Michael A. Kinsela

Extreme coastal erosion enhanced by anomalous extratropical storm wave direction

Extratropical cyclones (ETCs) are the primary driver of large-scale episodic beach erosion along coastlines in temperate regions. However, key drivers of the magnitude and regional variability in rapid morphological changes caused by ETCs at the coast remain poorly understood. Here we analyze an unprecedented dataset of high-resolution regional-scale morphological response to an ETC that impacted southeast Australia, and evaluate the new observations within the context of an existing long-term coastal monitoring program. This ETC was characterized by moderate intensity (for this regional setting) deepwater wave heights, but an anomalous wave direction approximately 45 degrees more counter-clockwise than average. The magnitude of measured beach volume change was the largest in four decades at the long-term monitoring site and, at the regional scale, commensurate with that observed due to extreme North Atlantic hurricanes. Spatial variability in morphological response across the study region was predominantly controlled by alongshore gradients in storm wave energy flux and local coastline alignment relative to storm wave direction. We attribute the severity of coastal erosion observed due to this ETC primarily to its anomalous wave direction, and call for greater research on the impacts of changing storm wave directionality in addition to projected future changes in wave heights.

Shoreface Controls on Barrier Evolution and Shoreline Change

Barriers exist in a continuum of forms, which are fundamentally governed by processes that shape the shoreface to determine the envelope available for sediment accommodation. This envelope is contained between the shoreface and underlying surface defined by the continental shelf and coastal plain (i.e., the substrate). Barrier form also depends on coastal change that is constrained, following reasonably well-established principles, by the volume and type of sediment supply (or loss) and rates of change in sea level that modify the shoreface and associated accommodation potential. While the shoreface is therefore significant to barrier form and behavior, processes that shape the shoreface itself remain poorly understood. In particular, systematic long-term evolution of the shoreface, which is evident in geological data, indicates not only a time-varying morphology, but also a lagged response to environmental change. Such shoreface evolution has implications for barrier evolution (and vice versa). In this chapter, we review (1) relations between shoreface and barrier form, (2) limits to knowledge on shoreface behavior and insights from depositional records from which systematic changes over time can be inferred, and (3) exploratory experiments on the morphodynamic timescale of shoreface change. The third part of the review derives from results of experimental modeling of combined shoreface and barrier evolution constrained by geologic data. The numerical experiments demonstrate that, on intermediate timescales (decades to centuries) that are most relevant to coastal management and planning, adjustments are dominated by sediment exchanges between the beach and shallower portions of the shoreface in response to rapid changes in boundary conditions, especially sea level. Significant morphodynamic hysteresis can be expected from the partial adjustment of lower shoreface geometry during sea-level change, resulting in ongoing barrier evolution and shoreline migration after the stabilization of boundary conditions.

Regional Scale Coastal Mapping to Underpin Strategic Land Use Planning in Southeast Australia

ABSTRACT Hanslow D.J.; Dela-Cruz J.; Morris B.D.; Kinsela M.A.; Foulsham E.; Linklater M., and Pritchard T.R., 2016. Regional scale coastal mapping to underpin strategic land use planning in southeast Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 987–991. Coconut Creek (Florida), ISSN 0749-0208. In the current study we develop spatial data to inform strategic land use and coastal planning which considers coastal hazards and the protection, maintenance and in some cases, restoration of our waterways. The benefits of sustainable coastal development underpinned by a well-established understanding of coastal processes and our ability to avoid negative impacts from inappropriate placement of development are well understood. With climate change and increasing use of coastal systems there is increasing need to identify and manage both current and possible future risk exposure. Mapping is undertaken to allow upfront identification of risks and benefits associated with potential future land use as well fundamental data to help inform coastal assessments. This will help ensure impacts on the coastal environment are minimised and impacts on proposed development from coastal hazards are avoided. The study has several parallel components addressing estuarine water quality and ecosystem health, sea level rise inundation, coastal erosion and sediment/geomorphic setting. Additional benefits of the project include risk based assessment to enable prioritisation of effort to address existing development which is either exposed to coastal hazards or having an impact on coastal waterways.

A Flexible Approach to Forecasting Coastline Change on Wave-Dominated Beaches

ABSTRACT Kinsela, M.A., Morris, B.D., Daley, M.J.A. and Hanslow, D.J., 2016. A Flexible Approach to Forecasting Coastline Change on Wave-Dominated Beaches. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 952–956. Coconut Creek (Florida), ISSN 0749-0208. Standard methods of predicting coastline change typically rely on the analysis and extrapolation of historical trends, or simple geometric rules that consider the response of idealised coastal morphology to environmental change – e.g. the Bruun rule of coastline retreat due to sea level rise. In practice, predictions based on such methods are often challenged, due to the limited capacity to characterise natural geomorphic complexity, and consequently, the inability to satisfy restrictive assumptions. While statistical simulation methods offer a means to manage uncertainty in environmental forcing, datasets, and predictive models, the continued reliance on simple geometric rules introduces unnecessary error into forecasts. The increasing coverage and detail of geomorphic datasets, provided by modern remote sensing techniques (e.g. LiDAR, GPR), means that more rigorous approaches are now achievable in many settings. This paper presents a simple yet flexible approach to forecasting coastline change on wave-dominated beaches. The method combines a Monte Carlo simulation approach with a volumetric coastline response model that features a parameterised sediment budget. Model complexity reflects the levels of topographic and geomorphic data typically available for beaches in southeastern Australia, allowing for the sediment budget parameterisation to be broad or refined. A volumetric implementation of all components of coastline variability and change ensures that forecasts are sensitive to the complex coastal geomorphology of individual beaches. Application of the method demonstrates the sensitivity of forecast coastline change to three-dimensional beach and dune morphology, irregular substrates comprising mixed hard and soft materials, and complex shoreface surfaces featuring submerged reef structures.

A Regional Scale Approach to Assessing Current and Potential Future Exposure to Tidal Inundation in Different Types of Estuaries

Broad scale assessments of impacts associated with sea level rise have mainly been undertaken using ocean water level data from tide gauges located in harbours and ports assuming that these can be applied directly in mapping inundation throughout estuaries. On many coasts, however, exposure to sea level rise comes about through inundation adjacent to rivers and estuaries, in many instances far from the ocean. In this study, we examine the potential impacts of sea level rise within the diverse estuaries of South East Australia. We use an extensive and long-term water level data set, which show that water levels within the different types of estuaries vary from ocean water levels. We map potential inundation scenarios for each estuary using an approach which improves on the commonly used bath tub method by allowing for variation in tidal processes both between and along estuaries. We identify considerable exposure to future sea level rise, and variable suitability of the bath tub method within different estuaries. Exposure is particularly high around tidal lake systems, where reduced tidal ranges have allowed development to occur in relative proximity to present sea level, and around larger coastal rivers, which feature extensive low-lying plains exposed to potential inundation.