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Central Wetlands

Location

The Central Wetlands are a 29,140-acre semi-impounded wetland, located in Orleans and St. Bernard Parishes, that is bordered by the Mississippi River Gulf Outlet (MRGO) and the Hurricane and Storm Damage Risk Reduction System (HSDRRS) Levee to the east and the 40 Arpent Levee and Canal to the west.

Location of the Central Wetlands in the Pontchartrain Basin. The wetlands are surrounded by impoundments including the MRGO, 40 Arpent Levee and the newly built hurricane protection system around New Orleans.

History

Historically, prior to European settlement, the Central Wetlands was made up of a combination of bald cypress (Taxodium distichum) and water tupelo (Nyssa aquatica) swamp, fresh marsh and bottomland hardwood forest. This provided natural storm surge protection for the people of New Orleans.  The Central Wetlands have been subjected to many anthropogenic and natural impacts over time including, leveeing of the Mississippi, construction of oil, gas, and navigation canals, construction of the MRGO, the construction of the levees around the Central Wetlands and logging. Currently, the Central Wetlands are a mosaic of wetland habitats that range from relatively healthy with stable soil and dense vegetation to open water and ghost swamps. They are subject to influences inside and outside the area that can have an effect on hydrology and salinity dynamics.

 

 

 

The Central Wetlands are made up of a mosaic of habitats of varying ranges of health. There area areas of solid and healthy brackish marsh, marsh that is breaking up, cypress tree growth, ghost swamp with dead cypress trees and open water. Because of the varying habitats and health, restoration solutions do not apply universally across the area.


 

The Central Wetlands are affected by man-made forces. Logging in the earl 1900’s removed swamp and left behind logging ditches scaring the landscape (left). Indicators of the dynamic salinity history can be seen with dead oysters (salty species) are present on a dead cypress stump (fresh species) (right).


Why Study the Central Wetlands?

There has been great interest and much conversation about the restoration and effective management of the Central Wetlands since Hurricane Katrina devastated much of the Ninth Ward community and surrounding area. The closure of the MRGO was a solid first step towards hydrologic restoration. Currently, there are several restoration plans and pilot projects in consideration that would impact the Central Wetlands and attempt to restore some of the natural habitat while strengthening ecosystem services. These plans and proposals include, but are not limited to: marsh creation, cypress‐tupelo and marsh plantings, and wastewater assimilation. The success of planting projects depends largely on the suitability of the planting location, namely soil salinity and hydrological conditions, to the vegetation being planted. An in-depth understanding of the environmental parameters found across the Central Wetlands will allow for more informed planning of restoration activities, which will increase the success and effectiveness of those restoration activities and their impact on the surrounding communities.

Monitoring and Research in the Central Wetlands

LPBF studied the Central Wetlands by collecting bathymetric, surface salinity, soil salinity and vegetation data. Bathymetric surveys were conducted in the Bayou Bienvenue Triangle, Bayou Bienvenue, the Back Levee Canal, the Violet Canal and Bayou Dupre in the spring of 2013. Surface water salinity was measured in canals periodically throughout the Central Wetlands from July 2012 through April 2014. Interstitial or porewater salinity (also referred to as soil salinity) is one of the major limiting factors of plant community composition in wetlands. Further, the spoil banks associated with the construction and maintenance of canals impound wetlands and hinder sheet flow, in effect dissecting the Central Wetlands into smaller, disconnected sub‐units. Spoil banks cause impoundments to remain flooded and stagnant, which affects seed germination, while disallowing water exchange that would naturally freshen an area impacted by salt water intrusion from storm surges. Soil salinity was collected on a biannual basis, beginning January of 2013 (winter 2012/2013) through August 2014 (summer 2014). Vegetation data was collected at each sampling point concurrently with data collection for porewater soil salinity. Total cover of vegetative species and cover of individual species was determined in randomly placed 1‐meter square plots at each sampling site.

Results

Detailed analysis, figures and conclusions for the Central Wetlands study can be found in our report entitled: Recommendations for restoration: Central Wetlands Unit, Louisiana.

The bathymetry data showed that bottom elevations ranged from approximately -1 ft. to -20 ft across the Central Wetlands. Most of the Bayou Bienvenue Triangle area had elevations of ‐3 ft. or less.

Generally, the surface water salinity was similar across the Central Wetlands with higher salinities in the late summer to early winter and lower salinities in the spring. Therefore, the waterways in the Central Wetlands are all connected but are not connected to the impounded wetlands due to the presence of spoil banks, which prevent exchange with the main waterways. For the duration of this project, mean surface water salinity was consistently lower than mean soil salinity (at soil salinity sample sites) by 2.0-3.7 ppt.

Fresher soil salinity sites tended to cluster together on the western edge of the Central Wetlands, near the Violet Siphon and storm water pump stations. Saline soil sites tended to cluster on the eastern edge of the Central Wetlands near the Bayou Dupre sector gate. A temporal trend of overall freshening soil salinity was not observed throughout the duration of this study.

The vegetation sampling identified 20 species over the entire survey period representing eleven plant families. The dominant species in all surveys and across most sites was Spartina patens. Other common species that were found in all surveys and at many sites were Spartina alterniflora and Distichlis spicata.

Restoration Recommendations

Based on the data collected, LPBF developed a series of recommendations for restoration in the CWU. Recommended restoration activities in the Central Wetlands include hydrologic restoration, swamp reforestation, marsh creation, submerged aquatic vegetation habitat and continued monitoring. In some areas, a phased approach to restoration is recommended. Hydrologic restoration should be conducted first and time allowed to observe if increase in tidal exchange to some impounded areas allow for the marsh to naturally recover. In areas where this is not sufficient to result in a healthy marsh community, further action may be needed. Detailed maps and descriptions of the recommendations can be found in our report: Recommendations for restoration: Central Wetlands Unit, Louisiana.

Restoration recommendations for the Central Wetlands. No restoration strategy can be applied across the entire area since it is cut up by numerous canals that created impoundments with differing hydrology.

Conclusions

This two-year study of the Central Wetlands Unit was conducted in order to ascertain the health of the region and what kinds of restoration are needed, feasible and realistic. Due to the many impoundments and hydrologic influences, it was determined that the CWU cannot be thought of as one large area under the same influences, and therefore restoration solutions do not apply universally across the entire area. Lastly, the restoration of the entire CWU should not be considered the highest priority in the Pontchartrain Basin. While providing some flood water storage capacity during heavy rains, the CWU is located entirely inside the levee system and therefore provides limited enhanced storm surge protection during hurricanes. There is much restoration needed outside of the levee system that would enhance storm surge buffering and provide additional protection to local communities and the Greater New Orleans area.

Resources

Soil and surface salinity: Winter 2013
Recommendations for restoration, Central Wetlands, 2015