System Time of Concentration

Thank you for the reply. However, the travel time in pipe P2 is only 0.13 min.  Actual system time is still below 5 min.  This is why I have posted the qiestion. One interesting thing is that the calculation for discharge uses the correct time.  In this instance the intensity is based on a 5min Tc, so this is just a reporting issue.  But leads to questions since the reported CA * reported time does not equal the reported discharge.

Hello Kevin,

Can you send a copy of the model where this occurs? We can test this and see what the expected calculation will be.

Regards,

Scott

Hello Kevin,

It seems you have only sent the .STSW file. We require two files to open your model viz. the .STSW and the .STSW.SQLITE files (which is the database where all model information is stored). In the article Scott shared with you, there is a section titled "Which files do I need to provide?" which lists the required files for each of our applications.

Please share the .STSW.SQLITE file also for the project so that we can take a look at your model and see where the issue lies.

A better way to share the required files is to use the Save to Package feature (File > Save to Package) which is available in all our products since CONNECT Edition Update 1.

Let me know when you share the required files.

Sorry for the oversight.  The requested files have been uploaded.  Best

Hi Kevin,

It looks like this may only be a reporting problem, related to the "actual uniform flow velocity" selection for the Average Velocity Method calculation option in combination with a surcharged pipe.

Per the definition seen here, if the pipe is in a surcharge condition (which the first pipe is), then it should use the full flow velocity when using that particular average velocity method. Based on the pipe diameter, flow and length, the travel time should be 0.33 minutes, yielding a system time of 5.33 minutes at the next downstream catchbasin. Since the catchment that also enters that catchbasin has a Tc of only 5.0 minutes, StormCAD selects the higher Tc (per this article) of 5.33, hence you see 5.33 as the system time in the next downstream pipe.

So, the model seems to be using the correct travel time for the first pipe, but for some reason it reports a different flow time of 0.117 minutes in that pipe. If you change to another method such as "simple average velocity", then the travel time in the first pipe correctly shows as 0.33 minutes. I have reported this issue to our developers as Defect #1093980, to be investigated for a future release.

Forgive me if I am being obtuse here but this still does not seem correct to me.  The 1st node has an actual time of 1 min, with a minimum time of 5 min.  If we then use the 0.33 above, wouldn't the actual system travel time be 1.33 min?  In which case the utilized travel time would be the 5 min minimum?   Why use one Tc for reporting but another in the calculations? 

Kevin White said:

If we then use the 0.33 above, wouldn't the actual system travel time be 1.33 min?

Although the catchment has a Tc of 1 minute, it adopts the minimum Tc of 5 minutes which you have set in the calculation options. So, at the first catchbasin the system time is 5.0 minutes. It takes 0.33 minutes to travel down the first pipe for a total of 5.33 minutes, and the Tc of the second catchment is 5.0 minutes, so StormCAD adopts the higher of the two, 5.33 minutes.

Kevin White said:

Why use one Tc for reporting but another in the calculations? 

This is the part that appears to be a defect. Since the calculations still appear to be correct, this could be considered a reporting issues. In other words, you could ignore the travel time result field shown in the first pipe. 

Kevin White said:

If we then use the 0.33 above, wouldn't the actual system travel time be 1.33 min?

Although the catchment has a Tc of 1 minute, it adopts the minimum Tc of 5 minutes which you have set in the calculation options. So, at the first catchbasin the system time is 5.0 minutes. It takes 0.33 minutes to travel down the first pipe for a total of 5.33 minutes, and the Tc of the second catchment is 5.0 minutes, so StormCAD adopts the higher of the two, 5.33 minutes.

Kevin White said:

Why use one Tc for reporting but another in the calculations? 

This is the part that appears to be a defect. Since the calculations still appear to be correct, this could be considered a reporting issues. In other words, you could ignore the travel time result field shown in the first pipe.