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.

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.

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.

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