airport drainage
TRANSCRIPT
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.
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
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.