The Caernarvon Freshwater Diversion is located 15 miles downriver from New Orleans in Plaquemines Parish near the Plaquemines/St. Bernard Parish line and was constructed in 1991. The control structure was designed to divert up to 8,000 cfs or the volume of 2,352 large crawfish pots per second, from the Mississippi River into the local estuary. Big Mar Pond, a failed agricultural impoundment, is part of the receiving area for diverted waters.

Location of the Caernarvon Freshwater Diversion and Big Mar in the Pontchartrain Basin (left) and the diversion structure (right) which can divert up to 8,000 cubic feet per second down the Caernarvon conveyance canal and into Big Mar.

Why Study the Caernarvon Freshwater Diversion and Delta?

The Caernarvon Diversion has been in operation since 1991 and therefore provides an area to study the long-term impacts of freshwater diversions on the receiving basin. Sediment diversions are an integral part of the future coastal restoration plan for southeast Louisiana and studying the basin side effects of the Caernarvon Diversion can inform the construction and operation of these future diversions.  Although not directly comparable, aspects of the study of the Caernarvon Freshwater Diversion can be applied to sediment diversions. Studying this emerging delta also offers an opportunity to observe and study land building processes as the result of a diversion. A thriving delta with a restored swamp enhances hurricane storm surge protection to nearby levees and communities following the Multiple Lines of Defense Strategy.

Research Program

PC has been conducting regular measurements (more often during high water) of the turbidity (cloudiness of the water) entering the basin through the diversion. The turbidity measurements can be used to approximate the sediment load entering Big Mar and the rest of the receiving basin. PC has also been investigating the growth of a delta complex in Big Mar as a result of the diversion. We continue to measure the expansion and consolidation of this delta and are beginning to expand these investigations outside of Big Mar into the larger receiving basin.  On the growing delta we have conducted numerous tree plantings to re-establish a swamp plant community in the area. We have been keeping close tabs on what contributes to the success or failure of these plantings. We have acquired an extensive list of lessons learned from all of the plantings we have conducted. We also monitor the planted trees closely for survival and growth rates. We also investigate the extent of the influence of the Caernarvon Diversion in the Breton Sound Basin.

Land Change in the 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 divert freshwater and limit the capture of sediment. Despite this, new land is building. Since the construction of the diversion in 1991 sediment has been deposited in the Caernarvon receiving area. 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. Since 2004, wetland growth (or land gain) has been significant and appears to be accreting annually. Over 700 acres of land growth has occurred since the diversion came on line in 1991. From 2005 to 2012 there was 153 acres of land gain and the delta continues to expand. There was significant land gain from 2010 to 2011, most likely due to the fact tha the diversion was opened to maximum capacity to combat the BP Oil Spill for two months, during which time, three sediment spikes in the Mississippi River occurred.  Sediment spikes in the river cause an increased sediment load to be carried through the diversion, if it is open. In 2014 and 2015, long periods of low water in the summer in the basin led to an expansion of the delta as vegetation expanded. Over time we have seen a delta form with classic delta morphology of crevasse splays and distributary channels.  The delta has become vegetated quickly with marsh species and an extensive black willow forest has developed.

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

In 2015, we conducted a study transect and sediment study to in the Caernarvon receiving area to see how far the reach the Caeranrvon Freshwater Diversion into the basin was. This study included 6 transects with 128 sampling points were 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 depostition 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

Bald cypress regeneration often does not occur in southeast Louisiana because of nutria herbivory, high interstitial soil salinity, altered hydrological regimes and slow germination rates. Under existing conditions, without human intervention, bald cypress seedling mortality is quite high.

PC has conducted various swamp tree plantings in and around Big Mar since 2010 in partnership with the Coalition to Restore Coastal Louisiana (CRCL) and with the help of many dedicated community volunteers. Our volunteers help transport, space and plant the trees, as well as outfit them with nutria protectors. Nutria protectors are a critical component of tree survival as trees without protection are readily eaten or destroyed by the prolific rodent.

The main goals of this project were to plant wetland trees to abate and reduce storm surge and to monitor the growth of these trees under the influence of the diversion, at different distances to the diversion, in order to use this information to inform future restoration projects. The work being done around the Caernarvon Delta Complex is a unique opportunity to test the effectiveness of many proposed State Master Plan restoration initiatives, which rely heavily on river diversions. This project also 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.

PC chose the Caernarvon area for tree planting because the diversion created conditions that were conducive to swamp tree species growth by lowering the effects of high salinity and fertilizing the trees with river nutrients leading to higher growth rates. We have been solely responsible establishing and conducting the monitoring of the planted trees. Under this program, a percentage of all the trees that are planted are tracked over time for survival and growth rates. This allows for accurate estimations of success of the plantings over time. The monitoring includes scientific data collection and analysis. For each tree, the survival is assessed and diameter at breast height (DBH) and height are measured. 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 77%!

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).

Resources

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To obtain archived (no hyperlink) reports and graphs please email info@ScienceForOurCoast.org