Located 15 miles downriver from New Orleans in Plaquemines Parish near the Plaquemines/St. Bernard Parish line is the Caernarvon Freshwater Diversion. Constructed in 1991, the diversion was designed to divert up to 8,000 cubic feet per second, approximately the volume of 2,353 large crawfish pots each second. It's intended purpose was to move fresh river water from the Mississippi into the local estuary, limiting the capture of sediment. Despite this, new land is building in the diversion's main catch basin, Big Mar Pond.
The Caernarvon Freshwater Diversion has been in operation since 1991 and therefore provides an opportunity to examine the long-term impacts of freshwater diversions on the receiving basin.
Although the diversion was not meant to transport sediment, the subsequent growth of land as a result of its opening resembles the activity of a sediment diversion. For that reason, understanding the effects of the Caernarvon Diversion on the surrounding basin may better inform the construction and operation of future diversions; an integral part of coastal restoration plans for the Southeast Louisiana.
Investigating this emerging delta also offers an opportunity to observe and analyze land building processes as the result of a diversion. It is important to remember that a thriving delta with a restored swamp enhances hurricane storm surge protection to nearby levees and communities (see Multiple Lines of Defense Strategy.)
Because the Caernarvon Freshwater Diversion deposits sediment in spite of its design, we want to know how much sediment is in the water it's moving into Big Mar. We have been conducting regular measurements (more often during high water) of the turbidity entering the basin, which can be used to approximate the sediment load flowing through the diversion.
Turbid, or murky, waters containing sediment have been depositing and developing into a delta complex within Big Mar. With the hope of better understanding the effect of future sediment diversions, we track the expansion and consolidation of this delta. We are beginning to expand these investigations outside of Big Mar and into the larger receiving basin, including any influence the diversion may have on the Breton Sound Basin.
To us, new land means more barriers between coastal communities and storm surge! And we can help ensure the longevity and stability of newly formed land by restoring the healthy swamp that once thrived there. That in itself is an intricate task, but luckily for us, the forming delta provides an outdoor laboratory, perfect for identifying best practices for swamp regeneration.
We plant species native to swamp environments, and continue to keep tabs on them in the following years, monitoring closely for survival and growth rates to inform future restoration projects. Since 2010, we have planted 25,594 trees and have restored more than 130 acres within Caernarvon receiving basin.
Land Change in Big Mar
In southern Louisiana, land loss rates are high and areas of delta growth have generally been limited to the Atchafalaya Delta and the Bird’s Foot Delta. The Caernarvon Diversion was built to regulate surrounding salinity levels with fresh water to improve fishery productivity in the Breton Basin. Doing so required moving river water from the Mississippi into the local estuary. Because the original intention of the diversion was to regulate salinity, it was placed at an area of the Mississippi River unlikely to be laden with sediments and constructed to avoid sediment capture by drawing water off the surface of the river.
Despite this, new land is building. Since the construction of the diversion in 1991, sediment has been deposited in Big Mar Pond and beyond. Over time, there has been enough accumulation in some areas to permanently support emergent wetland plant life. In Big Mar Pond, prior to 2004, wetland growth was negligible, but since then wetland growth (or land gain) has been significant and appears to be increasing annually. Over 700 acres of land growth has occurred since the diversion came on line in 1991!
Here's a breakdown: From 2005 to 2012, there was 153 acres of land gain. From 2010 to 2011, there was significant wetland growth most likely built during the two months that the Caernarvon Diversion was open at maximum capacity (8,000 cfs) to combat the 2010 BP Oil Spill. Coincidentally, while open at maximum capacity, there were also three sediment spikes in the river that feeds the diversion allowing for more sediment deposition than expected. In 2014 and 2015, long periods of low water in the basin during the summer led to delta growth as vegetation expanded.
Over time we have seen a delta form with classic delta morphology of crevasse splays and distributary channels. The delta has quickly become vegetated with fresh marsh species and an extensive black willow forest.
The changing landscape around the Caernarvon Freshwater Diversion from 1998 to 2015. Land gain was not visible until 2005. Prior to that Big Mar was filling in with sediment and a threshold was finally reached where land emerged above water and became vegetated. Also noticeable is the substantial land loss that occurred between 2004 and 2005 due to Hurricane Katrina. The delta continues to expand over time.
