(1) Locate appropriate nomograph for type of
culvert selected.
(2) Begin nomograph solution by locating
starting point on length scale. To locate
the proper starting point on the length
scale, follow instructions below:
(a) If the n value of the nomograph corre-
sponds to that of the culvert being used,
on the appropriate nomograph locate
starting point on length curve for the
for the culvert selected differs from that
of the nomograph, see (c) below.
(b)
intermediate between the scales given,
connect the given length on adjacent
scales by a straight line and select a point
on this line spaced between the two chart
(c) For a different value of roughness coef-
ficient nl than that of the chart n, use
the length scales shown with an ad-
justed length Ll, calculated by the for-
mula:
(See subpara b below for n values.)
(3) Using a straight edge, connect point on
length scale to size of culvert barrel and
(1) above to given discharge and read di-
mark the point of crossing on the "turning
ameter, height, or size of culvert required.
line." See instruction c below for size con-
c. To detemine discharge.
siderations for rectangular box culvert.
(4) Pivot the straight edge on this point on
(1)
the turning line and-connect given dis-
charge rate. Read head in feet on the head
scale. For values beyond the limit of the
chart scales, find H by solving equation
(2)
given in nomograph or by H = KQ2 where
K is found by substituting values of H and
Q from chart.
b. Table B3 is used to find the n value for the
culvert selected.
(3)
c. To use the box-culvert nomograph (fig B17)
for full flow for other than square boxes:
B6. Instruction for use of outlet-control nom-
(1) Compute cross-sectional area of the rec-
ography.
tangular box.
a. Figures Bn through B17 are nomography
Note: The area scale on the nomograph is calculated
to solve for head when culverts flow full with out-
for barrel cross sections with span B twice the
height D; its close correspondence with area
let control. They are also used in approximating
of square boxes assures it may be used for all
the head for some partially full flow conditions
sections intermediate between square and B
with outlet control. These nomography do not give
= 2D or B = 2/3D. For other box proportions
a complete solution for finding headwater. (See
use equation shown in nomograph for more
para B4.)
accurate results.
B-30
Previous Page
