Associated with R = 0.85 the area is 9.72 square feet and the depth (d) found in the column at the far left is 1.8

feet. Record these values under the appropriate headings in the ditch design work sheet.

STEP 7. Calculate the ditch Q using the Continuity Equation and the cross-sectional area (A) and the depth (d)

from the appendix.

Make a check to see if this particular ditch will meet the requirements of our design by performing the calculation

using the Continuity Equation.

Qc = AV

Qc = (9.72 sq ft) (4 fps)

qc = 38.9 cfs

Trial Number 1. This calculated quantity of flow (Qc) must fall within +5 percent of the deign Qd.

If this requirement is not met, then a new velocity should be tried. If the Qc is higher than Qd, then a lower

velocity than the previous trial should be used. For this example, since 38.8 cfs is greater than 28.35 cfs, the

upper limit of the acceptable range, a velocity of 3.5 fps would be appropriate for the next trial.

Trial Number 2

STEP 2 V = 3.5 fps

STEP 3 R = 0.72

STEP 4 R = 0.71

STEP 5 A = 6.75; d = 1.5 ft

STEP 6 Qc = (3.5 fps)(6.75 sq ft) = 23.6 ft

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