Record Area and Depth
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.
Table 4-8. Selecting the area and depth
Check for Q
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
Table 4-9. Q calculation
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