This study applied a time series of fully polarimetric synthetic aperture radar (SAR) images using RADARSAT-2 to analyze the polarimetric responses of the west coast of Taiwan. A total of seven images were acquired from 2009 and 2012, covering the low, high, and similar tide levels. A four-component target decomposition algorithm was used to investigate the tidal effects and dynamic ocean interactions of sandbanks near the coast. The experimental results showed that the Yamaguchi fourcomponent decomposition can extract different features in the coastal area efficiently and effectively. From the target decomposition image constructed using the Yamaguchi four-component model, strong double-bounce scattering and weak volume scattering were favorable indicators of oyster farms, whether they were densely arranged or rugged, and surface scattering delineated the water boundaries of sandbanks. Using the extracted water boundaries and tidal-level information from different times, a sandbank elevation model was estimated. The experimental results also revealed that a small offshore sandbank had a displacement of approximately 1.5 km under a tidal level of -1.14 m. Furthermore, large sandbanks had a counterclockwise displacement on the west coast and moved eastward on the eastern coast.