Add contraction losses at entrance to lagoon outlet channel

[RegMeasures]

Looking at the initial model results I'm not managing to get the lagoon water level high enough, even with an increased roughness coefficient. My previous experience trying to replicate the Rakaia outlet channel in 1D hydrodynamics packages indicates a contraction loss is necessary to get the model to calibrate. Two options spring to mind:

• Have a fixed contraction loss at the entrance to the channel (simple but a little unrealistic in some situations)
• Have contraction losses wherever there are contractions, calculated based on the degree of contraction - not sure what formula to use but there must be something out there... (more generally applicable but slightly more complex?)

I guess there are bend losses which are also relevant but maybe these can be somewhat rolled in with the contraction losses...?

[RegMeasures]

This could help stability? maybe?

[RegMeasures]

Maybe this is over complicating things and I should just increase the roughness (now that I have separated grain roughness and form roughness #76 )