TM 5-820-1/AFM 88-5, Chap. 1
is permitted. In designing shoulders, the first 10-foot strip of shoulder adjacent to the pavement edges of
runways, taxiways, or aprons should have a 5 percent slope. The elevation of the pavement edge and the
shoulder will coincide. The shoulder gradient beyond the 10-foot strip will conform to the minimum 2
percent and maximum 3 percent specified in AFR 86-14. Waivers will not be required for the 5 percent
slope discussed above. Paved shoulders will normally have the same transverse slope as that of the
contiguous runways and taxiways.
(3) Determination of drainage area. Use the completed grading plan as a guide and sketch the
boundaries of specific drainage areas tributary to their respective drain inlets. Compute the area of
paved and unpaved areas tributary to the respective inlets by planimetering.
(4) Drainage patterns. Drainage patterns consisting of closely spaced interior inlets in pavements
with intervening ridges are to be avoided. Such grading may cause taxiing problems including bumping
or scraping of wing tanks. Crowned sections are the standard cross sections for runways, taxiways, and
safety areas. Crowned sections generally slope each way from the center line of the runway on a
transverse grade to the pavement. Although crowned grading patterns result in most economical
drainage, adjacent pavements, topographic considerations, or other matters may necessitate other
pavement grading.
c. Classification of storm drains. Storm drains for airfields and heliports may be classified in two
groups, primary and auxiliary.
(1) Primary drains. Primary drains consist of main drains and laterals that have sufficient
capacity to accommodate the project design storm, either with or without supplementary storage in
pending basins above the drain inlets. To lessen construction requirements for drainage facilities,
maximum use of pending consistent with operational and grading requirements will be considered. The
location and elevation of the drain inlets are determined in the development of the grading plans.
(2) Auxiliary drains. Auxiliary drains normally consist of any type or size drains provided to
facilitate the removal of storm runoff, but lacking sufficient capacity to remove the project design storm
without excessive flooding or overflow. Auxiliary storm drains may be used in certain airfields to provide
positive drainage of long flat swales located adjacent to runways or in unpaved adjacent areas. During
less frequent storms of high intensity, excess runoff should flow overland to the primary drain system or
other suitable outlet with a minimum of erosion. An auxiliary drain may also be installed to convey
runoff from pavement gutters wherever a gutter capacity of less than design discharge is provided.
d. Storm-drain layout. The principal procedures in the determination of the storm-drain layout
follow:
(1) Preliminary layout. Prepare a preliminary map (scale 1 inch = 200 feet or larger) showing the
outlines of runways, taxiways, and parking aprons. Contours should represent approximately the finished
grade for the airfield or heliport. Details of grading, including pending basins around primary drain
inlets, need not be shown more accurately than with l-foot contour intervals.
(2) Profiles. Plot profiles of all runways, taxiways, and aprons so that elevations controlling the
grading of intermediate areas may be determined readily at any point.
(3) Drain outlets. Consider the limiting grade elevations and feasible channels for the collection
and disposition of the storm runoff. Select the most suitable locations for outlets of drains serving
various portions of the field. Then select a tentative layout for primary storm drains. The most
economical and most efficient design is generally obtained by maintaining the steepest hydraulic
gradient attainable in the main drain and maintaining approximately equal lateral length on each side of
the main drain.
(4) Cross-sectional profiles of intermediate areas. Assume the location of cross-sectional profiles of
intermediate areas. Plot data showing controlling elevations and indicate the tentatively selected
locations for inlets by means of vertical lines. Projections of the runways, taxiways, or aprons for limited
distances should be shown on the profiles, to facilitate a comparison of the elevations of intermediate
areas with those of the paved areas. Generally, one cross-sectional profile should follow each line of the
underground storm-drain system. Other profiles should pass through each of the inlets at approximately
right angles to paved runways, taxiways, or aprons.
(5) Correlation of the controlling elevations and limiting grades. Begin at points corresponding to
the controlling elevations, such as the edges of runways, and sketch the ground profile from the given
points to the respective drain inlets. Make the grades conform to the limiting slopes. Review the
tentative grading and inlet elevations and make such adjustments in the locations of drain inlets and in
grading details as necessary to obtain the most satisfactory general plan.
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