Co-evolution of wetland landscapes, flooding, and human settlement in the Mississippi River Delta Plain
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- Twilley, R.R., Bentley, S.J., Chen, Q. et al. Sustain Sci (2016) 11: 711. doi:10.1007/s11625-016-0374-4
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River deltas all over the world are sinking beneath sea-level rise, causing significant threats to natural and social systems. This is due to the combined effects of anthropogenic changes to sediment supply and river flow, subsidence, and sea-level rise, posing an immediate threat to the 500–1,000 million residents, many in megacities that live on deltaic coasts. The Mississippi River Deltaic Plain (MRDP) provides examples for many of the functions and feedbacks, regarding how human river management has impacted source-sink processes in coastal deltaic basins, resulting in human settlements more at risk to coastal storms. The survival of human settlement on the MRDP is arguably coupled to a shifting mass balance between a deltaic landscape occupied by either land built by the Mississippi River or water occupied by the Gulf of Mexico. We developed an approach to compare 50 % L:W isopleths (L:W is ratio of land to water) across the Atchafalaya and Terrebonne Basins to test landscape behavior over the last six decades to measure delta instability in coastal deltaic basins as a function of reduced sediment supply from river flooding. The Atchafalaya Basin, with continued sediment delivery, compared to Terrebonne Basin, with reduced river inputs, allow us to test assumptions of how coastal deltaic basins respond to river management over the last 75 years by analyzing landward migration rate of 50 % L:W isopleths between 1932 and 2010. The average landward migration for Terrebonne Basin was nearly 17,000 m (17 km) compared to only 22 m in Atchafalaya Basin over the last 78 years (p < 0.001), resulting in migration rates of 218 m/year (0.22 km/year) and <0.5 m/year, respectively. In addition, freshwater vegetation expanded in Atchafalaya Basin since 1949 compared to migration of intermediate and brackish marshes landward in the Terrebonne Basin. Changes in salt marsh vegetation patterns were very distinct in these two basins with gain of 25 % in the Terrebonne Basin compared to 90 % decrease in the Atchafalaya Basin since 1949. These shifts in vegetation types as L:W ratio decreases with reduced sediment input and increase in salinity also coincide with an increase in wind fetch in Terrebonne Bay. In the upper Terrebonne Bay, where the largest landward migration of the 50 % L:W ratio isopleth occurred, we estimate that the wave power has increased by 50–100 % from 1932 to 2010, as the bathymetric and topographic conditions changed, and increase in maximum storm-surge height also increased owing to the landward migration of the L:W ratio isopleth. We argue that this balance of land relative to water in this delta provides a much clearer understanding of increased flood risk from tropical cyclones rather than just estimates of areal land loss. We describe how coastal deltaic basins of the MRDP can be used as experimental landscapes to provide insights into how varying degrees of sediment delivery to coastal deltaic floodplains change flooding risks of a sinking delta using landward migrations of 50 % L:W isopleths. The nonlinear response of migrating L:W isopleths as wind fetch increases is a critical feedback effect that should influence human river-management decisions in deltaic coast. Changes in land area alone do not capture how corresponding landscape degradation and increased water area can lead to exponential increase in flood risk to human populations in low-lying coastal regions. Reduced land formation in coastal deltaic basins (measured by changes in the land:water ratio) can contribute significantly to increasing flood risks by removing the negative feedback of wetlands on wave and storm-surge that occur during extreme weather events. Increased flood risks will promote population migration as human risks associated with living in a deltaic landscape increase, as land is submerged and coastal inundation threats rise. These system linkages in dynamic deltaic coasts define a balance of river management and human settlement dependent on a certain level of land area within coastal deltaic basins (L).
