Satellite-Derived Relative Bathymetry Mapping of Kopa Dam Using Sentinel-2 Level-2A Imagery and GIS Techniques

Abstract

Satellite-derived bathymetry (SDB) provides a remote-sensing approach for characterizing spatial variations in water depth-related information without requiring complete conventional hydrographic coverage. This study developed a relative satellite-derived bathymetry map of Kopa Dam, Tafa Local Government Area, Niger State, Nigeria, using Sentinel-2 Level-2A imagery and ArcGIS 10.8. Sentinel-2 Bands 2 and 3, representing the blue and green portions of the visible spectrum, were used to derive a logarithmic spectral-ratio called Relative Bathymetry Index (RBI). The RBI is an uncalibrated, unitless spectral index representing relative bathymetric variation rather than measured water depth in metres. A water mask was developed and applied to restrict the RBI to the water body, after which descriptive statistics were calculated. The RBI was subsequently classified into four relative bathymetric classes and processed using a Majority Filter to improve spatial coherence. The resulting RBI ranged from 0.386808 to 1.000235, with a mean of 0.777053, standard deviation of 0.058869, and range of 0.613426. The final classification contained 37,845 valid water pixels, equivalent to 378.45 ha at 10-m spatial resolution. Moderate depth class was dominant, covering 249.60 ha (65.95%), followed by deep zone class with 127.29 ha (33.63%), while very shallow and shallow class accounted for 0.02 ha (0.01%) and 1.54 ha (0.41%), respectively. The study demonstrates the usefulness of Sentinel-2 multispectral imagery and GIS-based spectral-ratio processing for preliminary spatial characterization of relative bathymetric variation in an inland water body. Future studies should incorporate field-based bathymetric calibration to enable conversion of the RBI into quantitative water-depth estimates.