AbstractCoastal boulder deposits, within the inter- and supratidal zone represent sedimentary signatures of extreme storm wave and/or tsunami origin. However, owing to the infrequency of such highmagnitude events there is a limited understanding of the complex mechanisms that govern boulder transport upon rocky coasts. Existing studies frequently focus on geographically remote coastal locations which are exposed to considerable oceanic swell waves. Owing to the inhospitable nature of these sites they are typically unpopulous and lacking in associated infrastructure. By contrast, rocky coastal sites subject to moderate wave regimes that are regularly exposed to low-magnitude, high-frequency storm activity are generally more populated with increased housing provision and a greater level of municipality. Notwithstanding the reduced level of storm wave competence, such sites are rarely evaluated in terms of boulder transport response to contemporary storm events despite exhibiting sedimentary assemblages which are indicative of storm wave deposition.
Current data pertaining to boulder transport is often limited to qualitative observational assessment from isolated field sites or increasingly, via the use of remotely sensed data. Furthermore, the inability to regularly monitor sediment transport of boulder-sized clasts means displacement is rarely accurately quantified.
This thesis aims to document and accurately quantify boulder transport in response to contemporary storm waves at a coastal location subjected to low-magnitude, high-frequency storm events. Quantification was achieved by undertaking a sediment tracing field study which incorporated monitoring the displacement amongst an array of specific intertidal boulders. Using a novel field technique, Radio Frequency Identification tags (RFID’s) were embedded in selected boulders. Each RFID tag is pre-programmed with a unique serial number which enabled identification of tagged boulders in the field. Coordinate data for each tagged boulder was obtained using a Differential Global Positioning Navigation Satellite System (DGNSS). Repeated field surveys following storm activity were conducted to relocate the tagged clasts; once relocated the bolder location was rerecorded. On completion of the three-year study coordinate data for each of the 104 tagged boulders provided a spatial and temporal framework within which boulder transport pathways could accurately be quantified.
Additionally, the processes and mechanisms that facilitate boulder production, transport and deposition on the shore platforms at the field site are defined. This is undertaken with a view to better understanding the interrelationship between a host of boundary conditions which modify and ultimately regulate shore platform evolution
The chapters within this thesis present a range of the findings from the field study including an extensive review of the RFID tagging methodology (Chapter 2). This comprehensive account provides coastal researchers with the specific details required to implement a successful tagged boulder monitoring campaign.
The key findings arising from this empirical field-based study are presented in Chapter 3 which addresses the overarching aim of the thesis in terms of accurately quantifying boulder transport. The data identifies that despite the low/moderate wave climate at the field site, boulder displacement is widespread which suggests a need to reassess the perceived geomorphic docility of relatively sheltered coastal locations.
Additional noteworthy findings include: (1) the statistically significant difference between transport distances attributed to constrained and unconstrained boulders, suggesting the pre-transport morphological setting exerts considerable control over boulder transport potential; (2) boulder production is initiated by undermining which is enhanced by the presence of geological discontinuities within the boulder producing unit; (3) moderate storm waves are able to mobilise very coarse boulders above the calculated wave parameters derived from widely-cited hydrodynamic equations; (3) sediment-laden, low-energy waves facilitate platform modification with respect to the formative processes of intertidal pool development, the pools subsequently act to impede boulder transport (Chapter 4); (4) storm-induced reworking of intertidal boulders modifies wider landform morphology in terms of collective boulder displacement altering largescale landform features (Chapter 5); (5) the processes by which boulder production, transport and deposition occur are inextricably linked and are reflected at various coastal sites across a range of scales producing similar landform features and sedimentary assemblages (Chapter 5).
Given the anticipated increase in storm frequency and intensity arising from changing climate patterns the rate at which future geomorphic modification and landform evolution occurs on intertidal shore platforms is expected to accelerate. Therefore, it is necessary to establish the responsiveness of such coastal features with an emphasis on better understanding the ability and the extent to which storm waves are able to detach, transport and deposit boulder-sized clasts within the intertidal zone.
|Date of Award||Apr 2020|
|Supervisor||Rob Inkpen (Supervisor)|