TM 5-820-3/AFM 88-5, Chap. 3
necessary to establish the turf quickly or to provide
6-3. Design procedure.
supplemental protection by mulching or similar
a. This design procedure is based on the premise
means. For further discussion of turfing methods,
that the above empirical relation can be used to
see TM 5-803-13/AFM 126-8. Where excessive
determine the Froude number of flow in the chan-
velocities are to be encountered or where satisfac-
nel required to initiate or prevent movement of
tory turf cannot be established and maintained, it
various sizes of material. Relations based on the
may be necessary to provide a paved channel.
Manning formula can then be applied to determine
d. A channel design calling for an abrupt change
the geometry and slope of a channel of practical
in the normal flow pattern induces turbulence and
proportion that will convey flows with Froude
causes excessive loss of head, erosion, or
numbers within a desired range such that finer
deposition of silt. Such a condition may result at
material will be transported to prevent deposition
channel transitions, junctions, storm-drain outlets,
but larger material will not be transported to pre-
and reaches of excessive curvature, and special at-
vent erosion.
tention will be given to the design of structures at
b. Appendix D contains an example problem for
these locations.
the design of a channel using this procedure. It will
e. Channel design in appendix D must include
satisfy the conditions desired for the design
measures for preventing uncontrolled inflow from
discharge and one that will ensure ho deposition or
drainage areas adjacent to open channels. This local
erosion under these conditions.
inflow has caused numerous failures and is
particularly detrimental where, due to the normal
6-4. Drop structures and check dams.
irregularities experienced in grading operations,
a. Drop structures and check dams are designed
runoff becomes concentrated and results in exces-
to check channel erosion by controlling the effec-
sive erosion as it flows over the sides of the chan-
tive gradient and to provide for abrupt changes in
nel. A berm at the top edge of the channel will
channel gradient by means of a vertical drop. They
prevent inflow except at designated points, where
also provide satisfactory means for discharging
inlets properly protected against erosion are pro-
accumulated surface runoff over fills with heights
vided. The inlet may vary from a sodded or paved
not exceeding 5 feet and over embankments higher
chute to a standard field inlet with a storm drain
than 5 feet if the end sill of the drop structure
connection to the channel. Erosion resulting from
extends beyond the toe of the embankment. The
inflow into shallow drainage ditches or swales with
check dam is a modification of the drop structure
flat side slopes can be controlled by a vigorous
used for erosion control in small channels where a
turfing program supplemented by mulching where
less elaborate structure is permissible.
required. Where excavated material is wasted in a
b. There are numerous types of drop and grade
levee or dike parallel and adjacent to the channel,
control structures. They can be constructed of con-
provision will be made for frequent openings
crete, metal piling, gabions, riprap, or a combina-
through the levee to permit local inflow access to
tion of materials. Design of many of these struc-
the channel. A suitable berm (minimum of 3 feet)
tures is beyond the scope of this manual, and if the
will be provided between the levee and the top
designer needs design information for a specific
edge of the channel to prevent sloughing as a result
type structure, the publications in the bibliography
of the spoil bank load and to minimize movement
should be consulted.
of excavated material back into the channel.
c. Pertinent features of a typical drop structure
Example problems in channel design are shown in
are shown in figure 6-2. The hydraulic design of
appendix D.
these structures can be divided into two general
f. Field observations indicate that stable chan-
phases: design of the weir and design of the stilling
nels relatively free of deposition and/or erosion can
basin. It is emphasized that for a drop structure or
be obtained provided the Froude number of flow in
check dam to be permanently and completely
the channel is limited to a certain range depending
successful, the structure must be soundly designed
upon the type of soil. An analysis of experimental
to withstand soil and hydrostatic pressures and the
data indicates that the Froude number of flow
effects of frost action, when necessary. Also, the
(based on average velocity and depth of flow)
adjacent ditches or channels must be completely
required to initiate transport of various diameters
stable. A stable grade for the channel must first be
of cohesionless material, d50, in a relatively wide
ascertained before the height and spacing of the
channel can be predicted by the empirical relation,
various drop structures can be determined.
F = 1.88 (d50/D)1/3. The terms are defined in
appendix E.
6-4
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