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I have a channel network which I've intersected with a proposed road network to produce crossing locations. I need to find the channel slope averaged over a 20m stream length.

I also have a raster of elevations

Is there a way to 'look' upstream and downstream the channel network from a set of crossing locations?

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In three steps:

(1) Create 20m line segments

Line segments +20/-20 meters from the crossing point is generated by Service area (from layer) tool (Processing Toolbox | Network analysis)

  • Path type to calculate: Shortest
  • Travel Cost: 20m

then Multipart to singleparts (Processing Toolbox | Vector geometry) to split upstream/downstream segments.

(2) Create Slope layer

Go to menu Raster | Analysis | Slope

(3) Calculate average of Slope value along the streams

Use Add raster values to features (in SAGA | Vector <-> raster) on Slope raster and 20m line (i.e. Single part layer from the Step 1). It will automatically calculate average of the Slope along the stream.

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