Shore profile development on rock coasts: an exploratory modelling study

Abstract

A new numerical model of rocky shore profile evolution is developed and used to investigate two fundamental questions of rock shore geomorphology concerning: 1. The relative dominance between wave erosion and weathering processes in shore platform development; 2. The inheritance of rocky shore morphology due to relative sea level changes. An exploratory modelling approach is adopted which captures the key rocky shore processes that drive erosion (intertidal wave erosion and rock weathering), and the role of water level fluctuations and relative sea level changes (eustatic and tectonic) that modulate erosion processes. The model differs from existing numerical models of rocky shore evolution in several ways: first, it considers various wave types (unbroken, breaking, and broken waves); second, it employs a grid discretization scheme which allows generation of a wide range of profile geometries, and; third, it represents gradual weathering processes through the in-situ reduction of rock strength. Results from initial 2088 simulations show that the model is capable of producing a wide range of shore profile geometries, ranging from cliffs with/without a notch, cliffs with narrow ramps and benches, to cliffs with a variety of shore platform geometries including type-A and type-B shore platforms. Model outcomes are shown to be qualitatively consistent with previous field-based shore platform observations: for instance, positive and negative correlations exist respectively between tidal range and platform gradient, and between platform width and platform elevation. The model is further validated using geometric data obtained from a systematic and large scale (~700 km) analysis of LiDAR data from the rocky shore of southwest England. Analyses show qualitative consistency between model results and morphometric data: for instance, shore platform gradient increases with tidal range and rock strength, and decreases with wave height. The systematic and large scale analysis of rocky shores in southwest England is also used to derive a multi-variate empirical model which is able to predict shore platform gradient with an error 5% less than the error in a single-variate model used on the same dataset. Following model validation the model is used to investigate the two research questions. The first research question is concerned with the long-standing ‘waves versus weathering’ debate. The model considers a broad parameter space in which at one extreme waves can produce shore platforms without rock weakening, while at the other, waves require rock weakening to produce shore platforms. This approach enables an assessment of the relative dominance of wave erosion and weathering processes in shore platform development. Results show that with a constant sea level, in micro-tidal environments, wave erosion is more important in the development of sub-tidal shore platforms as well as sub-horizontal shore platforms with a receding seaward edge, but weathering dominates the development of sub-horizontal shore platforms with a stable seaward edge. In contrast, in mega-tidal settings, depending on process parameterisations, modelled platforms of similar geometry are sometimes wave dominated and sometimes weathering dominated. Clear recognition of equifinality in platform development is important, in part elucidating why there has been such longstanding debate regarding the relative importance of formative controls. The second rocky shore research area investigated in this thesis concerns the inheritance of rocky shore morphology due to relative sea level changes. Emphasis is placed on the development of uplifted Holocene marine terraces, with specific consideration of the interplay between wave erosion and weathering processes, relative sea level changes due to tectonic uplift events, and feedbacks relating to the emerging rocky shore morphology. A Monte Carlo simulation comprising 10,010 model runs enables analysis of a range of uplift scenarios. Analyses show that intertidal erosion process efficacy, amount of vertical displacement from tectonic uplift events, the emergent terrace morphology, and its morphological feedbacks on erosion processes are the key factors controlling process interplays, potentially altering evolutional pathways of uplifted marine terrace development. Both wave erosion and weathering processes are each important controls on the formation and preservation or destruction of terraces, whereas weathering processes can be critical when wave erosion efficacy is small, by deteriorating rocks and facilitating wave erosion, potentially leading to different uplifted marine terrace morphology. This is an important consideration for efforts globally to interpret marine terrace sequences, because existing terrace models do not suitably represent weathering processes. The findings of the two model application studies provide new insights into two key rocky shore issues, and demonstrate that this new exploratory model represents a useful tool for better understanding of rocky shore profile development.