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Wang, L.; Bianchette, T.A., and Liu, K.-b., 2019. Diatom evidence of a paleohurricane-induced coastal flooding event in Weeks Bay, Alabama, USA. Journal of Coastal Research, 35(3), 499–508. Coconut Creek (Florida), ISSN 0749-0208. Sand layers deposited in coastal backbarrier lakes and marshes have been taken as indicators of overwash processes generated by hurricane landfalls, but sand layers discovered in sediments from non-backbarrier estuarine settings have not been as thoroughly investigated. Sedimentological and diatom analyses of a 44-cm core (WB 08-04) taken from a seep forest on the north shore of Weeks Bay in southern Alabama contains an 8-cm-thick sand layer embedded in dark gray peat. As an aid to determining the provenance of the sand layer, diatoms were also analyzed from eight surface samples taken from five nearby environments representing different salinity conditions. The diatom assemblage in the sand layer is dominated by acidophilous (Eunotia spp., Frustulia krammeri) and freshwater (Pinnularia neomajor, Pinnularia latevittata) species, suggesting that they were transported by fluvial processes from the acidic and freshwater bogs above the banks of Fish River. The sand layer also contains small percentages of brackish and marine diatoms (Tryblionella marginulata, Planothidium delicatulum), suggesting that incursion of seawater and brackish water from Weeks Bay and Mobile Bay also played a role in its deposition. It is inferred that the sand layer was deposited by a hurricane that brought both intense precipitation to coastal Alabama and a storm surge into Weeks Bay. Heavy rainfall caused overbank flooding of Fish River, which transported sand from the river banks and diatoms from nearby bogs to the coring site. Concurrently, storm surge waters also deposited marine and brackish diatoms at the site. Historical records suggest that the Bernard Romans' Gulf Coast Hurricane in AD 1772 may have been responsible for the flooding that resulted in the deposition of the sand layer. [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]

To understand the geological processes induced by tropical cyclones, geochemical and sedimentological analyses were performed on a 3 m sediment core (basal 14C date of 940 +/- 50 years BP) extracted from a marsh adjacent to a backbarrier lake along Louisiana's Gulf of Mexico coast (USA). This study was conducted in order to identify the geologic changes within a subsiding coastal region in the light of coastal recession and past hurricane activity. Previous studies show that Bay Champagne, a semi-circular lake near Port Fourchon, Louisiana, is subjected to subsidence rates between 1.0 and 1.2 cm year-1, the highest rate of retreat within the entire northern Gulf of Mexico. Loss-on-ignition (LOI) and X-ray fluorescence (XRF) analyses, employed to generate lithological and geochemical core profiles, identified three distinct sand layers measuring up to 50 cm in thickness deposited by recent hurricanes. LOI shows large decreases in water, organic, and carbonate contents, indicating the occurrence of marine inundation. Within each marine incursion layer terrestrial elemental concentrations as determined by XRF display large depletions. Grain size analysis of a portion of the core (30-86 cm) indicates the presence of two series of sequential high-energy storm deposits followed by intense fluvial flooding within Bay Champagne. These events are attributed to Hurricanes Katrina/Rita in 2005 and Gustav/Ike in 2008. [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]

Liu, K.., McCloskey, T.A., Bianchette, T.A., Keller, G., Lam, N.S.N., Cable, J.E., Arriola, J. 2014. Hurricane Isaac Storm Surge Deposition in a Coastal Wetland along Lake Pontchartrain, Southern Louisiana. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 266-271, ISSN 0749-0208. 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]