The document discusses airport drainage systems. It explains that a well-designed drainage system is important for safety, efficiency, and pavement durability. The key aspects covered are: 1) Airport drainage systems use surface ditches, inlets, and underground pipes to remove runoff. 2) Estimating runoff involves calculating factors like rainfall intensity and runoff coefficient. 3) Drainage channels and underground pipes are designed using equations like Manning's, with velocities above 2.5 ft/sec to prevent deposits. Inlets are placed at low points with spacings depending on the airport type.

1. AIRPORT DRAINAGE
By Srinivas

2. 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

3. 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.

4. 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

5. 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.

6.  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.

7.  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.

8. Values of runoff coefficient

9.  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

10. For example., calculation of rainfall
intensities from charts can be done
as follows

11.  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

12.  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.

14.  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

15. 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

16.  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

17.  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).

19.  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

20. 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

21. 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.

22.  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.

23.  Placing drain inlets nearer to pavements
should be avoided as it might lead to ponding
and may cause flooding or saturation of sub-
grade

25. 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

26.  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

27. 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.

28.  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

30. 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.

31.  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

32.  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

34. 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

35.  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.

36. 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.

37.  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.

38.  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.