Cooper, Andrew

Green, Andrew

Loureiro, Carlos

English

Recognition of the fundamental role of the geological framework in coastal morphodynamics has increased over the past two decades, with significant contributions highlighting the importance of geological control on beach and nearshore morphology and dynamics. Although it is self-evident that the evolution of coastal systems is intimately linked to the antecedent or inherited geological framework, its influence on contemporary coastal processes is poorly represented in decadal to centennial (i.e. meso-scale) conceptual and numerical models of coastal change. This is partly driven by a limited quantitative understanding of how the geological framework controls modern coastal evolution and how it relates to hydrodynamic processes and morphodynamics, as well as difficulties in transforming qualitative geophysical observations into quantitative parameters. However, the poor incorporation of geological control in contemporary coastal models is mostly the outcome of widely accepted assumptions that equilibrium concave-upward profiles are a suitable representation of the nearshore, even when only covered by a veneer of sediment, and that barriers and beaches are unrestricted piles of sediment responding exclusively to hydrodynamic forcing.In this work, we explore a wealth of morphological, geophysical, sedimentary and hydrodynamic data to investigate the morphological configuration of nearshore profiles associated with various degrees of geological control along a broad range of coastal environments in the northern coast of Ireland. We analyse the adequacy of parametric limits for nearshore morphological evolution and compare nearshore profiles obtained from multibeam bathymetry of 15 sites with modelled equilibrium profiles based on a) uniform wave energy dissipation and b) balance of sediment transport relationships. Equilibrium profile models were applied considering the range of sediment sizes in the nearshore of each site and computed using the morphodynamics important wave conditions for nearshore sediment transport, determined by magnitude-frequency analysis.Results reveal a general disagreement between observed and modelled nearshore equilibrium profiles, also suggesting that the active nearshore zone extends seaward, deeper than indicated by parametric depth of closure approaches. Shallow sub-bottom seismic profiles from several of the study sites further demonstrate that the underlying geological framework exerts a fundamental control, which influences nearshore morphodynamics and large-scale coastal behaviour. Different modes of geological control on nearshore configuration are identified, based on the outline of the underlying bedrock surfaces and the wave ravinement surfaces. Our work expands the reasoning that coastal systems, more often than not, deviate significantly from simplified dynamic equilibrium concepts, and reaffirms that nearshore geological control is unavoidable in the prediction of future coastal evolution

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Geophysical Research AbstractsVol. 20, EGU2018-10523, 2018EGU General Assembly 2018© Author(s) 2018. CC Attribution 4.0 license.More than a concave-upward equilibrium profile: geological control onnearshore morphological configurationCarlos Loureiro (1,2,3), Andrew Green (2), Andrew Cooper (1,2)(1) Ulster University, School of Geography and Environmental Sciences, Coleraine, United Kingdom(c.loureiro@ulster.ac.uk; jag.cooper@ulster.ac.uk), (2) University of KwaZulu-Natal, Geological Sciences, Durban, SouthAfrica (greena1@ukzn.ac.za), (3) Universidade do Algarve, CIMA, Faro, PortugalRecognition of the fundamental role of the geological framework in coastal morphodynamics has increased overthe past two decades, with significant contributions highlighting the importance of geological control on beach andnearshore morphology and dynamics. Although it is self-evident that the evolution of coastal systems is intimatelylinked to the antecedent or inherited geological framework, its influence on contemporary coastal processes ispoorly represented in decadal to centennial (i.e. meso-scale) conceptual and numerical models of coastal change.This is partly driven by a limited quantitative understanding of how the geological framework controls moderncoastal evolution and how it relates to hydrodynamic processes and morphodynamics, as well as difficulties intransforming qualitative geophysical observations into quantitative parameters. However, the poor incorporationof geological control in contemporary coastal models is mostly the outcome of widely accepted assumptions thatequilibrium concave-upward profiles are a suitable representation of the nearshore, even when only covered bya veneer of sediment, and that barriers and beaches are unrestricted piles of sediment responding exclusively tohydrodynamic forcing.In this work, we explore a wealth of morphological, geophysical, sedimentary and hydrodynamic data toinvestigate the morphological configuration of nearshore profiles associated with various degrees of geologicalcontrol along a broad range of coastal environments in the northern coast of Ireland. We analyse the adequacy ofparametric limits for nearshore morphological evolution and compare nearshore profiles obtained from multibeambathymetry of 15 sites with modelled equilibrium profiles based on a) uniform wave energy dissipation and b)balance of sediment transport relationships. Equilibrium profile models were applied considering the range ofsediment sizes in the nearshore of each site and computed using the morphodynamics important wave conditionsfor nearshore sediment transport, determined by magnitude-frequency analysis.Results reveal a general disagreement between observed and modelled nearshore equilibrium profiles, alsosuggesting that the active nearshore zone extends seaward, deeper than indicated by parametric depth of closureapproaches. Shallow sub-bottom seismic profiles from several of the study sites further demonstrate that theunderlying geological framework exerts a fundamental control, which influences nearshore morphodynamics andlarge-scale coastal behaviour. Different modes of geological control on nearshore configuration are identified,based on the outline of the underlying bedrock surfaces and the wave ravinement surfaces. Our work expands thereasoning that coastal systems, more often than not, deviate significantly from simplified dynamic equilibriumconcepts, and reaffirms that nearshore geological control is unavoidable in the prediction of future coastalevolution.

More than a concave-upward equilibrium profile: geological control on nearshore morphological configuration

https://pure.ulster.ac.uk/ws/files/12501658/EGU2018_10523.pdf

More than a concave-upward equilibrium profile: geological control on nearshore morphological configuration

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