Development of New Restoration Strategies for Louisiana Barrier Island Systems, Northern Gulf of Mexico, USA
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Development of New Restoration Strategies for Louisiana Barrier Island Systems, Northern Gulf of Mexico, USA Syed M. Khalil†, Charles W. Finkl‡, and Richard C. Raynie† †Coastal Protection and Restoration Authority, Baton Rouge, LA 70801, USA
[email protected] [email protected]
‡ Coastal Education & Research Foundation (CERF) and Florida Atlantic University, Boca Raton, FL 33431, USA
[email protected]
www.cerf-jcr.org
ABSTRACT
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Khalil, S. M.; Finkl, C. W., and Raynie, R. C., 2013. Development of new Restoration strategies for Louisiana barrier island systems, northern Gulf of Mexico, USA In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 1467- 1472, ISSN 0749-0208. The rapid degradation of Louisiana’s barrier island (BI) systems adversely impacts the vitality of strategic economic and biological resources (including aquatic habitat). Louisiana’s BI systems have undergone landward migration through BI rollover, area loss and island narrowing as a result of complex interactions among land subsidence, sea-level rise, wave processes, inadequate sediment supply, and intense human disturbance. Consequently, the structural continuity of the BI shorelines has weakened as the islands narrowed, fragmented and, in some cases, submerged. Several multipronged restoration strategies are currently underway to develop effective interdisciplinary restoration projects that mitigate degradation of the Mississippi River delta plain and compensate for coastal land loss. Restoration of the BI systems requires strategies that protect interior wetlands and bolster the first lines of storm defense in a postHurricane Katrina-Rita-Gustav-Ike-Isaac and post-Deep Water Horizon era. Coastline degradation would have continued were it not for a series of massive barrier island restoration projects. Such efforts commonly consist of beach nourishment on the Gulfside and marsh creation on the bayside of barrier islands. Regionalizing monitoring, maintenance, and data management efforts support the still evolving scientific and engineering aspects of rebuilding BIs. ADDITIONAL INDEX WORDS: Beach renourishment, coastal erosion, coastal marine resource, geomorphic systems, restoration, wetland loss.
INTRODUCTION The Mississippi River, one of the largest fluvial systems in the world, has built an expansive deltaic plain. As for many of the world’s delta plains, natural and anthropogenic processes at many scales are responsible for severe erosion and rapid land loss. At the continental scale, management decisions upstream (e.g. construction of locks and dams for navigation) have limited the inriver sediment supply, which historically built and sustained the delta. At the regional-scale, natural processes such as subsidence and sea-level rise are exacerbated by anthropogenic actions. Subsurface fluid withdrawal (which may affect subsidence rates), dredging of access and navigation canals through wetlands (which alters hydrologic flow patterns and salinities, and intercepts the littoral transport of nearshore sediment transport), construction of local levees for flood control or spoil banks from channel construction (which affect hydrology but also limit sediment availability), have acted singularly or complementarily to accelerate ecosystem degradation (e.g. Bedford and Preston, 1988; Day et al., 2001; Penland and Ramsey, 1990; Roberts and Coleman, 1996;; Stone and McBride, 1998). At the local level, natural processes such as shoreline erosion along Gulf and barrier island bay sides, dune erosion, overwash, and inlet formation, contribute to the degradation of coastal barrier islands. These local processes are magnified by some of the same anthropogenic landscape alterations mentioned above (Penland et al., 2005; ____________________ DOI: 10.2112/SI65-xxx.1 received Day Month 2012; accepted Day Month 2013.
