TM 5-820-3/AFM 88-5, Chap. 3
CHAPTER 3
INLETS AND BOX DRAINS
construction. Although standard type structures are
3-1. General.
usually adequate, occasionally special structures
a. Inlet structures to collect storm runoff at air-
will be needed.
fields and heliports may be built of any suitable
b. Grating elevations for field inlets must be
construction material. The structures must ensure
carefully coordinated with the base or airport
efficient drainage of design-storm runoff in order to
grading plan. Each inlet must be located at an ele-
avoid interruption of operations during or fol-
vation which will ensure interception of surface
lowing storms and to prevent temporary or perma-
runoff. Increased overland velocities immediately
nent damage to pavement subgrades. Most fre-
adjacent to field inlet openings may result in ero-
quently, reinforced concrete is the material used
sion unless protective measures are taken. A solid
although brick, concrete block, precast concrete, or
sod annular ring varying from 3 to 10 feet around
rubble masonry have also been used. The material,
the inlet reduces erosion if suitable turf is estab-
including the slotted drain corrugated metal pipe to
lished and maintained on the adjacent drainage
handle surface flow if employed, should be strong
area. Prior to the establishment of turf on the ad-
enough to withstand the loads to which it will be
jacent area, silt may deposit in a paved apron
subjected.
around the perimeter or deposit in the sod ring
thereby diverting flow from the inlet. In lieu of a
b. Field inlets are usually those located away
sod ring, a paved apron around the perimeter of a
from paved areas. Box drains, normally more costly
grated inlet may be beneficial in preventing erosion
than field inlets, are usually located within paved
and differential settlement of the inlet and the
areas to remove surface drainage.
adjacent area as well as facilitating mowing
c. Local practices and requirements governing
operations.
field inlets greatly influence design and construction
c. Drainage structures located in the usable
details. Experience has indicated that the features
areas on airports should be designed so that the
described in paragraph 3-2 should be considered by
grating does not extend above the ground level.
the designer.
The tops of such structures should be 0.2 of a foot
below the ground line (finished grade) to allow for
possible settlement around the structure, to permit
3-2. Inlets versus catch basins.
unobstructed use of the area by equipment, and to
Catch basins are required to prevent solids and
facilitate collection of surface runoff.
debris from entering the drainage system; however,
d. A grating in a ponded area operates as a weir
their proper maintenance is difficult. Unless the
under low head situations. At higher heads, how-
sediment basin is frequently cleaned, there is no
ever, the grating acts as an orifice. Model tests of
need for catch basins. Since catch basins are not
a grating shown in the typical plan of a double inlet
necessary when storm drainage lines are laid on
grating (fig 3-1) indicate that vortex action
self-cleaning grades, proper selection of storm
influences the discharge characteristics when the
drain gradients greatly reduce the need for catch
head exceeds 0.4 foot. Hydraulically acceptable
basins. Whenever practical ordinary inlets should be
grates will result if the design criteria in the above
used instead of catch basins.
figure are applied. For the entire area, the system of
grates and their individual capacity will depend on
the quantity of runoff to be handled and the al-
3-3. Design features.
lowable head at the grates. Head limitations should
not exceed 0.5 foot.
a. Structures built in connection with airport
drainage are similar to those used in conventional
3-1
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