Step 1: Read the manuals.

In HEC-RAS help you will find two interesting books. First is "User manual" with chapter "Performing unsteady flow analysis". The second is "Application guide", where the last chapter includes also unsteady flow application

Setp2: Look at the forums.

But not in the general scientific forum at Reserchers Gate. There are such forums, where people using HEC-RAS are discussing. Ask them more detailed questions. I can recommend HEC-RAS Users group in the LinkedIn or RAS Solutions portal.

The two references given by my colleague are good as you need to the theoretical basis of  calculating "return period", of certain precipitation event/pattern "possible maximum precipitation, PMP" and "Possible Maximum Flooding, PMF". You need also to know the catchments, drainage pattern and types, soil characteristics, geomorphology and geology.

Dear A. M. Ebraheem, I'm affraid that you are writing about HEC-HMS. Using HEC-RAS  doesn't require knowledge on "the catchments, drainage pattern and types, soil characteristics, geomorphology and geology"

Please find a relatively simple tutorial at:

www.civil.uwaterloo.ca/cive770/tutorials/HEC-RAS_tutorial_flume_example.pdf

Thanks every one for answering i'll try to get the concept from theses shared references.

Dear Tomasz Dysarz

You are correct, I was answering and having in mind  the question of "Flood Hazard". Thank you for the correction. Kind Regards

You may have a look to such presentation in the link below. He applied the model for flood hazards or get in touch to the person:

https://www.researchgate.net/publication/258239528_URBAN_FLOOD_HAZARDS_MODELLING_IN_ARID_ZONE_%28Case_Study_in_Wadi_Qows_Jeddah%29

Thesis URBAN FLOOD HAZARDS MODELLING IN ARID ZONE (Case Study in Wa...