[GRASS-user] Re: Defining Partial Drainage Basin

[Rich Shepard]

On Wed, 10 Feb 2010, Daniel Victoria wrote:

> In order for a basin to be delineated correctly, the pour point has to
> be in a high accumulation area (I once heard the expression synthetic
> river...)

Daniel,

   Perhaps if I explain what I'm trying to do it will help everyone who's
assisting me.

   There's a small parcel of land several hundred meters below a dam. On
January 1, 2009, after almost 14 inches of rain in the past week (including
6.44 inches the day before), that property flooded, including the trailer
house sitting there. This is the first flat spot below the dam (and the dam
spillway operated properly); the creek is deeply incised with almost
vertical walls below the dam. The bank on the other side from the property
is higher so all the flooding was all on this one side. Of course, lower in
the drainage basin flooding was more extensive (including a brand-new house
that hasn't sold in two years because it was built on the floodplain.)

   What I want to show is where all that precipitation runoff came from, and
that it's not all from the reservoir overflowing the spillway. Immediately
west of the flooded property is a steep hill, so a lot of surface runoff
came from there and augmented what was transported in the stream channel.

> Well, put your pour point on top of the accumulation map and see if it's
> on top of the river. If not, change your easting and northing coordinates
> appropriately. If you come from ArcGIS world, they have a tool called Snap
> Pour Point that does that. I recall doing something similar in grass using
> v.distance and a stream raster...

   The last time I used ARC/Info was 1989. :-)

   The pour point is on the stream channel. See the attached .png that I
forgot to put on my response to Stephen. The topmost 'x' is the flooded
property, the one below it is the center of the dam.

Thanks,

Rich
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