6. Highway drainage
1. Introduction to drainage system
Highway drainage is the process of removing and controlling
excess surface and sub soil water within right of the way.
This includes interception and diversion of water from the road
surface and sub-grade
A part of the rainwater falling on the road surface and adjoining area is
lost by evaporation and percolation. The remaining water is called the
surface water. Removal and diversion of this surface water from
highway and adjoining land is called surface drainage.
Similarly, diversion or removal of excess soil water from the sub
grade is known as sub-surface drainage.
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11. 2. Causes of moisture variation in sub
grade soil.
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12. Causes of moisture variation in sub grade
soil.
By free water.
Seepage of water from higher ground adjacent to the road.
Penetration and percolation of water through pavement.
Transfer of moisture from the shoulders and pavement
edges.
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Initial Seepage level
13. Causes of moisture variation in sub grade soil
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13
By ground water.
Rise and fall of water table.
Capillary rise from lower soil level.
Transfer of water vapors through soil.
Condt…..
19. 30 August 2021
19 Absent of intercepting mechanism
so surface runoff along with debris reach road surface
20. 3. Importance of drainage system.
Road surface made of soil,
gravel or WBM will be soft and
losses strength if there is no
proper drainage facilities.
Increases in moisture causes
reduction in strength of many
pavement materials.
Due to poor drainage facilities
there will be problem of
pavement failures due to
formation of waves and
corrugation. 30 August 2021
20
21. 30 August 2021
21
Due to poor drainage for surface water, there occurs erosion of
soil from unsurfaced roads and slopes of embankment.
If the rain water is not properly drained and allowed to flow
along road side for long distance slips and landslides may
occur.
22. Importance of drainage system.
Excess water on shoulders and pavement
edges causes considerable damage.
Presence of water in sub grade in water causes
considerable damages to the road due to frost
action.
Excess moisture causes increase in weight and
thus increase in stress and simultaneous
reduction in strength in soil mass. This is one of
the main reasons of failure of earth slope and
embankment foundations
Erosion of soil from top of un-surface roads and
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22
Erosion of soil
damages to the road
due to frost action
Excess water on
shoulders and pavement
28. 4. Requirement of good drainage systems.
The surface water from the carriage way and shoulders should be
effectively drained off without allowing it to percolate to subgrade.
The surface water from the adjoining land should be prevented from
entering the roadway.
The side drain should have sufficient capacity and longitudinal slope to
carry away all surface water collected in side drain.
Seepage should be drained off by the sub surface drainage system.
Highest level of the ground water table should be kept well below the
level of sub grade probably by at least 1.2 m
Flow of surface water across the road and shoulders and along slopes
should not causes formation of cross ruts and erosion.
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29. 5. Classification of highway drainage
system.
Highway drainage system may be divided into
Surface drainage.
Sub surface drainage.
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31. a. Surface drainage.
The removal of rainwater from the road surface and road side ground is
called surface drainage.
The water is first collected in longitudinal drains and then water is
disposed off at the nearest stream or water courses.
Sometimes, cross drainage structures like culverts and small bridges
may be necessary for disposal of surface water from road side drain
Surface drainage system consists of following two operation.
1. Collection of surface water.
2. Disposal of collected surface water.
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40. In case of rural highway surface water from pavement
surface is diverted to side drains which are provided parallel
to road alignment. These drains are called longitudinal
drains.
Longitudinal drains in case of embankment is provided at
bottom of toe.
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Contd…
41. 30 August 2021
41
In case of urban roads it is necessary to provide under ground
longitudinal drains to collect surface water.
Similarly, in case of hilly road along with longitudinal drain,
we have to provide other drainage structures e.g. catch water
drain so as to control or collect water flowing down hill.
Contd…
42. After collecting surface water, sometimes
we have to provide cross drainage
structures to dispose off collected water in
stream or water courses.
Apart from the drainage of water from the
road formation, the efficient diversion and
disposal of water flowing down the hill slope
across the road and from numerous cross
streams is an important part of hill road
construction.
If drainage system in hill road is not
adequate and efficient, it will result in
complex maintenance problems.
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Contd…
43. 1. DESIGN OF SURFACE DRAINAGE
SYSTEM
The design of surface drainage system is divided in two
phases:
A. Hydrologic Analysis
B. Hydraulic Analysis
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44. A. HYDOLOGIC ANALYSIS: -
Q = Ci Ad
Q= run-off, meter cube/second
C= run-off coefficient
I= intensity of rain fall mm/second
Ad= drainage area in 1000 meter square.
