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What is the difference in runoff generated using CivilStorm / SewerGEMS / and OpenFlows FLOOD?
There are differences in the methodology adopted by FLOOD and storm-sewer products such as CivilStorm / SewerGEMS.
Using CivilStorm / SewerGEMS you have to explicitly provide data in the form of certain 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 OpenFlows FLOOD.
In OpenFlows FLOOD you don’t need to specify the area or time of concentration for a catchment since Digital Surface model together with 2D calculation (Saint-Venant and Manning-Strickler equations) will do this. Additionally, you don’t need to enter depression data as the 2D surface contains topographical data which is considered in runoff estimation. FLOOD also has tools to remove depressions from the ground surface if required.
OpenFlows FLOOD 2D modeling engine (MOHID Land) calculates the movement of water based on the parameterizations and process that a selected during analysis. Water that reaches the surface (from rain), can then infiltrate, a part of it can evaporate (evapotranspiration), enter river / surface channel drainage network or a subsurface stormwater drainage network or flow as runoff over the surface. All these movements can be computed internally (depending on the processes that you connect).
This shows that runoff generation (and other processes) is a much more complex process in case of OpenFlows FLOOD which is done primarily based on the 2D surface terrain and the modules selected to run. On the contrary the runoff hydrograph from a catchment in CivilStorm / 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.
OpenFlows FLOOD provides more realistic runoff hydrographs and estimations whereas CivilStorm / SewerGEMS rely on standard runoff estimation methods such as SCS CN, IDF Curves etc. for runoff estimation which can be useful for a more conservative design
Related Forum Post : comparison of run off hydro-graphs
Bentley OpenFlows FLOOD
Integrating SewerGEMS or CivilStorm with OpenFlows Flood for 2D simulations