## 2 Materials and Methods  
### Dataset  
The dataset employed in this research [16] comprises 446 images from Sentinel-1 and 446 images from Sentinel-2 satellites, both integral components of the European Space Agency's Copernicus program, which is facilitating open access to satellite data. This dataset encompasses imagery from 11 distinct global flood events, occurring in Bolivia, Ghana, India, Cambodia, Nigeria, Pakistan, Paraguay, Somalia, Spain, Sri Lanka, and the USA, as depicted in Figure 1.  
Sentinel-1 satellites employ C-band SAR instruments, characterized by longer wavelengths compared to optical sensors. This feature enables them to penetrate through challenging conditions such as clouds, storms, and dense vegetation. Additionally, SAR sensors provide their own source of illumination, rendering them operational at any time of the day. Nonetheless, SAR imagery is susceptible to speckle, involving random signal variations, which can potentially impede the ability of deep learning models to discern authentic signals. The images selected for this study were captured in the interferometric wide swath (IW) mode, offering extensive coverage with an enhanced swath width of approximately 250 km. This mode is particularly well-suited for large-scale applications, including flood inundation mapping. The ground resolution in this imaging mode ranges between 5 and 20 m, and the images utilized in our study feature a ground resolution of 10 m. This moderate resolution strikes a balance between high- and low-level detail, ensuring visibility of smaller water bodies while adequately covering larger areas. The Sentinel-1 satellite offers various polarization modes, with the selected images featuring dual polarization, comprising vertical transmit and vertical-horizontal receive (VV + VH). This dual polarization mode enriches information for enhanced water detection while maintaining computational efficiency.  
Conversely, Sentinel-2 satellites capture optical imagery using a multispectral instrument (MSI) with a 290 km field of view. This instrument records 13 spectral bands encompassing visible light, near-infrared, and shortwave-infrared spectra, offering detailed insights into vegetation, land cover, and water bodies. Such detailed information proves instrumental in distinguishing flooded areas from other land types. Among the 13 spectral bands, 4 feature a 10 m ground resolution, 6 have a 20 m ground resolution, and 3 exhibit a 60 m ground resolution. For the sake of consistency and improved analysis, the images were resampled to a uniform 10 m ground resolution for each band. A visual representation of the spectral bands from both satellites is illustrated in Figure 2.  
The dataset comprises flood events for which both Sentinel-1 and coincident Sentinel-2 images were available within a maximum time frame of two days. Specifically, five events featured imagery captured on the same day, four within a one-day interval, and two with a  
Figure 1: Map showing the locations of the flood events sampled in the dataset.  
two-day difference. Variations exist in the orbit direction, denoted as ascending (moving from the Southern to the Northern Hemisphere) or descending (moving from the Northern to the Southern Hemisphere), as well as in relative orbit values, signifying the satellite's position and movement relative to other satellites in the constellation. An overview of this information for each event is provided in Table 1.