Airport Drainage
# Function of airport drainage
(1) Removal of surface runoff from the airfield.
(2) Interception and diversion of surface and ground water flow originating from adjacent land to
the airfield area.
(3) Lowering of sub-grade water level in airfield area.
# Effect of poor drainage/ improper drainage
(1) Improper drainage results in ponding on the pavement surface which can be hazards for safe
landing or takeoff.
(2) Poor drainage also results early failure of pavement.
# Special characteristics of airport drainage
(1) Extensive area under consideration
(2) Varying soil condition
(3) Heavy concentrated wheel loads of aircrafts
(4) Wide runways, taxiways and aprons
(5) Flat longitudinal and transverse grades
(6) Shallow water course
(7) Absence of side ditch
(8) Concentration of out fall flow.
# Design Data
(1) A contour map of airport site and adjacent land showing all natural water courses, the area
counting contributing runoff.
(2) An additional map showing existing layout of various airport elements. The map should also
show all pipelines, manholes, inlets, outfalls, individual drainage, drain pipe slope etc.
(3) Rainfall data.
(4) Centre line profile of all the runways, taxiways and apron area with necessary X-section.
(5) Boring plans of soil strata along with ground water profile.
(6) Temperature data.
(7) Data on infiltration properties of soils.
(8) Necessary hydraulic data, graph, table for design.
# Surface Drainage Design
Estimation of runoff: the estimation of runoff depends upon the number of factors:
(1) Condition of soil affecting infiltration
(2) Temperature of air, water and soil.
(3) Extent and condition of vegetation cover.
(4) Perviousness or imperviousness
(5) Slopes of surface
(6) Moisture content of soil
Formula
Q=CIA
Where,
Q = runoff in cubic meters/sec
A = drainage area in 100 sq. meters
I = intensity of rainfall in mm/sec. for the time of concentration.
C = coefficient of runoff representing the ratio of runoff to rainfall.
# Time of concentration
Time of concentration is defined as the tame taken by storm water to reach the drain inlet6 from the
farthest point in tributary area.
It has two components:
(a) Inlet time: Time taken by storm water to flow overland from most remote point in drainage area
to the drain inlet.
(b) Time of flow: Time during which water flows through drainage system to the point under
consideration.
Formula to determine inlet time:
D=KT2
Where,
D = Distance (m)
T = Inlet time (min.)
K = Factor.
# Design Principle:
Each pipe is design into carry the discharge from the inlet at its upstream end plus the concentration
through all preceding inlets.
- A minimum mean velocity of 0.75 m/sec is maintained to prevent scouring.
- Dia of storm drain > 30 cm, to ensure adequate x-section for flow at all times.
- Inlet located at a distance not less than 22.5m from the edge of pavement.
- On aprons the inlets are usually placed in the pavement. This is only one way by which the large
area of apron can be drained efficiently.
- Drains inlets are usually spaced 60-120m apart in case of ponding and 52.5m in case of no
ponding.
# Design procedure:
(1) Indentify the structures and establish the length of pipe segments between the structures with
accuracy.
(2) Select value of ‘C’ for all type of surface over which flows.
(3) Compute the weighted value of ‘C’
C = (A1C1 + A2C2 + - - - -)/ ∑
(4) Determine the distance from inlet to the most distant point in the tributary.
(5) Using the distance in (Step- 4) determine the inlet time.
(6) Determine the time of concentration.
(7) Determine the rainfall intensity for the corresponding time of concentration.
(8) Compute the quantity of runoff using the formula Q = CIA.
(9) Calculate pipe size and slope using Manning’s formula:
Q = AV
V=
V=
# Ponding
Ponding is simply means of providing temporary storage of runoff prior to its entry into the
underground system. Ponding would protect the pavement areas from damage due to storms of
intensity greater than design value. Ponding may be permissible on open airfield areas and graded areas
where it would not saturate or damage the sub-grade or base course.
