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26 Oct, 2008

Civil 3D 2009 and the New Catchment Delineation Tool: Is it any good?

Posted by: Skylar In: Stormwater|technology

1.0 Intro:

When I first found out that AutoCAD was adding a catchment delineation tool to Civil 3D, I was pretty excited. In fact, this tool was one of the reasons why I thought it’d be a good idea to create my AutoCAD to EPA-SWMM script. If you don’t know where to find the catchment and waterdrop tools, they’re in Civil 3D under “Surfaces->Utilities”:

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2.0  The (Very) Simple Example:

Now, when I first tried out the catchment area tool, I used it on a large surface I’d found at random. To my dismay, the catchment areas were not all completed quick and clean. So, I decided to try it with the most simple of catchment scenarios: one makeshift parking lot with four quadrants, each with a catch basin in the middle. Here’s what the parking lot looked like:

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Now, I set it up so that each quadrant is level on the outside (at 100.15) and each catch basin is at 100.00. Using three feature lines and four points for the catch basins, I came up with this surface:

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And, when I tested AutoCAD’s built in catchment tool, I came up with this solution:

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So, it worked quite well in this over-simplified example. Now, let’s see how it performs in more complicated scenarios – the scenarios that you’d actually need some help with!

3.0 The More Complex Example:

I decided to use the surface from the drawing called “Intro-1.dwg”. If the surface looks familiar to you at all, then you’ve probably gone through a couple of the AutoCAD tutorials already. According to the documentation I read under the AutoCAD help section, it is recommend that prior to using the catchment tool, you add a lot of water drop locations to your surface. Originally I was unsure as to the purpose of this step, but I’ll demonstrate that in a bit. First, I tried it the AutoCAD way, with water drops and all:

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The first thing I notice with this delineation is that there are way too many catchments for my liking (they’re shown in red). This is because the watershed contains multiple flat spots. While Civil3D does a great job of finding the water divides, it cannot see the “bigger picture” to divide the area into exact drainage areas. Take for instance this area:

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Notice how the water drop just kind of stops in the middle of nowhere? That’s because there’s a low point in the middle of this “catchment area”. However, when I look at the low point, the North side has an elevation of 92.64, while the low point has an elevation of 92.58, and the south side is at 92.66. Sure, water will pool in that area for a time, but I wouldn’t think that 0.06m would constitute a separate catchment area. And this is only one example – there are numerous other scenarios where this will happen. So yes, the tool delineates water paths quite well, but it still requires someone’s good old fashioned common sense to see the bigger picture.

4.0 The Point of the Water Drop

So, on to my other question: Do waterdrops matter? I tried the delineation without any waterdrops. The AutoCAD documentation uses the waterdrop tool to show the user where the low points in a catchment area are located. It seems that the catchment area tool works best when the low point is selected as a starting point. Here’s what I got with a bunch of random clicks:

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And the original:

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It seems that the water drops are an important tool to show you where to click to make the most representative catchment area. The important thing to note here is that while the delineation without the water drops produces a much greater number of areas, the outer boundaries are consistent in both scenarios. So while the water drops are not a necessity, they will make your job much easier in the end when you go to decide on the final catchment borders. Because as I talked about previously, AutoCAD will delineate water paths, but it will not define the actual catchment for you.

The documentation suggests is the command “LineworkShrinkWrap”, which essentially creates a polyline around all of the polylines in your selection set. Of course, you need to have the right polylines in your selection set to begin with.

1 Response to "Civil 3D 2009 and the New Catchment Delineation Tool: Is it any good?"

1 | Ryan

June 13th, 2011 at 11:54 pm

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Great advice, I was very close to giving up completely on the catchment area tool until I started using the water drop. Very helpful!

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