KeywordsDeltas Human settlement Flood risks Sediment delivery Wetland loss Coastal basins
River deltas all over the world are sinking beneath increasing sea levels, causing significant threats to natural and social systems (Syvitski et al. 2009). This is due to the combined effects of anthropogenic changes to sediment supply and river flow, subsidence, and sea-level rise, posing an immediate threat to the 500–1000 million residents, many in megacities that live on deltaic coasts (Vörösmarty et al. 2009). Compounding the problem, most deltaic coasts are also important regions for agricultural production, fisheries, hydrocarbon production, and global shipments of commercial goods. Solving this problem requires understanding how human settlement patterns and economies have co-evolved with the physical system linked to major river basins (Syvitski and Saito 2007; Vörösmarty et al. 2009; Day et al. 2012; Chen et al. 2012). In the last 100 years, river-management decisions have affected human settlements on most major river deltas, including Nile, Po, Yellow, and Pearl (Syvitski et al. 2009). For instance, river-management projects—designed to stimulate navigation, reduce river flooding, enhance agriculture production and energy exploration, and protect increased human settlement—have had the correlated impact of increasing land loss and the threat of tropical cyclone inundation (Syvitski et al. 2009). Given observed long-term rates of land loss, it is not clear if human occupation on many coastal river deltas is sustainable (Tessler et al. 2015). This article addresses sustainability of deltas and associated human communities by advancing systems analysis that focuses on how historical river engineering decisions have reduced sediment supply, reduced wetland area, increased vulnerability to coastal flooding, and impact human settlement.
Coastal Louisiana, with its wealth of natural resources, has had a long history of humans attempting to manage the flood risks of occupying an extremely dynamic deltaic environment. There are examples of how environmental and social systems have adapted to sea-level rise, subsidence, and hurricanes to accommodate the sustainable development. Wetlands of the MRDP consist of marshes, forested wetlands, and barrier islands that account for 60 % of the coastal wetlands in the lower 48 states of USA (Boesch et al. 1994; Gosselink et al. 1998). Prior to significant human settlement, land building and deltaic processes over the last several thousand years resulted in a net increase of more than 2.5 million ha of coastal wetlands (Coleman et al. 1998). During this time, wetlands were able to expand across the delta even with the occurrence of sea-level rise, subsidence, and hurricanes, as sediment supply was sufficient to accommodate the net effects of a sinking delta. Following human settlement and key decisions on river management that will be described below, this coastal wetland landscape has been degrading for the past 50 years, owing to the fact that wetlands drown when insufficient sediment is supplied to counter relative rise in sea level (Boesch et al. 1994; Day et al. 2007; Paola et al. 2011; Edmonds 2012b). In response, wetland loss in coastal Louisiana increased dramatically over the last 50 years, with losses reaching a peak of around 102 km2/year in the mid 1970s (Britsch and Dunbar 1993). Since 1956, almost 4000 km2 of Louisiana’s coastal wetlands have been converted to open water (Britsch and Dunbar 1993; Coleman et al. 1998; Barras et al. 2004). The rate of wetland loss has declined over the last two decades, and currently coastal Louisiana is losing about 44 km2 of wetlands each year (Barras et al. 2008). It is estimated that an additional 1800 km2 of wetlands will be lost from the MRDP by the year 2050 (Barras et al. 2004; Barras 2009). At present, coastal waters have submerged 25 % of this productive delta, as the Gulf of Mexico moves slowly inland closer to human settlements that were developed in their present location nearly 300 years ago.
The survival of human settlement on MRDP is arguably coupled to a shifting balance between a deltaic landscape occupied by either land built by the Mississippi River or water occupied by the Gulf of Mexico. We argue that the balance of land relative to water in this delta will determine how people deal with increased flood risks from tropical cyclones, as this retreating coast experiences rising seas and subsiding landscapes. Ideally, river-management decisions should reduce flooding and promote navigation, while maintaining natural processes necessary to sustain the physical landscape upon which the safety of human settlement is so dependent. We describe how the coastal deltaic basins of the MRDP provide insights into how different river-management strategies, with varying degrees of sediment delivery to coastal deltaic floodplains, change the patterns of how deltaic coasts and human settlement co-evolve. This region of the MRDP has historically been manipulated by river engineering decisions to control river flooding and maintain navigation across a landscape with extensive human settlement, and has recently undergone substantial change in both land area and population. The focus on protecting human settlements from river flooding has lead to feedback effects on sediment distributions that have resulted in increased flood risks from coastal waters. We focus on flood risk reduction by wetland landscapes in the deltaic plain as an ecosystem service linked to river engineering options along deltaic coasts. Understanding the connections among river-management decisions, delta landform evolution, storm-surge risks, and human settlement decisions provide guidance to restoration, protection, and regional planning processes (Twilley et al. 2008). The coastal deltaic basins of the MRDP provide insights into how different river-management strategies, with varying degrees of sediment delivery to coastal deltaic floodplains, change the patterns of how human settlement co-evolve with flooding risks of a sinking delta. This region can be used to demonstrate and explore feedback effects among reducing sediment delivery with human river-management decisions in deltaic coast, corresponding landscape degradation and increased water area that lead to human perception of increased flooding risk (Fig. 1). Such feedback loops are common in deltaic coasts globally (Chen et al. 2012; Tessler et al. 2015), and understanding system behaviors will provide insights on approaches to restoration and protection strategies under a changing climate.
