TM 5820-4/AFM 885, Chap 4
(2) Curb type (on slope). In general, a curb
Such toleration levels will nearly always be influ-
inlet placed on a grade is a hydraulically ineffi-
enced by costs of drainage construction. With the
cient structure for flow interception. A relatively
flat street crowns prevalent in modern construc-
long opening is required for complete interception
tion, many gutter flows are relatively wide and in
because the heads are normally low and the di-
built-up areas some inconveniences are inevita-
rection of oncoming flows is not favorable. The
ble, especially in regions of high rainfall, unless
cost of a long curb inlet must be weighed against
an elaborate inlet system is provided. The
that of a drop type with potentially costly grate.
achievement of a satisfactory system at reason-
The capacity of a curb inlet intercepting all the
able cost requires careful consideration of use fac-
flow can be calculated by an empirical equation.
tors and careful design of the inlets themselves.
The equation is a function of length of clear open-
However, it must also be remembered that a lim-
ing of the inlet, depth of depression of flow line at
itation on types and sizes for a given project is
inlet in feet, and the depth of flow in approach
also desirable, for standardization will lead to lower
gutter in feet. Depression of the inlet flow line is
construction costs. Design of grated, curb, and
an essential part of good design, for a curb inlet
combination inlets on slopes will be based on prin-
with no depression is very inefficient. The flow
ciples outlined below.
intercepted may be markedly increased without
(1) Grated type (on slope). A grated inlet placed
changing the opening length if the flow line can
in a sloping gutter will provide optimum inter-
be depressed by one times the depth of flow in the
ception of flow if the bars are placed parallel to
approach gutter. The use of long curb openings
the direction of flow, if the openings total at least
with intermediate supports should generally be
50 percent of the width of the grate (i.e. normal
avoided because of the tendency for the supports
to the direction of flow), and if the unobstructed
to accumulate trash. If supports are essential, they
opening is long enough (parallel to the direction
should be set back several inches from the gutter
of flow) that the water falling through will clear
line.
the downstream end of the opening. The minimum
(3) Combination type (on slope). The capacity
length of clear opening required depends on the
of a combination inlet on a continuous grade is
depth and velocity of flow in the approach gutter
not much greater than that of the grated portion
and the thickness of the grate at the end of the
itself, and should be computed as a separate grated
slot. This minimum length may be estimated by
inlet except in the following situations. If the curb
the partly empirical formula:
opening is placed upstream from the grate, the
combination inlet can be considered to operate as
two separate inlets and the capacities can be com-
puted accordingly. Such an arrangement is some-
A rectangular grated inlet in a gutter on a con-
times desirable, for in addition to the increased
tinuous grade can be expected to intercept all the
capacity the curb opening will tend to intercept
water flowing in that part of the gutter cross sec-
debris and thereby reduce clogging of the grate.
tion that is occupied by the grating plus an amount
If the curb opening is placed downstream from
that will flow in along the exposed sides. However,
the grate, effective operation as two separate in-
unless the grate is over 3 feet long or greatly de-
lets requires that the curb opening be sufficiently
pressed (extreme warping of the pavement is sel-
downstream to allow flow bypassing the grate to
dom permissible), any water flowing outside the
move into the curb opening. The minimum sepa-
grate width can be considered to bypass the inlet.
ration will vary with both the cross slope and the
The quantity of flow in the prism intercepted by
longitudinal slope.
such a grate can be computed by following in-
e. Structural aspects of inlet construction should
struction 3 in figure 3-2. For a long grate the in-
generally be as indicated in figures 3-5, 3-6, and
flow along the side can be estimated by consid-
3-7 which show respectively, standard circular
ering the edge of the grate as a curb opening whose
grate inlets, types A and B; typical rectangular
effective length is the total grate length (ignoring
grate combination inlet, type C; and curb inlet,
crossbars) reduced by the length of the jet directly
type D. It will be noted that the type D inlet pro-
intercepted at the upstream end of the grate. To
vides for extension of the opening by the addition
attain the optimum capacity of an inlet consisting
of a collecting trough whose backwall is cantile-
of two grates separated by a short length of paved
vered to the curb face. Availability of gratings and
gutter, the grates should be so spaced that the
standards of municipalities in a given region may
carryover from the upstream grate will move suf-
limit the choice of inlet types. Grated inlets sub-
ficiently toward the curb to be intercepted by the
ject to heavy wheel loads will require grates of
downstream grate.
3-13
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