Achieving long term sustainability and carbon sequestration in Louisiana coastal wetlands using dredged sediment

Summary

Data from the Louisiana Coastwide Reference Monitoring System (CRMS) is indicating that a robust supply of sediment is being delivered to the saline and brackish marshes of the Terrebonne, Barataria and Breton estuaries by tidal exchange.  A majority of the CRMS stations in the study area are measuring rates of sediment accretion that exceed the combined rates of subsidence and sea level rise, and 70% of the stations in the delta region are measuring positive elevation trajectories relative to a fixed rod at the site. 

A detailed examination of a stable marsh platform in the Barataria estuary indicates that the source of the sediment is a combination of edge erosion of the emergent marsh and subaqueous erosion of recently submerged sediment along the perimeter of the platform. CRMS imagery data indicates that the magnitude of loss by edge erosion is partially offset be deposition within and infilling of ponds in the interior platform. This configuration suggests the edge erosion could be partially mitigated and the depositional process could be supplemented by placing berms of dredged sediment along the edges of the marsh platform. The berms would wash away over time, but they would serve to protect the platform edge from wave erosion and a portion of the sediment would be carried onto the platform by tidal exchange. The process could be repeated after the berms wash away. A proposed source of sediment would be material dredged from the Mississippi River channel and transported to the project site by barge. This process has the capability to offer long-term sustainability of these marshes.

Long-term sustainability

It is a common misconception that the coastal wetlands of southeast Louisiana were created in a continuous progression of land-building by the Mississippi River.  In fact the processes of land-building in the delta plain have been highly cyclical, and a majority of the new land created by the Mississippi River delta over the past 6,000 years has submerged below the surface.  Nearly all of the first ten delta lobes identified by David Frazier (1967) are no longer evidenced at the surface.  Some are buried below younger delta lobes and some have converted to large bodies of open water like Atchafalaya Bay, Terrebonne Bay, Barataria Bay and Breton Sound.  Subsidence has been the persistent driver of change across the delta plain.  Each delta lobe has followed a progression of change called the “delta cycle”.  During the active delta phase abundant supplies of river-borne sediment build new land in open bodies of water (often left by the submergence of a previous delta).  Annual overbank flooding also delivers mineral sediment to existing marshes in the vicinity of the active delta lobe.  After the avulsion of the active channel to a new lobe the abandoned lobe may be completely cut off from the supply of river-borne sediment.  The largely freshwater marshes of the recently abandoned delta lobe begin a succession of change from fresh to intermediate to brackish to saline marsh.  The transportation of sediment in these ecosystems is largely driven by tidal exchange.  The distributary mouth bars that had been deposited by the active delta are rolled inland by the tides after abandonment.  These develop into emergent headlands and flanking barrier islands.  Some of the marshes behind the islands submerge into open water, but a significant portion of the marshes in an abandoned delta lobe maintain elevation above sea level through sediment accretion and organic growth.

Although marshes in abandoned delta lobes are constantly challenged by subsidence and erosion, some portions may remain emergent for long periods of time.  Emergent marshes are sustained where the combined rates of sediment accretion and organic growth are greater than the combined rates of subsidence and sea level rise.  The age of emergent marshes across the delta plain today can be roughly approximated by the last time period in which there was an active delta building new land in their immediate vicinity.  Beyond that point emergent marshes have been sustained by sediment delivered by some combination of overbank flooding from a river channel and tidal exchange.  During periods when the active delta was far from an area of emergent marsh, it is likely to have maintained elevation entirely from organic growth and mineral sediment carried in by the tides.  Many areas across southeast Louisiana appear to have been sustained in this manner for at least several centuries. 

Fisk, 1944 illustrated the relationships among sediment deposition by an active delta lobe, subsidence, ecosystem succession of the marshes, and vertical accretion of saline and brackish marshes by organic growth.  Although the accumulated peat deposits are highly organic, they often contain more than 50% mineral sediment.  When the abandoned river channels have submerged below the surface, the entire mineral component is delivered by tidal exchange.  Coastal marshes can be sustained for centuries by organic growth and mineral sediment delivered by tidal exchange.  The most effective way to continue long-term sustainability would be to supplement these natural delivery processes with dredged sediment