Evolution of deltas and emergent ecosystems
The formation of deltaic lobes (L, Fig. 1) within a coastal basin (A, Fig. 1) is built by coastal accumulation of fluvial sediment as part of source-to-sink processes connecting large river basins to continental margins (e.g., Bentley et al. 2015). Newly emerged ecosystems develop on these landforms with specific vegetative patterns (E, Fig. 1) as a function of elevation controlled by the self-organization processes of geomorphic features (Paola et al. 2011; Nardin and Edmonds 2014). These coastal deltaic floodplains co-evolve with the geomorphic features forming extensive wetland landscapes along continental margins that prograde out to coastal ocean environments (Gosselink et al. 1998). Patterns of human communities (G, Fig. 1) living on deltaic landscapes also follow features of land area (L, Fig. 1) within coastal basins, forming a co-evolution of delta and human development, as communities seek higher elevations along natural levees (Davis 2000).
A coastal deltaic basin has a composition of land area and water area, depending on the relative supply of sediment (coastal deltaic basin area (A) = water area (W) + land area (L), Fig. 1). As L increases with sediment supply sufficient to compensate for RSLR, the land:water (L:W) ratio of the coastal deltaic basin will increase. If sediment supply in Eq. 1 is insufficient to compensate for RSLR, then L will decrease, resulting in corresponding reduction in L:W ratio. Delta mass balance is defined within Eq. 1 by denoting that there is a rate of sediment supply that compensates for RSLR, given a contribution of organic production to land elevation (ro). This mass balance suggests that there is rate of sediment supply that can sustain a stable L:W ratio in coastal deltaic basins that represent a measure of delta sustainability. The apparent simplicity of this delta mass balance condition between sediment supply and RSLR, leading to a constant L:W ratio, is deceiving, as terms on the right hand side of Eq. 1 are each complicated functions of multiple interacting physical, geological, and ecosystem processes. For example, the effects of sediment supply on land elevation change ecosystem types and these vegetative processes control marsh inputs of organic matter (ro) and sediment retention (fr) (Nardin and Edmonds 2014). Thus, both the land (L) and ecosystem features (E) in Fig. 1 represent positive feedback effects on land area formation in the coastal deltaic basin, contributing to increases in L:W ratio. If newly formed land of the MRDP does not receive new sediment from river floods to increase elevation equal to RSLR, then land becomes submerged and reverts back to the sea, resulting in decrease in L:W ratios.
The delta cycle is a fundamental concept that describes this pattern of coastal basin formation and degradation in the provinces of MRDP over 1000–2000 year increments of river avulsions (Fig. 2b; Coleman et al. 1998). The formation of land during fluvial-driven delta progradation (with respect to steady-state conditions of mass balance, or delta maintenance, Eq. 1) contrasts strongly with phases of delta deterioration resulting from decreased sediment flux as function of river abandonment (Fig. 2b). The importance of sediment supply to maintaining landform and wetland ecosystems in coastal deltaic basins is evident when the river abandons a coastal region and moves to another location during river avulsion, characteristic of river abandonment in the delta cycle (Fig. 2b; Penland et al. 1988). As sediment supply decreases in the abandoned coastal deltaic basin, land decreases and water area of the coastal deltaic basin increases, decreasing the L:W ratio (Gosselink et al. 1998). Along with subsidence, marine erosional processes rise in importance, reworking sandy sediments to form sandy coastal spits and barrier islands that transgress across the subsiding deltaic plain (Penland et al. 1988). If avulsion results in the reoccupation of a subsiding coastal basin by a river, then the delta cycle restarts landscape progradation, increasing L:W ratio, and restoring wetland ecosystems within the coastal deltaic basin (Fig. 2b).