Stone and McBride, 1998). Additionally, tropical weather systems (e.g. hurricanes Katrina and Rita in 2005; hurricanes Gustav and Ike in 2008; TS Lee in 2011 and Hurricane Isaac in 2012) exacerbate the degradation of mainland Gulf shorelines and delta-front barrier islands in the manner described by Boesch et al. (2006). Unless significant efforts are made to restore barrier islands and coastal wetlands, Louisiana will continue to lose marshland, swamps, and barrier islands (beaches, dunes). While it is appreciated and understood (Khalil et al., 2011) that coastal environments are inherently dynamic and subject to ongoing changes, the failure to forestall and redirect present degradational trends foreshadows a serious future risk to the nation. Fortunately, several multipronged efforts are developing effective interdisciplinary management strategies that can be applied to Louisiana’s coastal environments. The primary objective of coastal restoration is to stabilize landward retreat, interrupt the disintegration of barrier islands by adding sediment to the system, and translating the barriers into various types of modified morpho-sedimentary environments (Khalil et al., 2011). Amelioration of shoreline retreat and submergence of barrier and deltaic systems can be achieved through soft nonstructural engineering efforts or the “soft options” that involve placement of dredged sediments to rehabilitate degraded beaches, dunes, and marshes (Khalil et al., 2006). These morpho-sedimentary features are further stabilized by installing sand fences and facilitating vegetative growth on dunes and in marshes to protect and stabilize landforms. These restoration efforts primarily depend on the emplacement of sand and suitable
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sediment to build up barrier-deltaic systems. The objective of engineering and designing barrier islands in Louisiana is equally concerned with restoration of structure and habitat. The process basically includes establishing a design for beach nourishment on the Gulf-side, and a design for a marsh platform on the bayside, consistent with the existing system. Typically, there are four phases in a barrier island project: planning; engineering and design; construction; and operation and maintenance (Khalil et al. 2006). This paper describes the integrated approach that Louisiana has taken to barrier islands restoration and management.
GEOLOGIC CONTEXT OF LOUISIANA’S BARRIER ISLANDS The coastlines of the modern Mississippi River delta plain are bordered by numerous barrier islands from Raccoon Island in the west to Hewes Point in the northern Chandeleur Islands (Figure 1). These barrier islands could be grouped to represent fragmented remnants of distal extremities of several major delta lobes and headlands: from west to east they are the Teche Delta System, Lafourche Delta System, Modern Delta System, and the St. Bernard Delta System. The back-barrier lagoons are connected to the Gulf of Mexico by approximately 25 tidal inlets, which allow the exchange of diurnal tides. Environments occurring within the official Louisiana coastal zone boundary (http://dnr.louisiana.gov/assets/OCM/CoastalZoneBoundary/CZB 2012/maps/Overview_of_Revised_CZB.pdf) of the delta plain include: alluvium, marshes (fresh, intermediate, brackish and salt), coastal forest, bay, and barrier islands. Barrier island morphology is related to sediment supply and physical processes (Georgiou et al., 2005). Because barrier islands migrate and deteriorate over time (e.g. McBride and Byrnes, 1997), restoration of these habitats will require recurring emplacement of sediment dredged mostly from borrow areas (e.g. Finkl et al., 2006; van Heerden and DeRouen, 1997). Such maintenance events would be the most cost-effective way to
ensure the longevity of the barrier islands.
LOUISIANA BARRIER ISLAND DEGRADATIONAL PROCESSES Barrier islands and abandoned deltaic headland shorelines are rapidly migrating landward and degrading (e.g., Penland et al. 1988; Williams et al. 1992). Processes associated with the transgressive phase of the delta cycle (Roberts, 1997) that include high rates of delta plain subsidence, a paucity of sand, and interior wetland loss, coupled with climatic factors such as eustatic sealevel rise and frequent large-magnitude storm impacts, collectively contribute to barrier degradation (Fearnley et al. 2009; FitzGerald et al. 2007; List et al. 1994, 1997; McBride et al. 1992; Miner et al. 2009; Penland and Ramsey 1990). A large deficit in the coastal sediment budget, high rates of relative sea-level rise (~9 mm/yr), and storm-induced current and wave erosion are forcing barrier shoreface retreat along the periphery of the Mississippi River delta plain. Additionally, conversion of interior wetlands to open water has increased the bay tidal prism, resulting in degradation of barrier islands due to inlet widening, formation of new inlets, and sediment sequestration at ebb-tidal deltas. As a result, the structural integrity and continuity of the barrier shoreline weakens as the barrier islands narrow, fragment and submerge. In the past 100 years, total barrier island subaerial area has declined 55% at an average rate of 63 ha/yr (McBride and Byrnes 1997). This deterioration will continue to destroy Louisiana’s barrier coastline until coastal restoration techniques are implemented that complement natural processes to restore and fortify the shoreline and restore coastal processes that promote sustainability. The seemingly disparate and disjointed units of barrier islands and headlands occurring in a chain from Raccoon Point to Sandy Point (west of the Mississippi River) and from Hewes Point to Curlew Island (East of the Mississippi River) are parts of a formerly contiguous barrier island system (Figure 1). Conversion of land to open water and consequential enlargement of bays and
Figure 1. Locations of barrier islands and barrier island systems in coastal Louisiana.
