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City Cat

City Cat is a tool for 2D hydrodynamic modeling which is therefore appropriate for studying an urbanized area. It can be a tool to be used when assessing flood risks in cities and can also be used to analyze effect of applying different types of measures such as blue-green features.

 Input files – topography (DEM), rainfall for return period of 20 years with duration of 60 minutes, building data showing geometry of buildings and green areas.




1)      Initial Results - No Mitigation or Measures

Figure 1 : Simulation results after first trial - no measures or mitigations in place


1)      Roof Storage vs Changing Permeability

 Given the situation in the previous simulation, a green roof with 2.5 cm and 10 cm capacity were applied to the green areas.

Figure 2 : Simulation result with green roof storage at 2.5cm

Figure 3: Simulation result of changing impermeable surfaces to permeable


The results are the same whether the storage is 2.5cm or 10cm for this scenario of 20 years return period. This shows efficiency in this situation for either storage capacity would be the same.



1)      Combining Green Roof and Changing Permeability

Figure 4 : Simulation result using green roof and permeable surfaces


This result appears ideal since the velocity in general is low and the water depth has also significantly decreased with a maximum only at 0.3 m. In a theoretical scenario, one would choose this option of combining the two but the practicality of the measures should also be considered.


Conclusion and discussion


It appears that choosing green roofs with storage capacity of 2.5cm is enough and appropriate to dampen effects of flooding. 

 In this particular exercise however, the underground sewer network has not been taken into account which would have had a significant effect in the results. Having the sewer network would have decreased the depth and possibly velocity of the water. This exercise is also constrained to a 20-year event and it is suggested that when doing such studies, several return periods should be considered. The rainfall data provided has an interval of 300 seconds or 5 minutes. It would be better to have a more detailed rainfall data in order to have more detailed results as well. It is interesting how the buildings and surface has been represented.

The buildings have been excluded from the flow path and the rainfall is either distributed to the surface around the buildings or captured by the green roofs. This is a good aspect of the software since this generally mimics the reality of pluvial floods.