing in winter)—altering its ability to trap sediment and dampen waves and introducing spatial variation along with temporal evolution. These spatiotemporal changes alter the green infrastructure’s actual performance in the short and long term (Koch et al., 2009). Because of these natural discontinuities, green infrastructure intended to mitigate chronic or episodic inundation, wave, and erosion hazards may evolve to be more or to be less effective than intended (Feagin et al., 2010), and in some hazard conditions might become ineffective or even increase the hazard locally (e.g., Irish et al., 2014).
Improving such understanding is essential for ensuring resilient coastal communities in a future of increased human pressure and climate change. A better understanding of how to best combine green and gray infrastructure into hybrid approaches lies directly at the intersection of the natural and human systems in the Gulf Coast. However, more work is needed to evaluate the applicability, versatility, and effectiveness of these approaches.
Given that a large percentage of the U.S. energy infrastructure lies along the Gulf Coast, sea level rise, subsidence, increased hurricane intensity, and loss of wetlands and other ecosystems could lead to direct impacts on infrastructure, such as damage to equipment from coastal erosion or flooding. Other indirect impacts, such as increased costs from raising vulnerable infrastructure to higher elevations or building future energy projects farther
inland, thereby increasing transportation costs, can also occur (CCSP, 2007). In addition, vulnerabilities and risks associated with aging energy infrastructure cascade not just into transportation systems, but also into water infrastructure, ecosystems, agriculture, forestry, communities, and social systems (Wilbanks and Fernandez, 2013). With increased risks of coastal hazard impacts to aging energy infrastructure, social vulnerability for communities co-located with them has also increased (Bernier et al., 2017).
With the expected future impacts of changing environmental forcing on the Gulf Coast’s dynamic landscapes, it may become important to prioritize the most critical infrastructure. This is relevant not only for energy-related infrastructure, but also for other critical infrastructure such as water, power, and transportation infrastructure. There is a need for more knowledge regarding which infrastructure would need more protection (e.g., levees, floodwalls), which would require better physical resiliency (e.g., low-elevation buildings), which would need more flexibility (e.g., working operations), and which might need to be relocated or abandoned. Determining which option works for different types of infrastructure necessitates development of models that can couple the significant physical (e.g., flooding) and economic factors (e.g., profitability of the infrastructure). For instance, in the case of infrastructure that has to be abandoned, there needs to be discussion of how abandonment would occur, what type of demobilization would be needed, and where debris would be moved. For infrastructure that is to be relocated, there needs to be discussion of where it would be appropriate to move, and what the community, economic, and environmental impacts would be for both the original and the new location.
As an example, Port Fourchon is the largest deep draft oil and gas port along the Gulf Coast, providing services for the offshore oil and gas industry. Recognizing its vulnerability to storm surge and erosion, energy companies dependent on that infrastructure have previously invested in engineered protection, as well as beach and dune nourishment. In addition, Port Fourchon is conducting feasibility studies of a major expansion that would include deeper draft navigation channels, and beneficial use of the dredged sediment has been proposed to create wetland habitat areas and reduce storm surge impacts (Guidry, 2016, 2018; Duchmann, 2018).
EVOLUTION OF THE COUPLED SYSTEM IN THE PRESENCE OF ENERGY INFRASTRUCTURE
A comprehensive, field-based assessment of impacts to coastal habitats from the construction and operation of oil and gas infrastructure (e.g., pipelines, navigation channels and waterways, onshore facilities), emphasized the importance of a variety of interrelated factors in establishing causal relationships between the infrastructure and the response of the natural system (Wicker et al., 1989; Cranswick, 2001; Nairn et al., 2005; Johnston et al., 2009). Depending on the site physiography, the construction methodology employed, and mitigative measures, the impacts attributable to infrastructure ranged from an absence of
any impacts to significant direct and indirect impacts. Uncertainty still exists in accurately quantifying the relative contribution of human activities (such as oil and gas operations) to subsidence, versus other natural mechanisms such as tectonics (although natural mechanisms such as faulting may also be caused by fluid extraction), all of which vary across the Gulf Coast (Kolker et al., 2011).
Limited understanding of individual- and community-level decision making under different scenarios of coastal change hinders the ability to constrain the range of possible future outcomes. To model the long-term evolution of the coupled Gulf Coast natural-human system, relevant scenarios about individual and government decisions to invest in durable goods for climate change adaptation need to be developed. There is also a need to devise and evaluate the performance of novel mitigation strategies and emerging types of hazard mitigation infrastructure that are intended to be adaptive and resilient to change. Similarly, understanding the vulnerability of energy and energy-related infrastructure to relative sea level rise and storms, as well as the consequences of infrastructure failure, is limited. This is also true for understanding the direct and indirect causal relationships between energy-related infrastructure and the coupled response of the natural system. Taken together, these issues point to an overarching challenge to improve understanding of how the Gulf Coast’s built environment interacts with the natural system, including residential housing, commercial buildings, transportation and energy infrastructure, port infrastructure, and defensive structures such as seawalls and levees.
Research Gap 10: There is a need for better understanding of how coastal changes affect the built environment and which aspects of the built environment are most vulnerable to coastal changes.
- What strategies for coastal development are most cost-effective when considering future climate change, relative sea level rise, and episodic events?
- How do coastal engineering approaches or development restrictions feed back on continued coastal development and the tax base that supports engineering interventions?
