best way to put buildings in the inland flooding module

Hej!

I am carrying a flooding study with a 100-year rain over an urban area with the Mike 21 Inland Flooding module. I am wondering which is the best/smoothest way to put the buildings in the grid data in order to have the more accurate results as possible. I tried it modifying my DEM model previously in ArcGIS giving the buildings a value of 5 meters over the DEM, but the results of the simulations of Mike 21 show a lot of “Undefined values” around the borders of the buildings. Should I define the borders of the buildings as a dike?

The data and settings that I’m using are:
-Grid DEM of 1m. Created from the projected elevation points for a future urban area.
- Flooding and drying of 0.003 and 0.002 respectively.
-Manning number equal to 15.
-100 year rain during 6 hours, with a time step of 0.125s (when increasing the time step I get errors for too high water levels)

Best regards,

Xavier

Hi Xavier,
I've had successful simulations by placing buildings or other areas that you want to block out by putting them as your 'land value'.  I quite often use 99 m or 999 m for my land values in order to differentiate them from any other real elevation.  This also prevents them from being in the calculation, since they don't need to have any water on them.  I've never put a weir or dyke to join the DEM to a building. 

It might be worthwhile to check to see that your DEM does not have any gaps in it, between the simulated building and the actual land.  You could also check by doing a run without the simulated buildings, if the results show undefined values even without the building, then perhaps it is the underlying 1m DEM.  If you made a 1m DEM from some LiDAR points which maybe have a spacing of greater than 1m, you may have ended up with some gaps if there was no interpolation.  Also make sure your DEM creation is not interpolating between the building edge (either 5m higher like you've done, or 99 m land value) and the actual ground, otherwise you may have some very steep edges which are undesireable.

Good Luck!
Craig