Oyedotun, TDT; (2015) Estuary – coast interaction and morphodynamic evolution: a comparative analysis of three estuaries in southwest England. Doctoral thesis , UCL (University College London).

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Abstract

Investigations of geomorphology and morphodynamics within the coastal zone have tended to treat the open coast as an independent system to that of any neighbouring estuaries. This separation is also evident within shoreline management, which has traditionally been undertaken within the context of coastal cells or estuarine valleys. The focus of this research is a comparative analysis of morphodynamic behaviour and sedimentary characteristics of connected open-coast – estuary systems. The north coast of Cornwall, southwest England, is notably indented and dominated by bedrock cliff and shore platforms. However, it also comprises some broad embayments that accommodate estuarine valleys and open coast, typically sandy beaches. The region provides an ideal environment within which to assess broad-scale coastal change and the association between estuarine and open-coast morphodynamics. Furthermore, it provides an opportunity to consider regional coherence in coastal behaviour and to evaluate the relative importance of local physical context vs. regional climate forcing. The Hayle, the Gannel and the Camel estuaries that are located within St Ives, Crantock and Padstow bays respectively, have received considerable attention in terms of the impacts of mining on estuarine sedimentation. The impacts on sediment supply, sedimentology and mineraology have been explored extensively in these past studies, however, very little consideration has been given to the nature of coastal geomorphology and coastal system dynamics. This PhD research explores mesoscale coastal dynamics, and evaluates coastal behaviour over decades to centuries in the context of climate and sea-level change. Historical geomorphological evolution of these estuaries and their adjacent shorelines are examined to evaluate morphodynamic connectivity through the application of shoreline analysis tools (such as Digital Shoreline Analysis System (DSAS) and Location Probability Analysis). This study showed that low shoreline recession along the north Cornwall coast, where sediments are present, has attributed most to the significant sea-level rise in this region (no significant change was observed on rocky low water shorelines). The high water shoreline imposes a different pattern of change in response to constraining factors which are triggered by both environmental factors and historical human activities. Changes over contemporary time scales are focused on bedform movement into, within and landward of inlets and are primarily driven both by waves in the outer estuary/ebb delta region and by tides in the channels/flood delta region. The inlets, however, are largely fixed in position by the bedrock valley, and channel dynamics within the estuary are dependent on the accommodation space provided by the valley. Sedimentary linkages are also explored through the sedimentological and geochemical analysis of sediments sampled from the intertidal zone of these systems. Based on grain-size parameters, there is considerable homogeneity in the sediment populations specific to the sub-environments sampled and analysed. There is evidence of sediment mixing between estuarine and beach environments. Geochemical (XRF) and mineraological composition of sediment indicate contamination by mine waste tailings in the estuaries resulting from major historical mining activities in the region with Sn, Cu, As and Zn as predominant in the Hayle, Pb and Zn in the Gannel and Sn, W, and Zr in the Camel estuaries. This research presents a multidisciplinary approach that employs a range of computer and lab-based analyses to integrate geospatial resources (including published maps, chart archives, etc) and sedimentological characteristics (including grain size and XRF analyses). The thesis is the first comprehensive comparative investigation of the morphodynamic behaviour and sedimentology of these north Cornwall estuaries.

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