# [GRASS-user] elevation values of watershed basins

Dylan Beaudette dylan.beaudette at gmail.com
Mon Feb 5 16:51:32 EST 2007

```Hi -

On Monday 05 February 2007 02:19, Hamish wrote:
> temiz wrote:
> > As I see, you are a soil scientist, so you will understand me better.
>
> I'm not, but maybe there is are ideas from other fields which can help-
>
> > The logic of my question is based on finding slopes' length as
> > landslide susceptibility parameter ( I accept slope is waterhed
> > area).
>
> Perhaps the concept of "fetch"? "The unobstructed region of the ocean
> over which the wind blows to generate waves." The idea is the longer the
> fetch, the greater height waves can grow. Your slope length seems like a
> direct analogy, of sorts. Perhaps there are some fetch calculation
> algorithms which may be applicable?

This is a good analogy for the dynamics of hillslopes: longer slope length per
unit of water  = greater potential for erosion. Landslides would of course be
a bit more complex, but could follow a similar cascade-of-events type system:
like an avalanche in the snow.

> run r.watershed and get a basin and half-basin maps, r.to.vect,
> then isolate half basin borders not matching basin borders.
>
> This gives anti-ridges (obtain ridges with "r.param.scale par=feature").
> The anti-ridges will be the landslide path? (I guess ridges of flow
> accum river map will be the same thing?)
>
> then use the river paths as a MASK and maybe r.cost with slope as cost
> layer to see which basin (river) has the biggest cumulative slope?
>
>
> shooting in the dark,
> Hamish

Those are some interesting shots, and definitely something to try. Slope
length (RUSLE) and 'contributing upslope area' (a couple of definitions of
this are out there see [1] for some definitions).

1. Wilson, J.P. & Gallant, J.C. Terrain Analysis : Principles and Applications
Wiley, 2000

--
Dylan Beaudette