I was working on something today that brought up a question about the Catchment Area tool. The developer was kind enough to give me some insight into what is happening under the hood. (Thanks Chris M.!)
The catchment area tool asks you to pick a point on the surface, then it figures out which triangles slope down toward that point. The point must have at least pair of triangles that share an edge where the edge slopes towards your point. A "downhill channel" so to speak. Once it encounters triangles that no longer slope towards your selected point, the catchment tool stops.
I created an exaggerated example to help explain what this means. Here is the perfect drainage basin.


Everything slopes downward to the exaggerated "pond" in the middle.
Using the catchment area tool, I select the bottom of the pond. The tool returns, predictably, the entire surface as my catchment area.
Now, let's say I add a nearly negligible downward slope to the side of this drainage basin.
If I were doing a hydrologic analysis of this basin and manually drawing my subcatchment areas, I would still make the boundary the entire surface area, because though the water would theoretically stall here for a second, the general trend is still downward to the pond.
When I run the catchment tool, however, it sees a series of triangles that slope away from my selected point, and stops there.
This is something I want to talk about more so that I can show you the best way to leverage this tool when you are working through preparing your pre and post development stormwater management plans. I have a very simplified site that we'll use to illustrate the examples.
