AIRPORT DRAINAGEBy Teja Tallam

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 sections1. Estimation of runoff2. Design of basic system for collection and

disposal of runoff3. 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 equationQ = 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.