I am new to QGIS and just need it to do a hydrolocic analysis of an area of which I later make a flow model with HEC-RAS 2D.

I made it according to this tutorial: https://www.youtube.com/watch?v=Ro-RRzMMw-c&t=1774s

Unfortunately there is a railway with lower terrain nearly at the end of my river which is misunderstood as the stream so I can't make the catchment area to the very end of the river.

How can I edit the elevation of the DTM to get the correct stream (or is there another way)?

My river "Kraimser Bach" with the wrong end of the stream. It would go along the railway to north direction and not into the bigger river:

wrong catchment are

I tried to chagne the z-value of the DTM along this shapefiles to get the correct stream but it didnt worked the way I tried it. DTM with shapefiles i would like to edit the z-value in the DTM

I tried to do it with rasterize from polygons but that didn't work. (According to this: Modifying DEM elevation data in QGIS using a polygon) I also tried it with the SAGA tool "Burn streams into dem" but therefore I would need the real slope and so it didn't worked as well.

  • I cannot thank you enough for your answer MrXsquared, it worked pretty well. You saved me a lot of time and frustration, thank you!
    – Schneefux
    Jan 6, 2019 at 20:43
  • It worked pretty well, I cannot thank you enough for your help MrXsquared. You saved me a lot of time and frustration, thank you!
    – Schneefux
    Jan 7, 2019 at 9:57

1 Answer 1

  1. Create a new shapefile having the same CRS as your DEM. Create a numeric field (Whole number or Decimal number), like heightadd, and Add to Fields List.

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  1. Draw a polygon on the area you want to add height information. Add the value you want to add to the raster in the heightadd field.

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  1. Rasterize your polygon. Raster --> Conversion --> Rasterize (Vector to Raster)

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As input layer choose your polygon. For Field to use for a burn-in value choose heightadd field. Set the output raster size units to Georeferenced units and fill it with the original resolution of your DEM, in my example it is 20m. As Output extent choose Use Layer Extent... and use your DEM.

  1. Start r.null tool from processing toolbar. Choose your Rasterized polygon as input and for The value to replace the null value by choose 0. (Unfortunately this step has to be done as it seems the Rasterize tool is buggy and does not add 0 values instead of no data, even if I tell it to do so...)

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  1. Start the Raster Calculator. And enter the expression "YourOriginalDEM@1" + "NullRaster@1"

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Hit Ok. Done.

Using this way you can do all types of calculations, like -, +, *, /, and so on to modify your DEM.

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