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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