Why are my volumes for a basin different when calculating using surfaces vs. stage storage?

[heidi.schreiner]

I'm working on a site that has an existing and a proposed retention pond. For the proposed pond, I created a comparison surface boundary and set the elevation for the top of the proposed pond. This gives me a volume in cu yds of 124,805 cu yds. I then convert that to gallons by multiplying by 201.98 which gives me 25.2 mil gallons.

If I use stage storage calculations, it gives me 2,048,525 cu ft x 7.48 = 15.3 mil gallons. 

I've looked at the surfaces, but I can't see any obvious issues.

Is there any way to check my work?? A different way to calculate volumes? Why don't these 2 volumes match?

[Joe-Bouza]

interesting they are off by a factor of 16... not far from 12.  I will give your file a peek

[heidi.schreiner]

Could it have something to do with the drawing units being decimal?

[Udo_Huebner · Accepted solution]

The "StageStorage" command calculates the volume between the contour lines. If the enclosed areas of adjacent contour lines change abruptly (caused by the sump or the wall at the ramp), the calculation result is useless.

Remove once the feature lines  "Feature Ramp Top to Bottom 1" and "Feature SUMP 1" and "Feature SUMP 2", and the StageStorage result is close to the calculated volume of the volume Surface "Proposed Volume".

By the way, I changed the Units for volumes to "Cubic Foot" in Command EditDrawingSettings > Ambient Settings Tab to make it easier to compare the calculation results.

[Joe-Bouza]

dont know

[Joe-Bouza]

I concur. I just couldn't find a way to prove it.

the ramp and double basin throws it off.

[heidi.schreiner]

Wow. I'm stunned that this would be an issue -- surely the usefulness of the tool is negligible if this is the case. 

You're right in that once I removed the sump and the wall at the ramp, I am 'only' off by a million gallons. Actually much better than my first shot which was off by millions.

I do hope that Civil picks this up and solves it. There should be an easier way.

Thanks for your help!

[fcernst]

There's a few issues going on:

1. This pond has multiple contour areas at the same elevation. When you add those together and compute avg end area it's close to C3D Vol surface. The Stage-Storage tool won't do this for you.
2. The 1247 comparison surface does not completely cover the pond surface.
3. The pond's 1247 contour does not completely encapsulate the pond, giving a small contour area at the top for the Stage-Storage tool.

[Joe-Bouza]

the tool was designed to look at "a" hole in the ground not multiple holes with discontinuities. Your surface Volumes - provided the data is correct will give real volumes.

suggestion: you have some nice surface styles. in your drawing  Add some CutFill display surfaces for your volume surfaces. These can visually give you confidence in your data - as well as cut fill tic surface labels place on grid 

[fcernst]

The math for the avg end area and conical methods still works for walls and ramps, etc..

The problem is the  nonhomogenous pond with local high/low areas not being not being accounted for in the avg end area and conical methods analysis in this case.

[heidi.schreiner]

Hi, Fred

I appreciate your taking the time to reply. I have a better handle on it now, and my conclusion is this:

In an ideal world, the stage storage would flag the areas that cause it to give wrong data (of course, it doesn't think that it gives wrong data). When I took out small areas that have sharp contours, the data was much more reliable. Since I often have anomalies and there's no way for me to 'see' these problems, I will avoid using this tool.

What didn't change was the surface to surface volume calculation -- creating a flat surface that compares to the other surface and then creating a volume surface comparing the 2. Seems to be reliable although I'll be going through the rounds checking that today. 

[fcernst]

It's just the opposite actually..

• If you have a pond that drains to one low point you will get the typical stage-storage curve that has one area associated with each contour elevation (even if it has walls and ramps).
• If you have multiple low points in your pond, you will get multiple instances of the same contour elevation with specific associated areas.
  • If you're pond is not intended to have more than one low point...you just flagged some anomalies for yourself.
  • If you're having problems with pond anomalies, you may want to go down to a 0.1 contour interval for the stage-storage tool and use it as a QC check.The surface method in this does not tell you if you have multiple low points.

The surface method in this example does not give you any information about multiple low points, it just reports the volume.

[heidi.schreiner]

Yes, I think we're looking at the same thing from opposite sides because all I need is correct volume. 

[fcernst]

I'm more addressing the confusion that the avg end area and conical volume methods are not valid for ponds with walls, ramps, etc. for developing stage storage curves for people who may want to learn. This is industry standard methodology.