This thesis presents a comparison and validation of Digital Elevation Models for hydrological modelling and flood mapping in the Nairobi River Basin in Kenya. It addresses a real-world problem: flood risk in urban areas, as this city is quite prone to severe flooding during the onset of rain. It uses LiDAR data as a reference for validation to strengthen the results, which are evaluated using metrics such as RMSE, F1 score, and terrain parameters to distinguish vertical accuracy from hydrological performance. The software used includes SAGA GIS in QGIS, ArcGIS Pro, and Python, which we use to perform our everyday tasks.