I'm starting with a DEM, and looking to identify every point that can be directly "seen" from any point in a particular polygon on an overlaid layer defining an area on that DEM - like any other viewshed analysis, but with the additional stipulation that the observer points can only "see" at certain downward angles.
To help motivate what I'm looking for, my real goal is to identify run-out zones for avalanches. It's easy to identify "slideable slopes": just everything that's, say, 30-45 degrees. But if I want to identify the areas which snow would slide from the slideable areas into, the above quasi-viewshed starts to look like what I'm after.
My exploring the surface and hydrology tools (and help files for the tools) in Arc hasn't turned up anything that I think will do the trick. Any ideas - either in Arc or otherwise?
Thanks in advance for any insight.