This video tutorial is a good example to merge or mosaic raster image.
This video link (source) gives a good example on how to download free Landsat 8 by using Google Earth and EarthExplorer. The steps are as follows:
1. Download Landsat 8 KML file from: https://landsat.usgs.gov/landsat/ge/
2. Drag and drop it in Google Earth.
3. Navigate to the desired area and select the scene(s).
4. Select your required data from the pop-up interface.
5. If you click on Download Additional Products, the EarthExplorer site will open in a browser window.
6. Sign in using your free registered account.
7. Click on the download option on the product information page.
8. Choose your desired data product.
9. Open the downloaded file in ERDAS Imagine seeing the imagery.
This is a good explanation of considering the effect of tidal on the flood simulation using the HEC-RAS model.