STEP 7 Q. = 25.65, so the next trial velocity must be larger than 3.5 fps but smaller than 4.0 fps.
Try 3.6 fps.
Table 4-10. Second trial velocity
Trial Number 3
STEP 2 V = 3.6 fps
STEP 3 R = 0.76
STEP 4 R = 0.76
STEP 5 A = 7.68; d = 1.6 ft
STEP 6 Qc = (3.6 fps) (7.68 sq ft) - 27.6
STEP 7 27.6 > 25.65 < 28.35, so this depth is within the range and thus defines the ditch water
depth, area, and velocity.
Table 4-11. Third trial velocity
Determine if Ditch is Appropriate
The process tells us that a symmetrical triangular ditch, in GMd soil with a slope of .005 ft/ft and 3:1 side slopes,
that carries 27.0 cfs runoff will have a water depth of 1.6 feet and a velocity of 3.6 fps.
Since the velocity of the ditch is greater than 3 fps, the ditch can be considered self-cleaning. The peak runoff
will remove the silt buildup from the channel bottom; thus, it requires little maintenance. With V at 3.6 fps, there
will be some erosion of the ditch but it should not be a large maintenance problem. Therefore, the ditch is
deemed appropriate and should be used.