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Post-‐Emplacement Coastal Dune Blowout

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Post-‐Emplacement Coastal Dune Blowout Post-emplacement coastal dune blowout development and re-colonization Katherine C. Bolles Dr. Helene Burningham, Supervisor 2012 This research dissertation is submitted for the MSc Environmental Mapping at University College London. UCL DEPARTMENT OF GEOGRAPHY DEPARTMENT OF GEOGRAPHY M.Sc. in Environmental Mapping Please complete the following declaration and hand this form in with your M.Sc. Research Project. I, ................Katherine C. Bolles.............................................. hereby declare : (a) that this M.Sc. Project is my own original work and that all source material used is acknowledged therein; (b) that it has been prepared specially for the MSc in Environmental Mapping of University College London; (c) that it does not contain any material previously submitted to the Examiners of this or any other University, or any material previously submitted for any other examination. Signed : .................................................................................... Date : .......3 September, 2012............................................... Abstract Coastal dunes are a noteworthy habitat, comprising 20% of the world’s coastlines. Blowouts are a highly dynamic part of these systems, contributing to the overall stability of such environments. As such, they are a significant area of study for coastal dune dynamics, yet they are not well understood. This study examines blowout development and re-colonisation once initiated in a dune system in northwest Ireland in order to understand the ecogeomorphic influences at work. An analysis of aerial imagery from 1951 to 2009 was undertaken for the Magheramore dune system, located in County Donegal, examining changing blowout morphologies and vegetation coverage. Fieldwork employing a differential Global Positioning System (dGPS) was done to capture high-resolution data on the geomorphology of 30 blowout samples. For each blowout, a 1 x 1 meter vegetation quadrate was assessed for coverage and species on the north, south, east and west slopes. To assess regional wind climate, wind data from Met Éireann was analysed for storminess and wind regime. Analysis of aerial imagery and dGPS data was carried out in ArcGIS 9.3 and 10, as well as visualisations of morphological and vegetation data. A variety of multivariate statistics was carried out on both morphology and wind data in MATLAB, and vegetation data was analysed in TWINSPAN and CANOCO for Windows 4.5. A cycle of ecogeomorphic feedbacks is shown to control blowout evolution, mainly relating to the orientation of the blowout and the length of its dominant axis. The wind climate commands blowout initiation and growth, though the ultimate shape is a function of physical parameters. Blowout morphology in turn controls vegetation re-colonisation, including species and coverage, while vegetation determines how much a blowout can develop, while constraint by wind strength. This cycle is shown to be consistent across all blowouts, allowing for placement within a model of blowout evolution. This enables wider inferences to be made about the dune system as a whole, and a mode to understand coastal morphodynamics facing climate change. In total, this dissertation is 11,351 words. i Acknowledgements Many thanks are given to Dr. Helene Burningham for her excellent guidance and support with this project. I could not have asked for a better supervisor. Thanks are also due to my coursemates on the MSc Environmental Mapping and MSc Remote Sensing programmes, for the feedback and banter. The UNIX lab will miss us all. Thanks to the National Parks & Wildlife Service for providing accommodation for the duration of fieldwork. ii Table of Contents 1. Introduction ........................................................................................................................ 1 2. Literature Review ................................................................................................................ 4 2.1. Coastal dune morphodynamics .................................................................................................. 4 2.2. Studies of blowouts ................................................................................................................... 6 2.2.1 Blowout wind dynamics ............................................................................................................... 9 2.2.2 Vegetation growth and controls in coastal dunes and blowouts ............................................... 10 3. Methods ........................................................................................................................... 12 3.1 Study site .................................................................................................................................. 12 3.2 Data acquisition ........................................................................................................................ 14 3.2.1 Wind data ................................................................................................................................... 14 3.2.2 Aerial imagery ............................................................................................................................ 14 3.2.3 Differential Global Positioning System (dGPS) data ................................................................... 15 3.2.4 Vegetation data .......................................................................................................................... 16 3.3 Data analysis ............................................................................................................................. 17 3.3.1 Analysis of wind data .................................................................................................................. 17 3.3.2 Geomorphological analysis ......................................................................................................... 17 3.3.3 Ecological analysis ...................................................................................................................... 19 4. Results .............................................................................................................................. 20 4.1 Climactic variables and trends ................................................................................................... 20 4.2 Geomorphology of blowouts .................................................................................................... 22 4.2.1 Aerial imagery ............................................................................................................................ 22 4.2.2 dGPS data ................................................................................................................................... 26 4.3 Ecogeomorphological interactions ............................................................................................ 32 5. Discussion ......................................................................................................................... 40 6. Conclusions ....................................................................................................................... 44 7. Appendix 1 ........................................................................................................................ 45 8. Auto-critique .................................................................................................................... 46 9. References ........................................................................................................................ 48 iii Table of Figures Figure 1. Schematic of dune evolution (Ritchie 2008)………………………………………………………………………………5 Figure 2. Hysteresis curve related to wind power and vegetation cover (Tsoar 2005)……………………………..5 Figure 3. Diagrams of typical saucer- and trough-shaped blowouts (Hesp 2002). Typical windflow patterns are indicated by arrows………………………………………………………………………………………………..8 Figure 4. Location of the Magheramore dune system, Co. Donegal, NW Ireland……………………………………12 Figure 5. Dune ridge changed observed in aerial imagery from 1951, 1977, 1995 and 2000. From 2000, the ridge is largely stable. A recession rate of approximately 11.3m/year along the estuarine margin can be inferred……………………………………………………………………………………………………………..13 Figure 6. Representation of measurements taken for a (A) saucer- or cup-shaped blowout, where transects are along ordinal directions (in blue) and rim (in black), and for (B) a ‘mega-blowout’ where the dominant axis (red) is measured. Feature outlines (yellow) were also measured when present.…………………………………………………………………………………………………………………………………….16 Figure 7. Digital elevation model (DEM) of the Magheramore dune system..………………………………………..18 Figure 8. Wind roses for hourly wind data from Malin Head and Belmullet meteorological stations from 1956 to 2011…………………………………………………………………………………………………………………………….21 Figure 9. Geomorphological interpretation of aerial photographs for the period 1951-2009. A comparison of dGPS data to satellite imagery of the 30 sub-sampled blowouts was carried out for 2012.........................................................................................................................................23 Figure 10. Summary of association between blowout age and selected morphological variables………….27 Figure 11. Summary of association between blowout location and selected morphological variables.….27 Figure 12. Summary of associations between blowout activity grade and selected morphological variables…………………………………………………………………………………………………………………………………...28
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