topic Re: Civil 3D Storm and Sanitary - Rational Method Calculations in Civil 3D Forum

Civil 3D Storm and Sanitary - Rational Method Calculations

Anonymous — Thu, 15 Oct 2020 15:57:28 GMT

Hello,

This is a two part inquiry into the calculations behind SSA. Our office is currently running SSA 2019 version 13.0.94.0. We are using the rational method and our state DOT requires HEC-22 be followed for storm sewer design.

Our office is trying to use SSA to calculate peak flows for a proposed storm sewer. The first question involves the intensities calculated from the IDF curve. The intensity values were gathered from the NOAA website and were used to replace the generic values within SSA. The SSA model has incorrectly calculated the intensities for each sub-basin. Are there known issues with the interpolation of the input values for the IDF curves?? The correct intensity can be obtained but that requires placing incorrect values into the IDF curve in order to "trick" the software.

Our second question regards the calculations behind the rational method. Based on the peak flow outputs for each sub-basin we gather the ΣCA of the upstream areas are not used, rather the Q is calculated individually for each area. The following is a link to an older autodesk help form: https://forums.autodesk.com/t5/civil-3d-forum/hydraflow-vs-ssa-results/td-p/3415701. Can you confirm the discussion in this form still applies to the 2019 version our office is using, and that we will need to use another software (or update to the latest SSA) in order to stay compliant with HEC-22?

Thank you in advance for your time!

@Joe-Bouza @Todd_Rogers @wfberry

Re: Civil 3D Storm and Sanitary - Rational Method Calculations

tcorey — Thu, 15 Oct 2020 16:07:04 GMT

You pointed a link to a discussion on HydraFlow. That is not the same as Storm and Sanitary Analysis (SSA).

To run HydraFlow, go to Analyze tab, click the Design panel, select Launch Storm Sewers.

Re: Civil 3D Storm and Sanitary - Rational Method Calculations

Anonymous — Thu, 15 Oct 2020 19:41:33 GMT

Thank you for you quick response, it is very appreciated.

In regards to the first question, about SSA incorrectly interpolating the intensity values, is there any literature or personal experiences with this happening after the model has been ran?

Re: Civil 3D Storm and Sanitary - Rational Method Calculations

Todd_Rogers — Thu, 15 Oct 2020 20:04:35 GMT

They hydrology wiz @BrianHailey may be able to tell you the issue.

Re: Civil 3D Storm and Sanitary - Rational Method Calculations

tcorey — Thu, 15 Oct 2020 20:10:30 GMT

On this part of your question, sorry, I have no advice for you. I answered what I could. Someone with deeper knowledge of SSA and HydraFlow will need to answer that one.

Re: Civil 3D Storm and Sanitary - Rational Method Calculations

BrianHailey — Thu, 15 Oct 2020 20:27:55 GMT

As far as the incorrect calculations of the intensity, can you post up a sample file, say with a single basin, and explain what is happening and what you expect to happen?

I personally haven't run across this so rather than trying to recreate it, it will be a lot easier if you can post up a sample.

As far as the HEC-22 question, have you tried using the Analyze Gravity Networks tool? My understanding is that is based on HEC-22. If nothing else, it would be good to compare the results between the two programs.

Re: Civil 3D Storm and Sanitary - Rational Method Calculations

jmayo-EE — Thu, 15 Oct 2020 21:21:52 GMT

Test the interpolation by making a copy of the idf curve file and adding more data points closer together. If you use a min of 5 min interval add all those points. Run a few scenarios at the data points and in between for checking. If you have points of great curvature in your curves you may need extra data points to pick them up better.

You should be able to do the math in Excel and paste the results into SSA or Hydrograph tables to speed the processes.