I have not used it but it seems Q-GIS cloud is working very well. http://www.qgiscloud.com/

Thank you very much Mr Soheil for your answer

you can have a look at the open source portal http://www.osgeo.org/ (if you dont)... the common software used are GRassGIS, Quantum or GvSIg..but not sure for the context of flood modeling, simulation....

Thank you very much Hawani for your answer

Thanks George for your info

kindly check "Ushahidi" open source software

It very much depends on what you mean with "flood management". Do you 1) want to understand the dynamics of the flood through hydrological models and meteorological data or 2) manage the consequences on a socioeconomic side? I suggest you elaborate your question a bit. This will in turn give you more specific answers.

Have also a look to the SAGA open GIS http://www.saga-gis.org/. It has modules for hydrologic modelling

Thanks Mr Ragnvald Larsen, actually I would like to develop the system something similiar to Waze which all the inputs is from the user but my system is mainly for managing the occurance of flood in any area.

Thanks Harpinder and Jean for the suggestion of the software

I have a kind of  Disaster recovery Solution, it could be of a great support to your work

Article Incident Management System -Implementation Plan