TM 5-820-1/AFM 88-5, Chap. 1
(c) The average rate of supply over the area is obtained by reading vertically upward from the
point where the tC and L =
from this point. This value is found to be 3.6 inches per hour or 3.6 cfs/acre.
(2) Assume the critical duration of supply is 30 minutes:
(a) The average rate of supply is obtained by reading horizontally across from the point where
the duration of supply = 30 minutes and a intersect. This value is found to be 3.2 inches per hour or 3.2
(b) The effective length is obtained by reading the point where tc and the duration of supply =
30 minutes intersect. This is found to be 500 feet.
(c) The maximum rate of runoff is obtained by reading horizontally across from this point. This
is found to be 2.0 inches per hour or 2.0 cfs/acre.
9. Storage. The supply curves in figure 3 assume no surface storage. Where surface storage or ponding
is permitted, the overland flow will be stored temporarily and released as the pond drains. The discharge
rate from the pond will depend on the volume of storage provided and the extent to which the surface
area of the pond reduces the effective length of overland flow. Methods for designing with temporary
storage or ponding are given in appendix B.
10. Design procedures for the drainage system. Design-storm runoff must be efficiently removed from
airfields and heliports to avoid interruption of operations during or following storms and to prevent
temporary or permanent damage to pavement subgrades. Removal is accomplished by a drainage system
unique to each airfield and heliport site. Drainage systems will vary in design and extent depending
upon local soil conditions and topography; size of the physical facility; vegetation cover or its absence;
the anticipated presence or absence of pending; and most importantly, upon local storm intensity and
frequency patterns. The drainage system should function with a minimum of maintenance difficulties
and expense and should be adaptable to future expansion. Open channels or natural water courses are
permitted only at the periphery of the airfield or heliport facility and must be well removed from the
landing strips and traffic areas. Provisions for subsurface drainage, the requirements for which are
provided in TM 5-820-2/AFM 88-5, Chap. 2, may necessitate careful consideration. Subdrains are used
to drain the base material, lower the water table, or drain perched water tables. Fluctuations of the
water table must be considered in the initial design of the airfield or heliport facility.
a. Information required. Before proceeding with the design calculations, as illustrated in appendixes
B and C, certain additional information and data must be developed. These include:
(1) A topographic map.
(2) A layout of the helipad, runways, taxiways, aprons, and other hardstands with tentative
finished grading contours at l-foot intervals.
(3) Profiles of runways, taxiways, apron areas, and other hardstands.
(4) Soil profiles based on soil tests to include, whenever possible, infiltration properties of local
soils to be encountered.
(5) Groundwater elevation and fluctuation if known or obtainable.
(6) A summary of climatic conditions including temperature ranges, freezing and thawing patterns,
and depth of frost penetration.
(7) Snowfall records, snow cover depths, and convertibility factors to inches of rainfall.
(8) Runoff records for drainage areas in the same locality having similar characteristics and soil
b. Grading. Proper grading is the most important single factor contributing to the success of the
drainage system. Development of grading and drainage plans must be fully coordinated. Grading criteria
in AFR 86-14 for Air Force facilities and TM 58034 for Army airfields and heliports provide adequate
grading standards to insure effective drainage.
(1) Minimum slopes. For satisfactory drainage of airfield pavements, a minimum gradient of 1.5
percent in the direction of drainage is recommended except for rigid pavements where 1.0 percent is
adequate. In some cases, gradients less than 1.5 percent are adequate because of existing grades; arid or
semiarid climatic conditions; presence of noncohesive, free-draining subgrades; and locations of existing
drainage structures. Such factors may allow a lesser transverse slope; thus, construction economies are
effected and preferred operational grades are obtained.
(2) Shoulder slopes. In attachment 5 of AFR 86-14, transverse grades of shoulders are specified
for runways, taxiways, and aprons. In areas of moderate or heavy rainfall or excessive turf
encroachment, use of a steeper transition shoulder section immediately adjacent to the airfield pavement