b. Compute and record headwater for out-

let control as instructed below:

Select the culvert size by the following steps:

(1)

Step 1: List given data.

a. Design discharge, Q, in cubic feet per sec-

ond.

b. Approximate length of culvert, in feet.

c. Allowable headwater depth, in feet, which

is the vertical distance from the culvert

(2)

invert (flow line) at entrance to the water-

surface elevation permissible in the ap-

proach channel upstream from the cul-

vert.

d. Type of culvert, including barrel mate-

(3)

rial, barrel cross-sectional shape, and en-

trance type.

e. Slope of culvert. (If grade is given in per-

cent, convert to slope in feet per foot.)

f. Allowable outlet velocity (if scour is a

and Step 3b (inlet control and outlet con-

trol). The higher headwater governs and

indicates the flow control existing under

the given conditions.

d. Compare the higher headwater above

with- that allowable at the site. If head-

water is greater than allowable, repeat

the procedure using a larger culvert. If

headwater is less than allowable, repeat

the procedure to investigate the possi-

bility of using a smaller size.

Step 4: Check outlet velocities for size se-

lected.

a. If outlet control governs in Step 3c, out-

let velocity equals Q/A, where A is the

cross-sectional area of flow at the outlet.

If dC or TW is less than the height of the

culvert barrel, use cross-sectional area

corresponding to dC or TW depth, which-

ever gives the greater area of flow.

b. If inlet control governs in Step 3c, outlet

velocity can be assumed to equal normal

velocity in open-channel flow as com-

puted by Manning's equation for the

barrel size, roughness, and slope of cul-

vert selected.

Step 5: Try a culvert of another type or shape

and determine size and headwater by

the above procedure.

Step 6: Record final selection of culvert with

size, type, outlet velocity, required

headwater, and economic justifica-

tion.

Integrated Publishing, Inc. |