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

Periodic Reporting for period 1 - NEARCONTROL (NEARshore geological CONTROL on coastal morphodynamics: monitoring and modelling in high-resolution)

Reporting period: 2016-02-01 to 2018-01-31

The role of the geological framework in coastal morphodynamics has been increasingly recognized over the past two decades, with contributions highlighting the importance of geological control on beach and nearshore morphology and dynamics. Despite general acceptance that the evolution of coastal systems is intimately linked to the inherited geological framework, its influence on contemporary coastal processes is poorly represented in conceptual and numerical models of coastal change used worldwide and this has significant implications for the management of coastal areas. The poor incorporation of geological control in coastal models is driven, in part, by limited quantitative understanding of how the geological framework controls modern coastal evolution and how it relates to hydrodynamic processes and morphologic response, as well as by difficulties in transforming qualitative geophysical observations into quantitative parameters. Assumptions that simple equilibrium concave-upward profiles are a suitable representation of the nearshore and that beaches are unrestricted piles of sediment that respond exclusively to hydrodynamic forcing, have further contributed to the poor incorporation of geological control in contemporary coastal models.
In challenging these assumptions, NEARCONTROL project focusses on the role and impact of the underlying geological control in the evolution of nearshore areas, particularly during energetic conditions. We explore nearshore areas in eastern South Africa and in the north of Ireland, which are ideal natural laboratories given the exposure to energetic wave climates and complexity of the coastal geomorphology. Our research combines high-resolution geophysical surveying (multibeam bathymetry and sub-bottom seismic profiling) with exploratory numerical modelling in order to link the sedimentary and geological framework. The overall goal is to test the hypothesis that the geological framework exerts the fundamental control on the morphological configuration of the nearshore and, by influencing contemporary coastal processes, geological control is unavoidable in the prediction of future coastal evolution. Our work expands on previous research by analysing nearshore morphological response with unprecedented detail and investigating various modes of underlying geological control.
The NEARCONTROL work programme for the reporting period concentrated on training, acquisition, processing and analysis of high-resolution geophysical datasets (Work Packages 1 to 4). Formal training in high-resolution multibeam echo-sounding and hands-on training in shallow sub-bottom profiling (WP1) was completed and followed by the dedicated geophysical surveying campaigns of the project (WP2). The nearshore morphological response to energetic wave conditions in the eastern coast of South Africa during the austral winter was monitored in detail in one location (Isipingo, Durban). The characteristics of the nearshore wave field were further analysed using numerical wave modelling according to event-based conditions (WP3). Complications arising from vessel access and technical problems with the seismic instrumentation delayed the data acquisition and processing (surveying was postponed to the austral winter of 2017). To overcome the delays in data acquisition, high-resolution geophysical data for a range of locations along the northern coast of Ireland were explored (WP4). Key results in the highly-compartmentalized coast of Ireland reveal wide variability in nearshore configuration, almost always departing from equilibrium models and often presenting compound profiles. The degree of nearshore geological control, as determined by the outline of the acoustic basement differs between areas, while for most locations the nearshore configuration mirrors the underlying wave ravinement surface. The relevance of the wave ravinement surface highlights the importance of extreme erosional events in shaping the configuration of the nearshore. This has led to a dedicated analysis of winter time wave conditions and storms in the northwest of Ireland. Main results indicate significant positive temporal trends for winter wave heights, number of storms and their intensity, suggesting that winter wave conditions in the northwest of Ireland are becoming more energetic and stormier with attendant implications for nearshore morphology. Initial findings based on the geophysical datasets collected in South Africa also reveal that the geological and stratigraphic framework, as determined by the underlying bedrock and wave ravinement surface, is of paramount importance in determining nearshore configuration and influencing contemporary morphological response. This was evidenced by the development of nearshore erosional hotspots during the austral winter, which are related to the presence and configuration of a shoreface connected ridge and bathymetric irregularities and their influence on the nearshore wave field.
Given the increasing recognition of the role of geological control on coastal morphodynamics and the lack of a coherent evaluation of the various modes of geological control on decadal to centennial barrier behaviour, a review paper was developed to benchmark and integrate current knowledge. Alongside this review paper, the topics investigated in the project and the results obtained so far have been, and will continue to be, promoted and disseminated to scientific audiences and communicated to the public. Overall, 6 papers have been submitted for review to scientific journals, 9 abstracts and presentations have been delivered or submitted to national and international conferences and 1 MSc thesis has been completed. Regarding outreach and communication, contribution to one training event and organization of one outreach activity were also developed during this reporting period.
The work developed so far has revealed that the geological and stratigraphic frameworks exerts a fundamental control on nearshore configuration. For the locations analysed in Ireland and South Africa, erosion during extreme events is the dominant process in shaping the nearshore configuration, as it drives the development of the stratigraphic surfaces (wave ravinement and storm erosion surfaces) on which contemporary morphodynamics operate. As such, the nearshore configuration appears to be dominated by erosion and models of nearshore evolution based on uniform dissipation of wave energy or balance of sediment transport may be conceptually flawed. This may render decadal to centennial projections of coastal evolution due to sea level rise based on equilibrium profile models grossly inadequate, and yet these are the most commonly used models worldwide. The initial focus of the work was the eastern South African coast, but the widening of the spatial scope of the project has allowed investigation of the main hypothesis of NERCONTROL project in various locations. This will be further analysed using numerical modelling (WP5) in order to explore a varied range of conditions, potentially leading to revisions in the conceptualization and analysis of coastal evolution models with implications for coastal management.
Nearshore morphological configuration and underlying geological surfaces in Isipingo, South Africa