Mapping Shoreline Changes Using Digital Shoreline Analysis System in Coastal Areas of Bantul and Kulon Progo Regencies

Abstract

Wilayah pesisir Kabupaten Bantul dan Kulon Progo, yang berbatasan langsung dengan Samudra Hindia, sangat rentan terhadap dampak gelombang tinggi, arus, dan perubahan iklim. Penelitian ini bertujuan untuk menghitung panjang garis pantai, jarak dan laju perubahan garis pantai, serta luas abrasi dan akresi di wilayah pesisir tersebut selama periode 2019 hingga 2023. Data citra satelit Sentinel-2 digunakan untuk memantau perubahan garis pantai, dengan metode Modified Normalized Difference Water Index (MNDWI) dan algoritma Otsu Thresholding untuk ekstraksi garis pantai. Hasil ekstraksi tersebut kemudian dikoreksi terhadap data pasang surut menggunakan Mike-21 Tide Prediction, untuk mendapatkan posisi garis pantai yang lebih akurat. Analisis perubahan garis pantai dilakukan menggunakan Digital Shoreline Analysis System (DSAS) dengan metode Net Shoreline Movement (NSM) dan End Point Rate (EPR). Perhitungan luas abrasi dan akresi dilakukan dengan metode overlay garis pantai dari tahun terlama dengan tahun terbaru. Hasil penelitian menunjukkan bahwa abrasi tertinggi terjadi di Kabupaten Bantul pada periode 2019 hingga 2020 dengan laju rata-rata 31,874 m/tahun dan luas 36,839 ha. Sedangkan akresi tertinggi terjadi pada periode 2022 hingga 2023 dengan laju rata-rata 27,250 m/tahun dan luas 32,775 ha. Di Kabupaten Kulon Progo, abrasi tertinggi juga terjadi pada periode 2019 hingga 2020 dengan laju rata-rata 27,685 m/tahun dan luas 61,618 ha, sementara akresi tertinggi terjadi pada periode 2022 hingga 2023 dengan laju rata-rata 17,687 m/tahun dan luas 38,730 ha. Perubahan abrasi dan akresi ini dipengaruhi oleh faktor iklim, sedimentasi sungai, arus, gelombang laut, kecepatan angin, pasang surut, serta pembangunan infrastruktur pantai.

Abstract

The coastal areas of Bantul and Kulon Progo regencies, which are directly adjacent to the Indian Ocean, are highly vulnerable to the impacts of high waves, currents and climate change. This study aims to calculate the length of the coastline, distance and rate of shoreline change, as well as the area of abrasion and accretion in these coastal areas during the period 2019 to 2023. Sentinel-2 satellite image data was used to monitor shoreline changes, using the Modified Normalized Difference Water Index (MNDWI) method and Otsu Thresholding algorithm for shoreline extraction. The extraction results were then corrected against tidal data using Mike-21 Tide Prediction, to obtain a more accurate shoreline position. Shoreline change analysis was conducted using Digital Shoreline Analysis System (DSAS) with Net Shoreline Movement (NSM) and End Point Rate (EPR) methods. Calculation of abrasion and accretion area is done by overlaying the shoreline method from the oldest year with the latest year. The results showed that the highest abrasion occurred in Bantul Regency in the period 2019 to 2020 with an average rate of 31,874 m/year and an area of 36,839 ha. While the highest accretion occurred in the period 2022 to 2023 with an average rate of 27,250 m/year and an area of 32,775 ha. In Kulon Progo Regency, the highest abrasion also occurred in the 2019 to 2020 period with an average rate of 27,685 m/year and an area of 61,618 ha, while the highest accretion occurred in the 2022 to 2023 period with an average rate of 17,687 m/year and an area of 38,730 ha. Changes in abrasion and accretion are influenced by climatic factors, river sedimentation, currents, ocean waves, wind speed, tides, and coastal infrastructure development.