Caernarvon Freshwater Diversion Outside of Big Mar
As the sediment continues to flow from the freshwater diversion, we are interested in identifying the boundary of its influence on the surrounding area. We made an effort to uncover the extent of impact in 2015, when we conducted a transect and sediment study in the Caernarvon receiving area. Our goal was to find out how far the influence of the Caernarvon Freshwater Diversion reached into the surrounding basin.
This study included 6 transects with 128 sampling points, where elevation was measured with a high precision GPS. At 50 of the sites, soil cores were collected to test for bulk density (soil compaction) and organic matter content. In general, soils were more compact and less organic closer to the diversion, which would be expected since the sediment coming from the river is mineral rather than organic. The highest organic matter content was in hurricane shears created during Hurricane Katrina.
Many of the cores showed a grey mineral layer over a black organic layer, indicating deposition from the diversion on top of organic soils that were in place before the diversion came online.
Soil core from Caernarvon region showing 20 cm (7.8 in) of deposition from the diversion (left), coastal scientists collecting high precision elevation data and a core (middle), soil core from the Caernarvon region that is all organic material with no deposition, indicating that it was outside the area of influence of the diversion (right).
Swamp Restoration in the Caernarvon Delta
For all of us in south Louisiana, a healthy swamp plant community provides an additional defense against storm surge. Unfortunately, swamp regeneration often does not occur naturally in southeast Louisiana because of nutria herbivory, high interstitial soil salinity, altered hydrological regimes and slow germination rates. Under existing conditions, without human intervention, natural swamp regeneration is not easily possible.
However, we believe that south Louisiana would greatly benefit from a rebuilt swamp land. So, after monitoring soil salinity, marsh density, and other factors that affect swamp species, we identified several sites within the Caernarvon Diversion receiving basin that would be suitable for a swamp intervention. PC chose the Caernarvon area for swamp regeneration because the diversion created conditions that were conducive to swamp species growth by creating fresh habitat and possibly fertilizing the trees with river nutrients.
Since then we've conducted large scale swamp species plantings with our partners Coalition to Restore Coastal Louisiana (CRCL), Restore the Earth Foundation (REF), and with the help of many dedicated community volunteers. Our plantings have been on a roll since 2010 and to this date we have planted 25,594 trees and restored more than 130 acres! Together we transport, space and plant the swamp trees, as well as outfit them with nutria protectors. Nutria protectors are a critical component of tree survival; without them newly planted trees are readily eaten or destroyed by the prolific rodent.
In 2013, to estimate the success of our efforts, we instituted a monitoring program that tracks the growth and survival of a portion of the many trees we’ve helped to plant. Revisiting the swamp restoration sites every year, we collect data on survival as well as measure height and diameter (at breast height) of each tree in our monitoring program. Afterward, the data is analyzed to produce growth rates for trees by planting (over time), by species, and by location. The data provided from the monitoring program has proven to be invaluable for showing success of this project and providing lessons learned for future plantings to improve success. To date, that data shows an overall survival rate of 80%!
The main goal of this project is to restore the natural swamp forest of the Caernarvon area, and in doing so buffer existing levees and reduce the risk of storm surge for the surrounding coastal communities. Besides the tangible progress this project has made, the work being done around the Caernarvon Delta Complex provides a unique opportunity to test the effectiveness of many proposed State Master Plan restoration initiatives, which rely heavily on river diversions. This project shows what is possible in the footprint of future sediment diversions.
Map showing the location of the 26,000 swamp trees planted through 2016 in the Caernarvon Delta Complex. The trees are planted in front of the newly built federal levee system (HSDRRS) and local Plaquemines Parish levee. The goal is to protect our levees which protect our people.
Volunteers having fun in the mud (top left), tags used to identify individual trees in the monitoring program (top right), nutria protector being installed by volunteers (bottom left) and a site just after planting showing all the trees with their protectors (bottom right).
Caernarvon Delta and Swamp Restoration
Caernarvon Delta Report 2011
Caernarvon Delta Planting July 2012
Caernarvon Delta Planting Dec 2012
Caernarvon Delta Planting June 2013
Caernarvon Delta & Diversion Study April 2014
Caernarvon Delta Planting June 2016
Caernarvon Turbidity Sampling Dec 2009-Oct 2010
Caernarvon Turbidity Sampling Feb 2011-Dec 2011
Caernarvon Turbidity Dec 2011-2012
Caernarvon Turbidity 2013
Caernarvon Turbidity 2014
Caernarvon Turbidity 2015