Monsoon-Driven Geomorphological Changes Along the West Coast of Sri Lanka: A Combined Approach Utilizing ‘CoastSat’ and Google Earth Engine

Long-term field monitoring of shoreline changes is time-consuming, expensive, and labor-intensive. Instead, satellite images can be used as an alternative method to collect field data. The time-series satellite images are available at any location in the world that can be processed with the Google Earth Engine cloud environment. This study primarily focuses on shoreline change detection and describing the coastal geomorphology of three urban beaches on the west coast of Sri Lanka. The study extended from 2015 to 2021 during which large-scale coastal development projects were carried out in the study area. The ‘CoastSat’ toolkit was used to extract the time-series of shoreline positions. Time-series shoreline position obtained through ‘CoastSat’ was compared with the field measurements carried out using the Global Navigation Satellite System technique with a horizontal accuracy of 7 mm. The results indicate that the average horizontal difference of shoreline positions obtained by ‘CoastSat’ and field observation was 7.5 ± 1 m in Agulana-Ratmalana on 19 August 2019, and was 8.3 ± 1 m in Kalutara on 29 July 2020. The extracted shoreline changes show erosion and deposition patterns affected by monsoon seasonality and anthropogenic events. The results further show that North of Mount Lavinia Beach was accreted from 18 to 27 m, while South of Mount Lavinia Beach was eroded from 12 to 17 m. Accretion was mainly due to sand nourishment in the area during the 1st quarter of 2020. Furthermore, Agulana-Ratmalana Beach predominantly accreted from 22 to 30 m, while the northernmost transect (AR1) had a steady-state beach condition. This was again due to nourished sand during the 1st quarter of 2020. In contrast, accretion and erosion trends in Kalutara Beach are mainly due to the breakup of the river mouth sand bar to control flooding by the artificial cutting open of the sand spit bar in 2017. The transect (KL2) near the broken sand spit bar at the north of Kalutara shows severe erosion (56 m), since northward longshore transport of sediment has stopped with the breakage of the sand spit bar. In contrast, Kalutara south transects show an accretion to steady-state condition due to the existing hard engineering structures. Consequently, the study suggests that the CoastSat: A Google Earth Engine-enabled Python toolkit can be used to extract shoreline positions and to detect medium-to-large-scale coastline changes with appropriate tidal corrections, when and where there