The construction of levees, locks, dams and river control structures over the past century has all but eliminated the supply of river-borne sediment being delivered to the coastal marshes.  The locks and dams across the drainage basin have cut the sediment load being carried by the river to a fraction of what it was in pre-industrial times.  Levees have prevented overbank flooding, and the small delta lobes building new land in Atchafalaya Bay are miniscule compared to what they would have been in an entirely natural system.  It is important to recognize that in an entirely natural system the river almost certainly would have made a complete avulsion to a course into Atchafalaya Bay.  The marshes of southeast Louisiana would have been cut off from river-borne sediment by this avulsion.  A portion of these marshes would have been lost to submergence in areas prone to high subsidence, and a smaller portion would have been sustained in tectonically more stable areas by natural accretion.  A natural system would have maintained “no net loss” of wetlands area across the delta plain because the rate of land-building at the new delta lobe would have offset the rate of land loss due to submergence in the marshes to the east.  The reality is that the marshes of southeast Louisiana are receiving no less sediment today than they would have in a natural system. In the absence of significant new land-building, the most effective way to sustain emergent marshes across the area will be to mimic and supplement the natural processes of sediment accretion.  The idea that the marshes in the eastern portion of the delta plain need to be sustained by “reconnecting the river to the marsh” is not supported by the science of delta evolution.  Forcibly diverting river water into saline and brackish marshes would be likely to do more harm than good, and would cause more wetlands loss due to erosion and flooding than would be created by sediment deposition.  A much more efficient and effective way to sustain these wetlands would be to supplement the natural tidal exchange processes with dredged sediment.

West Plaquemines Marsh Platform

An excellent area to examine these natural processes can be found in the marshes west of the Mississippi River in southern Plaquemines Parish.  A relatively stable area of saline and brackish marsh, referred to here as the West Plaquemines Marsh Platform, reveals the patterns of natural sediment accretion that hold the key to long-term sustainability.  These marshes were probably created about 500 to 600 hundred years ago in Frazier’s delta lobe #13.  They probably received some river-borne sediment during the time period that delta lobes #14 and #15 were active, and it is even possible that distributary channels from

all lobes remained open and received a portion of the flow of the river throughout this time period.  The area certainly received sediment from overbank flooding over the last 200 years when delta lobe #16 was active.  The construction of levees along the west bank of the river in the 1930s prevented any further overbank flooding, and the marsh platform has been sustained with tidal-borne sediment for most of the last century.  One of the keys to the sustainability of the West Plaquemines Marsh Platform is its tectonic stability.  The platform is bounded by the surface traces of faults along its south and west sides and by the natural levees of Mississippi River and Bayou Grand Chenier along its north and east sides. 

Marsh submergence due to fault movement

The faults along the south and west boundaries of the marsh platform connect the Lake Washington and Bay de Chene salt domes.  Together they are a part of the Terrebonne Linked Tectonic System (Causes and Effects of a Late 20th Century Fault Slip Event).  Elements of this tectonic system have moved interactively for at least the last 15 million years, and into the 20th century.  The most obvious impacts of movement on these faults have been the submergence of marshes along the hanging wall, or downthrown sides, of their surface traces.  The sharply defined edge of the marsh platform that coincides with the Adams Bay fault was first defined by Gagliano et al, 2003 using 2-D seismic and boring profiles (they called this fault the Empire fault).  A portion of the surface fault trace was subsequently delineated by Armstrong et al, 2013 using 3-D seismic data.  Couvillion et al, 2017 showed that the submergence of the hanging wall side of the Adams Bay fault occurred primarily between 1973 and 1985. 

The surface trace of the Barataria fault generally coincides with the southwestern flank of the marsh platform, but there is no sharply defined marsh edge along fault trace, nor is there any measurement of land area change in the 20th century.  It is likely that the submergence of the marshes of the Bayou des Families delta (Frazier lobe #7) took place in the centuries after the delta lobe was abandoned (Interactions of the Bayou des Families delta and the Barataria linked tectonic system) .  The progress of the building and submergence of the delta lobe was depicted by Flocks et al, 2006.  The surface trace of the Barataria fault has been superimposed on their illustrations here.  Based on the pattern of the recent submergence of marshes along the Adams Bay fault, it is reasonable to assume that the initial submergence of marshes along the Barataria fault took place between 2,000 and 1,000 years ago, after the abandonment of the delta lobe.  The initially sharp edge of the marsh platform along the fault trace has been subsequently deformed by an unequal advancement of edge erosion. 

