There are differences in the methodology adopted by FLOOD and storm-sewer products such as CivilStorm and SewerGEMS. Using CivilStorm or SewerGEMS, you must explicitly provide data in the form of specific parameters such as surface roughness values, time of concentration and depression data (if the surface has areas of ponding). All these factors are automatically considered using FLOOD.
You do not need to specify the area or time of concentration for a catchment in FLOOD. Digital Surface model together with 2D calculation (Saint-Venant and Manning-Strickler equations) will do this for you. Additionally, you do not need to enter depression data as the 2D surface contains topographical data which is considered in runoff estimation. FLOOD has tools to remove depressions from the ground surface if required.
Runoff generation and other processes are complex. With FLOOD, it is primarily generated based on the 2D surface terrain and the modules selected to run. The runoff hydrograph from a catchment in CivilStorm or SewerGEMS is based on a more simplified approach such as the SCS method which only takes the area, permeability factor (like the SCS CN) and Tc into consideration.
FLOOD provides more realistic runoff hydrographs and estimations whereas CivilStorm, SewerGEMS, or StormCAD rely on standard runoff estimation methods such as SCS CN, and IDF Curves for runoff estimation, useful for a more conservative design.
FLOOD can generate advanced water quality and pollutant dispersion modeling studies in water environments.