Land area gains in the Louisiana wetlands
Several stations in the Coastwide Reference Monitoring System (CRMS) have measured net gains in land area in a 1-square kilometer area around the station over the past two decades. These same stations have also measured significant rates of sediment accretion and positive surface elevation change as well.
Four example stations are represented here. The location of each is shown with respect to the USGS Land Area Change Map (Scientific Investigations Map 3381, Couvillion et al, 2017). Each of these stations is in a belt the runs just north of a belt of land loss hot spots discussed in the last blog. It appears likely that the land area gains measured at these stations is due in part to a redistribution of sediment that was lost to submergence in the belt to the south.
The values of sediment accretion rates and surface elevation change rates recorded at these four CRMS stations are higher than both the rates of subsidence and sea level rise for coastal Louisiana. A majority of the nearly 400 CRMS stations across the coast have measured rates of accretion and surface elevation change greater than either subsidence or sea level rise. New wetlands area appear to be created in many of these naturally accreting areas. In other areas accretion is helping to mitigate the rate of wetlands loss.
CRMS was establish under the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA), which passed in 1990. Significant data from CRMS has accumulated within the past two decades, and is now allowing for detailed evaluation of accretion and other important processes. A typical CRMS marsh location includes elevated boardwalks in an H-shaped configuration. Accretion data is measured by a set of procedures in which a white feldspar powder is spread on the surface of the marsh at designated locations around the boardwalks. At regular intervals sediment samples are recovered by inserting a tipped slim copper tube into the soil column and filling it with liquid nitrogen. This process freezes the sediment around the tube and allows for the extraction of an undisturbed core-like sample. The surface created by the feldspar powder is obvious as a white layer within the core. The thickness of sediment accreted at the site can be measured above the white layer. Over time multiple feldspar markers at multiple locations around each site provide enough data to allow for accurate estimates of long and short term accretion rates through statistical analysis. The figure below illustrates the essential elements of accretion measurement process and a typical graphical representation of the data from one site.
The methodology for measuring surface elevation change was established by Cahoon et al, 2000. This methodology has been employed by CRMS to record data over the past two decades. As shown in figure below, a rod surface elevation table (R-SET) is established by driving the rod to refusal and surveying its elevation. A set of fiberglass rods are suspended from an arm with is perpendicular to the rod. This allows for the direct measurement of the elevation of the surface of the marsh relative to the R-SET. Changes in surface elevation are not measurements of positive or negative velocities relative to a fixed datum such as sea level, but only to the surveyed elevation of the R-SET. In order to relate changes in surface elevation measured at a CRMS site to subsidence the site would have to be surveyed on a repeated basis to determine changes in elevation relative to the fixed datum, or the site would have to be equipped with a GPS device.
An evaluation of CRMS surface elevation change data was presented by Leigh Anne Sharp and Camille Stagg at the 2018 State of the Coast Conference. They found “… 332 [CRMS sites] have surface elevation data that can be assessed along with land change data to infer which processes are influencing recent land change … In the Deltaic Plain land loss, in general, is not associated with elevation loss, suggesting that erosion, not subsidence, may be responsible for continuing land loss there. Over the last decade, wetland surface elevation trajectories have been positive at 75% of sites, including those in coastal areas near the Mississippi River delta that have seen the most historic land loss. Land change data reveal continuing land loss at just 14% of CRMS sites and land gain at 10% of CRMS sites including those in the birdsfoot delta and in the upper Mermentau basin. Furthermore, at least eight sites that were classified as floating marsh at project inception are now attached, including sites downstream from diversions and near the Atchafalaya delta. In general, CRMS data reveal that the coast of Louisiana is dynamic, that much of it has been stable in recent years, and that restoration efforts in concert with natural processes have increased stability in some areas”.
The belt of the wetlands in which gains in land area have been measured follows the trend of what Shea Penland called the “Teche Shoreline”. This boundary is a near perfect coincidence of ecological, geological and soil composition boundaries. Geologically, the Teche Shoreline coincides with a line of faults across the marsh surface. These faults appear to have played a significant role in causing the wetlands loss event described in the previous blog. Hot spots of wetlands loss from this event line up along the hanging wall or downthrown side of the faults. Ecologically, the Teche Shoreline coincides with a sharp transition from freshwater marsh to intermediate, brackish and saline marshes. The succession of marsh type over time appears to be associated with subsidence. The coincidence of the faults and the ecosystem boundary suggests that subsidence caused by fault slip is driving the succession of ecosystems as the marshes are progressively submerged in more saline waters. The Teche Shoreline also coincides with a distinct boundary in composition of the marsh soils. Marshes to the north of the boundary generally have organic contents greater than 90%, while those to the south have lower organic compositions. It appears likely that long term accretion to the north of the boundary has promoted the retention of organic content over many centuries. To the south of the boundary marshes have been repeatedly submerged and removed by erosion, then rebuilt by new deltas. This cyclical pattern of marsh creation and submergence south of the Teche Shoreline has resulted in a lower organic content relative to the more stable area to the north.
The CRMS data indicates that areas north of the Teche Shoreline are naturally building new wetlands area in the face of subsidence and sea level rise. It seems logical that the sediment source with which new land area is being created is the same marsh material that was lost to submergence and erosion in the last wetlands loss event. This sediment is naturally carried to the north over time by tidal flux. The rates of sediment accretion and surface elevation change associated with land area gains suggests that these marshes can continue to maintain elevation under future scenarios of subsidence and sea level rise for many decades to come.
