Impacts of sea level rise on flooding in an estuarine environment

Impacts of sea level rise on flooding in an estuarine environment

Coastal communities are commonly located at the interface between river catchment, estuary and ocean. This location, while providing many of the values that make our coastal towns unique, also makes them vulnerable to environmental phenomena. Coastal towns are often susceptible to flooding both from the seaward direction, as storm tide inundation, and from the landward, as riverine flooding. The combination of these two flooding mechanisms results in high flood risks for coastal communities.

Vulnerable as they already are, projected sea level rise, due to climate change, poses high risks to towns located on coastal floodplains. Under sea level rise, not only does the risk of storm tide inundation rise, but also the risk of riverine flooding, due to a higher tailwater elevation. To ensure that future flood risks to coastal communities are minimised, the Victorian Coastal Strategy (VCS) 2008 recommended a policy of planning for sea level rise of not less than 0.8 metres by 2100. This recommendation was based on information arising from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (IPCC, 2007), which projected a sea level rise of 0.18-0.59 metres by 2090-2099, with an additional 0.1- 0.2 metres from potential ice sheet melt. Adopting a precautionary approach, the Victorian Coastal Committee recommended that a minimum sea level rise of 0.8 metres (to 2100) be adopted for planning purposes. The VCS 2008 also recommended that the combined effects of tides, storm surges, coastal processes and local conditions be considered when assessing the
impacts of climate change.

In this study, current and potential future flooding behaviour in Port Fairy, a coastal town in south-west Victoria, Australia, is investigated. In 2008, Water Technology completed the Port Fairy Regional Flood Study (PFRFS) and in 2010 undertook further modelling to incorporate the 0.8 m sea level rise recommended by the VCS 2008. The sea level rise scenario was based on modelling by McInnes et al (2009), and also accounted for a 19% increase in wind speed causing increased storm surge levels. The results of the modelling allow us to quantify the effect of projected sea level rise on design flood levels on a coastal floodplain. We identify the locations most affected by sea level rise and the upstream extent of this effect. We also investigate the balance of risks due to storm tide inundation and riverine flooding, the parts of the floodplain dominated by each mechanism and the areas subject to high risk from both.

Through this modelling we demonstrate a method of using a dynamic coastal boundary condition to simulate the combined effects of tide, storm surge and projected sea level rise. Most flood studies in coastal environments currently use a static downstream water level for design flood modelling. However, this gives an overly conservative tailwater elevation, as peak storm tide levels are generally short-lived and may not fully propagate up an estuary. A dynamic ocean water level boundary may provide a more accurate, while still conservative, estimate of tailwater conditions.

Read the full publication here: Bishop et al PRCC Sept 2010

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