Wetland Accretion Rates Along Coastal Louisiana: Spatial and Temporal Variability in Light of Hurricane Isaac’s Impacts

Abstract

Abstract:The wetlands of the southern Louisiana coast are disappearing due to a host of environmental stressors. Thus, it is imperative to analyze the spatial and temporal variability of wetland vertical accretion rates. A key question in accretion concerns the role of landfalling hurricanes as a land-building agent, due to their propensity to deposit significant volumes of inorganic sediments. Since 1996, thousands of accretion measurements have been made at 390 sites across coastal Louisiana as a result of a regional monitoring network, called the Coastal Reference Monitoring System (CRMS).We utilized this dataset to analyze the spatial and temporal patterns of accretion by mapping rates during time periods before, around, and after the landfall of Hurricane Isaac (2012). This analysis is vital for quantifying the role of hurricanes as a land-building agent and for understanding the main mechanism causing heightened wetland accretion. The results show that accretion rates averaged about 2.89 cm/year from stations sampled before Isaac, 4.04 cm/year during the period encompassing Isaac, and 2.38 cm/year from sites established and sampled after Isaac. Accretion rates attributable to Isaac's effects were therefore 40% and 70% greater than before and after the event, respectively, indicating the event's importance toward coastal land-building. Accretion associated with Isaac was highest at sites located 70 kilometers from the storm track, particularly those near the Mississippi River and its adjacent distributaries and lakes. This spatial pattern of elevated accretion rates indicates that freshwater flooding from fluvial channels, rather than storm surge from the sea per se, is the main mechanism responsible for increased wetland accretion. This significance of riverine flooding has implications toward future coastal restoration policies and practices.

Relationships Between Salinity and Short-Term Soil Carbon Accumulation Rates from Marsh Types Across a Landscape in the Mississippi River Delta

“…Each plot was then sampled bi-annually for 2 years and less frequently in subsequent years (Folse et al 2014). See other literature (Cahoon and Turner 1989;Folse et al 2014;Bianchette et al 2015) for more details on the feldspar methodology including limitations and benefits as well as details about downloading accretion data from the Coastal Information Management System (CIMS).…”

“…Accretion is commonly measured via marker horizons, such as feldspar, and particle-reactive radiotracers, such as 7 Be, 137 Cs, and 210 Pb (Callaway et al 1996;Taylor et al 2013). These methodologies are complementary: feldspar marker horizons measure accretion on a short time scale, months to years, (Cahoon and Turner 1989;Bianchette et al 2015) whereas 137 Cs and 210 Pb measure accretionary processes over decadal timescales to a century or more (Delaune et al 1978). Primary production processes dominate over decomposition processes in the surface layers (top~10 cm) of the wetland soil column, which significantly influences the accumulation of organic matter and carbon content of marsh soils (Hatton et al 1983;Nyman et al 2006;Turner et al 2006).…”

Hurricanes as a Major Driver of Coastal Erosion in the Mississippi River Delta: A Multi-Decadal Analysis of Shoreline Retreat Rates at Bay Champagne, Louisiana (USA)

“…Multiple hurricanes, including Katrina (2005), Rita (2005), Ike (2008), Gustav (2008), and Isaac (2012) have affected the Mississippi River delta region within the last few decades alone [8,36,37]. Several studies have shown that hurricanes are important sediment sources to coastal environments and play a crucial role in the erosion, transport, and deposition of coastal sediments in Louisiana [36][37][38][39][40]. Hurricanes can cause storm surge in excess of 6 m in the northern Gulf of Mexico [10,23], which is more than high enough to overtop any of the barrier beaches on the Caminada-Moreau headland [23] and can rapidly cause both ephemeral and permanent alterations to coastal features.…”

Assessing Resilience and Sustainability of the Mississippi River Delta as a Coupled Natural-Human System

“…The physical processes can strongly affect sediment transport and deposition, which would result in spatial and temporal variability of vertical accretion rates in coastal wetlands. The study by Bianchette and others [14] utilized an existing database (CRMS-Coastal Reference Monitoring System) with thousands of wetland accretion measurements at 390 sites across coastal Louisiana to analyze the spatial and temporal variability of their elevation changes. Specifically, the study mapped accretion rates in the region during time periods before, around, and after the landfall of Hurricane Isaac of 2012.…”