Evolution of deltas and human settlement
Change in land area in a coastal deltaic basin over the last several decades may have direct impacts on migration rates of coastal human populations through this mechanism, which may be modified by the construction of protection levees. Thus, dynamic deltaic landscapes, both in physical structure (land area, elevation, and vegetation cover) and changes in flood risks (hazard vulnerability), explain how reductions in wetland loss and L:W ratio may shape patterns of human settlement, as driven by increased risks of coastal inundation. As stated above, the long history of human behavior in the MRDP to reduce risks to river flooding has also reduced sediment supply leading to reduced land formation in coastal deltaic basins, producing a negative feedback to sustaining land area (L, Fig. 1). Thus, reductions in sediment supply (Qs) as a result of river management (construction of protective levees) to reduce risks from river floods can contribute significantly to increase flood risks from coastal storms by removing the capacity of wetlands to reduce wave and storm-surge that occur during extreme weather events. Tradeoffs between river and coastal flood risks will determine population migration as human risks for those living in a deltaic landscape increase, as land is submerged and coastal inundation threats rise. These system linkages in dynamic deltaic coasts define a balance of river management and human settlement dependent on a pattern of environmental succession that sustains a certain level of land area within coastal deltaic basins (L) as evidence by constant L:W ratio over time.
The effect of withholding by the levees from the great areas of the delta of the annual contributions of sedimentary matters, and the steady, though slow, subsidence of these areas, is one which should be considered in deciding the important question of how to protect the people from the flood waters of the river. (Corthell 1897, p. 354).
No doubt, the great benefit to the present and two or three following generations accruing from a complete system of absolutely protective levees, excluding the flood waters entirely from the great areas of the lower delta country, far outweighs the disadvantages to future generations from the subsidence of the Gulf delta lands below the level of the sea and their gradual abandonment due to this cause. (Corthell 1897, p. 354).
The risks associated with decisions to manage flood control of the Mississippi River are clearly defined as tradeoffs to the economic benefits associated with opening up the river to navigation and protecting the region’s rich agricultural lands from devastating floods. The economic drivers to minimize flood damages to crops and increase the capacity of commerce along the river were substantial relative to the risks of a delta sinking under the sea, at least for three future generations.
Today, modified deltaic processes (reduced Qs) and altered conditions of the landscape demonstrate that some adjustments to present river-management decisions are critical to allow human settlements and critical infrastructure to safely occupy the MRDP. The fourth generation is now struggling to develop restoration plans within the constraints imposed by the needs to provide river flood control and maintain navigation in the Mississippi River (Barry 1997; Galloway et al. 2009). A goal of the proposed restoration plan, as defined in the Coastal Louisiana Master Plan (Peyronnin et al. 2013), is to build land that will reduce risks to coastal inundation and sustain the economic wealth of this delta region. Diverting freshwater and sediment from the Mississippi River into adjacent coastal wetlands and estuaries is one approach to the comprehensive restoration plan being implemented in Louisiana (Boesch et al. 1994; Day et al. 2007; Twilley and Rivera-Monroy 2009; Paola et al. 2011). The pulsing water-flow of the Mississippi River is believed to be critical for providing sediments necessary to stabilize wetland structure and function in the delta. The manipulation of controlled floods into coastal deltaic basins using river diversion structures may be an important tool for supplying coastal wetlands with freshwater, sediments, and nutrients that can enhance productivity, vertical accretion, and marsh stability. The challenge to implementing such aggressive restoration projects is limited by large-scale testing of how effective such river-management practices will provide critical needs to the fourth generation inhabiting MRDP. We propose that the coastal basins and river engineering practices in the last century provide experimental landscapes to calibrate the outcomes of such projects as way to develop better formulations of how deltas and human settlement co-evolve (Eq. 3).
