DESIGN A CHANNEL FOR A ROADSIDE DITCH
Given the following information. (The next pages follow the design of an open channel step-by-step.)
Design Example. Problem: Design a diversion ditch using the given information.
Known: Location-Seoul, Korea
Q = 27.0 cfs (from Rational Method)
Soil = GMd
STEP 1. Determine Q for the basin, given 27 cfs.
STEP 2. Determine the slope of the channel.
Slope = 0.5 percent = 0.005 ft/ft (invert elevation of No 4 = 52.0 ft; mean elevation of swamp = 49.5 ft; change in
elevation = 52.0 - 49.5 = 2.5 ft; length = 500 ft; therefore, slope = 2.5500 = 0.005 ft/ft)
STEP 3. Select the trial value for "n" and V from table 4-1, page 4-5.
NOTE: For "n" and V, use mean values.
n = 0.024 (from table 4-1; use average of "n" values range, unless additional information indicates otherwise)
V = 3-5 fps (from table 4-1 on page 4-6) (Use the mean value for Vmax.)
STEP 4. Determine the hydraulic radius using "n" and V from STEP 3 and slope (S) from STEP 2. Using figure
4-5, page 4-7 or the equation, determine the hydraulic radius.
Table 4-4. Ditch selection
STEP 5. Determine the type of ditch based on the Q design.
Select Ditch. Since the Q design (QD) is less than 60 cfs, the ditch should be triangular. Side slopes for the
triangular ditch should be 3:1 as a first trial. They can be changed if the ditch is found to be not suitable. Enter
this information in the columns under the ditch selection in the ditch design work sheet. Also, the ditch should be
symmetrical (Sym), since it is not adjacent to a road.
Select Velocity. The erosion velocity for the GMd soil is 3-5 fps. This means that at 3 fps the soil in the ditch
will begin to erode and at 5 fps the whole ditch will be eroding. Since we do not want to exceed the Vmax of 5
fps, the best initial choice usually is 1 fps lower than Vmax or 4 fps.