TM 5820-4/AFM 88-5, Chap 4
CHAPTER 3
carrying capacity is required. Ditches are com-
3-1. General. Hydraulic design of the required
monly used for collection of surface water in out-
elements of a system for drainage or for protective
lying areas and along roadway shoulders. Larger
works may be initiated after functional design cri-
teria and basic hydrologic data have been deter-
open channels, which may be either wholly within
the ground or partly formed by levees, are used
mined. The hydraulic design continual y involves
principally for perimeter drains, for upstream flow
two prime considerations, namely, the flow quan-
diversion or for those parts of the drainage system
tities to which the system will be subjected, and
within a built-up area where construction of a cov-
the potential and kinetic energy and the momen-
ered drain would be unduly costly or otherwise
tum that are present. These considerations re-
impractical. They are also used for rainfall drain-
quire that the hydraulic grade line and, in many
age disposal. Whether a channel will be lined or
cases, the energy grade line for design and per-
not depends on erosion characteristics, possible
tinent relative quantities of flow be computed, and
grades, maintenance requirements, available
that conditions whereby energy is lost or dissi-
space, overall comparative costs, and other fac-
pated must be carefully analyzed. The phenom-
tors. The need for providing a safety fence not less
ena that occur in flow of water at, above, or below
than 4 feet high along the larger channels (es-
critical depth and in change from one of these flow
pecially those carrying water at high velocity) will
classes to another must be recognized. Water ve-
be considered, particularly in the vicinity of hous-
locities must be carefully computed not only in
ing areas.
connection with energy and momentum consid-
erations, but also in order to establish the extent
b. The discussion that follows will not attempt
to which the drainage lines and water-courses may
to cover all items in the design of an open channel;
be subjected to erosion or deposition of sediment,
however, it will cite types of structures and design
thus enabling determination of countermeasures
features that require special consideration.
needed. The computed velocities and possible re-
sulting adjustments to the basic design layout often
c. Apart from limitations on gradient imposed
affect certain parts of the hydrology. Manning's
by available space, existing utilities, and drainage
equation is most commonly used to compute the
confluences is the desirability of avoiding flow at
mean velocities of essentially horizontal flow that
or near critical depths. At such depths, small
occurs in most elements of a system:
changes in cross section, roughness, or sediment
transport will cause instability, with the flow depth
varying widely above and below critical. To insure
n
reasonable flow stability, the ratio of invert slope
to critical slope should be not less than 1.29 for
The terms are defined in appendix D. Values of n
supercritical flow and not greater than 0.76 for
for use in the formula are listed in chapters 2 and
subcritical flow. Unlined earth channel gradients
9 of TM 5-820-3/AFM 88-5, Chapter 3.
should be chosen that will product stable subcrit-
ical flow at nonerosive velocities. In regions where
3-2. Channels.
mosquito-borne diseases are prevalent, special at-
a. open channels on military installations range
tention must be given in the selection of gradients
in form from graded swales and bladed ditches to
for open channels to minimize formation of breed-
large channels of rectangular or trapezoidal cross
ing areas; pertinent information on this subject
section. Swales are commonly used for surface
is given in TM 5-632/AFM 9116.
drainage of graded areas around buildings and
d. Recommended maximum permissible veloci-
within housing developments. They are essen-
ties and Froude numbers for nonerosive flow are
tially triangular in cross section, with some bot-
given in chapter 4 of TM 5-820-3/AFM 88-5, Chap-
tom rounding and very flat side slopes, and nor-
ter 3. Channel velocities and Froude numbers of
mally no detailed computation of their flow-
3-1
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