Submerged marshes as a source of sediment

The submergence of marshes on the hanging walls of the faults that bound the West Plaquemines Marsh Platform introduced a significant amount of sediment into the subaqueous environment of the Barataria Basin.  Gagliano et al, 2003 estimated that 31.2 million cubic yards (23.9 million cubic meters) of marsh sediment was submerged along the hanging wall of the Adams Bay fault.  This mixture of mineral sediment and organic peats would have been subjected to erosion by waves and tidal currents.  Morton et al, 2009 documented erosion of a submerged marsh deposits below the surface of the water with a boring profile north of Fourchon. 

Re-depostion of submerged marsh sediment

Wilson and Allison, 2008 showed that a significant portion of sediment that was submerged and eroded in the marshes of Plaquemines Parish was re-deposited onto the adjacent marsh platform. The re-deposition of eroded sediment supplemented the vertical accretion of the marsh surface from organic growth.  Their figure depicts a natural berm of sediment deposited along the edge of the platform tapering toward the interior.  Sediment is also carried into the interior of the West Plaquemines Marsh Platform by tidal channels, which can form small deltas in the interior ponds.  Through these processes tectonically stable marsh platforms receive a robust supply of sediment in the absence of the delivery of river-borne sediment.  The most effective way to sustain these marshes would be to supplement the natural processes of sediment delivery by tidal exchange.

Footwall uplift

It may be possible that the bounding faults of the marsh platform caused a relative upward movement along their footwall, or upthrown sides, in addition to a downward movement of the hanging wall of their surface traces.  There is a small but growing body of evidence for “footwall uplift” on faults across south Louisiana, and the phenomenon may be an essential part of the formation of stable marsh platforms.  Apparent evidence for footwall uplift on the Lake Five fault on the western boundary of the West Plaquemines Marsh Platform can be seen in a LIDAR digital elevation model.  The model indicates that the elevations of marshes on the footwall side of the fault are 1.5 to 2.0 feet higher than on the hanging wall side.  Elevation differences may be due in part to higher rates of sediment accretion.  Data from CRMS station 0237 indicates an accretion rate of 16.6 mm/yr since 2017 and a rate of surface elevation change of 11.8 mm/yr.  Surface elevation change is measured relative to a fixed rod, and does not directly indicate that the marsh is gaining elevation relative to sea level.  If footwall uplift is playing a role in causing a higher elevation of the marsh surface along the footwall side of the fault, it may be because the relative upward movement is expressed as stability compared to the rate of subsidence on the hanging wall side.

Wilson and Allison, 2008 documented differential subsidence across marsh edges in Plaquemines Parish.  Differential subsidence along the edges of the West Plaquemines Marsh Platform may be the result of a combination of downward movement on the hanging wall and upward movement on the footwall of the faults.  This would create a stable marsh edge that would be optimum for the accretion of sediment carried by tidal exchange.

Marsh-edge sediment berms

The natural berm of sediment along the edges of the West Plaquemines Marsh Platform is clearly visible on LIDAR digital elevation models.  Rates of vertical sediment accretion have been measured across the West Plaquemines Marsh Platform by CRMS stations.  Accretion rates are generally greater toward the edges of the platform and decrease toward the interior.  One station (CRMS0174) is on the edge of the marsh adjacent to a recently submerged area.  It has recorded a long-term accretion rate of 76 mm/yr.  This provides a reasonable measure of sediment accretion rates in edge berms along the south and west flanks of the platform.  The point sources of accretion rate data provided by CRMS stations can be combined with the LIDAR digital elevation model to create an accretion model across the platform.  In this model sediment accretion rates are generally greater than 50 mm/yr along the sediment berms and taper toward the interior of the platform to an average of about 15 mm/yr.  This model can be used to calculate that approximately 6 million cubic meters of sediment accretion occurs on the marsh platform every year.

