atmospheric conditions affecting aircraft performance · 2016. 12. 29. · the functions of an...
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Airport Drainage
An adequate drainage system for the removal of surface and subsurface water is vital for the safety of aircraft and for the longevity of the pavements.
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Remarks
The material in this chapter is principally
concerned with estimating the amounts
of surface and subsurface runoff and not
with the hydraulics of pipes or details of
installation. These latter items are
adequately covered in texts on
hydraulics and literature provided by pipe
manufacturers.
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Purpose of Drainage
The functions of an airport drainage system are as follows:
1. Interception and diversion of surface and groundwater flow originating from lands
adjacent to the airport
2. Removal of surface runoff from the airport
3. Removal of subsurface flow from the airport
In very few cases will the natural drainage on a site be sufficient by itself to satisfy these functions; consequently artificial drainage must be installed.
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Design Storm for Surface Runoff
The selection of the severity of the storm which the drainage system should accommodate involves economic consideration.
An extremely severe storm occurring very infrequently would undoubtedly cause some damage if the system were designed for a storm of lesser severity.
However, if serious interruptions in traffic are not anticipated, a system designed for the larger storm may not be economically justified.
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Design Storm for Surface Runoff
Determining the Intensity-Duration
Pattern for the Design Storm
Rainfall intensity-duration curves (Corps of Engineers).
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Design Storm for Surface Runoff
Determining the Amount of Runoff by
the FAA Procedure.
1 Acre = 4046.85642 Square Meters
= 43560 sq. ft
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Design Storm for Surface Runoff
Coefficient of Runoff.
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Design Storm for Surface Runoff
Time of Concentration. The time of concentration is defined as the time taken by water to reach
the drain inlet from the most remote point in the tributary area. The most
remote point refers to the point from which the time of flow is the
greatest.
The time of concentration is usually divided into two components:
inlet time and time of flow.
The inlet time is the time required for water to flow overland from the
most remote point in the drainage area to the inlet.
The time of flow is the time taken by the water to flow from the drain
inlet through the pipes to the point in the system under consideration.
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Design Storm for Surface Runoff
Time of
Concentration.
3
1.18.1S
DT
c
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Design Storm for Surface Runoff
Time of Concentration.
Inlet time curves (Federal Aviation Administration)
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Design Storm for Surface Runoff
Coefficient of Runoff.
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Design Storm for Surface Runoff
Coefficient of
Runoff.
ACA
i
iiC
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Example
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Example
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Weighted Runoff
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Weighted Runoff
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Weighted Runoff
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Weighted Average Runoff
Coefficient
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Weighted Runoff
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Weighted Runoff
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Example
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Example Calculations
0.51 * 3.1 *2.88 = 4.55 ft3/s
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Example Calculations
Q = VA 4.55*12*12*12 = 4.0*12*pr2 r = 7.22 then d =15”
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Time and Intensity
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Subsurface Drainage
The functions of subsurface drainage are
to:
(1) remove water from a base course,
(2) remove water from the subgrade beneath a pavement, and
(3) intercept, collect, and remove water flowing from springs or
pervious Ground layers.
Base drainage is normally required where: (1) where frost action occurs in the subgrade beneath a pavement, (2)
where the groundwater is expected to rise to the level of the base course, and (3) where the pavement is subject to
frequent flooding and the subgrade is highly impervious.
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Subsurface Drainage
Subgrade subdrainage details (Corps of Engineers).
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Subsurface Drainage
Perforated metal, concrete, or vitrified clay pipe. Porous concrete pipe
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Subsurface Drainage
Pipe Sizes and Slopes: Experience has shown that a 6-in-diameter drain is adequate, unless
extreme groundwater conditions are encountered.
The recommended minimum slope for subdrains is 0.15 ft in 100 ft.
A minimum thickness of 6 in of filter material should surround
the drain.
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Subsurface Drainage
Gradation of Filter Material: The term filter material applies to the granular material which is used
as backfill in the trenches where subdrains are placed. To permit
free water to reach the drain, the filter material must be many
times more pervious than the protected soil.
Yet if the filter is too pervious, the particles of soil to be drained will
move into the filter material and clog it.
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Subsurface Drainage
Gradation of Filter Material:
Soil
Filter % Passing
Size
85
D85 D15
15