[GRASS-user] floodplain creation

[Vaclav Petras]

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*
>
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> grass-user at lists.osgeo.org
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