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Despite the large number of tsunamis that impact Mexico’s Pacific coast, stratigraphic studies focusing on geological impacts are scanty, making it difficult to assess the long-term risks for this vulnerable region. Surface samples and six cores were taken from Laguna Mitla near Acapulco to examine sedimentological and geochemical evidence for marine incursion events. Sediment cores collected from behind the beach barrier are dominated by intercalated layers of peat and inorganic sediments, mostly silt and clay, with little or no sand. Sand- and shell-rich clastic layers with high levels of sulfur, calcium, and strontium only occur adjacent to the relict beach ridge remnants near the center of the lagoon. With the exception of one thin fine sand layer, the absence of sand in the near-shore cores and the predominance of the terrigenous element titanium in the inorganic layers, evidently eroded from the surrounding hillslopes, suggests that these large-grained intervals do not represent episodic marine incursions, but rather were likely formed by the erosion and redeposition of older marine deposits derived from the beach ridge remnants when water levels were high. These results do not support the occurrence of a large tsunami event at Laguna Mitla during the Late Holocene. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Hurricanes play an important role in shaping the coast of Louisiana. Although the sedimentary signatures of hurricane deposits have been documented in several different coastal environments along the northern Gulf coast, no studies have as yet documented the signatures in wetlands adjacent to large, inland brackish water bodies. In this paper we present results of a case study documenting the distribution and characteristics of storm surge deposits related to Hurricane Isaac (2012) in a wetland on the western shore of Lake Pontchartrain, Louisiana. Hurricane Isaac, a category1 storm, made landfall near the mouth of the Mississippi River on August 28, 2012. Due to its large size and slow movement, Isaac generated strong easterly winds across Lake Pontchartrain, producing a large storm surge along the west shore of the lake and unprecedented flooding in the surrounding lowlands. Loss-on-ignition, XRF, radioisotopic, and grain-size analyses conducted on sediment cores and surface samples from the area identify two distinct sedimentary signatures for the Hurricane Isaac deposits. Near the lake shore the signature is characterized by a laminated silty sand with a geochemical profile closely resembling that of lake bed material. Storm deposits located in a brackish swamp ~ 1km inland consist of a dark, low-organic mud with low concentrations of terrestrial metals and elevated concentrations of Br, S, and Cl. Differences in the storm signal are explained by the differing effect of topographical features on the depositional and transportation processes occurring at the two sites. Utilizing the geochemical/compositional signatures as a hurricane-generated storm surge proxy indicates the possible occurrence of a similar event predating the historical record. [ABSTRACT FROM AUTHOR]

The Louisiana shoreline is rapidly retreating as a result of factors such as sea-level rise and land subsidence. The northern Gulf of Mexico coast is also a hotspot for hurricane landfalls, and several major storms have impacted this region in the past few decades. A section of the Louisiana (USA) coast that has one of the highest rates of shoreline retreat in North America is the Caminada-Moreau headland, located south of New Orleans. Bay Champagne is a coastal lake within the headland that provides a unique opportunity to investigate shoreline retreat and the coastal effects of hurricanes. In order to examine the influence of hurricanes on the rate of shoreline retreat, 35 years (1983–2018) of Landsat imagery was analyzed. During that period of time, the shoreline has retreated 292 m. The overall rate of shoreline retreat, prior to a beach re-nourishment project completed in 2014, was over 12 m per year. A period of high hurricane frequency (1998–2013) corresponds to an increased average shoreline retreat rate of >21 m per year. Coastal features created by multiple hurricanes that have impacted this site have persisted for several years. Bay Champagne has lost 48% of its surface area over the last 35 years as a result of long-term shoreline retreat. If shoreline retreat continues at the average rate, it is expected that Bay Champagne will disappear completely within the next 40 years. [ABSTRACT FROM AUTHOR]

A pressing question facing the Mississippi River Delta (MRD), like many deltaic communities around the world, is: Will the system be sustainable in the future given the threats of sea level rise, land loss, natural disasters, and depleting natural resources? An integrated coastal modeling framework that incorporates both the natural and human components of these communities, and their interactions with both pulse and press stressors, is needed to help improve our understanding of coastal resilience. However, studying the coastal communities using a coupled natural-human system (CNH) approach is difficult. This paper presents a CNH modeling framework to analyze coastal resilience. We first describe such a CNH modeling framework through a case study of the Lower Mississippi River Delta in coastal Louisiana, USA. Persistent land loss and associated population decrease in the study region, a result of interplays between human and natural factors, are a serious threat to the sustainability of the region. Then, the paper describes the methods and findings of three studies on how community resilience of the MRD system is measured, how land loss is modeled using an artificial neural network-cellular automata approach, and how a system dynamic modeling approach is used to simulate population change in the region. The paper concludes by highlighting lessons learned from these studies and suggesting the path forward for analysis of coupled natural-human systems. [ABSTRACT FROM AUTHOR]