Coastal deltaic basins as experimental units of delta instability
In contrast to these public work projects that restricted sediment delivery to most of the coastal deltaic basins of the MRDP over the last century, a major connection to river sediment supply was maintained in Atchafalaya Bay, where an outlet with Mississippi River was constructed, known as the Old River Control Structure (Fig. 4a). The Old River Control Structure represents the only location where an outlet has been maintained in the lower deltaic plain of the Mississippi River, emptying water and sediment into Atchafalaya River (along with discharge from Red River) down to Atchafalaya Bay (Roberts 1998; Roberts et al. 2003; Wellner et al. 2005). These flood control structures were designed to provide a floodway to the coast as an “outlet” that would help to protect urban centers downstream at Baton Rouge and New Orleans from flooding conditions. During the 1950s, it became clear that the Mississippi River was slowly migrating to the west and would soon follow the river basin of the Atchafalaya River (Trotter et al.1998; Reuss 2004; Edmonds 2012a). The Old River Control Project was designed for 30 % of the combined Mississippi and Red River’s total flow passing down the Atchafalaya River on an annual basis and 70 % down the Mississippi River to New Orleans. Two deltas are currently forming at the mouth of the Atchafalaya River, the Atchafalaya Delta, and the Wax Lake Delta (WLD); however, only the WLD has been allowed to form naturally without any major dredging manipulation. The Atchafalaya Basin is one of the few coastal deltaic basins where land has emerged above mean sea level in the last four decades (the other at mouth of Mississippi River), where subaerial Wax Lake Delta formed after the unusually high spring flood of 1973 (Roberts 1998). Land in this coastal deltaic basin has experienced rapid subaerial growth throughout the last 35 years (1–2 km2 year−1).
We can consider the Atchafalaya and Terrebonne Basins as experimental coastal deltaic basins to test the concepts described above about how land area (L) responds to varying degrees of sediment delivery (Qs) in Eq. 1. The Atchafalaya Basin continuously received sediment, even though structures were built on the Atchafalaya River in 1944–1963 to allow control of water and sediment (Reuss 2004). Terrebonne Basin is an experimental basin where sediment supply from Mississippi River was eliminated in 1903 (LBSE 1904). A satellite image during the 2011 river flood (Fig. 4a) demonstrates present sediment delivery patterns where plumes are evident in Atchafalaya Basin compared to no sediment plume evident in Terrebonne Basin. For reference, the Atchafalaya River discharged 40 Mt/year of sediment in 2008–2010, ~31 % of total Atchafalaya and Mississippi discharge (Allison et al. 2012).
This is the only region of coastal Louisiana that is building deltaic wetlands and confirms the ability of the river to sustain delta landscape if sediment delivery is allowed to occur across the coastal floodplain. Wetlands have colonized the emerging lands of the Atchafalaya and Wax Lake Deltas, including Sagitarria platyphylla as the dominant vegetation in the summer and fall. Older lobes of the Wax Lake Delta have a mixed community composed of Colocasia esculenta, Phragmites australis, Polygonum punctatum, Typha spp., Schoenoplectus spp., and Zizaniopsis miliacea. Salix nigra is the dominant vegetation present at levees of the older lobes, with an understory of C. esculenta and P. punctatum (Johnson et al. 1985; Shaffer et al. 1992; Holm and Sasser 2001). Marine and estuarine ecosystems have become less prominent as salinities decrease, and freshwater ecosystems expanded in this coastal deltaic basin, as has been observed in Fourleague Bay over the last several decades (Madden et al. 1988).
