[GRASS-user] Question about r.watershed and flow accumulation grid

[Thayer Young]

Tom,
Hopefully the higher spatial resolution data will also have higher vertical resolution.  
That said, if you have any idea where the stream channels are, for example a vector file, in the flat areas you can "burn" them into the raster using r.mapcalc.   So you can rasterize vector streams (v.to.rast) and then thin the raster streams (r.thin) and use r.mapcalc to subtract a constant value where the raster stream map is. If you do not thin the raster you will get an error, as there will be ambiguous flow paths (endless loops).
QGIS has a nice feature in its vector to raster tool. It will do the burn for you directly from the vector file into your DEM.  You should make a backup of your DEM though first, since it edits the original file.  You just add a field to the vector streams for a desired elevation.  Then you select that elevation field in the vector to raster dialog and select your DEM as the output raster.  After saying that yes it is OK to overwrite your DEM, it will burn the vector elevation. I have only used this to edit small areas on a raster, so I can not tell you if the burnt channel is sufficiently thinned or not. One of the big problems with using flow algorithms is that bridges and roads will frequently act like dams, causing the flow to be rerouted outside of the actual channel. To prevent this you can digitize a vector map of places where you want to cut through these "dams" and set a field with desired elevations as described above.
-Thayer

      From: Thomas Adams <tea3rd at gmail.com>
 To: Thayer Young <thayeray at yahoo.com> 
Cc: "grass-user at lists.osgeo.org" <grass-user at lists.osgeo.org> 
 Sent: Friday, February 13, 2015 5:48 PM
 Subject: Re: grass-user Digest, Vol 106, Issue 34
   
Thayer,
Thank you for the suggestion; I just tried it with the same result. I think the area just happens to be a difficult area, that is pretty flat…
Cheers!Tom


On Fri, Feb 13, 2015 at 1:54 PM, Thayer Young <thayeray at yahoo.com> wrote:

I don't know if you have tried this yet, but you may also want to look at r.terraflow.  It does both D8 (single flow direction) and multiple flow direction (flow is partitioned, according to the steepness of slope, to all directions that are lower than the central cell).  Supposedly you can also switch from MFD to SFD once flow exceeds a threshold, but I have not been successful at doing this.  It would be a simple simulation of channelization though. 

Just set the D8 flag in the options tab, otherwise it will give you MFD by default.  

-Thayer

    Date: Fri, 13 Feb 2015 11:12:30 -0700
From: Thomas Adams <tea3rd at gmail.com>
To: "grass-user at lists.osgeo.org" <grass-user at lists.osgeo.org>
Subject: [GRASS-user] Question about r.watershed and flow accumulation
    grid
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Hello all!

I'm making use of the flow accumulation grid in GRASS 6.4.5 generated from
r.watershed using the SFD (D8) flow algorithm. The DEM has a 250m spatial
resolution. What I'm getting is a break in the flow accumulation in a few
locations which is causing me serious problems with subsequent processing
(with help from some here, I have put together some scripting to generate a
pixel connectivity file for a distributed hydrologic model).

Besides going to a higher resolution DEM, are there any thoughts as to how
I can eliminate these flow accumulation breaks?

Thank you,
Tom

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