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I am trying to simulate Natural flood management in a river catchment, particularly the role of log jams/LWD in the channel. I have created polylines where I want to put my log structures and given them a value field, in meters. I then buffered these polylines and converted them to raster.

What I want to do is add these log jam rasters into the DEM, so they simulate structures in the channel. I have read around the best way to do this and have found two possible methods, but I am unsure if they have worked.

Method 1

  • Mosaic to new Raster
    • My issue with this is that the DEM ranges from 5m to 238m, so I am wondering whether the mosaic tool changes a value from, for example 138m, to 1.5m, instead of adding this on. Or am I overthinking it?

Method 2 - Raster calculator using the following equation: con(IsNull("New_DEM"),"Existing_DEM",("New_DEM"+"Existing_DEM"))

  • This layer now has a very low minimum value, and I am wondering if the same concerns as the Mosaic to new Raster are happening.
  • Screenshots of your inputs and outputs would help here, especially a shot of the rasterized logjams, but it sounds like Method 2 is what you want. As far as telling if it worked, zoom to the location of a logjam on your new DEM and see if you can see a "raised" area.
    – jcarlson
    Mar 8, 2018 at 16:08
  • Thanks for your reply, I have added an image to the original post.
    – user116036
    Mar 8, 2018 at 16:14

1 Answer 1


Mosaic operations are for merging adjacent rasters. What you want to do is quite simple:

  1. set your dem as the mask and extent raster;
  2. add a field with values representing your obstruction height offsets;
  3. convert your polygons to a raster;
  4. convert the background nodata values to 0;
  5. sum the two rasters.

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