Abstract
Abstract. The natural wetlands of coastal Louisiana are experiencing rapid subsidence rates averaging 9±1 mm yr−1. Recent measurements based on GPS data and CRMS surface elevation tables (SETs) have shown that most of the subsidence is shallow and occurs in the uppermost 5 meters. Sources of subsidence and the origin of their spatial variability are strongly debated. Here we use CRMS SETs together with historic maps of coastal Louisiana to explore two hypotheses: (i) shallow subsidence is a result of accommodation created by (long-term) deep subsidence processes and self-weight consolidation, and (ii) changes in marsh hydrology (groundwater and surface water flows) have led to a recent increase in shallow subsidence. First, we find that, although self-weight consolidation would result in generally high observed shallow subsidence rates, it does not explain the rates nor the spatial variability of the CRMS SET data. Second, based on historic maps, we find that shallow subsidence rates are significantly higher for CRMS sites where shipping canals have reduced their distance to the marsh edge. This is potentially a result from increased sediment deposition, but CRMS data also show altered groundwater levels near the marsh edge. We find some indication that prolonged periods of low water could have led to increases in effective stresses that explain some of the rapid rates of shallow subsidence observed along Louisiana's coastline.
Highlights
Many low-lying coastal areas including coastal Louisiana experience elevated rates of relative sea-level rise because of subsidence (Jankowski et al, 2017; Minderhoud et al, 2018; Nienhuis et al, 2017; Teatini et al, 2011)
Based on digitized land loss maps between 1932 and 1990 (Penland et al, 2000), we find that the median distance from Coastwide Reference Monitoring System (CRMS) sites to open water has been reduced considerably (Fig. 5)
Using the CRMS data together with land loss maps, we investigate the relationship between shallow subsidence and local hydrology
Summary
Many low-lying coastal areas including coastal Louisiana experience elevated rates of relative sea-level rise because of subsidence (Jankowski et al, 2017; Minderhoud et al, 2018; Nienhuis et al, 2017; Teatini et al, 2011). Recent data collection through the Coastal Reference Monitoring System (Steyer et al, 2003; https://lacoast.gov/crms/, last access: 1 August 2019) (Fig. 1a) has allowed for unprecedented analyses of subsidence rates and patterns in coastal Louisiana (e.g., Jankowski et al, 2017; Nienhuis et al, 2017; Sanks et al, 2019), but subsidence mechanisms are still poorly understood. We investigate two hypotheses: (1) shallow subsidence is caused by self-weight consolidation, and (2) shallow subsidence is caused by surficial hydrological change
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