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You're trying to use Chezy Manning with an unknown depth which can result in many instances of "d" in your equation.  You can eliminate calculation interpolations by using Kings tables, which can provide d/b ratios given everything else required in a flow scenario (Q, n, base or depth, etc.).


Use the equation:  K = (Q x n) / (k x b^8/3 x S^0.5)


I got K = 0.0307.  In the Kings tables, use m = 3 (for the side slopes), and find the closest d/b value.


Just by crude interpolation, I found that the corresponding d/b value is 0.11, which means that d = 1.1 meters.


Are you sure the question is asking for the Reynolds number (Re) and not the wetted perimeter ®?  Please check.  Reynolds number is applied to closed conduit flows to determine laminar or turbulent conditions.


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