Civil 3D and the Catchment Area Tool

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.

Comments

[David Perrings, PE]

Dana,

It has been a number of years ago now and it was probably autocad 2000 version of LDD that i attempted to use the catchment tool to determine a drainage area boundary. It is a neat idea but i had absolutely no luck in being able to use it for the same reasons you point out now with the 3D version. By the time i got all of the "Micro Ponds" out of the dtm..... well lets just say that i might still me messing with it 10 year later.

I would like to hear more feedback from you on this one.

David Perrings, PE
dperrings@padesignresources.com

[Ann]

I can usually get SOME use out of even the LDD watersheds. When I first start a project, I would create the watersheds and use the lines as aids to help me find high & low points where the water breaks. I would trace over some with end point Osnap or split down the middle the multi areas. then I would erase them & forget them.

[Ann]

Are these new catchments areas supposed to be able to be added together and split apart as a 'parcel' database? That would be more useful.