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Figure 2. Generalized design template of a barrier island along with key geomorphic features and construction elements. widening of passes have disrupted this system. Adequate sediment supply was the thread that held the chain of barrier islands together. Once the sediment supply was disrupted, the barrier island chain became increasingly degraded. Grouping these barrier islands, headlands, and sandy shorelines into coastal segments, for management purposes, will facilitate the development of a regional long-term strategy for shoreline maintenance, including project prioritization and development. It should be noted that any alteration to an area within a segment will affect the remainder of the segment due to coastal processes and morphodynamics, and, consequently, will be reflected by the sediment budget.
RESTORATION STRATEGIES FOR BARRIER ISLANDS The restoration of Louisiana’s barrier islands has been a priority for a number of programs over the past several decades. Beginning in the mid-1980’s, Terrebonne Parish completed a small restoration project on Trinity/East Island demonstrating the potential for successful barrier island restoration. In the 1990’s, barrier island restoration was a priority for the Coastal Wetlands Planning, Protection, and Restoration Act (CWPPRA) program, which has since constructed more than ten barrier island restoration projects. More recently, the Coastal Protection and Restoration Authority is planning to construct a large number of additional projects to restore barrier islands and headlands. The constructed projects have been studied and performance has been assessed to understand variances in resilience and persistence of these projects against the collection of stressors discussed above. For a sustainable restoration, it is imperative that these islands are considered as a system of islands rather than independent islands. No matter what strategy is proposed, it is imperative to consider the connectivity of adjacent islands, the availability and characteristics of sediment for construction, maintenance of the system, and the impacts of dredging sediment on the adjacent ecosystems. Strategies have been adopted to develop resilient design templates of barrier islands with an understanding of differences and contrast in geomorphological and geotechnical properties of Gulf and bayside, along with the importance of sediment management including sediment exploration (Delta Sand Search Model (DSSM: Finkl and Khalil, 2005)), sediment evaluation, sediment availability, sediment data management (which includes Louisiana Sand Resources Database (LASARD: Khalil et al., 2005)), Borrow Area Monitoring & Management (BAMM), and an overall Louisiana Sediment Management Plan (LASMP: Khalil et al., 2010). These strategies are discussed in more detail below.
Design Template of a Barrier Island
In general, a "soft engineering" approach is preferred over “hard structures”, though in a few cases hard structures (e.g. breakwaters in Raccoon Island Restoration Project and seawall in East Timablier Restoration Project) were used to address projectspecific issues (Knotts et al., 2006). The entire barrier island profile/cross-section is generally restored by constructing dune, berm, and beach on the Gulf side using compatible sand, and a marsh platform on the bayside using suitable sediment, to result in the appropriate desired elevation (Figure 2). In this context, there is an evolving trend towards using sandy/siltier sediment with less percentage of clay to build the back-barrier marsh. It is expected that ‘mixed sediment’ is suitable for stability of the island as a whole. The use of such sediment is better for predicting elevations and is helpful in overall constructability of marsh platform vis-avis vegetation. Several restoration projects (e.g. Whiskey Back barrier, and Pelican Island Restoration) were designed and constructed on this understanding. Also in these two cases, utilization of sediment resources was optimized by using the overburden (siltier sediment) for marsh platform creation and the underlying sand for beach and dune creation. However, there is no single standard template for construction of barrier islands in Louisiana. The crucial understanding in design of a barrier island is to replicate coastal geomorphs, which are reworked by coastal morphodynamics. The design concept has evolved by monitoring and evaluating project performances. The design of a barrier island involves an initial phase of data collection followed by design of project features (mainly beach, dune, and marsh platform). This design is mainly based on the prediction obtained from numerical modeling, lessons learned, and analyses of topographic, bathymetric, and geotechnical data (Knotts et al., 2006). Because most models were developed for sandy coasts, their application in mixed deltaic sediment is a major impediment in prediction of barrier island performances (Khalil et al., 2006). Various numerical models are being developed for appropriate prediction in a muddy deltaic environment (CPRA, 2012). Sediment budget, sediment suitability analysis, and overfill/cut to fill factors, are parameters related to coastal engineering applications that must be evaluated to accurately design a barrier island (Khalil et al., 2006). Sediment budget is an estimate of sediment transport in and out of a specific area. Its calculation depends on longshore and cross-shore transport, relative sea-level rise, overwash, and ebb-tidal and flood-tidal shoals. Topographicbathymetric surveys, aerial photography interpretation, and digitized shoreline locations also facilitate sediment budget computation. Sediment budgets are used in predicting the sediment movement around coastal structures or design templates. They are also used to predict erosion and estimate renourishment cycles.
