The fidelity of paleo-storm deposits and their implications for future flood risk
Creators: name: Joyse, Kristen Marie
Publisher: name: No Publisher Supplied
Year: 2022

description: Coastal storms generate storm surges that have historically been and remain some of the most devastating natural disasters in terms of infrastructural damage and loss of life worldwide. Yet, the expected return periods of storm surge events are derived from short instrumental and historical records that contain too few observations of these rare, extreme water levels to precisely describe their frequency over time. Geological records, on the other hand, can contain many thousands of years of extreme water level observations in the form of overwash deposits in back-barrier coastal environments. However, the preservational bias of the geologic record in capturing these overwash deposits and how this bias changes in relation to relative sea-level through time are unknowns.In this dissertation, I investigate which geological proxies have the highest preservation potential to identify paleo-storm overwash deposits in back-barrier environments in order to determine the resolution of the geologic record of extreme water level events compared to that of instrumental records. I subsequently investigate the utility of these geologic observations in estimating the return frequency of extreme water level events over using short, instrumental records alone.In Chapter 1, I characterize the stratigraphic, sedimentological (grain size and organic content), geochemical (δ13C and C/N), and microfossil (foraminifera and diatom) characteristics of the modern overwash deposit generated by Hurricane Irma’s storm surge in southern Florida mangrove islands two to three months after Irma’s landfall. I then track the evolution of these characteristics over twenty-two months to determine which proxies have the highest likelihood of identifying Hurricane Irma in the geologic record. Hurricane Irma’s overwash deposit was composed of light gray carbonate muds and sands up to 11 cm in thickness and overlayed the organic-rich, red mangrove peats that constituted the substrate of the mangrove islands. Sedimentological characteristics including mean grain size and organic content in conjunction with foraminiferal assemblages provided the strongest evidence for differentiating Irma’s overwash deposit from underlying mangrove peats twenty-two months post-storm. Therefore, these proxies should be utilized in paleo-storm reconstructions in the region.In Chapter 2, I analyze the stratigraphic, sedimentological, and microfossil characteristics of sediments cored from a back-barrier marsh in Cheesequake State Park, New Jersey, to identify overwash deposits from extreme water level events. I then compare the geologic record to nearby instrumental tide gauge records from Sandy Hook, New Jersey, and the Battery, New York, to determine the preservational potential of the geologic record. Eight overwash deposits were identified within the stratigraphy of the back-barrier marsh. Seven of the eight overwash deposits were identified by their fan-shape morphology in which they were thickest towards their origin at the coast and thinned landward. The exception was the overwash deposit generated by Hurricane Sandy, which appears to have originated by overtopping the banks of a tidal creek adjacent to the marsh. The overwash deposits had a larger mean grain size (2.7 ± 0.4 φ) when compared to the autochthonous coastal sediments (5.1 ± 1.2 φ) in which the overwash deposits were embedded. An age-depth model informed by bulk Pb, 206Pb:207Pb, Cs-137, and pollen chronohorizons constrained the ages of the overwash sediments and determined four of the eight overwash events overlapped with the instrumental tide gauge records. Comparison of extreme water level events within the tide gauge records and the geologic record yielded the conclusion that the geologic record at the Cheesequake marsh has a 40 to 50 % rate of capturing extreme water level events that exceeded the 10 % annual expected probability of occurrence level.In Chapter 3, I produce a Poisson Point Process model within a peak-over-threshold framework to test how the inclusion of geologic data of extreme water level events improves the precision of return period estimates of those extreme events over using tide gauge data alone. I generate synthetic data to test the performance of the model and demonstrate that the inclusion of geological data reduces the uncertainty on extreme water level return periods by up to ~50 %. I also perform a case study using tide gauge data from the Woods Hole tide gauge and a geological record from Mattapoisett Marsh, Massachusetts. In doing so, I demonstrate the inclusion of geological data produces more precise estimates of extreme water level events such as the 0.2 % annual probability of exceedance water level, which has a 5th to 95th percentile estimate range of 1.35-2.38 m using Woods Hole tide gauge data alone but a 1.38-2.17 m range when geologic data from Mattapoisett Marsh is included. Thus, this dissertation illustrates the utility of geological data for improving our understanding of paleo-overwash events and the precision of extreme sea level distributions.