[Lih]

Thanks everyone for the ideas. Model is up and running now. In the end I did the following:

* Took out all bridges to simplify the model. Then added them in one by one and addressing any instability issues as they came up.

* Refined pilot channel to be continuous with smooth transitions between reaches

* Due to irregularity of channel invert, a number of inline weirs wer added at locations displaying sharp drops

* hotstart file created by running a constant flow from top of model at a rate that approx half filled the channel. starting tailwater level filled the entire model domain and was ramped down to realistic levels.

* I did not change any of the calculation tolerances, except for maximising the number of iterations.