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Full text release has been delayed at the author's request until May 13, 2025

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Assessing Vulnerability to Watershed Erosion and Coastal Deposition in the Tropics

Browning, Trevor Nulton
http://orcid.org/0000-0003-2006-7248
http://rave.ohiolink.edu/etdc/view?acc_num=osu1586964925152273

Abstract Details

2020, Doctor of Philosophy, Ohio State University, Geological Sciences.
The tropics (±23.5° latitude) are a rapidly developing and highly populated region, expected to contain 50% of the Earth’s population by 2040. Rapid growth has led to deforestation and the removal of soil-binding vegetation, which exacerbates erosion issues. Erosion and deposition also dampen aquatic ecosystem function, degrade water quality, and disrupt food webs. Given the projected increased rainfall, anthropogenic development, and population, it is critical to understand connected watershed erosion and coastal deposition in tropical systems. However, many risk assessment methods are on the regional or watershed scale and none endeavor to connect watershed erosion and coastal deposition. Here, I present the first erosion and deposition vulnerability index to connect watersheds and coastal zones while covering all tropical regions and using open-source data focused on anthropogenic land use. Importantly, coastal deposition cannot be quantified on a tropical scale, but is calculated at the watershed scale. I find that tropical watersheds with high occurrences of earthquakes, steep slopes, and high annual precipitation are most vulnerable to erosion, and that these combined characteristics are most prevalent in the Caribbean and Southeast Asia. The overall vulnerability of a watershed and coastal zone system can change drastically when connecting watershed erosion to coastal deposition. The erosion and deposition vulnerability index is effective for larger scale analyses, but small tropical islands are often overlooked or misclassified due to the low-resolution of global datasets used. There are ~45,000 small tropical islands (<1,000 km2) scattered across the Pacific, Atlantic, and Indian Oceans with ~115M people living on them. On these islands erosion and deposition issues are of paramount importance because of the impact on their sediment-intolerant coastal ecosystems, which sustain their economies. I focus on one such system on St. John in the US Virgin Islands, which is threatened by recent land use change. I conduct a detailed characterization the island’s geology from source to sink (watershed to coastal zone) to provide high-resolution datasets to create a more accurate erosion and deposition vulnerability index. I find that the island of St. John is composed primarily of terrestrial volcaniclastic minerals and, despite steep slopes and short pathways to the sea, marine carbonates dominate in the coastal zone away from outlets. Using this and other data from St. John, I compare the accuracy of my global, low-resolution erosion and deposition vulnerability index with locally collected high-resolution datasets. I find that the high-resolution erosion and deposition vulnerability index results differ slightly from the low-resolution erosion and deposition vulnerability index at the island scale (50km2), but drastically in some watersheds. This is driven by accuracy of the anthropogenic development dataset used in the low-resolution erosion and deposition vulnerability index. Thus, high-resolution data for the anthropogenic development variable is most important when calculating the vulnerability index. Anthropogenic land use change can impact the vulnerability index drastically, but hurricanes also affect erosional and depositional processes. In 2017, two Category 5 hurricanes, Irma and Maria, passed St. John within two weeks of each other. Irma was the strongest hurricane on record to hit the Leeward Islands. Pre- and post-storm bathymetric surveys show significant deposition throughout a protected bay in eastern St. John two months after the storms passed. Despite the island receiving its average annual rainfall between the surveys and massive degradation of the watershed, terrestrial sediments represented <1% of the overall depositional volume. Hurricane wind driven waves resuspended marine carbonate sediments from offshore St. John to make up >99% of the overall depositional unit. Results from these analyses will be used to further develop the low- and high-resolution vulnerability indices, with the goal of better understanding datasets needed to complete an accurate vulnerability analysis of these threatened, fragile, and often small, connected systems.
Derek Sawyer, PhD (Advisor)
Anne Carey, PhD (Committee Member)
Michael Durand, PhD (Committee Member)
Lawrence Krissek, PhD (Committee Member)
202 p.
Geology; Geomorphology; Marine Geology; Oceanography
erosion, risk assessment, deposition, tropics

Recommended Citations

Citations

  • Browning, T. N. (2020). Assessing Vulnerability to Watershed Erosion and Coastal Deposition in the Tropics [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1586964925152273

    APA Style (7th edition)

  • Browning, Trevor. Assessing Vulnerability to Watershed Erosion and Coastal Deposition in the Tropics. 2020. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1586964925152273.

    MLA Style (8th edition)

  • Browning, Trevor. "Assessing Vulnerability to Watershed Erosion and Coastal Deposition in the Tropics." Doctoral dissertation, Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1586964925152273

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.