GEOPAK Drainage error message

I am getting the following error message on a certain drainage project:

"Time of Concentration Outside Rainfall Table"

I have checked all of my drainage areas and all have a manually entered Tc of anywhere between 4.9 and 15.2 minutes, well within my library's rainfall data.  My minimum Tc is set at 5 minutes.  The one area that I have a Tc of 4.9 processes using the minimum of 5 minutes, which is what I expect.  All of the others seem to produce normal results with their larger Tc's.

Should I be concerned about this error message?

Josh Mauritz

MS v08.11.05.17

GPK v08.11.05.39

  • Josh,

    I'm not a drainage expert but I asked Alex Mabrich and this is what he said:

    "The time of concentration is not only for the drainage areas but it gets accumulated as runoff travels inside the pipe. E.g. Drainage area A starts with 5 minutes and thru a piping system travels to another inlet/drainage area B for 15 minutes. Then, the new time of concentration to calculate the intensity for drainage area B is 15 minutes.

    If the system is too long, it could happen that the travel time/time of concentration falls outside the intensity table definition. Please, check the table since Agencies usually provide this tables and the time axis is quite enough."

    Derricke 


    This is a test

  • Josh

    Yes you should be concerned since this indicates that one or more nodes is probably receiving an invalid flow.  If you can't find the rogue then you should submit a support call to track down the problem.  You can send me your data but it might take me a a few days to get a chance to look at it.

    Here's a hint that I would try first:  For each drainage area go to the computations tab and click the compute button.  See if the reported flow is a reasonable number.  If one of the computations looks too large or too small then investigate this area more closely.

    Another idea:  Are you using the automatic Cvalue computations to determine a weighted C from multiple sub areas?  Iwonder (guessing really) what would happen if one of the subareas was extremely tiny??????

    Robert Garrett
    Senior Product Engineer
    Bentley Systems Inc.



  • Derricke and Robert,

    My library has times of 5 minutes to 14400 minutes (10 days), so I don't think that is the problem.  I will try Robert's suggestions to pinpoint the problem.

    Thanks for your help,

    Josh

  • Well in Texas, 14 thousand minutes isn't a long time to wait for it to rain...    ;-)

    But if your Tc is actually less than 5 minutes or more than 10 days, the easiest way to locate it is run the Report Builder and output the Tc values into a csv file, then use Excel to highlight the times outside that range.

    Based on Robert's suggestion, output the  Area variables to check that a drainage area isn't the offender. Based on what Alex says, ouput the Links's "From Node"  and "To Node" cumulative Tc values as the offending Tc value could be inside a pipe or ditch.

    Sometimes we use "fake" areas just to get a link or node connected to the system and we have to use a Q=.001cfs, if we forget the to leave that in and design/analyze the network the resulting Tc is huge.   An excel conditional formatting statement should highlight the offender for you.

  • Dan,

    Thanks for your help.  I took a look at my file using your/other's suggestions.  Here is what I have concluded:

    An Area report shows all Tc's used to be between 5 and 15.2 minutes - Seems OK, but error message still appears.

    Links From and To node Cumulative Tc report show Tc' s of between 5 and 15.77 minutes - Seems OK, but error message still appears.

     

    My normal preferences use the "Weight Time of Concentration" under Intensity Options.  If I uncheck this and then design my network, the error message goes away.  However, then I don't think my results are correct.

    Any suggestions?

    Josh