Comprehensive Monitoring and Assessment of South African Shoreline Dynamics Using Satellite Imagery

Abstract

South Africa’s beaches lack sufficient monitoring, hindering the holistic understanding of shoreline dynamics under increasing environmental and anthropogenic pressures. This study addresses this critical knowledge gap by using an enhanced, semi-automated CoastSat algorithm to create the first comprehensive shoreline database for South Africa, spanning nearly all sandy coastal areas from 1984 to 2023 (an average of 750 data points per transect, every 100m alongshore). The satellite-derived data demonstrated good accuracy (R = 0.95) against surveyed data, with an overall root-mean-square-error (RMSE) of 8.5m to 13.7m, and bias reduced by up to 79% when wave runup was additionally accounted for. Despite relatively inaccurate beach-face slope estimations at some sites (R = 0.45), the satellite-derived shoreline positions proved to be accurate. Assessments of all South Africa’s sandy coastal areas revealed that the west and south coasts showed net shoreline movements indicative of a 3- to 7-year period of medium-term dynamic stability, while the east coast exhibited ongoing progressive changes. Between 2000 to 2023, 98% of beaches on the west and south coasts experienced net movements of less than 40 meters, while the east coast experienced nearly double that amount. Year-on-year shoreline changes (m/year) were similar in magnitude across all three regions, with both accretion and erosion having a similar occurrence at mild and extreme rates. This first-of-its-kind resource for South Africa will guide informed, effective and sustainable coastal management strategies and engineering design.