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
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 GarrettSenior Product EngineerBentley 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,
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?