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The morpho-sedimentary features (beach, berm, dune, and marsh platform) are further stabilized by installing sand fences and facilitating vegetative growth on dunes and in marshes (Khalil et al., 2006). Sand fencing /vegetative plantings are construction techniques used to help capture wind-blown sand after construction is complete. Khalil and Lee (2004) observed in Isle Dernieres that significant loss of sediment may occur prior to colonization and maturation of vegetation on the recently placed sediments and sand fences appear to have made substantial contributions to maintenance of sediments within other project areas during an important post-construction morphologic adjustment period. Sand fences not only help trap sand (which otherwise would have blown away) but also facilitates formation of much needed dunes (Khalil, 2008). Sand Fences are very inexpensive way of conserving sand post construction provided they are installed in appropriate manner within a few months after emplacement of sand.
Sand/Sediment and Sediment Management The most important factor affecting the long-term performance of the beach and dune portion of a barrier island restoration project is not only the grain size of the sand placed in the restored shoreline template but also a thorough understanding of sediment grain size and composition in the borrow area (Finkl and Khalil, 2005). This understanding of regional geological framework, along with the geological setting of barrier islands (geomorphology, coastal dynamics, coastal processes, etc.) is important in exploration for sand/sediment. Because barrier island restoration efforts depend on emplacement of sediment to build barrier and deltaic systems, qualitatively compatible and quantitatively adequate sediment/sand is the vital factor in any restoration effort. Almost 80-90 % of total cost of restorationbudget is allocated for exploration, exploitation, and emplacement of sand/sediment (Finkl and Khalil, 2005). This cost is directly proportional to the distance of the borrow sources from the project area. Therefore success of Louisiana restoration efforts depend on not only locating sufficient volumes of sand and mixed sediments that are suitable for placement on beaches, for building dunes, and for creating marshes but also managing it efficiently. In muddy deltaic environments of the Mississippi Delta Plain, sand deposits occur mainly as offshore sand shoals and buried paleo-channel deposits, though other types of sand deposits may also occur. Sand occurs as surficial sand deposits in shoals but normally these deposits are far from the barrier islands and their transport is expensive. Cost effective use of sand from more proximate, buried paleo-channel often includes using the overburden fine or mixed sediment for back barrier marsh restoration and sand for beach and dune restoration. A protocol (Finkl and Khalil, 2005; Khalil 2004, 2010) was developed to ensure that exploration of offshore sand and suitable sediment is undertaken efficiently and cost-effectively. The State of Louisiana has adopted aforementioned standard operating procedure/protocol for the exploration, characterization and identification of offshore and fluvial sand resources cost effectively in a systematic and phased manner for use in coastal restoration projects. This protocol has a phased approach which systematically utilizes geophysical surveys, geotechnical investigations, and cultural resources surveys to cost-effectively delineate a borrow site (Finkl et al., 1997) and facilitated the development of the Delta Sand Search Model (DSSM). An integral element of the sand search protocol and DSSM is the review of previous geoscientific/geological investigations. Numerous such investigations were undertaken in the past and are also conducted currently to generate a huge amount of
geoscientific data. In order to manage these data, the LouisianA SAnd Resource Database (LASARD) program was established on a GIS-platform to develop a spatial database of coastal and offshore geological, environmental, and associated data relevant to sediment resources that can be easily accessed (Khalil et al., 2005). The objective of LASARD is to centralize relevant data from various sources to promote better coordination of barrier island projects with available sediment sources. Relevant information consists predominantly of geophysical (seismic, sidescan sonar, magnetometer, and bathymetry) and geological (vibracore, jet probe, grab sample, isopach) data. Restoration of barrier islands is critically dependent on availability of suitable sand/sediment and hence it is imperative that a comprehensive