The variables in these equations are:

Q = Quantity of flow (cfs)

V = Velocity of flow (fps)

Aw = Cross-sectional area of water (square feet (sq ft))

"n" = Roughness coefficient

To design an open channel for a known area, the military engineer works within the constraints of his known

variables to decide on the best values for the unknown variables.

The known values are-

Q-The value of Q is the first step in determining the shape of the ditch. If Q is greater than 60 cfs, then

the ditch should be trapezoidal. If Q is less than 60 cfs, then the ditch should be triangular.

S-Slope generally follows the natural terrain. Small modifications may be made for short sections.

V-Velocity is determined initially by the type of soil. Check table 4-3 on page 4-14 for the point at which

erosion occurs.

Figure 4-4 shows the velocity (V) relationships.