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Date of Award

Spring 2024

Document Type

Masters Thesis

Department or Program Affiliation

Department of Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Grossman, Eric E.

Second Advisor

Mitchell, Robert J. (Geologist)

Third Advisor

Crosby, Sean Christopher

Fourth Advisor

Pfeiffer, Allison

Abstract

Sea level rise in the coming century is projected to cause substantial changes along the world’s coastlines and adverse effects to communities and environments. Among these changes, questions about the extent, rate, and uncertainty of coastal bluff retreat relative to historical observations and in response to sea level rise are critical to evaluate. Coastal bluffs comprise ~43% of the shoreline across Puget Sound in the Salish Sea and their recession is a growing concern to human safety, property, infrastructure, and diverse ecosystem services for culturally and commercially important fisheries. This project assesses bluff erosion using field observations, structure from motion photogrammetry, and evaluation of previous regional multivariate regression erosion models to project bluff retreat given three estimates of sea level rise. Field studies at four sites across Puget Sound found that bluffs recede through shallow landslides initiated on the upper bluff and through undercutting and failure at the base. Multi-parameter regression models demonstrated that long-term bluff recession in Puget Sound was best explained by offshore wave power, tidal range, and characterization of coastal shoretypes with wave and water level data accounting for approximately 38.5% of variation in historic bluff recession. A previously established multi-variate model was modified to remove a site-specific variable that depended on bluff recession measurement methods, which improved the regional applicability of the model. Historical erosion rates modeled using the multivariate model were then used to calibrate two schematized models to project future bluff erosion rates over 200 km of shoreline. The results indicate that bluff retreat may increase up to 61%, 83%, and 114% under sea level rise scenarios of 0.58 m, 0.76 m, and 1.01 m, respectively, corresponding to the 83-17% probability of exceedance of sea level rise for the region. While the new model captures 38.5% of the variance in historic bluff erosion rates, the remaining variance is likely due to sparse data characterizing bluff erodibility, beach dynamics, and bluff failure thresholds with respect to rainfall intensity.

Type

Text

Keywords

geology, bluff, erosion, recession, sea, level, rise, coastal, shoreline

Publisher

Western Washington University

OCLC Number

1439290379

Subject – LCSH

Cliffs--Washington (State)--Puget Sound; Erosion--Washington (State)--Puget Sound; Coast changes--Washington (State)--Puget Sound; Sea level--Washington (State)--Puget Sound

Geographic Coverage

Puget Sound (Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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