Assessing long-term changes in shoreline position using multi-decadal remote sensing datasets in Queensland (Australia)

Abstract:

Subtropical to tropical sandy beaches are subject to a wide variety of processes that influence shoreline position, though they receive little attention. The eastern to northeastern coast of Australia provides a unique setting to investigate shoreline change in tropical environments that are exposed to a range of different climate and oceanographic processes. An extensive, yet underutilised, record of historical aerial photography (HAP) provides an excellent opportunity to investigate geomorphological development of this region over a multi-decadal scale. Here, we present a study of beach dynamics for a selection of beaches in Queensland, Australia, over a period of up to 90 years using high resolution (0.5-5m) aerial photographs. HAP has been collected by the Queensland Government for a large section of the coast since as early as 1930. However, this data remains largely unused, largely due to a lack of accurate spatial referencing. These images were orthorectified and the shorelines and dune vegetation lines were digitised using GIS software, to extract shoreline change statistics. Shoreline change rates and net movement, with and without the effects of tidal variability, were calculated as were the confidence limits for the observed shoreline change. In addition to natural beach variability associated with seasonal adjustments to forcing conditions, there is a trend of statistically significant net accretion over the entire HAP record for all but the one location in the far north of Queensland. Most sites present a long-term pattern of beach rotation on varying temporal scales. However, this rotation is in a consistent direction along the entire coastline with accretion (erosion) occurring on the northern (southern) most sections of each beach. These observed changes may be a result of long-term adjustments in the offshore wave climate or sediment dynamics on the coast, though further work is to be completed to rigorously define the driving forces for these observations. This study provides insights into a variety of coastal settings through the subtropics and tropics and allows for further investigation into the mechanisms that control beach change over management oriented time scales.