Berm migration and edge erosion

During the time span over which CRMS station 0174 measured a long-term sediment accretion rate of 76 mm/yr the sediment berm can be seen to be migrating across the site.  Berms along the edges of the West Plaquemines Marsh Platform are all migrating inland, and the migration is accompanied by erosion of the marsh edge.  Sapkota & White, 2019 measured rates of edge erosion at three sites along the southern boundary of the platform.  These rates of edge erosion ranged from 142 to 242 cm/yr.  They also estimated the volume of marsh lost to edge erosion annually to be about 1.8 cubic meters of marsh material per meter of length of marsh edge.  This is consistent with the value of 1.7 cubic meters of volume per meter of length measured by Wilson and Allison, 2008.  The total length of the southern edge of the West Plaquemines

Marsh Platform represented by the green dashed line is about 8800 meters.  This means that the total volume of marsh lost annually to edge erosion is about 15,800 cubic meters.  This volume is a small fraction of the estimated 6 million cubic meters of sediment accretion occurring on the platform. This indicates that majority of the sediment being transported onto the platform by tidal exchange is sediment being eroded from the bottom of the open bodies of water along the platform.

Sediment deposition and carbon sequestration

One of the net results of the transportation and accretion of sediment onto the marsh platform is that the loss of wetlands to edge erosion is partially offset by the infilling of interior ponds by sediment deposition.  This can be seen in a comparison of the land-water area estimates for CRMS site 0173 for 2005 and 2016.  Land-water measurements are made for a one-square kilometer area around each CRMS station using the same evaluation techniques used to estimate total land area change in Couvillion et al, 2017.  The visual representations for land and water clearly show that the size and number of interior ponds diminished over the 11-year time span.  The supply of tidal-borne mineral sediment carried onto the marsh platform is accompanied by organic growth.  Fisk, 1944 illustrated that as sediment accretion and organic growth keep up with subsidence and sea level rise, a significant volume of organic material is buried in the process.  Organic peat layers up to 20 feet thick have been measured at various sites across the delta plain.  Baustian et al, 2021 examined the significant role of coastal wetlands in “producing, accumulating and storing organic carbon” and “their potential to sequester carbon and influence greenhouse gas emissions, climate change, and the ‘blue carbon’ economy.”  They used cores taken at CRMS sites across the Terrebonne and Barataria Basins to estimate rates of carbon sequestration including three sites on the West Plaquemines Marsh Platform.  An average annual rate of carbon accumulation based on two evaluation methods is shown in the figure.  These values indicate an average annual rate of carbon sequestration for the West Plaquemines Marsh Platform of about 300 grams of total carbon per square meter.  Using a reasonable estimate of 300 million square meters for the size of the platform, it can be estimated that it is sequestering carbon at the rate of about 90,000 metric tons per year. Baustian et al,

2021 used an estimate of 15 square kilometers of total wetlands area for coastal Louisiana to calculate a total rate of carbon sequestration of about 4.3 million metric tons per year.  The carbon sequestration capacity of the West Plaquemines Marsh Platform represents about 2% of this total.  An effective plan to sustain this platform for the next 25 years would allow for the sequestration of about 2.25 million metric tons of carbon.  The most effective sustainability plan should strive to mimic and supplement the natural processes of sediment accretion that have been occurring on the marsh platform over the past century.

Sustaining the West Plaquemines Marsh Platform with Dredged Sediment

The wetlands of the West Plaquemines Marsh Platform have been sustained since their creation by a combination of the deposition of mineral sediment and organic growth.  Measurements at CRMS stations over the past two decades indicate that vertical accretion rates are greater than the combined rates of subsidence and sea level rise for this area.  The distribution pattern of these accretion rates indicate that sediment is being carried onto the marsh platform by the tides.  Edge erosion appears to be the primary cause of wetlands loss in the area, and some portion of the marsh material lost to edge erosion is re-deposited onto the platform.  The total volume of sediment that could be provided by edge erosion cannot account for the volume of sediment that is accumulating on the platform.  The primary source of sediment must be that which is being eroded from the bottoms of the surrounding bodies of water.

Sediment carried on the marsh platform by tidal flux creates a natural berm that migrates inland over time as edge erosion advances.  The most effective way to sustain the marsh platform would be to introduce new sediment into the system in a manner that most closely mimics the natural processes.  Zapp & Mariotti, 2021 proposed a combination of thin-layer deposition through “spray dredging” and the deposition of dredged material at the mouths of tidal channels.  The principles of the latter approach are that the sediment would be carried into the marsh platform by the tidal channels and supplement the natural deposition in small tidal deltas in interior ponds.  Another approach would be to create an artificial berm of dredged sediment immediately adjacent to the natural berm.  The focus of edge erosion would be sifted outward to the edge of the artificial berm.  This would supplement the natural supply of sediment coming from the bottoms of the water bodies and mitigate losses due to erosion of the natural edge of the marsh platform.

