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

[Thomas Adams]

Thayer,

Great suggestions! I'll give it a try...

Thank you,
Tom

On Fri, Feb 13, 2015 at 4:56 PM, Thayer Young <thayeray at yahoo.com> wrote:

> 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
> Message-ID:
>     <CAGxgkWgefeTtcjZWKi3JP8wj79Eb0j3fZsf6hGzT8m9-dgce6w at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> 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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> --
> Thomas E Adams, III
> 718 McBurney Drive
> Lebanon, OH 45036
>
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