ORCID

Abstract

Coastal boulder deposits are found on coastlines worldwide, transported inland by extreme storm or tsunami events. These deposits are understudied, yet hold unique information about the magnitude of the waves which emplaced them, enabling the addition of new events to the storm and tsunami record. However, boulder transport by storm waves is commonly misidentified as tsunami events, traditional methods for calculating boulder size are outdated, the influence of boulder shape on transport is understudied, there are difficulties making comparisons between physical experiments and field data, and commonly used equations for estimating wave flow velocities from boulders have been shown to be inaccurate. To investigate these challenges in coastal boulder research, field studies were conducted on case study examples of extreme wave deposits on the tsunami deposits of the Sanriku Coast of Japan and storm deposits in Shetland, Scotland. At these sites modern field methods including handheld LiDAR, unmanned aerial vehicle Structure-from-Motion 3D modelling, and a new boulder digitisation methodology were applied, and tested against traditional techniques at the Grind o’ da Navir, Shetland. A physical experiment was also conducted to establish connections between the offshore wave environment and boulder transport distance on varying boulder shapes, providing data to inform a novel dimensional analysis technique for comparing physical experiments.The results showed that when analysing a whole site, the new digitisation method was efficient and comparable to traditional measurement techniques. Handheld LiDAR enabled more accurate boulder volumes to be extracted, with calculated values 37% smaller than when using the traditional axis multiplication technique. In Shetland, local geomorphology had a large influence on sedimentological statistics such as boulder size, orientation, and spatial distribution, despite the sites being affected by the same storm waves. In Sanriku, tsunami emplaced boulders were isolated from the storm wave overprint, showing the very largest boulders at each site were the most likely to be tsunami emplaced. In the physical experiment the elongate irregular boulder was consistently transported further than the regular cuboid boulder model, undergoing primarily rolling transport whilst the cuboid predominantly underwent sliding transport. By comparing the storm and tsunami emplaced boulder data, sedimentological and geomorphological characteristics were identified as indicators of emplacing wave type. Tsunami deposits appear to show poorer sorting and less orientation alignment with larger transported clasts, while storm deposits are more rounded, better sorted, and show stronger orientation alignment. Deposit morphology and spatial distribution can also be used as evidence, with tsunami deposits likely widely distributed while storm-emplaced boulders are more clustered, sometimes forming ridge systems. However, coastal geomorphological context is necessary as topography also has a strong control on boulder statistics. For coastal net-zero energy infrastructure projects, site-specific modelling validated with local boulder field evidence is recommended to more reliably estimate the flow velocities of past extreme wave events. Ultimately, integrating coastal boulder deposits into hazard assessments provides an additional line of evidence for reconstructing extreme wave impacts and improving coastal risk planning.

Awarding Institution(s)

University of Plymouth

Supervisor

Sarah Boulton, Alison Raby, Irene Manzella

Keywords

Geomorphology, Tsunami Wave, Storm, Coastal Processes, Japan

Document Type

Thesis

Publication Date

2026

Embargo Period

2026-06-12

Deposit Date

June 2026

Creative Commons License

Creative Commons Attribution-NonCommercial 4.0 International License
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License

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