Multitemporal analysis of tropical cyclone impacts on the iSimangaliso Wetland Park sea turtle nesting beach using geospatial technologies.

Abstract

Increase in the intensity and frequency of tropical cyclones due to changing climatic conditions, poses a threat to sea turtle nesting beaches. In general, tropical cyclones increase the rate of coastal erosion along sandy beaches, potentially disrupting Sea turtle nesting when tropical cyclone seasons coincide with Sea turtle nesting seasons. The iSimangaliso Wetland Park Sea turtle nesting beach is situated along the coast in north-eastern KwaZulu-Natal, South Africa. Its unique location adjacent to the southwest Indian Ocean means that it experiences seasonal flooding due to tropical cyclones. Nevertheless, the impact of these tropical cyclones on the iSimangaliso Wetland Park sea turtle nesting beach remains uncertain. There is, therefore, a need to examine the intensity and frequency of tropical cyclones in this Ocean basin in order to understand their potential impact on the adjacent sea turtle nesting beaches. In this study, tropical cyclone Track Archive data was downloaded from the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Centre for the years 1980 to 2020. Space-time pattern mining tools were then used to analyse the tropical cyclone data in ArcGIS 10.6. Medium resolution multi-spectral Landsat 7 and 8 satellite images were also collected from the USGS and were used in the Digital Shoreline Analysis System to calculate tropical cyclone induced changes in the position of the shoreline along the iSimangaliso Wetland Park sea turtle nesting beach. The results indicate that: (1) the intensity of tropical cyclones within 1000 km off the coast of the iSimangaliso Wetland Park sea turtle nesting beach have increased by 13.80% from 1980 to 2020. (2) Between the year 1980 and 2020 the frequency of tropical cyclones exhibited a spatiotemporal trend that is not statistically significant (z = 0.56 and p = 0.58 (>.05), suggesting that there is no noticeable increase or decrease in the frequency of tropical cyclones within 1000 km off the coast of the iSimangaliso Wetland Park sea turtle nesting beach despite rising sea surface temperatures. (3) The iSimangaliso Wetland Park sea turtle nesting beach is situated along an oscillating tropical cyclone Cold Spot, suggesting that tropical cyclones rarely reach the study area and generally dissipate further north towards southern Mozambique. (4) From 1999 to 2020, the iSimangaliso Wetland Park sea turtle nesting beach has experienced an average seaward shoreline movement by 68.73 m, averaging 0.76 m/year despite the increase in the intensity of tropical cyclones. (5) The results of this study suggest that the frequency of tropical cyclones have a significantly negative relationship (p < 0.01; r2 = -0.69) with the rate of shoreline movement along the iSimangaliso Wetland Park sea turtle nesting beach. (6) The intensity of tropical cyclone has a

moderate correlation (p < 0.076, r2 = 0.39) with the rate of shoreline change along the iSimangaliso Wetland Park sea turtle nesting beach. (7) Distance of tropical cyclones to the study area has a very low negative relationship with the rate of shoreline change (p < 0.2, r2 = -0.2). Generally, these results suggest that unlike other sea turtle nesting beaches, the iSimangaliso Wetland Park sea turtle nesting beach is relatively safe from tropical cyclones activity despite the rapid increase in sea surface temperatures. The impact of tropical cyclones on the study area is attenuated by the presence of steep dunes, coastal vegetation, mangroves, and flood induced sediment deposition from the Mfolozi River and the St. Lucia estuary. A limitation to this study was inaccessibility to high spatial resolution satellite images due to cost. As a result, Landsat images with a medium spatial resolution of 30 m were used as these are freely available. Future research should consider the use of higher resolution satellite or drone and lidar images to study shoreline changes in relation to tropical cyclone activity and possible sea-level rise along the iSimangaliso Wetland Park sea turtle nesting beach.