The Response of Coastal Systems to Anthropogenic Climate Change

Abstract

<p>Over the coming century, coastal systems around the Pacific Islands will be the subject to the effects of global climate change, notably sea-level rise and extreme precipitation events, and the associated impacts caused by a combination of these events. This study investigates the likely effects of rising sea levels and increased fluvial flood magnitudes in different emissions scenarios on the coastal systems. The study is focused on coastal systems in Nadi, Fiji Islands. The aim of this thesis is to explore the impacts of future projected changes driven by climate change, that is, sea-level rise and fluvial discharge changes that would a result of precipitation changes, in order to understand more fully the vulnerability and response of coastal systems to the effects of climate change.</p> <p>The vulnerability of the coastal systems is studied using high-resolution aerial imagery that has recorded shoreline change for the past three decades, flood-inundation maps, digital elevation models, hydrological modelling, and conceptual models. These data and models are used to establish past shoreline changes, and flood extents generating predictions of projected future sea-level rise (0.25 m, 0.5 m, 0.75 m, 1 m, and 1.25 m) and changes in flood discharge magnitudes and frequencies (2, 5, 10, 20, 30, 50, and 100-year ARI) for three different climate change emissions scenarios (Current Emissions, Future Low Emissions or RCP2.6, and Future High Emissions or RCP8.5). The impacts of these combined changes on coastal systems are explored by overlaying land-use and cover mapping on modelled future shoreline change and flood-hazard maps.</p> <p>First, the results of this study indicate that the coastal system in Nadi has experienced net shoreline retreat during the last few decades. The land most susceptible to retreat are northern Nadi Delta and the delta areas close to the Nadi River mouth, as well as areas north and south of Nadi Bay. Significant wetland loss has also been observed in Nadi Delta over the past three decades. The history of retreat and loss of wetlands suggests that the existence of coastal processes will allow continued shoreline retreat in these areas as sea level rises in future.</p> <p>Second, based on a high-resolution digital elevation model, the study suggests sealevel rises from 0.25 m up to 1.25 m will, unsurprisingly, result in progressively greater areas of inundation, but more importantly the increase in inundation area is likely to be non-linear, with the greatest increase in inundation area predicted from 0.5 m to 0.75 m, and from 0.75 m to 1 m sea-level rise. The results also indicate that while sealevel rise of up to 0.25 m does not have a significant impact on the Nadi Delta, the Nadi Delta is very vulnerable to sea-level rise of 0.5 m and more. In addition to this, the wetland communities, consisting of mangrove and marsh, were the most susceptible land cover and vegetation type to rising sea levels.</p> <p>Third, the results of hydraulic modelling suggest that the biggest impacts of future emissions scenarios, in terms of degree of change and the land cover types affected, are going to occur as a result of the highest frequency events, that is, the 2- and 5-year ARI events. The analysis for the combined effects of fluvial flooding and sea-level rise suggests that the increase in the area inundated by fluvial flooding when sea-level rise is factored in ranges from around 2.5% up to 17%. The general trend of the combined impact on inundation extent is non-linear, where the greatest impacts are observed with sea-level rise of 0.5 to 1.0 m when compounded with fluvial flooding. Furthermore, these combined events impact both high-value land uses, such as agricultural, dense commercial, industrial, residential, as well as important natural vegetation communities, such as mangroves and marshes.</p> <p>Finally, the results and the conceptual understanding achieved from this study and other empirical studies were used to develop a conceptual model of delta response to future climate change. The stability of deltas is dependent on sediment flux and the condition of the wetland communities present on the deltas, which are influenced by sediment trapping, fluvial flooding, sea-level rise and mangrove management. It is essential that wetland communities are proactively managed around coastal systems to improve their resilience, and sediment flux is improved to lower catchment areas to minimise the effects of climate change.</p> <p>The results of this thesis, thus provide observations of hypothetical situations and the general patterns of change across a broad range of sea-level rise, discharge and emissions scenarios, and indicate the potential flood-hazard areas in anticipation of future climate change. It also highlights the importance of active catchment management actions to reduce the vulnerability of coastal systems to climate change.</p>