System CA Computation

Hi Kelsey,

This appears to be related to a known defect (#333602) where the "System CA" at the outfall element is not correct and appears to be double the expected value.  If you  are seeing that the calculated value is greater than your manually calculated value, here is a Wiki article with a workaround.

If you are seeing that the calculated value is lower than your manually calculated value, here is a Wiki article with a workaround.

Please let me know if neither of these solutions work for you.
Thank-you,
Larry

Hi Larry,

Thank you for your response. I realize there was a mistake in my original post and meant to say that my calculation for System CA is as follows: System CA = (Total Area Tributary to Node X Composite Runoff Coefficient) - Bypass Flow + Carryover Flow

Using the above formula, my manually calculated CA value is greater than the calculated value from StormCAD for some segments of my model. The second wiki article you sent confirms that bypass, intercepted, and I believe carryover flows are parameters in the System CA calculation. The above formula seems to match StormCAD's results for smaller catch basin to catch basin pipe networks, however the issues more so arise at manhole nodes. For example, in the photo below, System CA manual calculations and StormCAD calculations generally match up with the above formula for Catch Basins D-54 and D-55.  The System CA manual calculations are greater than the StormCAD calculations at manhole D-8. Is part of the problem that manholes are sealed and therefore bypass and intercepted flows aren't accounted for in the model like at catch basins? 

Thank you,

Kelsey

Hi Larry,

Thank you for your response. I realize there was a mistake in my original post and meant to say that my calculation for System CA is as follows: System CA = (Total Area Tributary to Node X Composite Runoff Coefficient) - Bypass Flow + Carryover Flow

Using the above formula, my manually calculated CA value is greater than the calculated value from StormCAD for some segments of my model. The second wiki article you sent confirms that bypass, intercepted, and I believe carryover flows are parameters in the System CA calculation. The above formula seems to match StormCAD's results for smaller catch basin to catch basin pipe networks, however the issues more so arise at manhole nodes. For example, in the photo below, System CA manual calculations and StormCAD calculations generally match up with the above formula for Catch Basins D-54 and D-55.  The System CA manual calculations are greater than the StormCAD calculations at manhole D-8. Is part of the problem that manholes are sealed and therefore bypass and intercepted flows aren't accounted for in the model like at catch basins? 

Thank you,

Kelsey

Kelsey, are you referring to the CA values, or the flow, or both? Can you provide a copy of the model files (File > Save to Package) for review? (with details on exactly where to look).

Note that at junction points, StormCAD selects a controlling time to determine the intensity to use to recalculate the flow based on the Rational Method, but the CA should not be impacted by this.