PINPOINTING THE SOURCES OF VARIABILITY IN SURFACE ELEVATION CHANGE IN RAPIDLY DEGRADING WETLANDS OF COASTAL LOUISIANA

High rates of subsidence and relative sea-level rise (RSLR) in coastal Louisiana drive severe land loss in the region. Subsidence rates are primarily associated with the compaction of Holocene deposits, and estimates of shallow subsidence (SS) are highly variable in space. These estimates are derived from average rates of measured surface elevation change (SEC) and vertical accretion (VA) at hundreds of stations monitored by the Coastwide Reference Monitoring System (CRMS) program. For the coming decades, SS estimates are foundational for (a) forecasting the best-, intermediate-, and worst-case scenarios of coastal land loss in Louisiana, and (b) informing planned intervention efforts to rebuild the coast. However, we observe significant temporal variability in SEC measurements across CRMS stations; this variability is likely to contribute to accrued error in forecasts of SS rates and coastal restoration outcomes. In this work, we investigate the causes of the variability in SEC measurements. The success of wetland restoration projects hinges upon robust estimates of (a) surface elevation change, (b) potential error associated with these estimates, and (c) the processes responsible for introducing the observed variability.

We use seasonally-recorded SEC data throughout the Mississippi River Delta and the adjacent Chenier Plain. At each station, we quantify best-fit SEC rates and characterize the associated residuals in space. We separate stations based on statistically characterized SEC residuals and potential sources of variability, e.g., elevation change rates, marsh type, storm surge, and water levels. The median of the 25^th and 75^th percentiles of residuals measured at all stations (n=301) is -/+ 0.7 mm; the median of the 10^th and 90^th percentiles of residuals measured at all stations is -/+ 1.3 mm. These residual values represent a significant fraction of the projected average annual increase in global sea level (~4 mm) and local sea level (12 -/+ 8mm). Characterizing factors that introduce spatial and temporal variability in our estimates of surface elevation change is therefore of paramount importance.