17:15 〜 19:15
[U03-P05] Satellite-Based Shoreline Change Detection in the Tamsui River Estuary Using Otsu’s Dynamic Thresholding
キーワード：Shoreline Change Detection, Satellite Imagery, Otsu’s Dynamic Thresholding, Digital Shoreline Analysis System
Coastal infrastructure often alters sediment dynamics, leading to significant shoreline change through erosion or accretion. The Tamsui River, located in northern Taiwan, flows from south to north into the Taiwan Strait. Since its construction, Taipei Port has significantly impacted the Tamsui River estuary, especially increasing sedimentation on the left bank, which poses a challenge for vessel navigation. To address these concerns, this study integrates multi-temporal SPOT satellite imagery (2022–2024) with historical tidal data to analyze shoreline change along the left bank of the Tamsui River estuary.
The methodology consists of the following steps: (1) Selecting SPOT satellite images with cloud-free conditions and high tide levels. (2) Automatically extracting the shoreline using Otsu’s thresholding applied to the Normalized Difference Water Index (NDWI). (3) Recording shoreline positions over time with perpendicular transects through the Digital Shoreline Analysis System (DSAS). Key shoreline change indicators include the End Point Rate (EPR), which calculates the average annual shoreline movement rate between two dates, and the Net Shoreline Movement (NSM), which measures the total shoreline displacement between two dates.
The results show that the left bank of the Tamsui River estuary is mostly undergoing sediment accumulation, with 70.6% of the transects indicating seaward movement. Further analysis, through regional breakdown, provides a clearer understanding of shoreline change. Between Taipei Port and the Tamsui River estuary, EPR (11.7 m/yr) and NSM (27.8 m) both indicate accretion, with sediment accumulation intensifying northward, peaking at an NSM of 89.8 m. Conversely, the estuary to the left-bank tidal flat exhibits erosion, with negative values for EPR (-1.8 m/yr) and NSM (-4.3 m), indicating shoreline retreat, with the most severe erosion reaching an NSM of -17.4 m. The left-bank harbor area within the main river channel remains relatively stable, with minimal changes (EPR: 1.7 m/yr, NSM: 3.9 m).
This study uses automated shoreline detection from satellite imagery and shoreline measurements to systematically assess how Taipei Port affects sediment deposition in the Tamsui River estuary. Future work could expand the temporal analysis, integrate field measurements or hydrodynamic models, further investigate sediment transport dynamics to support coastal management.
The methodology consists of the following steps: (1) Selecting SPOT satellite images with cloud-free conditions and high tide levels. (2) Automatically extracting the shoreline using Otsu’s thresholding applied to the Normalized Difference Water Index (NDWI). (3) Recording shoreline positions over time with perpendicular transects through the Digital Shoreline Analysis System (DSAS). Key shoreline change indicators include the End Point Rate (EPR), which calculates the average annual shoreline movement rate between two dates, and the Net Shoreline Movement (NSM), which measures the total shoreline displacement between two dates.
The results show that the left bank of the Tamsui River estuary is mostly undergoing sediment accumulation, with 70.6% of the transects indicating seaward movement. Further analysis, through regional breakdown, provides a clearer understanding of shoreline change. Between Taipei Port and the Tamsui River estuary, EPR (11.7 m/yr) and NSM (27.8 m) both indicate accretion, with sediment accumulation intensifying northward, peaking at an NSM of 89.8 m. Conversely, the estuary to the left-bank tidal flat exhibits erosion, with negative values for EPR (-1.8 m/yr) and NSM (-4.3 m), indicating shoreline retreat, with the most severe erosion reaching an NSM of -17.4 m. The left-bank harbor area within the main river channel remains relatively stable, with minimal changes (EPR: 1.7 m/yr, NSM: 3.9 m).
This study uses automated shoreline detection from satellite imagery and shoreline measurements to systematically assess how Taipei Port affects sediment deposition in the Tamsui River estuary. Future work could expand the temporal analysis, integrate field measurements or hydrodynamic models, further investigate sediment transport dynamics to support coastal management.