[GRASS-user] Advice needed on how to do something that seems straight-forward, but...

[Thomas Adams]

All,

I'm trying to help a colleague with a project; on the face of it, the issue
seems straight-forward, but I can't seem to find the right combination of
GRASS commands to do this.

In lieu of doing a full-blown hydraulic modeling analysis, as a first
approximation, my colleague wants to use a GIS-based approach. Ultimately,
he wants to create a detailed flood inundation map and to do this, create
the ascii input file for r.inund.fluv that is comprised of:

x,y,wsel,sta_id, where wsel is the water surface elevation of a stream
channel centerline.

He has:

(1) a detailed DTM
(2) vector lines approximating the left and right lateral extent of
flooding (presumably without elevations of the nodes, so, just x,y
locations)
(3) using watershed analysis, he has the approximate stream centerline

The thought was that if a surface that intersects the DTM along the left-
and right- banks could be found -- which should be the vector lines (2),
above -- (with the assumption that perpendicular to the channel centerline
the water surface elevation is constant across the stream channel, but
sloping upwards in the upstream direction, then the intersection of this
vector surface with the vertical projection of the stream centerline,
should yield the wsel along the stream channel centerline.

My colleague considers the vector lines in (2) to be rough approximations
and that by using r.inund.fluv and the x,y,wsel,sta_id formatted ascii
input file, he could get a more refined flood inundation map.

How can this best be done? Do you have a suggestion for a better approach.
I should add that I believe my colleague used an approach similar to that
suggested by using r.hazard.flood.

Best regards,
Tom
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