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A New Subsidence Map for Coastal Louisiana

Jaap H. Nienhuis, Torbjörn E. Törnqvist, Krista L. Jankowski, Department of Earth and Environmental Sciences, Tulane University,
New Orleans, Louisiana 70118-5698, USA; Anjali M. Fernandes, Department of Earth and Environmental Sciences, Tulane
University, New Orleans, Louisiana 70118-5698, USA, and Center for Integrative Geosciences, University of Connecticut, Storrs,
Connecticut 06269, USA; and Molly E. Keogh, Department of Earth and Environmental Sciences, Tulane University, New Orleans,
Louisiana 70118-5698, USA

Coastal Louisiana has experienced cata- Mississippi River) from its delta plain and rate of surface-elevation change from the
strophic rates of wetland loss over the past the adjacent coastal zone due to the con- vertical accretion rate at each site (Cahoon,
century, equivalent in area to the state of struction of flood-protection levees. As a 2015). Recently published GPS time series
Delaware. Land subsidence in the absence result, the majority of the sediment carried (Karegar et al., 2015) complement this
of rapid accretion is one of the key drivers by this system is funneled into the deep information; because these GPS stations
of wetland loss. Accurate subsidence data waters of the Gulf of Mexico, rather than (n = 13) are typically anchored >15 m
should therefore form the basis for esti- offsetting the naturally occurring high below the land surface, they capture the
mates of and adaptations to Louisiana’s subsidence rates. A landmark study (Blum “deep” subsidence component that
future. Recently, Jankowski et al. (2017) and Roberts, 2009) has shown that this includes glacial and sedimentary isostatic
determined subsidence rates at 274 sites problem is likely to worsen in the future adjustment (Wolstencroft et al., 2014) plus
along the Louisiana coast. Based on these due to limited sediment loads and acceler- compaction and faulting in deeper strata.
data we present a new subsidence map and ated sea-level rise.
calculate that, on average, coastal A NEW SUBSIDENCE MAP
Louisiana is subsiding at 9 ± 1 mm yrí. SUBSIDENCE DATA
Our subsidence map (Fig. 1) shows a
COASTAL SUBSIDENCE Tide gauges are frequently used to spatially continuous pattern of subsidence
obtain records of RLSR. However, tide rates as recorded at the land surface, based
Low-elevation coastal zones (LECZs) gauges in coastal Louisiana, and likely on the sum of the two data sources dis-
are among the most vulnerable landscapes many other LECZs, have major limitations cussed above. While spatial variability
within the context of climate-driven accel- because they typically measure RSLR with between our discrete monitoring sites is
erated sea-level rise, often exacerbated by respect to benchmarks anchored tens of high, the map shows that the expected
other human impacts as well as high sub- meters below the land surface. Subsidence average subsidence rate is relatively uni-
sidence rates. Predictions of rates of rela- rates are highest in the uppermost 5–10 m, form across coastal Louisiana, with a
tive sea-level rise (RSLR) in such settings but the average depth of the benchmarks mean rate of 9 mm yrí and a standard
depend to a considerable extent on our associated with National Oceanic and error of the mean of 1 mm yrí. It should
ability to monitor present-day subsidence Atmospheric Administration (NOAA) tide be noted, however, that uncertainties at
rates—including their spatial pattern—at gauges in coastal Louisiana (n = 31) is individual monitoring sites are signifi-
the land surface. Obtaining such data is ~23 m. Tide gauges therefore do not capture cantly higher, and we therefore stress that
challenging; space-based techniques (e.g., the component that accounts for 60%–85% both model (Fig. 1C) and data (Fig. 1D)
InSAR) struggle in non-urbanized land- of the total subsidence as observed at the uncertainties should be taken into account
scapes and to date only few of such studies land surface (Jankowski et al., 2017). when estimating subsidence rates at spe-
have provided useful results (e.g., Strozzi cific localities, including those that coin-
et al., 2013). Here we combine recently Our recent work (Jankowski et al., 2017) cide with CRMS sites. The map predicts
published subsidence data, collected by offers a novel approach to determining slightly higher than average subsidence
different yet complementary methods, to total subsidence rates at 274 sites along the rates in the eastern Chenier Plain, the
produce a novel subsidence map for coastal Louisiana coast, based on data collected Atchafalaya and Wax Lake Deltas, and
Louisiana, one of the world’s most vulner- through the Coastwide Reference along the Mississippi River downstream of
able LECZs. Monitoring System (CRMS) program. The New Orleans. The lowest rates are found in
centerpiece of this analysis consists of rod the western portion of the Chenier Plain,
While a variety of factors have contrib- surface-elevation–marker horizon records, the region with the lowest vertical accre-
uted to Louisiana’s wetland loss problem, 6–10 years long, enabling us to calculate tion rates (Jankowski et al., 2017). These
the fundamental culprit is the isolation of present-day shallow subsidence rates (i.e., two findings are in all likelihood related;
the sediment-delivery system (the shallow compaction) by subtracting the

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