TM 5-820-4/AFM 885, Chap 4
(TW) depth is equal to hO, and the relation of
headwater to other terms in equation 2 is illus-
trated in figure B18.
c. Tailwater elevation below the top or crown of
the culvert barrel outlet. Figure B1OB, C, and D
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-
swers with some error since the only correct
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
B-20
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