TM 5-820-4/AFM 88-5, Chap. 4
Table B-3. Roughness Coefficients for Various Pipes
n = 0.012 for smooth interior pipes of any size, shape or type*
n Value for Annular Corrugated Metal
Unpaved
0.011-0.014
24-30 inches
0.016-0.018
36-96 inches
0.019-0.024
0.017-0.021
*Includes asbestos cement, bituminized fiber, cast iron, clay, concrete (precast or cast-in-place) or fully paved (smooth interior) corruga -
ted metal pipe.
U.S. Army Corps of Engineers
NOTE: Limitations helical coefficient - The designer must assure that fully developed spiral flow can occur in his design situa-
tion before selecting the lower resistance factor. Fully developed spiral flow, and the corresponding lower resistance factors, can only
occur when the conduit flows full. For conduits shorter than 20 diameters long, it is felt that the full development of spiral flow cannot
be assured. Bed load deposition on the culvert invert may hinder the development of spiral flow until sediment is washed out. When
these conditions exist, the resistance factors for annular C.M .P. of the same size and corrugation shape should be used.
(2) Connect proper point (see para B-6b ) on
control equals the discharge with inlet control. For
culverts with free flow and control at the outlet,
length scale to barrel area and mark point
on turning line.
values is permitted in the range of headwater depths
(3) Pivot the straight edge on this point on
equal to or less than twice the barrel height. The
t h e turning line and connect given dis-
upper limit of this range of headwater depths is
charge rate. Read head in feet on the head
designated by a horizontal dotted line on the charts.
scale.
do not impose any limitation; merely read the solid-
B-7. Culvert capacity charts. Figures B-26 through
B-43, prepared by the Bureau of Public Roads,
line curves. The symbol AHW means allowable
headwater depth. The charts permit rapid selection
present headwater discharge relations convenient
of a culvert size to meet a given headwater limitation
for use in design of culverts of the most common
for various entrance conditions and types and shapes
types and sizes. The solid-line curve for each type
of pipe. One can enter with a given discharge and
and size represents for a given length: slope ratio
read vertically upward to the pipe size that will carry
the culvert capacity with control at the inlet; these
the flow to satisfy the headwater limitation of the
curves are based generally on model data. For those
design criteria. The major restriction on the use of
culvert types for which a dashed-line curve is shown
the charts is that free flow must exist at the outlet.
in addition to a solid-line curve, the dashed line
In most culvert installations free flow exists, i.e.,
represents for a given length: slope ratio the dis-
flow passes through critical depth near the culvert
charge capacity for free flow and control at the out-
outlet. For submerged flow conditions the solution
let; these curves are based on experimental data and
can be obtained by use of the outlet control nomo-
backwater computations. The length: slope ratio is
graphs.
Change 1 B-31
case is the value at which the discharge with outlet
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