India has such a diverse geographical area that there are floods in some parts and droughts in other parts of the country and same time they co-exist. Large numbers of severe and devastating floods are endangering life and properties. In the state of Odisha, flooding is caused primarily due to Mahanadi River. The flow of water and its level in the river Mahanadi having a catchment area 141000 km² is controlled by Hirakud dam. The entire deltaic region of Mahanadi River intercepting a catchment of 48700 km² gets affected by medium to severe flood almost every year causing immense loss to life and property. Study was done to find out reduced level of flood water in different locations of Mahanadi river reach between Hirakud dam and Naraj for 10, 25, 50, and 100 years return period using Hydrologic Engineering Center-River Analysis System (HEC-RAS) model. The study has been accomplished by preparing the basin map for Mahanadi river in HEC-RAS readable format and computing the peak flood for 10 years, 25 years, 50 years, and 100 years return period using Gumbel's distribution. Improvements for the channel cross-section, bank embankment modification, height of flood protection structure such as dikes, levees in the flooded zone were suggested. The peak flood discharge of Mahanadi river at 10 years, 25 years, 50 years, 100 years return have been calculated as 37535.026 m³/s, 45067.19m³/s, 50656.19 m³/s, and 56203.24 m³/s, respectively. Flood flow hydrograph has been prepared for the year 2008 by unsteady flood flow simulation. Around 10 marked location of river stations are prone to flood under 20 year return period. Eight such points were observed to be overtopping the existing banks with a varying height of 1 m to 10 m with reference to existing banks reduced level in subsequent 10 years to 100 years return period in increasing order. The findings of present study suggest the minimum height of dikes, levees to provide protection from flood at different locations of existing banks to be 2 m to 10 m.