There are 4 required inputs for this addon (r.damflood) in GRASS.

  • elev= elevation raster map (raster file with elevation data)
  • lake= water depth raster map (raster file of only the lake area)
  • dambreak= Name of dam breach width raster map
  • manning= Name of Manning's roughness coefficient raster map

For the last two (dambreak and manning), I have had some trouble when trying to get them.

First, for the dambreak file, the idea is to do 'breach = DTM - your map'. But how can I do this with only one raster file?

Second of all, for the manning map, I am supposed to use the r.reclass tool. But what could I possibly classify in an elevation map?

1 Answer 1


In order to obtain a dambreak map, you can digitize (raster digitizer wxGUI.rdigit, and in new versions g.gui.rdigit) an area on top of the dam and assign it a depth (i.e., negative value). Then use r.mapcalc to add this dam map on top of the elevation model. Since the dam depth value is negative, it will lead to the desired dambreak raster map.

Concerning manning you may turn the Manning's n friction coefficients into a map, see



  • Petrasova, A., Harmon, B., Petras, V., & Mitasova, H. (2015). Tangible modeling with open source GIS. Cham: Springer International Publishing (DOI; authors' page; related section).
  • Discussion here
  • Thank you so much for your answer. However, the tool in Grass or QGis tool to digitise the raster (g.gui.rdigit) is not available anymore... Do you have any other suggestions on how I could assign a negative value to the map? Thanks, I really appreciate it
    – Monica NM
    Commented Dec 19, 2020 at 11:45
  • The tool is new :-) in any case there is also grass.osgeo.org/grass-stable/manuals/wxGUI.rdigit.html (I have edited my answer accordingly).
    – markusN
    Commented Dec 19, 2020 at 17:44

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