We developed a technique to more clearly define the location of the 50 % L:W isopleths using query methods of images available in 1932, 1973, 1999, and 2010. A neighborhood moving window operator is applied to a binary land/water raster image of the coastal region derived from aerial or satellite imagery. The function calculates the ratio of land-to-water inside a macro-sized floating analytical processing window and applies the result to the center pixel. The output is a continuous representation of the land–water ratio from which intervals are derived. The L:W model shows results across two dated landscapes in 1932 and 1999, with the observations and predictions by Gagliano et al. (1970) overlain upon the results (Fig. 5b, c). The comparison of results from our technique to those of Gagliano et al. (1970) using the 1932 data yields strong similarities (Fig. 5b). This was also the case when comparing results of the 1973 data sets (data not shown). However, the isopleths predicted for 2000 by Gagliano et al. 1970 differ from our analysis of actual migrations based on an image in 1999 (Fig. 5c). Our 50 % L:W isopleth has migrated further inland in Terrebonne Basin than predicted by Gagliano, compared to less migration inland from 1970 to 2000 in Barataria Basin. The migration of Gulf of Mexico in Terrebonne Basin shows that the 50 % L:W ratio isopleth is nearly 10 km farther inland in 1999 compared to the prediction of 2000 by Gagliano et al. (1970).
Metrics describing the relative change in land-to-water ratios between Atchafalaya and Terrebonne Basins from 1932 to 2010 using the analysis of isopleths that describe the 50 % land:water ratio
Land migration metric
Total migration (m)
Migration rate (m/year)
Area <50 % L:W (km2)
We argue that most of the changes observed between these two experimental basins are due to reduced sediment delivery (Qsfr) and not differences in subsidence (σ) between the two coastal deltaic basins, which could also explain differences in L based on Eq. 1. In general, subsidence rates in our study area decrease inland, as a function of the relative age and thickness of the local deltaic sediments (CPRA 2012). Inland sediments are older and the thickness of a particular deltaic package decreases inland (e.g., Blum and Roberts 2009). These two factors contribute to (but do not entirely control) the observed pattern of inland decrease of subsidence rates. For example, the median subsidence rates for the lower Terrebonne and Atchafalaya basins are 13 ± 6 and 7 ± 3 mm/year, respectively, whereas median subsidence rates for inland Terrebonne and Atchafalaya Basins (mapped as one unit in CPRA 2012) are 6 ± 4 mm/year. Based on this analysis, the inland decrease in subsidence rates should produce an inland deceleration of isopleth migration over time, which is not the pattern we observe. Subsidence certainly contributes to land loss, but in these cases, we propose that sediment supply exerts stronger influence.
Patterns of delta instability and flooding risks
This analysis demonstrates how coastal deltaic basins with and without sediment delivery change land area relative to water area, along with shifts in ecosystem types, in Atchafalaya Basin and Terrebonne Basin, respectively. There was minor landward migration in the L:W isopleth from 1930 to 2010, where river floods have not abandoned a coastal deltaic basin. Newly emergent landscapes in the Atchafalaya Basin with L:W ratio >50 % appear at the mouth of Atchafalaya River and Wax Lake Outlet, in response to sediment delivery from a river diversion constructed over 50 years ago. As described above, we propose that reduced sediment delivery along Bayou Lafourche that was initiated during the late 1800s and completed in 1903 can explain the landward migration of water from the Gulf of Mexico in Terrebonne Basin. These measurements of L:W ratio are indicative of delta instability in coastal deltaic basins as a function of reduced sediment supply from river flooding, as predicted by the delta cycle concept when the river abandons a coastal region.
Environmental succession of coastal deltaic basins linked to river sediment supply describes a gradient in ecosystem services that occur during distinct stages of delta cycle. The succession of these ecosystem services is linked to the ecosystem sequence that occurs as landform, salinity, and elevation change, and relative L:W ratio shifts with sediment input. The cumulative measure of ecosystem area and type determines many of the ecosystem attributes in a coastal deltaic basin. For example, at peak L:W ratio, salinities are low, and thus mostly freshwater marshes and coastal forests occupy the total area of a coastal basin. Under these conditions, ecosystem services, such as storm-surge reduction is high. In contrast, a coastal deltaic basin with decreasing L:W ratio as result of wetland loss and salinity increase is more susceptible to coastal inundation, as forested wetland area declines and water area increases. At some point, the continued reduction in L:W ratio increases the risks of flooding with longer fetch lengths that lead to increased wetland erosion and wave formation during extreme weather events that continue to threaten human settlement (Karimpour et al. 2015). These conceptual conditions of ecosystem services during the stages of the delta cycle can be described by the L:W ratio that is a function of sediment delivery.
