Hello Amits,
I went ahead and split your original thread, since this is a new issue. Generally we recommend posting one topic per thread, to make things easier to read and search for.
Regarding the "root sought..." error - this is a generic error that can happen in a number of different unusual situations where the model is not able to solve for some reason. Is it possible that the tank is overflowing at some point? Have you entered valid values for the weir length and coefficient? Also, are you using the latest version of HAMMER? (08.11.04.50) You can check this under Help>About.
It may help for us to see your model in this situation. If posting it publicly is acceptable, please compress the files into a zip file and attach to your reply via the "attachments/options" tab within the Advanced Reply Editor. If the model is confidential, please consider using Bentley Secure Files Upload, where only Bentley colleagues can access it.