Borrow Area Monitoring and Management (BAMM) plan is put in place to meet this challenge. The critical element of BAMM is improving our ability to link projects with the most appropriate borrow sources. There have been instances where, in the absence of any integrated approach to sediment management, the sand reserves were not fully utilized. Tools, such as LASARD and BAMM, are components of a comprehensive sediment management program that will optimize the utilization of available resources in the most cost-effective way. Sand and mixed sediment are needed for restoration of barrier islands, and in addition, for long-term sustainable restoration of barrier island systems, it is important to develop a sediment management plan. It is observed that for efficient planning and management of limited sediment deposits, a regional understanding of sediment/sand resources would be beneficial. This regional scale understanding would aid in protecting and preserving the sand deposit once explored/delineated. It would only be possible by coordination among various agencies that such potential locales are preserved and not traversed later by oil and gas pipelines or infrastructures. Basically, any sediment management plan in a given geological framework comprises sediment deposits and its inventory on regional scales and encompasses understanding of regional sediment budgets of the system along with records of dredging activities in the region (Khalil et al., 2010). A comprehensive sediment management plan provides a tool and an opportunity to proactively identify and minimize conflicting uses for sediment such that more sediment could be made available by proper management. Louisiana Sediment Management Plan (LASMP) is an overarching sediment management effort which encompasses various components of sediment evaluation (using DSSM), sediment data management (via LASARD), optimization of sediment from borrow areas (via BAMM) and protection and preservation of the delineated borrow area from oil and gas pipelines by coordination with various agencies.
Barrier Island Comprehensive Monitoring (BICM) Program A monitoring framework to assess shoreline processes and resulting habitats, and the changes in these ecosystems over time was developed. Initial goals of the BICM program were to establish baseline conditions for Louisiana’s barrier shoreline after hurricanes Katrina and Rita, as well as to refine the methods and products for use in restoration projects/programs. The advantage of a regional perspective provided by BICM over project-specific monitoring is the ability to provide integrated long-term data on all of Louisiana’s barrier shorelines, instead of just those areas with restored projects. As a result, a greater amount of long-term data became available to evaluate barrier islands, facilitate planning and design of future barrier island projects, assist operations and maintenance activities, and determine storm
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impacts. Because data were collected for the entire barrier island chain concurrently, BICM data are more consistent and complete than previous barrier island data collection efforts. Initial BICM datasets collected include (1) post-storm damage assessment photography and videography, (2) shoreline position, (3) land/water analysis, (4) topography, (5) bathymetry, (6) habitat composition, and (7) surficial sediment characteristics.
Barrier Island Maintenance Program (BIMP) In general, barrier island projects have a design life of twenty years; however, scheduled maintenance of these projects has not been incorporated into their funding or design. Design of these projects relies heavily on numerical models for predicting their longevity and ultimate success. Inherent in these models are certain assumptions and the realization that there are significant uncertainties about the physical processes that affect the stability of these geomorphic features. If the project is impacted by more high-energy events (hurricanes) than assumed in the model, the condition of the barrier island or headland deteriorates considerably, thereby reducing the life of the project. The project then requires maintenance to sustain the predicted design template. Restoration costs can increase exponentially when maintenance is not performed in a timely manner. A tipping point in barrier island restoration is breaching. Once breached, it takes a lot more sand, time, and money to restore the island. Therefore, it is important and cost effective to maintain and renourish before an island breaches. BIMP is a tool that can be used to formulate a much-needed component of maintenance planning for existing projects.
