Unsteady flow analyses could be suitable. However, you need to have a very fine DEM and enter unsteady flow data. HEC-RAS dose not allow users to input precipitation. I suggest to use KINEROS2 for generating the unsteady flow data.

Hope that it could help!

Quang

If you are in a city or have high value downstream areas, most dams are not designed for the 100 year flood.  You may want to assume both flood routing through dams (the normal approach), and then dam failure and the effects of that.  We had that severe October 2015 event in the SC area, including Columbia, SC.  Many of the old dams within communities failed, leaving downstream areas inundated.  It surprised many people.  I cannot tell you exactly how to do this in a few words, but hydrology books discuss this considering inflow, outflow and changes in dam elevation and water storage.  Hydrologists have training in routing floods through dams, but I don't remember if we discussed dam failures, but I have seen some of the results -- catastrophic to peoples lives, but most got out in time and to safety.  Some did not.  Emergency plans are sufficient or in place, that is something to work on too.

I think it depends on your upstream and downstrram available data for your boundary condition.The upstream BCs must be flow data and downstrram BCs must be water election data. If your downstrram elevation data variasi hourly or daily,you had better use unsteady flow analysis.

If the objective of your study is to map the inundation risk ( (in terms of crossing hazard and vulnerability layers) with respect to a set of defined flood discharge return periods a steady flow computations would be convinient, unless the flow is influenced by a presence of hydraulic structures.

The flood frequency analysis of discharges (which will provide boundary conditions for the hydraulic model ) should take into account the rainfall pattern. Otherwise for the low frequency values there is a possibly of discharge underestimation. It may stem from the possible phenomenon of saturation of the basin, which occurs for various return periods. In fact, it depends on the quantity of rainfall, soil occupation pattern and on the geology.

It would be useful to know for example the 1% and 0.1% daily rainfall (annual maxima) what will allow having some order of magnitude of the quantity of rainfall necessary for soil saturation so preconditioning overland flow and floodings.

in order to compute the inundation for a specific rainfall event, I would suggest you to use HEC-HMS to compute the flow hydrograph. Use this flow hydrograph for mapping the inundation of the city. Use the un-steady flow analysis for the case you are analysing.

@ Thank you all for your valuable suggestions

https://www.researchgate.net/publication/294089869_Assessing_flood_inundation_extent_and_landscape_vulnerability_to_flood_using_geospatial_technology_A_study_of_Malda_district_of_West_Bengal_India

Article Assessing flood inundation extent and landscape vulnerabilit...