Can RS/GIS Techniques Identify Morphological Changes in the Mahaweli River Due to River Sand Mining? A Case Study of Mahiyanganaya Section

River sand mining represents significant environmental threats in the Mahiyanganaya Mahaweli (Sri Lanka) river section, which causes significant upheaval to both the river biodiversity and morphology. Consequently, the structure of the local (Weragantota) bridge was found to be in danger, promoting the government to implement a rectification process for the piers 2,3,4 and 5. This study focuses on identifying, measuring and visualizing the morphological changes of the river and the consequent impact on the bridge using Remote Sensing and GIS techniques. Morphological changes were analyzed through centerline migration using Digital Shoreline Analysis System (DSAS), using time series images of Landsat 5, Landsat 8 images. Validation with Sentinel-2 data resulted in an RMSE of 53.86m. The average river width of the Mahaweli river in the Mahiyanganaya Weragantota section is approximately 161.209m. That explains the RMSE represent about 33.41% of the river width and that is 1 pixel from Landsat image, providing context for the measurement uncertainty. The sand mining permit records were used to build a correlation between sand mining and centerline migration from 2009 to 2023, while bridge deformation monitoring was investigated using Planet Scope (PS) imagery and Object Based Image Analysis (OBIA). Due to the low resolution of the images, the bridge area extraction was unsuccessful. Therefore, image enhancement methods of fusion and interpolation were tested, but they proved to be ineffective due to the absence of higher resolution or panchromatic data in PS images. However, the results showed that centerline migration has reduced in recent years, and in parallel, the number of sand mining permits issued near the bridge area has also decreased.