TM 5820-4/AFM 88-5, Chap 4
sults of laboratory research concerning soil infil-
increase will be matched, the invert grade drop-
ping across the junction structure; similarly, the
tration through pipe joints and the effectiveness
crowns of incoming laterals will be matched to
of gasketing tapes for waterproofing joints and
that of the main line. If the downstream conduit
seams are available.
is smaller as on a steep slope, its invert will be
matched to that of the upstream conduit. Some
36. Underground hydraulic design.
additional lowering of an outgoing pipe may be
a. The storm-drain system will have sufficient
required to compensate for pressure loss within a
capacity to convey runoff from the design storm
junction structure.
(usually a 10-year frequency for permanent in-
d. Manholes or junction boxes usually will be
stallations) within the barrel of the conduit. De-
provided at points of change in conduit grade or
sign runoff will be computed by the methods in-
size, at junctions with laterals of branches and
dicated in chapter 2. Concentration times will
wherever entry for maintenance is required. Dis-
increase and average rainfall intensities will de-"
tance between points of entry will be not more
crease as the design is carried to successive down-
than approximately 300 feet for conduits with a
stream points. In general, the incremental con-
minimum dimension smaller than 30 inches. If the
centration times and the point-by-point totals
storm drain will be carrying water at a velocity
should be estimated to the nearest minute. These
of 20 feet per second or greater, with high energy
totals should be rounded to the nearest 5 minutes
and strong forces present, special attention must
in selecting design intensities from the intensity-
be given such items as alinement, junctions, an-
duration curve. Advantage will be taken of any
chorage requirements, joints, and selection of ma-
permanently available surface ponding areas, and
terials.
their effectiveness determined, in order to hold
design discharges and storm-drain sizes to a min-
imum. Experience indicates that it is feasible and
37. Inlets.
practical in the actual design of storm drains to
a. Storm-drain inlet structures to intercept sur-
adopt minimum values of concentration times of
face flow are of three general types: drop, curb,
10 minutes for paved areas and 20 minutes for
and combination. Hydraulically, they may func-
turfed areas. Minimum times of concentration
tion as either weirs or orifices depending mostly
should be selected by weighting for combined paved
on the inflowing water. The allowable depth for
and turfed areas.
design storm conditions and consequently the type,
b. Storm-drain systems will be so designed that
size and spacing of inlets will depend on the to-
the hydraulic gradeline for the computed design
pography of the surrounding area, its use, and
discharge in as near optimum depth as practicable
consequences of excessive depths. Drop inlets,
and velocities are not less than 2.5 feet per second
which are provided with a grated entrance open-
(nominal minimum for cleansing) when the drains
ing, are in general more efficient than curb inlets
are one-third or more full. To minimize the pos-
and are useful in sumps, roadway sags, swales,
sibility of clogging and to facilitate cleaning, the
and gutters. Such inlets are commonly depressed
minimum pipe diameter or box section height will
below the adjacent grade for improved intercep-
generally be not less than 12 inches; use of smaller
tion or increased capacity. Curb inlets along slop-
size must be fully justified. Tentative size selec-
ing gutters require a depression for adequate in-
tions for capacity flow may be made from the nom-
terception. Combination inlets may be used where
ography for computing required size of circular
some additional capacity in a restricted space is
drains in appendix B, TM 5-820-l/AFM 88-5,
desired. Simple grated inlets are most susceptible
Chapter 1. Problems attending high-velocity flow
to blocking by trash. Also, in housing areas, the
should be carefully analyzed, and appropriate pro-
use of grated drop inlets should be kept to a rea-
visions made to insure a fully functional project.
sonable minimum, preference being given to the
curb type of opening. Where an abnormally high
c. Site topography will dictate the location of
curb opening is needed, pedestrian safety may re-
possible outlets and the general limiting grades
quire one or more protective bars across the open-
for the system. Storm drain depths will be held to
ing. Although curb openings are less susceptible
the minimum consistent with limitations imposed
to blocking by trash, they are also less efficient
by cover requirements, proximity of other struc-
for interception on hydraulically steep slopes, be-
tures, interference with other utilities, and veloc-
cause of the difficulty of turning the flow into them.
ity requirements because deep excavation is ex-
Assurance of satisfactory performance by any
pensive. Usually in profile, proceeding downstream,
system of inlets requires careful consideration of
the crowns of conduits whose sizes progressively
3-9
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