Fluid Mechanics Open Channel Problem

[Horizons1]

The problem: A canal cut in rock (n= 0.030) is a trapezoidal in section with a bottom width of 6.10m and the side slopes of 1 on 1. The allowable average velocity is 0.76 m/s. What slope will produce 5.66 m^3/s? The books answer is 0.000675

from the mannings equation

V=(1.486/n)R^2/3 x S^1/2 i get... 0.76=(1.486/0.030)? I am confused on where to go

 

[LawrenceC]

Use of the Manning formula necessitates knowing the slope. But that is what is sought. So how did you do justify you did?

Hint:
Start out by determining the area of the open channel. Then use the Manning formula to determine the slope.