Reply To: How is the characteristic length of open channel flow calculated?

[Rob]

In much the same way as it's calculated in any other model. However, y+ is very much not the only criteria to worry about hence my comment. Depending on what the open channel is you may also want to think about how wall roughness interacts with the near wall cell. 

I'll also remind you about the T&Cs. Specifically sections regarding politeness. Written language is often misunderstood, so extra care should always be taken.