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NEARshore geological CONTROL on coastal morphodynamics: monitoring and modelling in high-resolution

Periodic Reporting for period 2 - NEARCONTROL (NEARshore geological CONTROL on coastal morphodynamics: monitoring and modelling in high-resolution)

Reporting period: 2018-02-01 to 2019-01-31

Understanding the fundamental controls and drivers of coastal evolution at event to decadal scales is paramount for successful management of sedimentary coastlines, particularly in the current context of increasing coastal development and the challenges posed by global environmental change. Despite general acceptance that the evolution of coastal systems is intimately linked to the inherited geological framework (i.e. coastal orientation, configuration of bedrock and older sedimentary surfaces, sediment type and supply), its influence on contemporary coastal processes is poorly represented in conceptual models of coastal change and generally disregarded in numerical models used worldwide to predict coastal evolution under sea-level rise (SLR). The poor incorporation of geological controls in coastal models is driven, in part, by a limited quantitative understanding of how the geological framework determines modern coastal evolution and how it relates to hydrodynamic processes and morphologic responses, as well as by difficulties in transforming qualitative geophysical observations into quantitative parameters. Long-held assumptions that simple equilibrium profiles are a suitable representation of the nearshore and that beaches are unrestricted piles of homogenous sediment that respond exclusively to hydrodynamic forcing, have further contributed to the poor incorporation of nearshore geological control in contemporary coastal models.
Challenging these oversimplified assumptions about coastal evolution and improving the quantification of geological control is essential to understanding event to decadal scale evolution of sedimentary coastlines and improving our ability to predict future coastal response to environmental, climate and sea-level change. NEARCONTROL focusses on the role and impact of the underlying geological control in the evolution of the nearshore, particularly in response to energetic storm conditions. The overall objective is to develop an approach that integrates high-resolution geophysical surveying and exploratory numerical modelling in order to test the hypothesis that the geological framework exerts the fundamental control on nearshore configuration and evolution. Our results highlight that the underlying geological and stratigraphic surfaces determine the overall shape of the nearshore and, by constraining sediment dynamics and morphological change, their impact in contemporary and future coastal evolution is unavoidable. Understanding these interactions and incorporating the associated uncertainties in coastal modelling frameworks will enhance quantitative and qualitative assessments of future coastal evolution and contribute to improved coastal adaptation to SLR.
During NEARCONTROL we synthesized the current knowledge on the geological influences in coastal barrier behaviour at decadal to centennial scales (mesoscale), which allowed to benchmark the challenges in describing and understanding mesoscale coastal change and develop a hierarchy of geological controls that individually or collectively determine the behaviour of coastal barriers. We identified nearshore (or shoreface) morphology as an intermediate-level control, as although sedimentary nearshores are recognized as a dynamic feature, morphological changes occur at much longer timescales rendering nearshore morphology an effective geological control on contemporary and future coastal barrier behaviour. Detailed analysis of nearshore morphology and stratigraphy in various coastal sites in South Africa and Ireland using high-resolution geophysical data revealed wide variability in nearshore configuration, which consistently departed from equilibrium models and often presented compound profiles. Our results show that the underlying bedrock and stratigraphic surfaces exert spatially variable nearshore geological control, with the wave ravinement surface (i.e. erosional surface created by wave erosion and scour as the coastline migrates landward with SLR) being prevalent in determining the morphological configuration of sedimentary nearshores. The dependence of the modern nearshore surface on the underlying wave ravinement surface confirms that contemporary nearshore dynamics are underpinned by antecedent wave erosion, highlighting the importance of extreme coastal storms in shaping the past, present and future nearshore morphology. For Ireland in particular, our results show positive temporal trends in the frequency and intensity of coastal storms over the past 60 years, indicating that winter wave conditions are becoming more energetic and stormier with attendant implications for nearshore morphological behaviour and future evolution. We also quantified storm-induced nearshore morphological change with unprecedented detail in a South African embayment, and these results were integrated with hydrodynamic numerical models to explore the coupling between inherited large-scale bedforms and nearshore waves in the development of nearshore erosional hotspots.
The response of sedimentary barriers and nearshore areas to accelerated SLR in a changing climate will be overwhelmingly determined by the geological and stratigraphic framework of the coastal area to be transgressed and the extent to which future wave erosion is effective in modifying the nearshore morphology. The synchronous barrier and nearshore landward translation under SLR that is still assumed in most mesoscale coastal evolution modelling approaches is far from ubiquitous, and the conceptual and analytical approach that underpins the equilibrium-profile model grossly oversimplifies the processes and mechanisms of coastal evolution. Our results demonstrate that decoupled barrier-nearshore evolution over an irregularly erodible surface is to be expected for most sedimentary coasts and that progress in predicting mesoscale coastal evolution requires an improved understanding of nearshore erosion during extreme coastal storms.
The research developed and results obtained have been and will continue to be disseminated in various ways. So far, 10 manuscripts have been published or submitted for peer-review in scientific journals, 16 presentations have been delivered to national and international conferences, 1 MSc thesis has been completed, a conference session and a workshop have been organized, as well as a training event and one outreach activity.
The work and results produced during NEARCONTROL will improve the conceptualization and representation of nearshore morphology and behaviour in coastal evolution models, moving beyond simplistic equilibrium-profile approaches. The dependence of the modern nearshore morphology on the underlying erosional surfaces identified in our results implies that contemporary nearshore dynamics is significantly controlled by antecedent erosional processes, with decoupled barrier-nearshore transgression over irregularly erodible surfaces being a dominant response to SLR along sedimentary coastlines. Improved predictions of coastal response to SLR require an explicit integration of geological controls and associated uncertainties in models of coastal change, rendering most commonly used numerical models of coastal evolution inadequate. This has wide-ranging implications for the management of sedimentary coasts in a changing climate and results of NEARCONTROL will feed into initiatives to revise decadal to centennial models of coastal evolution.