AIRPORT DRAINAGE

By Srinivas

Introduction

A well-designed airport drainage system is a

prime requisite for operational safety and

efficiency as well as pavement durability

Inadequate drainage facilities may result in

costly damage due to flooding

Constitute a source of serious hazards to air

traffic

Erosion of slopes

Saturated and weakened pavement foundations

Airport Drainage - Introduction

Airport drainage system is similar to street and

highway drainage design.

Airports are characterized by large areas of

relatively flat gradient.

Airports require prompt removal of surface and

subsurface water.

Hence, they need an integrated drainage

system.

Removal of water should be done from

runways, taxiways, aprons, parking lots etc.

Airport drainage

Runoff is removed from airports by means of

surface ditches, inlets and an underground

storm drainage system

Airport drainage can be described into

following sections

1. Estimation of runoff

2. Design of basic system for collection and

disposal of runoff

3. Provision for adequate subsurface drainage

1. Estimation of Runoff

No of formulas and analytical procedures exist

for finding runoff

All methods are not precise and accurate

Of the available methods Rational Method is

widely used one

Co-efficient of runoff, rain fall intensity, duration

and frequency are the factors on which this

method depends on.

Only a portion of the rainfall flows as runoff Some water evaporates

Some intercepted by vegetation

Some infiltrates into ground

Airport drainage channels and structures must be designed for the precipitation – losses

Losses depends on slope, soil condition, vegetation and land use. Some of these factors change with time

Important to assess the effects of planned development works on the runoff as they may disturb the existing pattern of runoff.

Runoff coefficient indicates the hydrologic

nature of the drainage area

It is defined as the ratio of quantity of runoff to

the total precipitation that falls on the drainage

area

The below table gives the recommended

values of runoff coefficient.

Values of runoff coefficient

Rainfall intensity is the rate at which rain falls,

typically expressed in inches per hour.

Because of the probabilistic nature of weather, it

is discussed in terms of its frequency and

duration

Procedures for the construction of rainfall

intensity–duration curves have been published

by the FAA.

Making use of such charts we can determine

the intensity of a 5 year rainfall of desired

duration

For example., calculation of rainfall

intensities from charts can be done

as follows

To obtain values for short-duration rainfalls, the

following relationships between a

30-min rainfall and 5-, 10-, 15-min amounts

may be used:

Duration (min) Ratio

5 0.37

10 0.57

15 0.72

In the previous example we took, the 30 mm rainfall intensity that is 1.37 inch will be multiplied by the ratios given to yield the rainfalls of smaller durations as follows.

0.51 in. in 5 min

0.78 in. in 10 min

0.99 in. in 15 min

After that values will be converted into inches per hour to get the intensity

Now the above obtained values can be used for drawing a 5 year storm intensity duration curve.

Similarly for other periods like 2, 10, 20 we can develop curves.

A designer must choose correct curve for the

design

This involves the weighing and judging various

factors related to physical and social damages

that might result from the flood of a given

frequency.

Normally a return period of 5 years is used for

the design of drainage systems at airports

Choosing a higher return period will return a

costly design which is not economical

Time of concentration

In the design of airport drainage facilities, a rainfall duration equal to the time of concentration is chosen.

It is the time taken by the water droplet from the remotest area of the catchment to reach the inlet of drainage.

It consists of 2 components

Time of surface flow or inlet time

Time of flow within the structural drainage system

Inlet time can be obtained using the formula

D = KT2

D – distance in mts

T – inlet time in minutes

K = a dimensional emperical factor depends on terrain, and extent of vegetation, distance to drain inlet

Following is the formula recommended by FAA for finding the time T

where

T = surface flow time (min)

C = runoff coefficient

S = slope (%)

D = distance to most remote point (ft)

Or else following figure can be used for finding the approximate inlet time

The time of flow within the structural system

can be determined by dividing the structure

length (in feet) by the velocity of flow (in feet

per minute).

The rational method is recommended for the calculation of runoff from airport surfaces, especially for drainage areas of less than 200 acres

The method is expressed by the equation

Q = CIA

Q = runoff (cfs)

C = runoff coefficient (typical values are given in Table 12.1)

I = intensity of rainfall (in./hr for estimated time of concentration)

A = drainage area (acres); area may be determined from field surveys, topographical maps, or aerial photographs

COLLECTION AND DISPOSAL

OF RUNOFF

The hydraulic design of a system for the

collection and disposal of surface runoff is

discussed in the framework of four subtopics:

Layout of drainage system

Design of underground pipe system

Design of open channels

Design of inlets, manholes, and other

apparatus

1. Layout of Drainage System

A generalized topographical map showing existing

2-ft ground contours should be obtained or

prepared.

