Patu River flowing through the western part of Tulsipur city in Nepal has been encroached severely by human settlements during the last fifteen years. The settlements and infrastructures are at high risk when the river gets heavily flooded. With the help of images obtained from Google Earth, Landsat images, and application of ARC GIS, the land cover changes during the years 2004, 2010, 2015, and 2019 were observed. Since Patu is an ungauged river, the peak flood was estimated at 109.9 m3/s, 132.9 m3/s and 158.1 m3/s for 25, 50 and 100 years return period by Catchment Area Ratio method referring historical flood data of gauged Sharada River. Through the application of HEC-RAS modeling, the flood mapping was obtained for 25, 50, and 100 years return period. The inundation of settlement area back in 2004 was observed as just 0.009 km2 which was found to have been increased to 0.043 km2, 0.070 km2, and 0.096 km2 in the years 2010, 2015, and 2019 respectively following the linear trend with an R2 value of 0.99. The risk of flood in the settlement has been accelerating with increasing urbanization around the river. It can be concluded that the inundation of settlement could be increased to 0.16 km2 in 2024 if a similar trend follows for the next five years. Before the situation worsens, it is necessary to provide enough embankments on both sides of the river and control the haphazard urbanization around the river. In conclusion, river training and proper planning of the city has been found to be critical for controlling inundation which could increase more in the coming years.