TM 5-820-1/AFM 88-5, Chap. 1
(6) Trial drainage layouts. Several trial drainage layouts will be necessary before the most
economical system can be selected. The first consideration will be the tentative layout serving all of the
depressed areas in which overland flow will accumulate. The inlet structures will be located, during the
initial step, at the lowest points within the field areas. The pipelines will be shown next. Each of the
inlet structures will be connected to the field pipelines, which in turn will be connected to the major
outfalls.
(7) Rechecking of finished contours. Before proceeding further, recheck the finished contours to
determine whether the surface flow is away from the paved areas, that the flow is not directed across
them, that no field structures fall within the paved areas (except in aprons), that possible ponding areas
are not adjacent to pavement edges, and that surface water will not have to travel excessively long
distances to flow into the inlets. If there is a long, gradually sloping swale between a runway and its
parallel taxiway (in which the longitudinal grade, for instance, is all in one direction), additional inlets
should be placed at regular intervals down this swale. Should this be required, ridges may be provided to
protect the area around the inlet, prevent bypassing, and facilitate the entry of the water into the
structure. If the ridge area is within the runway safety area, the grades and grade changes will need to
conform to the limitations established for runway safety areas in other pertinent publications.
(8) Maximum pending area and volume. Estimate the maximum elevation of storage permissible in
the various ponding areas and indicate the elevations on the profiles referred to in (4) and (5) above.
Scale the distances from the respective drain inlets to the point where the elevation of maximum
permissible ponding intersects the ground line, transfer the scaled distances to the map prepared in (1)
above, and sketch a line through the plotted points to represent the boundary of the maximum ponding
area during the design storm. Determine the area within the various ponding areas and compute the
volume of permissible storage at the respective drain inlets. All ponding area edges will be kept at least
75 feet from the edges of the pavement to prevent saturation of the base or subbase and of the ground
adjacent to the pavement during periods of ponding.
(9) Ditches. A system of extensive peripheral ditches may become an integral part of the drainage
system. Ditch size and function are variable. Some ditches carry the outfall away from the pipe system
and drainage areas into the natural drainage channels or into existing water courses. Others receive
outfall flow from the airport site or adjacent terrain. Open ditches are subject to erosion if their
gradients are steep and if the volume of flow is large. When necessary, the ditches may be turfed,
sodded, stabilized, or lined to control erosion.
(10) Study of the contiguous areas. After the storm drain system has been tentatively laid out and
before the actual computations have been started, the areas contiguous to the graded portion of the
airport which may contribute surface flow upon it should again be studied. A system of open channels,
intercepting ditches, or storm drains should be designed where necessary to intercept this storm flow
and conduct it away from the airport to convenient outfalls. A study of the soil profiles will assist in
locating porous strata which may be conducting subsurface water into the airport. If this condition
exists, the subsurface water should be intercepted and diverted.
e. Typical design procedures. The procedures in paragraphs 2 through 10 are illustrated and
annotated in the design computations contained in appendix C. Comparative designs with and without
provisions for temporary ponding have been prepared for the airfield shown.
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