DISCUSSION Louisiana’s barrier islands could be considered the epitome of integrated coastal protection and restoration. In addition to the protective value that barrier islands provide for coastal communities as the first line of defense to storms, erosion, and marsh degradation, they also protect valuable oil and gas infrastructure, separate the estuaries from the open Gulf of Mexico, protect and maintain valuable nursery grounds for fisheries, and among other ecosystem services, these islands support habitats for threatened and endangered species, such as piping plovers and several species of sea turtles. These islands are unique, as several habitats are compressed in a relatively small area. Restoring and protecting these island systems, therefore, is a priority for Louisiana. Evaluation of the performance of various constructed barrier islands indicates that restoration efforts, during which sand was emplaced to construct beach and dune, and mixed sediment was emplaced to create the marsh platform, increase longevity of the barrier islands. Regular monitoring after high-energy events helps to identifying weak spots. Maintenance and repair of barrier islands in a timely manner is a cost effective proposition. Over the past 25 years, many barrier island restoration projects have been constructed in Louisiana. These projects have been constructed independently from each other and largely without consideration of synergies, however that is no longer the case. In recognizing the importance of collective synergies of chains of islands, and to maximize the efficient use of limited sediment and funding, Louisiana’s 2012 Coastal Master Plan (CPRA, 2012), which is the state’s roadmap for protecting and sustaining the coast, has presented future barrier projects as groups. In doing so, it promotes the understanding of sediment transport, of more efficiently utilizing limited sediment supplies, and of minimizing costs based on economies of scale. The 2012 Master Plan projects
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include (refer to Figure 1): Isles Dernieres Barrier Island Restoration (from Raccoon Island to Wine Island); Timbalier Islands Barrier Island Restoration (from Timbalier Island to Belle Pass); Belle Pass to Caminada Pass Barrier Island Restoration; and Barataria Pass to Sandy Point Barrier Island Restoration. In order to successfully achieve this systematic approach, it is important to efficiently manage resources. The systematic sediment investigations (i.e., DSSM) and data management (i.e., LASARD) protocols are essential tools to strategically identify and utilize available sediments while minimizing transportation and mobilization costs (i.e., LASMP). The development of systematic monitoring (i.e., BICM), and maintenance (i.e., BIMP) programs are critical to maintain barrier island integrity and ensure that storm damages and breaches are repaired. Louisiana has built the intellectual and institutional capacity, and has fostered the development of a competitive industry to evolve from barrier island restoration to the restoration of barrier island systems.
CONCLUSIONS Restoration of barrier islands is a complex process. The key lies in the understanding of the framework geology and the geological setting of these barrier islands in relation to the Mississippi River Deltaic Plain and then restoring them as a system. The other key factors involve appropriate understanding of various geological phenomena like subsidence, global sea-level rise, coastal hydrodynamic processes and coastal geomorphology. This understanding coupled with cost-effective management of sandsediment resources will help in restoring sustainable barrier island systems. For the last two decades, individual barrier island projects have been constructed and restored. It is realized that for a sustainable restoration the whole barrier island system needs to be restored. Grouping these apparently disparate and disjointed units of barrier islands and headlands into coastal segments will facilitate the development of a regional long-term strategy for shoreline maintenance, including project prioritization and development. It is appreciated that alteration to an area within a segment will affect the remainder of the segment due to coastal processes and morphodynamics and, consequently, the sediment budget. Sand deposits in Mississippi Delta Plain are scarce and limited and almost all the easily accessible deposits have been used. Presently, sand is been dredged from offshore shoals and transported almost 30-40 kilometers. The main cost of BI restoration is in dredging and transporting sediment. To make future barrier island restorations cost-effective, the best methodology would be to restore/repair several islands at the same time using one mobilization of heavy equipment. LITERATURE CITED Bedford, B.L. and Preston, E.M., 1988. Developing the scientific basis for assessing cumulative effects of wetland loss and degradation on landscape functions: Status, perspectives, and prospects. Environmental Management,12(5), 751-771. Boesch, D.; Shabman, L.; Antle, L.; Day, J.; Dean, R.; Galloway, G.; Groat, C.; Laska, S.; Luettich, R.; Mitsch, W.; Rabalais, N.; Reed, D.; Simenstad, C.; Streever, B.; Taylor, R.; Twilley, R.; Watson, C.; Wells, J.; and Whigham, D., 2006. A new framework for planning the future of coastal Louisiana after the hurricanes of 2005, University of Maryland Center for Environmental Science, MD, 48 pp.Coastal Protection and Restoration Authority (CPRA), 2012. Louisiana’s Comprehensive Master Plan for a Sustainable Coast. Coastal Protection and Restoration Authority of Louisiana. Baton Rouge, LA.
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