I am getting the following error and I cannot figure out why. It has something to do with my "User Defined TD" because when I set it to "False" The model runs.
The message in question indicates that the predevelopment target peak flow is greater than the postdevelopment peak flow, which would mean that a pond is not needed. With the Modified Rational method in PondPack, the assumption is that your postdevelopment peak flow is greater than the predevelopment peak, so a pond is needed to attenuate the postdevelopment peak down to the predevelopment. The program determines the "critical" storm duration (Td) resulting in the maximum requires storage volume.
In your case the issue is likely that the predevelopment and postdevelopment information is not entered correctly in the catchment properties, resulting in the condition where the peak flow after development is already less than the predevelopment peak, so the program is unable to continue and you get the error in question.
If you do not want PondPack to calculate the critical storm duration, you can enter it manually by setting "user defined TD?" to "true" and then enter the TD for the program to use for the duration of the Modified Rational hydrograph peak and for the peak itself (using Q=CIA). The model will run in this case because the program no longer needs to look at the predevelopment information since that is only used by the process that calculates the critical TD.
You can read more about how the Modified Rational Method works in the following article from our Wiki: Understanding the Modified Rational Method
If you need further assistance or a more specific answer as to why your predevelopment peak is greater than your postdevelopment peak, please provide a copy of the model for review: Sharing Hydraulic Model Files on the OpenFlows Forum
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.