Insights into how artificial sediment berms would work were provided by the construction of berms in the immediate aftermath of Deepwater Horizon oil spill in 2010.  The intention of these berms was to block the flow of oil into the wetlands.  It is unclear to what degree these berms achieved their intended effect, but they provide clear insights into how similar berms constructed along the edges of marsh platforms could provide sediment to the marsh.  A before and after comparison of the berm constructed on Pelican Island in southern Plaquemines Parish shows that by the end 2010 a majority of the sediment from the berm had been carried inland resulting in sediment plumes extending to the north.  A berm constructed on the beach face of a barrier island would be subjected to much higher levels of wave and tidal energy than one constructed on the edge of a marsh platform, but the inland migration of sediment should be expected in both cases.  It should be reasonable to expect that a sediment berm along a marsh platform edge would wash away more slowly, perhaps over a period of several years.  Replacing eroded berms on a regular basis would provide a continual supply of sediment to the marsh, and may even result in an increase of total wetlands area.

Sources of Dredged Sediment

Navigational waterways across the Louisiana coast are routinely dredged, and the sediment is deposited in beneficial use sites.  The primary objective of these sites is ease of disposal of the dredged material, as they are generally immediately adjacent to the dredged channel.  Little thought has been given to how the strategic placement of the dredged sediment may improve their beneficial use.  The introduction of a transport component to the beneficial use program could allow for the sediment to be used in the construction of channel seeding deposits and artificial berms along the edges of marsh platforms. 

In 2020 funding was approved for the Mississippi River Ship Channel deepening project.  The objective of the project is to deepen the channel to 50 feet to allow for new Panamax ships to reach ports on the river.  Sediment dredged at the deep draft crossings between Baton Rouge and New Orleans will not be captured for beneficial use in this project.  Instead, dredging will consist of lowering the elevation of the channel bars at the crossings, and releasing the sediment into the current.  Sediment released into the current will eventually settle out, probably on the next bar down river.  It is likely that maintaining the channel depth by this means will require repeated dredging operations.  Over the long term it will be more effective and efficient to capture and transport dredged sediment out of the river.  The transportation component of this operation could be extended to carry the sediment to strategic sites in the coastal marshes to be used in the construction of sediment berms along the edges of marsh platforms. 

The technology to capture and transport dredged sediment from the Mississippi River is not currently available, but it is certainly within the realm of our capabilities.  With a little creative thinking the process of transporting sediment from the bottom of the Mississippi River channel to marshes of coastal Louisiana could make their long-term sustainability a reality.

REFERENCES

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Baustian, M.M., Stagg, C.L., Perry, C.L., Moss, L.C., and Carruthers, T.J.B., 2021, Long-Term Carbon Sinks in Marsh Soils of Coastal Louisiana are at Risk to Wetland Loss, Journal of Geophysical Research: Biogeosciences, 126, e2020JG005832. https://doi.org/10.1029/2020JG005832

Fisk, H.N., 1944, Geological Investigation of the Alluvial Valley of the Lower Mississippi River, Mississippi River Commission.

Flocks, J.G., Ferina, N.F., Dreher, C., Kindinger, J.L., Fitzgerald, D.M, Kulp, M.A., 2006, High-Resolution Stratigraphy of a Mississippi Subdelta-Lobe Progradation in the Barataria Bight, North-Central Gulf of Mexico, Journal of Sedimentary Research, v. 76, p. 429-443

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Inman, B., 2021, Status of Mississippi River Ship Channel Deepening Project and Beneficial Use of Dredged Materials, CPRA Board Meeting presentation, June 16, 2021

Morton, R.A., Bernier, J.C., and Kelso, K.W., 2009, Recent subsidence and erosion at diverse wetland sites in the southeastern Mississippi delta plain: U.S. Geological Survey Open-File Report, 2009–1158, 39 p., plus app. (p. 41-221).

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Zapp, S.M. and Mariotti, G., 2021, Evaluating direct and strategic placement of dredged material for marsh restoration through model simulations, Shore & Beach, v. 89 No. 4, p. 33-39