Date of Award

2012

Degree Type

Thesis

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

John King

Second Advisor

Rebecca Robinson

Abstract

The barrier-lagoon system along the Rhode Island south shore is a vital natural resource that provides critical habitat and protects the state's southern communities against storm damage. The response of this system to changes in global climate is therefore of great interest to those who live along and manage this coastline. Responsible planning and accurate assessment of coastal vulnerability will require consideration of barrier spit evolution on different time scales. Accordingly, this dissertation presents evidence from geologic and instrumental records that are used to examine how the south shore responds to changes in the extent and frequency of coastal storms, and to long-term processes such as sea level rise.

Overwash layers present in a transect of sediment cores from Quonochontaug Pond, RI are used to construct a record of major hurricane landfall spanning the last 2200 years. An annual probability for intense tropical cyclone landfall in Rhode Island was calculated to be 0.45% - a value that is notably similar to other proxy-based reconstructions throughout the western North Atlantic. The record indicates that New England has experienced changes in tropical cyclone climatology during this time, with periods of increased activity during the past ~400 years, and between 1400-2150 cal. yr BP. Similarity in the timing of overwash events between Quonochontaug Pond and sites throughout the western North Atlantic suggests that millennial-scale variability may be the result of basin-wide climatic forcings.

A long-term dataset of beach profiles is used to construct a high-resolution record of shoreline change at eight transects along the Rhode Island south shore. Shoreline positions were estimated by intersecting a local tidal datum with ~6000 coastal profiles collected over 49 consecutive years. When compared to digital vector shorelines coincident with the survey period, the time-series of profile-derived shorelines demonstrate how sampling frequency and the choice of time-scales for analysis can bias calculated rates of shoreline change. A comparison of rate-of-change methods demonstrates that a weighted regression of vector shorelines depicts true shoreline behavior more accurately than other commonly used techniques. Annual variability resulting from changes in beach morphology in response to storms is quantified, providing the first estimates of such uncertainty for future studies of shoreline change. In addition, the offset between proxy-based, and datum-based shorelines is compared for recent years and is shown to be more than twice as large as previously estimated.

Sea level rise is predicted to be one of the largest and most sustained climate change impacts to coastal environments and populations during the next century. As a result, mitigation strategies that incorporate accelerated rates of sea level rise into coastal planning, design, and habitat restoration are increasing. In Rhode Island, estimates of sea level rise up to and exceeding 1-meter by 2100 will result in dramatic changes on the state's barrier-lagoon coastline. A range of geospatial data are already available for planning - including several high resolution elevation models -- yet future trends will require sustained research to monitor sea level changes and impacts in Rhode Island. We recommend a comprehensive program of coastal monitoring that utilizes high-resolution, sequential topography, high-accuracy geodetic control, and tools to quantify relative sea level change in Rhode Island. Coastal vulnerability can then be gauged with predictive models that integrate multiple datasets and include probabilistic estimates of shoreline response to climate change.

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