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AIRPORT DRAINAGE
By Srinivas
Introduction
 A well-designed airport drainage system is a
prime requisite for operational safety and
efficiency as well ...
Airport Drainage - Introduction
 Airport drainage system is similar to street and
highway drainage design.
 Airports are...
Airport drainage
 Runoff is removed from airports by means of
surface ditches, inlets and an underground
storm drainage s...
1. Estimation of Runoff
 No of formulas and analytical procedures exist
for finding runoff
 All methods are not precise ...
 Only a portion of the rainfall flows as runoff
 Some water evaporates
 Some intercepted by vegetation
 Some infiltrat...
 Runoff coefficient indicates the hydrologic
nature of the drainage area
 It is defined as the ratio of quantity of runo...
Values of runoff coefficient
 Rainfall intensity is the rate at which rain falls,
typically expressed in inches per hour.
 Because of the probabilist...
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-m...
 In the previous example we took, the 30 mm rainfall
intensity that is 1.37 inch will be multiplied by the
ratios given t...
 A designer must choose correct curve for the
design
 This involves the weighing and judging various
factors related to ...
Time of concentration
 In the design of airport drainage facilities, a rainfall
duration equal to the time of concentrati...
 Following is the formula recommended by FAA for
finding the time T
 where
 T = surface flow time (min)
 C = runoff co...
 The time of flow within the structural system
can be determined by dividing the structure
length (in feet) by the veloci...
 The rational method is recommended for the
calculation of runoff from airport surfaces, especially
for drainage areas of...
COLLECTION AND DISPOSAL
OF RUNOFF
 The hydraulic design of a system for the
collection and disposal of surface runoff is
...
1. Layout of Drainage System
 A generalized topographical map showing existing
2-ft ground contours should be obtained or...
 Each drainage subarea should be outlined on the
plan
 pipe sizes, lengths, and slopes should also be shown.
 The gradi...
 Placing drain inlets nearer to pavements
should be avoided as it might lead to ponding
and may cause flooding or saturat...
2. Design of Underground Pipe
System
 After the location of ditches, pipes, inlets, and manholes on the layout next
step ...
 It is important that sufficient velocities be
maintained to prevent the deposition and
accumulation of suspended matter ...
Ponding
 When the rate of runoff inflow at a drainage
inlet exceeds the capacity of the drainage
structure to remove it, ...
 Study involves the computation of runoff that flows
into ponding system, then the runoff removed by
the drainage facilit...
3. Design of Open Channels
 Open channels, ditches play a major role in
airport drainage system
 Size, shape, and slope ...
 To solve the Manning equation directly, the
depth and cross-sectional area of flow and the
slope, shape, and frictional ...
 Channel slope should not be steeper than
2.5:1 (H:V)
 To prevent erosion flow velocities should be
restricted to standa...
Design of inlets, manholes and
headwalls
 Where high heads are permissible, the
capacity of an inlet grating can be deter...
 With the equations given above, the number
and size of grates needed to accommodate a
given runoff and head can be readi...
4.Subsurface Drainage
 Special drainage systems are required to control
and avoid the undesirable effects of sub-surface
...
 The pipes should be bedded in a minimum
thickness of filter material.
 Subsurface drainage systems are most likely
to b...
 Subsurface drainage systems must be inspected
and maintained. To allow for this manholes should
be placed at intervals o...
Airport drainage
Airport drainage
Airport drainage
Airport drainage
Airport drainage
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Airport drainage

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airport drainage

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Airport drainage

  1. 1. AIRPORT DRAINAGE By Srinivas
  2. 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. 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. 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. 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. 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. 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. 8. Values of runoff coefficient
  9. 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. 10. For example., calculation of rainfall intensities from charts can be done as follows
  11. 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. 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.
  13. 13.  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
  14. 14. 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
  15. 15.  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
  16. 16.  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).
  17. 17.  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
  18. 18. 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
  19. 19. 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.
  20. 20.  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.
  21. 21.  Placing drain inlets nearer to pavements should be avoided as it might lead to ponding and may cause flooding or saturation of sub- grade
  22. 22. 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
  23. 23.  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
  24. 24. 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.
  25. 25.  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
  26. 26. 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.
  27. 27.  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
  28. 28.  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
  29. 29. 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
  30. 30.  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.
  31. 31. 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.
  32. 32.  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.
  33. 33.  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.

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