Hello Bentley,
I am having some issue with volume calculation in pondpack.
How to calculate volume for multiple landcovers outleting at same outlet without using weighted CN because in my case weighted CN is not acceptable.
The calculated volume from Pondpack is more or less 125000 CF with weighted CN and volume calculated in excel is more or less 142000 CF with individual CN. Every parameters are same in both platform except weighted CN and individual CN method. I am trying to get same result 142000 in pondpack.
Any suggestions?
see below.
Hari, in the last row of the spreadsheet, you have a CN of 84, but in the PondPack CN collection screenshot it is 82 for the corresponding entry. There could also be a difference between the storm depth in the PondPack model and the depth assumed in the spreadsheet, and possibly rounding due to the value in the spreadsheet vs. the value entered in PondPack. Where are you looking to see the calculated depth? See this article.
If this does not help, please provide a copy of the model and spreadsheet (zipped) for review, with any steps needed to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
That's my mistake however the volume is still different. But, I figured out something when I put those individual landcover and CN as a different catchment. In that way the volume nearly match with few hundred cfs different.
before I put the same thing in table as a composite CN and found different result. Is there any good way to calculate volume in two different case. Please consider that 84 as 82 for now.
It may be due to the way that the composite CN is weighting or rounding - when clicking Ok on that dialog, it will calculate the total area and weighted CN, which you can see in the catchment properties. I believe the overall weighted CN that you see in the "SCS CN (Composite)" field in the catchment properties is rounded to the nearest whole number.
I checked in different programs.My excel sheet is good. Only the thing is that excel sheet is calculating volume with individual CN. When I did the same thing in pondpack with individual catchment assignment, the result is more or less equal.
Conclusion: pondpack is using composite CN.
my question is , this is the only way. How to tell program not to do composite/ weighted my CN when you are assigning multiple landcovers in one table for one outlet.
If you use the composite CN method with a single catchment, it will always use weighting to determine a single CN representative of the entire area. This approach may be more appropriate if there is a mixture of land types within a given catchment area that drains to a common point. Additionally, the percentage columns in the composite CN dialog give you control over how runoff is directed between impervious and other areas as mentioned here, which can further adjust the overall CN.
If you have a situation where there are distinct, separate areas of land with their own CN, then having separate catchments may be more appropriate since they are calculated separately with their own collection points. This seems to be what you are assuming with your spreadsheet.
You can find more information about CN and weighting with respect to modeling practice in our book Stormwater Conveyance Modeling and Design.