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11/07/2019 15:21:46

Hi, im running flood modeller with both ADI and TVD 2d solutions for a catchment (google map of the catchment). Keeping the model simplistic i am using a user entered rainfall and a global roughness. The only difference between the model runs is the addition of a shapefile of all buildings with an associated roughness of 1 (mannings).

Results when using roughness values of 1 for buildings: lower flood depths for roads besides buildings and higher flood depths for areas where the buildings are. This is in comparison to a simulation not representing buildings in anyway. I would assume that the opposite would happen with higher flood depths being modelled when increasing friction for buildings. Is there any reason why this may be?


thanks, josh


11/07/2019 15:34:00

Best answer

Hi Josh,


Are you applying your rainfall to the entire catchment including the buildings etc. ?

It strikes me that a mannings roughness of 1 is huge and I wonder if the rainfall cant leave the buidling areas it falls on and hence is building up, and then this storage would make the roadways adjacent less wet? I think I might investiagte what happens if you set the mannings to very low for buildings instead, and as a different test what happens when you just increase the threshold height of the buildings without changing the roughness- such that flows can only 'leave' buildings unless they build up to a huge degree.

Someonelse may have a better idea as I admit i am not overly familiar with Rainfall inputs, but this was my first thought as to the issue and how I might troubleshoot it.

Hope this is helpful.



Flood Modeller Support

11/07/2019 15:46:17

Hi Josh

I agree with Ben's suggestion as a possible cause of the results you're seeing. What do the different timesteps through your simulation look like? Are there any significant flows onto or off the buildings?

One other thing to check is the roughness diagnostic check grid that should have been produced for your run that included a roughness shapefile (it's not produced if the only source of roughness is a global value). This will verify your roughness data is being read in and interpreted correctly by the 2D solver.

Hopefully looking at these data will explain why your model is producing the results it is.





14/07/2019 10:46:18

Thanks both, the water was storing in the buildings due to the high roughness, Ive reduced the roughness and am getting more resonable results.

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