Iterate for a Solution with an Excel Macro

The Manning formula for open-channel flow is:   whereV= Velocityn = Manning’s “n”,r = Hydraulic radius = area / wetted perimeter = b*y / (2*y + b) for a rectangular channel (where y = depth and b = width), ands = slope of channel (m/m).  Discharge Q = V*A = V*b*y.  We cannot solve this…