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Quantifying Rocky Coastline Evolution in North Torbay, Devon Quantifying Rocky Coastline Evolution in North Torbay, Devon, using 36Cl Exposure Dating and Structure-from-Motion Photogrammetry Drone image of Hopes Nose captured with an eBee drone, used within this study. Submitted by Victoria Rose Naylor, to the University of Exeter as a dissertation for the degree of Masters by Research in Geography, April 2019. This dissertation is available for Library use on the understanding that it is copyright material and that no quotation from the dissertation may be published without proper acknowledgement. I certify that all material in this dissertation which is not my own work has been identified and that any material that has previously been submitted and approved for the award of a degree by this or any other University has been acknowledged. ……………………………………………………………………………… i Abstract Around 70-80% of the world’s coastline, and around 60% of the UK’s coastline, can be considered as ‘rocky’. Rocky coasts erode much slower than their softer sedimentary counterparts, but their rates of erosion and their evolutionary history are poorly known. In this dissertation I use a new combination of methods, cosmogenic nuclide exposure dating, structure-from-motion photogrammetry and sea-level modelling, to study a typical stretch of rocky coastline in north Torbay, Devon, southwest England. Torbay’s coast is characterised by the presence of shore platforms and raised beaches above modern sea level, situated on the north headland peninsula, named Hopes Nose. These elevated landforms must relate to a previous interglacial period, with warmer environments and higher sea- levels, and their preservation indicates very slow rates of coastal evolution within the area. I apply exposure dating using 36Cl to determine the degree of geomorphological inheritance from previous high sea-level stands, along north Torbay’s rocky cliffs and across the main body of the raised shore platform at Hopes Nose. I combine this analysis with the measurement of a new digital surface model, collected via drone imagery and structure-from-motion photogrammetry, across the headland to perform a morphometric analysis of the modern and elevated interglacial platform. Lastly, I determine a new estimate of relative sea-level change at the site, considering glacio-isostatic adjustment, using the SELEN sea-level model. Cosmogenic nuclide exposure dating reveals that the rocky coastline around north Torbay has been actively eroding throughout the late Holocene, through a series of stochastic mass movements and some incremental loss. Similarly, the exposure dating of the raised shore platform at Hopes Nose reveals it has been covered by distinctive sediments during the late Pleistocene and hence survived surface erosion. Morphometric ii analysis of the raised interglacial shore platform and the modern shore platform shows a similar evolutionary history, highlighting the changes in marine and aerial influence over the shore platforms formation. An analysis of the raised platform’s elevation, evaluated relative to modelled relative sea-level change, is most consistent with the platform being formed during the last interglacial period (Marine Isotope Stage 5e). As a result, this research also puts into question the overall height of the sea-level during MIS 5e, or the presence of a double peak within the record. Overall, this research demonstrates that the unique combination of methodologies can quantify coastal erosion and help decipher a rocky coastline’s history under both present and previous sea levels. iii Acknowledgements I would like to thank my fantastic supervisory team, Dr Steven Palmer, Professor Timothy Barrows, Dr Matteo Vacchi and Olly Bartlett, who have supported me greatly throughout my Masters by Research at Exeter. Their knowledge and passion for the subject area and the methodologies employed within this research was truly inspiring, infectious and immensely valuable. I am truly grateful for their frequent availability and ability to always reignite my passion for the research, even at my lowest points in my research and other areas of my life. I am also grateful for the support of Dr Paolo Stocchi (Royal Netherlands Institute of Sea Research, Netherlands), who assisted me in sea-level modelling within this research. I am also grateful for Dr Keith. L Fifield (Australian National University, Australia), who provided his expertise to measure 36Cl concentrations within the collected samples. Furthermore, I greatly appreciate Dr Karen Leslie and Dr Sharon Turners (Research Technicians in Geography, University of Exeter, UK) for assisting with sample collection within the early stages of this research. I would like to acknowledge and thank Dr Richard Jones, without his early support and belief in my ability to persue a Masters by Research in Geography, I would not have reached this point. Finally, thanks also to the University of Exeter Geography department, as well as the Cryosphere, Coastal and River Dynamics (CCoRDs) research group for their support and friendship throughout my time at the university. iv Contents Title Page..……………….....…………………………………………….i Abstract.…….……………….……………………………………………ii Acknowledgements..….……….………………………………………iv Contents…………………………………………………………………..v Figures ........................................................................................ viiii Tables ............................................................................................. ix Equations ....................................................................................... ix Author’s Declaration ..................................................................... x List of Abbreviations ..................................................................... xi CHAPTER 1: INTRODUCTION ........................................................ 1 1.1. Why are coastlines important? .......................................................... 1 1.2. Rocky Coastlines ................................................................................ 5 1.2.1. What are rocky coastlines? ......................................................... 5 1.2.2. Mechanisms and theories of coastal erosion and shore platform formation ..................................................................................... 9 1.2.2.1. Horizontal erosion: Hydraulic erosion and mass movements............................................................................................ 11 1.2.2.2. Vertical erosion: mechanical erosion and subaerial processes .............................................................................................. 15 1.2.2.3. Other processes at work ..................................................... 20 1.3. Coastline evolution ........................................................................... 23 1.3.1. Existing Coastal Evolution Research .............................................. 25 1.3.1.1. In-field measurements and observations ................................. 26 1.3.1.2. Modelling coastline evolution .................................................... 28 1.3.1.3. Dating coastal evolution ............................................................ 33 1.3.1.3.1. Cosmogenic nuclide exposure dating................................ 37 1.3.1.4. Aerial imagery, LiDAR and Photogrammetry ........................... 40 1.4. Sea-level change ............................................................................... 45 1.4.1. Eustatic and steric sea-level changes ...................................... 45 1.4.2. Relative sea-level changes ........................................................ 46 1.4.2.1. Glacial-isostatic adjustment ................................................ 47 1.4.3. Palaeo sea-levels ........................................................................ 50 1.4.3.1. Marine isotope stages (MIS) ................................................ 51 1.4.4. Palaeo sea-level Indicators ........................................................ 54 1.4.4.1. Rocky coastline inheritance ................................................ 54 v 1.4.4.2. Measurement of palaeo sea-level indicators ..................... 55 1.5. Study Site: North Torbay .................................................................. 61 1.5.1. Torbay, Devon ............................................................................. 61 1.5.2. North Torbay coastline ............................................................... 66 1.5.2.1. Meadfoot Beach (Figure 1.9B) ............................................. 68 1.5.2.2. Beacon Cove (Figure 1.9A) .................................................. 68 1.5.2.3. Peaked Tor Cove (Figure 1.9A) ........................................... 68 1.5.3. Hopes Nose (Figure 1.9B) .......................................................... 68 1.5.4. Existing coastal management and protection .......................... 70 1.6. Study aims and objectives ............................................................... 72 CHAPTER 2: METHODOLOGY ..................................................... 76 2.1. 36Cl Cosmogenic Nuclide Exposure Dating ................................... 76 2.1.1. Background to cosmogenic nuclide exposure dating ............ 76 2.1.2. 36Cl Cosmogenic nuclide dating of Torbay’s coastline ........... 80 2.1.2.1. Sample collection and analysis .......................................... 80 2.1.2.2. CRONUScalc and production rates .................................... 84 2.2. UAV-SfM Photogrammetry ............................................................... 88
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