Table 4-1. Values for Manning's "n" and maximum permissible
velocities for channel design (continued)
For a given rate of runoff and a given longitudinal slope, the velocity of flow may be controlled by the shape of
the ditch, specifically the ditch side slope, and the ditch lining. Generally, a broad, shallow cross-section is
desirable.
HYDRAULIC SLOPE, S
The hydraulic slope will be considered as the slope of the channel bottom, or longitudinal slope. S is normally
determined by the natural slope of the site being drained. The minimum grade satisfactory for clear channels is
0.5 percent of S/0.005 ft/ft. On grades not exceeding 2 percent, checkdams may be used. On grades exceeding 8
percent or where erosion is a major problem, as in sand cuts, ditches should be paved.
4-5
EN5465
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