PART E - DESIGN OF PIPE CULVERTS
WITH UNSUBMERGED INLETS
The factors to be applied to the design of these culverts are determined as follows:
STEP 1. Determine the rate of runoff. Use the area the culvert must drain. This will be the required capacity,
Qp, of the culvert.
STEP 2. Determine culvert use. Will it be used for a road or an airfield?
STEP 3. Calculate the critical dimensions from the cross sections. See figure 5-10. Determine the length in
place (LIP) and the fill critical (Fc).
STEP 4. Determine the largest pipe for the fill. Begin from the cross section at the outside edge of the shoulder
on the inlet side of the culvert. Consider only pipes that are available and for which cover is adequate.
For roadway loadings, the maximum culvert diameter is equal to two-thirds of the minimum fill (FT from figure
5-10). The cover required for culvert protection is equal to one-half the diameter of the culvert or 12 inches
(whichever is greater).
For runways and taxiways sustaining aircraft wheel loads, refer to table 5-1, page 5-9, for the loads, and table 5-2,
page 5-10, for the minimum cover required.
STEP 5. Determine the culvert capacity, Qp, and outlet velocity using table 5-3, page 5-16.
CMP ("n" = 0.024) (CMP)
STEP 6. Determine the number of culvert pipes required. Divide the area runoff, Q by the pipe capacity, Qp.
Round up to the next whole number. Use the smallest diameter pipe that can be used with the least number of
pipes side by side.
STEP 7. Determine the order length (OL). The OL is calculated by multiplying the number of pipes (NP) times
the LIP times a waste and grade correction factor of 1.15. (See Step 3 for the LIP and Step 6 for the NP).
STEP 8. Determine the maximum permissible discharge velocity, Vmax. Use table 4-1, page 4-5, to calculate
Vmax for the channel lining into which the culvert outlet will discharge. Determine the correct pipe to be used.
Apply the following criteria in your calculations: