Sea-level rise is causing coastal inundation events in estuaries, harbours, bays, and tidal rivers to happen more often as predictable daily high and low tides reach higher levels. This can lead to coastal inundation happening under benign weather conditions, as flood thresholds are exceeded due to tides alone without the influence of storm surges or other phenomena. As such, changes in frequency of this 'tide-only' inundation may be a useful metric to quantify the role that sea-level rise plays in modulating the risk of coastal inundation from high still water levels. Here we present a conceptual model for 'tide-only' inundation and propose a practical methodology to formulate tide-only inundation statistics: estimates, historical trends, and future projections. This enables this emerging natural hazard to be fully incorporated into new and existing coastal risk assessment frameworks and considered in coastal management and planning strategies at local and national levels. Further, it leads to a framework that can quantify the role that tides play in coastal flooding as sea levels rise.

An approach to reduce forecast data to coastal waveguide coordinates is described and demonstrated, informed by the literature on coastally trapped waves (CTWs). All discussion is limited to the Australian mainland but the approach is generally relevant to regions where CTWs influence sea level, including the Americas and Africa. The approach does not produce new forecasts, but aims to focus forecaster attention on aspects of sea level forecasts prominent on the long Australian coast. The approach also explicitly addresses spatial issues associated with measuring coastal paths. Coastal paths are scale dependent and forecast models discretize the coastal boundary differently. A well-defined coastal path is required for the quantitative application of CTW concepts such as propagation distance and offshore direction. The relevance of coastally trapped signals and remote forcing is documented in the oceanographic literature, but is effectively unknown to the general public and rarely mentioned in press reports of sea level events such as nuisance flooding. Routine presentation of forecast guidance in waveguide coordinates could contribute to the transfer of oceanographic research understanding into forecast narratives. In addition, the approach can facilitate quantitative forecast evaluations that target CTW properties. Two ocean forecast systems are contrasted in this framework for the Australian mainland. One year of daily forecasts are compared, with indications that model baroclinicity is of practical relevance.

The operational Australian Bluelink ocean forecast system is used to transform physical oceanographic observations into coherent analyses and predictions. These analyses and predictions form the basis for information services about the marine environment and its ecosystem, and can provide boundary data for weather predictions. Bluelink information services are available to marine industries (e.g. commercial fishing, aquaculture, shipping, oil and gas, renewable energy), government agencies (e.g. search and rescue, defence, coastal management, environmental protection), and other stakeholders (e.g. recreation, water sports, artisanal and sport fishing) who depend on timely and accurate information about the marine environment. This review highlights the last 15 years of Bluelink achievements delivering mesoscale (eddy-resolving) to sub-mesoscale and short- to medium-range (days to weeks) ocean forecasts and reanalyses. Key achievements include the development of a global ocean forecasting and reanalysis system, a relocatable ocean-atmosphere model and a littoral zone analysis and forecasting capability. Beyond the traditional short-term forecasting of physical ocean properties (temperature, salinity, surface height, currents, waves), marine activities such as water quality and habitat management as well as climate monitoring increasingly rely on operational oceanographic data and products. These are areas of active research of the Bluelink team in collaboration with national and international partners.

An approach to reduce gridded forecast data to novel waveguide coordinates is demonstrated; informed by the literature on coastally trapped waves. This does not produce new forecasts per se, but reduces data to a useful model-independent physically ordered array. Discussion is limited to the Australian mainland and forecast systems currently maintained in national operations. Heterogenous forecast models are considered with regard to the development of "seamless" sea level services across timescales.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/jlJO_dxHwuw

This study presents the first assessment of the observed frequency of the impacts of high sea levels at locations along Australia’s northern coastline. We used a new methodology to systematically define impact-based thresholds for coastal tide gauges, utilising reports of coastal inundation from diverse sources. This method permitted a holistic consideration of impact-producing relative sea-level extremes without attributing physical causes. Impact-based thresholds may also provide a basis for the development of meaningful coastal flood warnings, forecasts and monitoring in the future. These services will become increasingly important as sea-level rise continues.The frequency of high sea-level events leading to coastal flooding increased at all 21 locations where impact-based thresholds were defined. Although we did not undertake a formal attribution, this increase was consistent with the well-documented rise in global sea levels. Notably, tide gauges from the south coast of Queensland showed that frequent coastal inundation was already occurring. At Brisbane and the Sunshine Coast, impact-based thresholds were being exceeded on average 21.6 and 24.3 h per year respectively. In the case of Brisbane, the number of hours of inundation annually has increased fourfold since 1977.

A system for providing routine seven-day forecasts of sea level observable at tide gauge locations is described and evaluated. Forecast time series are aggregated from well-established operational systems of the Australian Bureau of Meteorology; although following some adjustments these systems are only quasi-complimentary. Target applications are routine coastal decision processes under non-extreme conditions. The configuration aims to be relatively robust to operational realities such as version upgrades, data gaps and metadata ambiguities. Forecast skill is evaluated against hourly tide gauge observations. Characteristics of the bias correction term are demonstrated to be primarily static in time, with time varying signals showing regional coherence. This simple approach to exploiting existing complex systems can offer valuable levels of skill at a range of Australian locations. The prospect of interpolation between observation sites and exploitation of lagged-ensemble uncertainty estimates could be meaningfully pursued. Skill characteristics define a benchmark against which new operational sea level forecasting systems can be measured. More generally, an aggregation approach may prove to be optimal for routine sea level forecast services given the physically inhomogeneous processes involved and ability to incorporate ongoing improvements and extensions of source systems.

The Bureau of Meteorology maintains several operational forecasting systems; these include routine global models to forecast weather, climate, and now ocean circulation (OceanMAPS). The Bureau is also the lead federal agency responsible for the provision of a flood warning service within Australia. Some important locations used for flood forecasting and warning are near enough to the ocean that river height forecasts cannot disregard variations in nearby coastal sea level. At such locations the Bureau's operational event-based flood forecasting system employs official harmonic tide predictions together with a dependant rating curve to account for this effect.The relatively recent availability of routine operational OceanMAPS non-tidal sea level forecasts for the Australian region has lead to the development of a modified sea level forecast that includes both tidal and non-tidal effects. The non-tidal effects represented by OceanMAPS include both local effects (e. g., surge, boundary current and eddy impingement) and non-local effects (e. g., coastal trapped waves). This paper demonstrates incorporation of routine sea level forecasts into coastal river height predictions at three sites within the flood forecasting system. The quantified results are in the form of a simple case study based on recent flood events on the NSW North Coast. A promising aspect of the present concept is the low implementation effort and scalability as OceanMAPS is operationally maintained and provides routine forecasts available around the entire Australian coastline.The case study method consisted of using the existing configuration of the river forecast system (URBS) and substituting three viable sea level input into the existing dependant rating for each coastal river gauge. The three sea level inputs were: [1] the reference tide gauge harmonic prediction, [2] the reference tide prediction plus a manually inferred anomaly adjustment and [3] the alternative sea level forecast. The results indicate that the accuracy of the peak river level forecast using URBS was not sensitive to the difference between current best practice and the proposed new method; i.e. very similar peaks resulted from using either input [2] or [3] above. However, decreasing the extent to which operational forecasters are required to utilise the manually inferred adjustment is considered advantageous and thus it is suggested that the sea level forecasts from OceanMAPS be incorporated for operational flood forecasts in NSW.