Shoreline Response to Relative Sea Level Change: Culbin Sands, Northeast Scotland

Comber, Darren P. M (1993) Shoreline Response to Relative Sea Level Change: Culbin Sands, Northeast Scotland. PhD thesis, University of Glasgow.

Abstract

The aims of this research are to establish shoreline responses to relative sea level change in the Moray Firth. Using both the sea level history and contemporary coastal sediment budgets, a "palaeosediment budget" is presented, which is used to assess the response of the shoreline to changing conditions of relative sea level and sediment supply during the Holocene. The area around Culbin and Burghead Bay in NE Scotland comprises a series of relict shoreline and associated unconsolidated deposits which have formed since the last glacial maximum ca. 18 000 years ago. The Culbin foreland is composed of a suite of shingle storm beach ridges abandoned at up to 3 km inland. The storm ridges were deposited under falling relative sea level after the peak of the Holocene sea level maximum ca. 6500 BP as a staircase of features between 11 and 3 m OD, and are backed landwards by fine grained sediments which have been deposited in their lee. Using modern analogues from The Bar, a barrier beach 3 km downdrift, these are successfully used in conjunction with more standard dated indicators in the reconstruction of relative sea level trends for the area. Detailed morphological investigation of the shingle ridges demonstrates relationships between crest spacing and the regularity of sediment supply, which demonstrate rapid changes in ridge altitude to be directly related to sediment supply, and indirectly related to relative sea level change, An allied study of contemporary processes operating along the Culbin foreshore and at The Bar was also undertaken. Wave data from commercial sources was supplemented by data from a directional wave recorder constructed specifically for the project. Modal wave height and period in the Culbin area are low, at only 1 m and 4 s respectively. The wave recorder demonstrates the under-representation of low (< 0.5 m) waves in commercial records and the presence of converging wave trains. The incidence of swell from the NE sector creates a dominant westerly longshore current, responsible for the strong sediment sorting and drift-aligned landform assemblage at the coast. Tidal currents were found to be very weak in the Culbin area, and are not an important sediment transport mechanism. The primary mode of contemporary foreshore sedimentation has altered radically from a shingle to a sand dominated environment, with shingle now only actively deposited at the proximal end of The Bar. Quantification of the sedimentary inputs and outputs to the Culbin area allowed two sediment budgets to be calculated, one for the sandy Culbin foreshore, and a second shingle budget for The Bar. These both indicate that longshore transport remains the dominant mode of transport, with up to 3.3 x 104 m3 a-1 of sand transported along the Culbin foreshore, and 0.1 x 104 m3 a-1 of shingle along the proximal flank of The Bar. The calculation of a palaeosediment budget aimed to produce a first order quantification of the volumetric development of Culbin during the Holocene period. Adopting a similar methodology as the contemporary sediment budget, the inputs and outputs of sediment to the coastal zone during the Holocene were quantified, although at an order-of-magnitude scale. The supply of shingle from the neighbouring River Findhorn was insufficient to account for the development of Culbin alone, and an additional updrift supply of shingle from the Spey helps to explain the genesis of a composite landform the size of Culbin. A three stage developmental model is proposed, which attempts to explain the variable nature of sediment supply to Culbin under different relative sea levels at Culbin throughout the Holocene. This is based upon the identification of an operational water depth of 6 m around the northern flank of the present Covesea-Burghead area, defining a critical depth below which shingle is no longer actively transported in the nearshore zone. Phase 1 relates to the period 9500-7200 BP, when operational depths were below 6 m, allowing the free passage of shingle from the River Spey to augment the supply from the River Findhorn reaching Culbin. Phase 2 occurs between 7200 and 4300 BP, when operational depths exceeded 6 m and the supply of shingle from the Spey to Culbin was halted. Phase 3 is dated post-4300 BP, when the Spey link was re-established, but shingle supplies were beginning to fail from both rivers. From this, a developmental model is described, which attempts to explain the evolution of the Culbin area under conditions of varying relative sea level and sediment supply. Having considered the past and present evolution of Culbin, an assessment of the future development of the area is made in the light of potentially rising relative sea level as eustatic sea level begins to match the rate of isostatic rebound in the Moray Firth.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Geomorphology, Physical geography
• Date of Award: 1993
• Depositing User: Enlighten Team
• Unique ID: glathesis:1993-75781
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/75781