Thesis Defense – Josh Kelly (PhD)

Climate-driven coastal morphodynamics along Southeast Queensland, Australia

Abstract

Understanding how shoreline positions change in response to the variability of multitemporal climate and weather phenomena is essential for predicting the impact of future climate extremes. This dissertation aims to provide the first quantitative assessment on the relative influence of climate phenomena operating in the southwest Pacific region, i.e. El Niño-Southern Oscillation, on coastal morphodynamics along the southeast Queensland coast on a yearly to decadal timescale. Morphodynamic changes occurring at the timescale of days are also assessed in response to Tropical Cyclone Oma striking the region in February 2019. Previous attempts at directly correlating climate to natural shoreline evolution elsewhere were limited in their ability to observe the geomorphic changes as they relied on infrequent aerial photography, isolated beach profile data, or morphodynamic modeling results with no ground trothing. In an effort to capture intra-annual shoreline variability across large spatial scales (100’s km), shorelines are mapped using historical Landsat imagery. The accuracy of shorelines mapped automatically using spectral water indices was assessed by comparison to two contemporaneous GPS-surveyed intertidal zones, which represents the true position of the shoreline interface, which were conducted in Brazil and Massachusetts. The Modified Normalized Difference Water Index was determined to be the best index for automated shoreline mapping based on its superior accuracy and repeatable threshold value. MNDWI was used to automatically map over 9,000 km of historical shoreline positions along Queensland using Google Earth Engine. Shoreline change curves were generated using the USGS Digital Shoreline Analysis System and corrected for tidal variations using in situ gauge data and seasonal intertidal widths estimated from equilibrium beach profiles. Cross correlations between the shoreline dataset and the various climate indices were calculated using a 20 month maximum lag. Correlations suggest a bimodal climate control on shoreline change whereby El Niño-Southern Oscillation has the largest influence during negative Interdecadal Pacific Oscillation with the Subtropical Ridge becoming dominant during positive IPO. As observed using revolutionary PlanetScope satellite imagery, longshore transport along SE Queensland turned northwards in response to a significant anti-clockwise rotation of incident wave direction during Oma with erosion peaking at 41 m along the Sunshine Coast.