External changes around urbanized areas can created new hazard problems, which some times completely unpredicted before. An example of an external change of a city that may create a new hazard is disturbances in the upstream river network that carry water stream into the city. The newly established river flow configuration can change the characteristics of water that entering the city, including volume, velocity, viscosity, etc. This book explores how to analyze possible impacts due to changes in the upstream river network to flood problems in an urbanized area using 1D2D flood modelling in SOBEK software, which was developed by Delft Hydraulics. Different flood scenarios were developed by adjusting the flood modelling parameters, and then, Flood Hazard Analysis and Flood Risk Analysis were performed to determine the extent of flood problems caused by changes in the upstream river network. Results of the flood modelling show that indeed, urban areas are facing an increasing flood hazard due to changes in upstream river network.