- Under what circumstances might radically different ways to develop the coastal zone emerge, and how might those approaches affect evolution of the coupled natural-human system?
- In the near-decadal timescale, what Gulf Coast energy infrastructure is vulnerable to factors such as sea level rise, subsidence, increased hurricane intensity, wetland and other habitat loss, or age-related failure?
- How will construction and operation of infrastructure associated with coastal development and energy infrastructure (e.g., pipelines, waterways) affect evolution of the natural system across different temporal and spatial scales, and vice versa?
Although study of human migration has advanced in general terms, less understood is the relative importance of driving factors for migration decision making across different regions, groups of people, and types of coastal change processes. There is also disagreement and a knowledge gap with respect to the relative importance of long-term, gradual changes (e.g., relative sea level rise driving the movement of the community of Isle de Jean Charles [HUD, 2017]) as opposed to episodic events (e.g., households moving from New Orleans to Houston and other cities following Hurricane Katrina or evacuation of coastal Florida residents in response to Hurricane Irma) in influencing migration (Colten et al., 2008; Gutmann and Field, 2010; Hauer, 2017). Complicating the problem further are the compounding effects of economic and social stressors and multiple interactive and simultaneous sources of environmental change (e.g., sea level rise over the long term, increasing frequency of nuisance flooding, the sudden shock of a major storm). Some economic and social stressors are external to the coupled system; for instance, major changes in energy markets or the decline of entire economic sectors. Someone employed in a declining sector might be more vulnerable to environmental change or less able to adapt to change due to socioeconomic status.
Some economic and social stressors, however, are internal to the coupled system; a natural hazard that destroys an individual’s property could directly trigger a decision to migrate. Scientists and planners are not yet able to project or predict the long-term push- and-pull factors that influence human migration on the Gulf Coast in response to coastal change. This lack of understanding, particularly with respect to the direct and indirect relationships between environmental change and human decision making, make it challenging to incorporate these factors into models of the future coupled natural-human system. Consequently, current modeling of long-term coastal change and human response does not include migration decisions in response to coastal change and associated feedbacks on the Gulf Coast system.
Despite growing attention to migration in response to climate change, there is limited understanding of the drivers of human relocation and migration (both external and internal) in response to coastal change along the Gulf Coast, as well as how these decisions create feedbacks in the coastal environment. To include individual, household, and community migration decisions in response to coastal change in modeling scenarios for long-term coastal zone planning, significant advances in understanding are needed. Developing a long-term, longitudinal5 dataset to follow Gulf Coast residents, in-migrants, out-migrants, and geocoded information on coastal changes would significantly improve the ability to
5 Longitudinal studies collect repeated observational data on cross-sectional units (e.g., individuals, groups of people, cities) over time.
understand complex migration decisions and could also provide information on rebuilding decisions and household-level changes in response to coastal change.
Research Gap 11: There is an incomplete understanding of the vulnerability of different Gulf Coast communities to coastal dynamics, how coastal dynamics trigger migration and relocation decisions, and how these decisions create feedbacks to the natural system.
- How will people respond to unprecedented accelerations in environmental change, and how can this understanding be used to identify potential tipping points of human response (e.g., when might at-risk coastal communities across the Gulf Coast be abandoned)?
- How do migration decisions influence the tax base and the ability of communities to fund local public goods and defensive expenditures in response to coastal change?
- How do community-level investments in defensive capital, social capital, and local public goods influence migration decisions, including decisions to migrate into or out of a community?
- When do migration decisions (in or out) decrease or increase existing social capital, trigger further migration, and potentially reach a tipping point that undermines community viability or leads to complete abandonment?
- Which household characteristics, economic factors, and types of social capital drive household decisions to stay or migrate in response to coastal change, and how do these decisions aggregate up to larger population dynamics?
- When does evacuation in response to an acute storm event become a permanent migration decision?
- When people opt to rebuild rather than migrate after an acute storm, how do these decisions influence patterns in housing and urban development?
The research gaps previously discussed primarily involve one-way interactions among the various system components—how one aspect of the system (i.e., physical, ecological, or human) is influenced by or responds to other components. As such, these research gaps can be addressed while being viewed primarily through the lens of a specific component. In contrast, this research gap involves the consideration of the entire coupled system. Addressing this research gap entails an adequate understanding of the individual components, but the emphasis is on the understanding of the feedbacks among the components and the resulting evolution of the entire system, which can be best understood and projected through integrated modeling.
Research Gap 12: Understanding how decisions about the built environment and human migration will affect the coupled natural-human coastal system is limited and can be furthered through integrated modeling.
- What are the feedbacks among the coupled natural-human system, and how can they be incorporated into models?
- When will feedbacks reach tipping points that fundamentally change the coupled system?
- How can models account for household decisions in response to novel environmental changes (e.g., types or magnitude of changes that households have not yet experienced, such as unprecedented levels of sea level rise)?
- What are the impacts of gray, green, and hybrid engineering measures on the long-term evolution of coastal systems?
- How can modeling of the coupled natural-human system in the Gulf Coast incorporate the deeply uncertain possibilities of novel infrastructure or technologies that could radically change the experience of living along the coast?
- How do emerging or novel engineering approaches perform relative to traditional structures such as seawalls, groins, and beach nourishment?
- How does the performance of coastal engineering approaches in reducing flood hazards influence people’s attitudes or behaviors toward further coastal development?