(TW) depth is equal to hO, and the relation of

headwater to other terms in equation 2 is illus-

trated in figure B18.

are three common types of flow for outlet control

with this low tailwater condition. In these cases

hO is found by comparing two values, TW depth

d +D

in the outlet channel and c

and setting hO

2'

d +D

equal to the larger value. The fraction C

is

2

a simplified mean of computing hO when the tail-

water is low and the discharge does not fill the

culvert barrel at the outlet. In this fraction, dC

is critical depth as determined from figures B

18 through B23 and D is the culvert height. The

value of DC should never exceed D, making the

upper limit of this fraction equal to D. Figure

B19 shows the terms of equation 2 for the cases

discussed above. Equation 2 gives accurate an-

swers if the culvert flows full for a part of the

barrel length as illustrated by figure B23. This

condition of flow will exist if the headwater, as

determined by equation 2, is equal to or greater

than the quantity:

If the headwater drops below this point the water

surface will be free throughout the culvert barrel

as in figure B1OD, and equation 2 yields an-

method of finding headwater in this case is by

a backwater computation starting at the culvert

Fairly regular section:

Some grass and weeds, little or no brush ---------------------------------

0.0300.035

Dense growth of weeds, depth of flow materially greater than weed

height

-------------------------------------------------------------------

0.035-0.05

Some weeds, light brush on banks ----------------------------------------

0.0350.05

Some weeds, heavy brush on banks ---------------------------------------

0.050.07

Some weeds, dense willows on banks -------------------------------------

0.060.08

For trees within channel, with *branches *submerged at high stage, in-

crease all above values by -----------------------------------------------

0.010.02

Irregular sections, with pools, slight channel meander; increase values

given above about ------------------------------------------------------------

0.010.02

Mountain streams, no vegetation in channel, banks usually steep, trees

and brush along banks submerged at high stage:

Bottom of gravel, cobbles, and few boulders -----------------------------

0.040.05

Bottom of cobbles, with *large *boulders -----------------------------------

0.050.07

U.S. Army Corps of Engineers

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