Drainage Analysis runoff grossly wrong

[kevincurrey]

I'll assume this is a Me problem, not a software problem, but I'm attempting to use the drainage analysis tools in 2026, and I'm getting runoff results that are way too small, by a factor of 30. I have a simple drainage network of 7 inlets, with contributing areas for each inlet ranging from 10,000 sq ft to 44,000 sq ft. The catchments are set to the Rational Method, and I'm using NOAA IDF curves. Time of Concentration is manually set at 5 minutes, as the small areas gives something lower than that.

For a rational method runoff of Q=CIA, if I have a C of 0.9, intensity for a given 5 minute storm of 1 inch, and an area of 1 acre, shouldn't my runoff be 0.9 cfs? Instead I'm getting runoff of around 0.03.

I'd like to think its something I'm doing wrong, but I don't see how to check.

Relatedly, what's everyone else using for drainage as we begin 2026? I'll also give the Storm Sewers tool a spin, but this one seemed like its supposed to be more integrated with C3D.

[AdammReilly]

Can you post your file so we can look at the data more closely?

For your last question:

I've been testing the new drainage tools for a while but I'm still not fully using them for production because our review agencies are requiring other tools, but I am using it for preliminary designs. We also use Pond Pack/Storm CAD for our designs.

[fcernst]

'Drainage Analysis runoff grossly wrong'

This usually means the User has units and logic issues.. Should be using intensity (in/hr) , etc.

[kevincurrey]

That's what I'm trying to figure out, is what is going on with the inputs. Running the "Time Area" method, (which looks like SCS but I'm not sure how its different) gives results that seem reasonable. If I run the "Analyze Gravity Network," which looks like it solely analyzes using Rational Method, it gives results that are consistent with my inputs. I can follow the time of concentration calcs, and it the math for runoff checks out. 

Setting the Catchments to Rational Method and re-running Drainage Analysis is where it falls apart though. I figured out what it's doing - using a storm duration of an hour - but I still can't figure out WHY.

Here are screenshots of my results - Drainage Analysis "Time Area" method (good), Analyze Gravity Network (good), Drainage Analysis Rational Method (no good), and I've attached the Excel file exported from Drainage Analysis. 

Drainage Analysis - Time Area (SCS) Results

Analyze Gravity Network Results

Drainage Analysis - Rational Results

IDF Table

Drainage Analysis - Rational Criteria

Drainage Analysis - Rational Inputs

[robvanderveen]

I second @kevincurrey 's findings with Analyze Drainage System when using the rational method.  The peak flow results output by the analysis are off by a factor of about 30.

[fcernst]

I had not tried this extension yet.. So I created a quick IDF curve from NOAA 14 for my area..and I see what you're saying now..

[AdammReilly]

I had run into this on a recent project and reported it to the dev team. It looks like it might be a metric conversion issue but that's the best I can help with on what's going on.

Now, if you try it in metric, it probably works fine.

[fcernst]

Seems like it .. 35.3 conversion factor. Also that field should read peak runoff for a catchment result tab.. not inflow

[Charlie.Ogden]

Thank you for bringing this to our attention and for the detailed investigation.

We’ve identified an issue where imperial IDF intensity values were not being converted correctly for the Rational Method calculations. The underlying issue has been addressed and is currently going through our normal release process. While we can’t share release timing or version details yet, we’ll update this thread once the fix is part of a publicly available release.

In the meantime, as a workaround, you can convert imperial IDF intensities from inches per hour to millimeters per hour and enter those metric values as if they are the inches per hour values.

Apologies for the inconvenience, and thank you for your patience.

[dflynn3XV9C]

Has this issue been resolved? Is a release upcoming or available?

In addition, simply converting the in/hr to mm/hr is not a sufficient work around unless the metric rational method formula is being used and then that solution is being converted back to imperial. This doesn't appear to be the case in testing this work around.

[AdammReilly]

 This has been resolved in the current release versions.

[RussellCL4SFL]

As of 06/09/2026 for Civil 3D 2026 Imperial this issue has not been fixed. I've been going through the process of learning how the drainage analysis works to see if I can implement it for hydraulic computations. I created a test drawing and I'm getting the same runoff discrepancy as the original post, all the other pats of the program seem to be working fine.

[AllenJessup]

The current version as of 5/02/2026 is 2027.