All natural and man made objects influencing the

drainage should be represented

Existing water courses, canals, irrigation ditches,

roads etc,

In addition, a more detailed or grading and

drainage plan, which shows the runway–taxiway

system and other proposed airport features,

should be prepared.

Each drainage subarea should be outlined on the plan

pipe sizes, lengths, and slopes should also be shown.

The grading plan makes it possible to select appropriate locations for drainage ditches, inlets, and manholes.

Storm drain inlets are placed as needed at low points

FAA recommends that inlets be located laterally at least 75 ft from the edge of pavements and also at air carrier airports and 25 ft from the edge at general aviation airports.

Placing drain inlets nearer to pavements

should be avoided as it might lead to ponding

and may cause flooding or saturation of sub-

grade

2. Design of Underground Pipe

System

After the location of ditches, pipes, inlets, and manholes on the layout next step is to determine the size and gradient of pipe

The Manning equation is the most popular formula for determination of the flow characteristics in pipes.

Its use is recommended by the FAA in the design of underground airport pipe systems

where

Q = discharge (cfs)

A = cross-sectional area of flow (ft2)

R = hydraulic radius (ft: area of section/wetted perimeter)

S = slope of pipe invert (ft/ft)

n = coefficient of roughness of pipe

It is important that sufficient velocities be

maintained to prevent the deposition and

accumulation of suspended matter within the

pipes.

a mean velocity of 2.5 ft/sec will normally

prevent the depositing of suspended matter in

the pipes

Ponding

When the rate of runoff inflow at a drainage inlet exceeds the capacity of the drainage structure to remove it, temporary storage or ponding occurs in the vicinity of the inlet.

Excessive ponding is not desirable

Operational hazards

Damage of pavement subgrades

Kills grass

Hence probability of ponding and its magnitude must be understood.

Study involves the computation of runoff that flows

into ponding system, then the runoff removed by

the drainage facilities

Vin = QCIAt

Vout = qct

qc = capacity of drainage system

Capacity is independent of time hence varies linearly

It is also possible determine the amount of ponding at

different times.

Cumulative runoff graphs can be used to evaluate

ponding

3. Design of Open Channels

Open channels, ditches play a major role in

airport drainage system

Size, shape, and slope of these channels must

be carefully determined

Avoid overflow, flooding, erosion

Flow in long, open channels is assumed to be

uniform

Energy losses due to friction are balanced by

slope

Hence, mannings equation can be applied

here.

To solve the Manning equation directly, the

depth and cross-sectional area of flow and the

slope, shape, and frictional characteristics of

the channel must be known.

By solving the manning equation, we can

design the cross-section of channel according

to our requirement

Nomographs and charts are used for

eliminating the use of mannings equation

Generally, wide and shallow open channels are

preferred

Channel slope should not be steeper than

2.5:1 (H:V)

To prevent erosion flow velocities should be

restricted to standard values

When flow velocity exceeds 6ft/s special

treatment and lining should be done to edges

and sides

Design of inlets, manholes and

headwalls

Where high heads are permissible, the

capacity of an inlet grating can be determined

by the orifice formula

For low heads, the discharge conforms to the

general weir equation

With the equations given above, the number and size of grates needed to accommodate a given runoff and head can be readily determined.

The general weir formula should be applied for aircraft servicing aprons and other areas where significant ponding depths would be unacceptable.

The orifice formula normally applies to grates in turfed areas

A safety factor 1.25 for paved areas and 1.5 –2.0 for turfed areas should be applied.

4.Subsurface Drainage

Special drainage systems are required to control

and avoid the undesirable effects of sub-surface

moisture

Subsurface drainage has three functions

to drain wet soil masses

to intercept and divert subsurface flows, and

to lower and control the water table.

Subsurface drains consist of small pipes (typically

6–8 in. in diameter) which are laid in trenches

approximately 1.5–2.0 wide and backfilled with a

pervious filter material.

The pipes should be bedded in a minimum

thickness of filter material.

Subsurface drainage systems are most likely

to be effective in sandy clays, clay silts, and

sandy silts.

The finer grained materials (predominantly

silts and clays) are more difficult to drain

whereas the coarser grainer materials (gravels

and sands) tend to be self-draining.

Subsurface drainage systems must be inspected

and maintained. To allow for this manholes should

be placed at intervals of not more than 1000 ft and

at principal junction points in base and subgrade

drainage systems.

Inspection and flushing holes (risers) are normally

placed between manholes and at dead ends.

It is recommended that subsurface drains be laid

on a slope of at least 0.15 ft/100 ft.

the drain pipes must be backfilled with a carefully

graded filter material.