[GRASS-user] floodplain creation

[Shane Carey]

Hi All,

This works well, but doesn't capture every river - is it a case of making
the threshold value smaller or making a deeper carve in the rivers???

Thanks in advance - I think this method would will work well, if it were
able to "flood" all rivers.

Thanks

Le gach dea ghui,
*Shane Carey*
*GIS and Data Solutions Consultant*

On Sun, Sep 23, 2018 at 12:55 PM, Shane Carey <careyshan at gmail.com> wrote:

> Thanks, I will compare them and let you  know the differences.
> Thanks
>
> On Domh 23 MFómh 2018 at 02:09, Vaclav Petras <wenzeslaus at gmail.com>
> wrote:
>
>> These are steps based on:
>>
>> https://grasswiki.osgeo.org/wiki/From_GRASS_GIS_novice_to_
>> power_user_(workshop_at_FOSS4G_Boston_2017)#Hydrology:
>> _Estimating_inundation_extent_using_HAND_methodology
>>
>> You need r.stream.distance module from Addons:
>>
>> g.extension r.stream.distance
>>
>> Get drainage and streams from your DEM (your carved DEM):
>>
>> r.watershed elevation=dem accumulation=flowacc drainage=drainage
>> stream=streams threshold=100000
>>
>> Compute height above nearest drainage/stream (HAND):
>>
>> r.stream.distance stream_rast=streams direction=drainage
>> elevation=elevation method=downstream difference=hand
>>
>> Use r.lake not on the original DEM, but on the HAND and start flooding
>> ("lake") from the streams:
>>
>> r.lake elevation=hand water_level=3 lake=flood_3m seed=streams
>>
>> Convert to vector if desired:
>>
>> r.to.vect -s input=flood_3m output=flood_3m type=area
>>
>> The difference to the r.grow+r.mapcalc method [1] is that this uses an
>> addon module (there should be no problem installing it) and that r.grow
>> uses euclidean distance for what is later used for height difference while
>> r.steam.distance follows drainage and further that r.lake floods only the
>> cells accessible to water unlike the r.mapcalc expression which just looks
>> at height. The two methodological differences can be summarized as "not
>> respecting the surrounding terrain enough." Anyway, the r.grow+r.mapcalc
>> method can get you quite far and I would be interested in the comparison
>> (will differ for different terrains).
>>
>> Best,
>> Vaclav
>>
>> [1] https://lists.osgeo.org/pipermail/grass-user/2018-
>> September/079134.html
>>
>> On Fri, Sep 21, 2018 at 11:39 AM Shane Carey <careyshan at gmail.com> wrote:
>>
>>> Hi All,
>>>
>>> I have used r.carve to carve out the rivers of a DTM - a really super
>>> job. I now need to pour 3meters of water into every cell in the river and
>>> see how for this water extends out - onto the floodplain.
>>>
>>> I was trying to use r.lake to do this, but unsure as to how r.lake will
>>> work to pour 3 meters of water in every cell along the river network.
>>>
>>> Any advice on this would be great. It is for the creation of a
>>> floodplain.
>>>
>>> Thanks
>>>
>>>
>>> Le gach dea ghui,
>>> *Shane Carey*
>>>
>>> _______________________________________________
>>> grass-user mailing list
>>> grass-user at lists.osgeo.org
>>> https://lists.osgeo.org/mailman/listinfo/grass-user
>>
>> --
> Le gach dea ghui,
> *Shane Carey*
> *GIS and Data Solutions Consultant*
>
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