Coastal wetlands, including forests, play an important role in mitigating damages from extreme events, such as tropical storms and hurricanes. Coastal wetlands act as a buffer to protect coastal communities by attenuating strong winds, waves, and storm-surges. The impact of coastal wetlands and forests on storm-surge depends on many factors, such as vegetation properties that lead to resistance to flow of water (e.g., stem/trunk height, rigidity, diameter, density, and coverage) and vegetative properties that lead to reduction of momentum transfer of wind (e.g., canopy, height, density, and coverage), landscape characteristics (e.g., land/water configuration, bathymetry, topography, local geometry, levee, channels, and other features), and storm parameters (e.g., storm track, storm size, duration, forward speed, and wind intensity), as well as the interaction of these factors (Dietsche et al. 2007; Chen et al. 2008; Medeiros et al. 2012; Sheng et al. 2012; Zhao and Chen 2014; Hu et al. 2015; Bilskie and Hagen 2013; Bilskie et al. 2014; Passeri et al. 2014, 2015; Medeiros et al. 2015; Nardin and Edmonds 2014). State-of-the-art numerical models with a realistic representation of land use and land cover (LULC) allow us to quantify the influence of wetlands on inundation of tropical cyclones (Bilskie et al. 2014). Tropical cyclone surge height diminishes with increasing travel distance over land and wetland for a given set of storm and LULC conditions (Bilskie and Hagen 2013). However, small strips of wetlands that effectively attenuate wind waves are insufficient to reduce storm-surge (Chen and Zhao 2012; Jadhav and Chen 2013; Jadhav et al. 2013). Potential benefits of land, wetland, and coastal forests for mitigating flood risks strongly depend on the size of the land area and what is built or growing on it. As shown in Figs. 7 and 8, there is an increase in forested wetlands and freshwater marsh, as sediment supply enriches the Atchafalaya coastal deltaic basin. The former vegetation type has particular influence on storm-surge reduction in this coastal basin; whereas the loss of these habitats in Terrebonne Basin, since 1949, is another factor contributing to increased flooding risk.
We propose that human population dynamics also respond to these landform changes, such that as L:W ratios decrease migrations due to increased flood risk increase (Fig. 1). Our hypothesis is that population decreases with increasing flood risks associated with decreases in L:W ratio in coastal deltaic basins. When the risks are greater than the benefits (amenities), people will move away from the region, and the community will become unsustainable. In other words, if there is an increase in flood risks due to the lack of sediment (which leads to a decrease in L:W ratio), then populations are expected to decline, and vice versa. We can test this hypothesis by comparing patterns in Atchafalaya Basin and Terrebonne Basin, treating them as experimental coastal deltaic basins with and without sediment delivery. It will also be very useful to understand the conditions when the hypothesis will not hold, such as why some areas have an increase in flood risks but population remains constant or increases, whereas other areas have no increase of flood risks, but population keeps declining.
Based on our model of land and human co-evolution (Fig. 1) the reductions in delta landscape area (L, Eq. 1) in Terrebonne Bay over the last 78 years have caused population decrease due to increased risks from storm-surge as L:W ratios decrease. We provide evidence that land loss response to changes in Qs in coastal shorelines leads to proportional changes to increased storm-surge risks by increased RSLR (investigating both ‘H’ and ‘σ’, Eq. 1), as wetlands are drowned, migrate landward, and forested wetlands are lost due to encroachment of humans and the built environment. This is the case in much of Terrebonne Bay, compared to Atchafalaya Bay, where wetlands are maintained by sufficient Qs and ro, potentially diminishing storm-surge risks. We argue that this balance of land relative to water in this delta provide a much clearer understanding of increased flood risk from tropical cyclones rather than just estimates of areal land loss. The coastal deltaic basins of the MRDP can be used as experimental landscapes to provide insights into how varying degrees of sediment delivery to coastal deltaic floodplains change flooding risks of a sinking delta using landward migrations of 50 % L:W isopleths. The nonlinear response of migrating L:W isopleths as wind fetch increases is a critical feedback effect that should influence human river-management decisions in deltaic coast (Fig. 1). Changes in land area alone do not capture how corresponding landscape degradation and increased water area can lead to exponential increase in flood risk to human populations in low-lying coastal regions. Reduced land formation in coastal deltaic basins (measured by changes in the land:water ratio) can contribute significantly to increase the flood risks by removing the negative feedback of wetlands on wave and storm-surge that occur during extreme weather events. Increased flood risks will promote population migration as human risks associated with living in a deltaic landscape increase, as land is submerged and coastal inundation threats rise. These system linkages in dynamic deltaic coasts define a balance of river management and human settlement dependent on a certain level of land area within coastal deltaic basins (L).
