Most use ArcMap or other GIS software to help. Your local, university or country agencies that deal in resource management may already have this data. They make a digital elevation map (DEM) from area topographic map in a raster data base, then use the GIS hydro modeling tools. For more topographic detail, aerial LiDAR mapping can help develop a much greater definition to the DEM. Most hydrologists, geographers, geomorphologists, etc. would have little or no trouble creating hydrologic boundaries that help with this type assessment, but most urban areas have impervious surfaces, hydrologic and channel modifications and land use changes that can greatly complicate this type analysis. Also any historic stream gauges, flood water marks, rainfall and stream channel type, modifications and sediment loading can help with this type evaluation of flooding, past and potential for changes as well as considerations for mitigation, early warning, evacuation and other planning. A course of maybe a week in ArcMap software use and applications would probably be sufficient to learn many of the basic GIS skills. Hydrology, flooding and geomorphology and associated topics are more complex subject areas that would probably require a greater commitment of time and effort. But a search associated for any available data, reports, etc. is a good starting point. Due to the potential hazards and resource management issues, flood frequency or severity maps would be typically reviewed or certified by licensed engineers and qualified professionals.

I looked up the area from a satellite photo. The Ganga River most likely affects part of this area with flooding. To clarify, the Ganga River is braided, with obvious sediment deposits, an indication of an aggrading river. As the channel aggrades from sediment deposits, the channel may adjust location from time to time, and channel capacity may be decreased, contributing to more frequent flooding. If substantial increase in plant establishment, the channel roughness can increase, also contributing to lower flow velocity and more potential for sedimentation. Of course, the Ganga River is a large basin, containing many smaller hydrologic sizes, depending on how it is subdivided hydrologically. Before GIS and LiDAR and perhaps some models, the mapping of hydrologic units was typically done manually using a topographic map. In well definer terrain, manual mapping of hydrologic units not that difficult. The manual hydrologic or topographic skills are fairly easy to pick up, but relative flat areas as coastal plains and as well urban area hydrology are more difficult to define, and LiDAR improves the detail needed.

If you need vector shapefile for the area. I would recommend to give a look at openstreetmap available freely.

Patna | OpenStreetMap

if you need already processed shapefiles, try out this link

Geofabrik Download Server

You can also download DEM to extract drainage network but in urban areas it will be challenging.

DiVA-GIS

Try to find it via this link. Most of the shapefiles of different countries include here.

https://www.diva-gis.org/gdata

Thank you all for your comments and suggestion but still my query is unsolved.....Will try to mask from the available recourses.