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.
Figure 4-4. Velocity relationships
4-13
EN5465
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