Models of delta-human landscapes need to focus on integrating models of deltaic morphodynamics, wetland ecology, and storm-surge dynamics with human population risks to quantify how decreases in sustainable land area in deltaic coasts influence population dynamics (Chen et al. 2012; Li 2015). Such modeling needs to understand if there are thresholds in land loss in deltaic landscapes (L, Eq. 1) that may shift the significance in how flooding risks are realized by human populations (V, Eq. 2). We expect thresholds will exist, whereby, as the L:W ratio decreases, there is corresponding increase in fetch resulting in larger tidal surges and wave heights causing nonlinear increase in wetland edge erosion (Mariotti and Fagherazzi 2013; Karimpour et al. 2015). This physical feedback has an impact on the human system, because land loss also produces responses in ‘G’ (Eq. 2) along deltaic coasts. There is greater awareness of flooding risks, as coastal deltaic basins lose land coincident with an increase in water area, resulting in increased costs for protection and flood risk mitigation (taxes, insurance, and levee construction). In deltaic coasts around the world, these interactions are compounded by how subsidence (σ) and climate change (through H) will threaten sustainable landscapes (Syvitski and Saito 2007), infrastructure, and coastal communities in the future.
Collectively, these issues facing deltaic coastlines have been termed the multi-trillion dollar problem that will have global impacts to public safety, trade, and regional wealth (Foufoula-Georgiou et al. 2011). The combination of reduced sediment supply and increased subsidence, along with the predictions for accelerated sea-level rise, forecast a challenging situation for continued human settlement in most deltas around the world (Tessler et al. 2015). Major investments in engineering strategies and human adaptations, including migrations, are urgently needed given the nonlinear changes in these processes as L:W ratio decreases in these deltas, as indicated in this study for the MRDP. The economic consequences of no action or status quo are unimaginable, but real given the patterns that have been observed today for major deltas that can be used to define future scenarios of risks (Tessler et al. 2015). MRDP is used as an example of how engineering capacity may be able to reduce risks in future scenarios, but given factors that may reduce investment effectiveness (such as costs associated with energy constraints), ‘management strategies that address the drivers of RSLR, particularly sediment supply and deposition, will be a core determinant of long-term sustainability over the next century’ (Tessler et al. 2015). The comparative response of Atchafalaya and Terrebonne Basins in this study reflects the boundaries of such future responses of deltas, given the engineering of Mississippi River at Old River Control Structure has stabilized this region of the MRDP. In comparison, the increased coastal flooding investments are required in the Terrebonne Basin to compensate for the lack of sediment supply to reduce risks of RSLR associated with landward migration of Gulf of Mexico. The behavior of these coastal deltaic basins to engineering, river sediment supply, and coastal risks will continue to demonstrate the comparative effectiveness of river management and coastal defenses that will control human settlement patterns and investments in the future. These contrasts in strategies will provide insights to similar situations along deltaic coasts around the world.
This work was supported by the Coastal SEES program of National Science Foundation under the Agreement Number EAR-1427389, and Frontiers of Earth Surface Dynamics (FESD) under the Agreement Number OCE-1135427. Support was also provided by Sea Grant Laborde Chair in Science and Technology Transfer. The authors would like to thank Keith Maung-Douglass and the LSU Coastal Sustainability Studio with assistance on graphics.
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