Shoreline Dynamics of South Africa Using Satellite Imagery

Most South African beaches lack sufficient monitoring, which impedes a holistic understanding of shoreline dynamics amid increasing environmental and anthropogenic pressures. This study addressed this critical knowledge gap by utilising a satellite-derived shoreline algorithm (CoastSat) to rectify years of inadequate monitoring and to contribute to a thorough understanding of South African shoreline dynamics. Enhancements were made to the open-source CoastSat algorithm to enable a semi-automated, nationwide application. As a result, a pioneering database was created, spanning from 1984 to 2023 and covering nearly all sandy areas of the South African coastline. This extensive and coherent database represents the first of its kind for South Africa. The accuracy of the satellite-derived shoreline data (SDS) was assessed by comparing it with Lidar-surveyed data from 27km of beach area across six different beaches in the eThekwini Municipality. The results showed a very strong correlation (R = 0.95) between the SDS and the surveyed data, although an overall landward bias of 11.2m was observed. By incorporating wave runup in the analysis the accuracy was significantly improved, reducing bias by up to 79%. These findings were consistent with previous CoastSat studies from abroad.In addition to developing this extensive shoreline dynamics database, four local case studies and four regional assessments were carried out. These efforts served two primary objectives: to further the understanding of South African coastal dynamics both locally and regionally, and to demonstrate the utility of the database. For example, (i) A study of the Tugela River Mouth revealed shoreline erosion of several hundred metres from 2005 to 2023, which is important information for ongoing and planned catchment projects, such as large dams, that impact fluvial sand yield to the coast. (ii) The consistent extreme accretion south of the Richards Bay port entrance sharply contrasted with the extreme erosion to the north. This highlighted the impacts of various coastal engineering interventions, providing valuable insights into their effectiveness and guiding future coastal management strategies based on the lessons learned. (iii) Studies of the seasonal shoreline responses at St Helena Bay and Cape Town bays (Table Bay and False Bay) showed how the magnitude of these responses was related to the degree of wave exposure. (iv) Regional investigations found interesting distinctions in shoreline evolution: for instance, the west coast typically experienced shoreline retreat during winter, the south coast had less extreme winter erosion, and the east coast, particularly from Port St Johns northward, saw the greatest erosion shifting from winter to spring. This information is invaluable for informing local, regional, and provincial vulnerability assessments and guiding resource allocation more effectively.This study successfully established the first comprehensive database of shoreline dynamics for the entire South African sandy coastline. The data and insights provided could serve as a valuable resource for coastal managers, policymakers, engineers, researchers, and other stakeholders, facilitating the development of informed, effective, and sustainable coastal management strategies that address both current and future challenges. Future research can build on these data and insights by exploring new, unresearched avenues or enhancing methods and technologies to mitigate the identified errors and limitations.

南非绝大多数海滩缺乏充分的监测，这阻碍了在日益加剧的环境与人为压力下对海岸线动态的全面认知。本研究针对这一关键知识空白，利用卫星衍生海岸线算法（CoastSat）弥补了多年来监测不足的缺陷，助力全面阐明南非海岸线动态。
研究团队对开源CoastSat算法进行了优化，以实现半自动化的全国范围应用。由此构建了一套开创性的数据库，时间跨度为1984年至2023年，覆盖南非海岸线几乎全部沙质区域。这套规模庞大且结构完整的数据库为南非同类研究中的首例。
本研究通过将卫星衍生海岸线数据（SDS）与伊泰奎尼直辖市6处海滩共27公里岸段的激光雷达（Lidar）实测数据进行比对，对SDS的精度展开了评估。结果显示，SDS与实测数据间存在极强的相关性（R=0.95），但整体存在11.2米的向陆偏差。通过在分析中纳入波浪增水参数，精度得到显著提升，偏差最多可降低79%。这一发现与国外此前开展的CoastSat相关研究结果一致。
除构建这套大规模海岸线动态数据库外，本研究还开展了4项本地案例研究与4项区域评估。这些工作达成了两大核心目标：一是进一步加深对南非本地及区域尺度海岸线动态的认知，二是验证该数据库的应用价值。例如：
(i) 对图盖拉河口的研究显示，2005年至2023年间岸线后退达数百米，这一信息对正在推进及规划中的流域项目（如大型水坝）具有重要参考价值——这类项目会影响入海河流的沙量供给。
(ii) 理查兹湾港入口南侧持续出现极端淤积，与北侧的极端侵蚀形成鲜明对比，这凸显了各类海岸工程干预的影响，为评估工程有效性提供了宝贵见解，并可基于经验教训为未来海岸管理策略提供指导。
(iii) 对圣赫勒拿湾与开普敦周边海湾（桌湾及福尔斯湾）的季节性岸线响应研究表明，这类响应的幅度与波浪暴露程度密切相关。
(iv) 区域调查发现了海岸线演化的显著差异：西海岸冬季通常发生岸线后退，南海岸冬季侵蚀程度相对较弱，而东海岸（尤其是圣约翰斯港以北区域）的侵蚀最为严重，且侵蚀时段从冬季转向春季。这些信息可为地方、区域及省级海岸脆弱性评估提供宝贵依据，同时更高效地指导资源分配。
本研究成功构建了南非首个覆盖全部沙质海岸线的综合性海岸线动态数据库。本研究提供的数据与研究成果，可为海岸管理者、政策制定者、工程师、研究人员及其他利益相关方提供宝贵资源，助力制定兼顾当前与未来挑战的科学、高效且可持续的海岸管理策略。未来的研究可基于这套数据与研究成果，探索全新的未涉足方向，或优化相关方法与技术以缓解已识别的误差与局限。