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45. Run-off coefficient
SN Type of surface and slope C
1 Bituminous 0.8
2 concrete pavement 0.9
3 Gravel 0.35
4 WBM 0.70
5 Impervious soil 0.40-0.65
6 Soil covered with turf 0.30-0.55
7 Pervious soil 0.05-0.30
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46. B. HYDROULIC ANALYSIS: -
Q = AV
The allowable velocity of flow depends upon the soil type
S
N
Types of soil Velocity(m/s)
1 Sand and silt (fine sand, clay, or other material carried by
running water and deposited as a sediment,)
0.3-0.5
2 Loam (a soil with roughly equal proportions of sand, silt, and
clay)
0.6-0.9
3 Clay 0.9-1.5
4 Gravel 1.2-1.5
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47. Considering uniform and steady flow through channel of
uniform cross section and slope Manning’s formula is
used to calculate velocity of flow or longitudinal slope
which is given by relation
V=
2
1
3
2
1
S
R
n
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48. The roughness coefficient depend upon the type of
soil in unlined channel.
SN Type of soil n
1 Ordinary soil 0.02
2 Earth with grass and
vegetation
0.05-0.1
3 Well finished concrete 0.013
4 Rough rubble and rip rap 0.04
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49. DATA FOR DRAINAGE DESIGN
The following data are to be collected for the design of road side
drain:
Total road land and width of land from where water is expected to
flow on the stretch of the side drain.
Run-off coefficient of different types of surface in the drainage area
and their respective areas.
Distance from farthest point in the drainage area to the inlet of the
side drain along the steepest gradient and the average value of the
slope.
Type of soil of the side drain, roughness coefficient allowable
velocity of flow in the drain.
Rain fall data including average intensity and frequency of
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50. numerical
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50
The distance between the farthest point in the turf covered drainage area (with an average slope of 1%
towards the drain) and the point of entry to side drain is 200 m. the weight average value of the run off
coefficient is 0.25. The length of the longitudinal open drain in a sandy clay soil from the inlet point to
the cross drainage is 540 m. Estimate the design quantity of flow on the side drain for ten year period of
frequency of occurrence of the storm.
Inlet time for average turf with 1.5% slope corresponding to
200 m distance = T1 = 33 mins (By interpolation)
Time for water to flow through 540 m length of drain at 0.6 m/sec
= T2 =
Time of concentration = T = 33 + 15 = 48 min
Drainage area = 540 x 200 = 108000 m2
Ad =
Design value of rainfall intensity for 10 year frequency of occurrence
and corresponding to 48 mins is 70 mm/hr
i =
s
x
min
15
60
60
540
108
1000
108000
sec
/
3600
70
m
sec
/
525
.
0
108
60
60
70
25
.
0 3
m
x
x
x
CiA
Q
51. The maximum quantity of water expected in open longitudinal drain on clayey soil is 0.9 m/s3. Design
the cross section and longitudinal slope of trapezoidal drain. Take Manning’s roughness coefficient as
0.02. hints……….
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Allowable velocity of flow through the clay soil = v = 1.2 m/sec
Assume bottom width = B = 1m and side slope 1:1.5
A = 0.9/1.2 = 0.75 sq. meter
Cross sectional area = A =
A =
With free board = 0.15 m
Total depth = 0.45 + 0.15 = 0.6 m
2
3 2
d
d
m
x
x
d
onsolving
d
d
d
d
45
.
0
5
.
1
2
)
75
.
0
(
5
.
1
4
1
1
0
75
.
0
5
.
1
75
.
0
2
3
2
2
2
2
1
3
2
1
S
R
n
v
B=1m (Suppose)
d
1
1.5
1.5d 1.5d
1
1+3d
2
*
)
3
1
1
(
d
d
52. 30 August 2021
52
n=0.02
v = 1.2 m/sec
A = 0.75
Wetter perimeter = m
x
x 62
.
2
1
2
)
45
.
0
5
.
1
(
45
.
0 2
2
5
.
322
1
0553
.
0
286
.
0
02
.
0
2
.
1
286
.
0
62
.
2
75
.
0
3
2
3
2
2
1
S
x
R
vxn
S
P
A
R
53. b. Sub surface drainage.
The drainage which is constructed for diversion or
removal of excess soil water from the sub grade
is called sub surface drainage. The main purpose
of sub surface drainage are
To lower water table.
To control seepage.
To control capillary rise.
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54. Lowering water table.
If underground water table is more than 1.5 m below sub grade
of the road, it does not require any sub soil drainage.
But if it is closer than this, it is necessary to raise road
formation to such a height that sub grade remains at least 1.2 m
above the highest water table.
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55. Contd…
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In below fig1 soil is permeable. So, ground water table is lowered
by constructing longitudinal drains trench with drain pipe and filter
media.
56. 30 August 2021
56
But in fig below in order to lower ground water table at center
where soil is impermeable in such case transverse drains are
provided along with longitudinal drains.
59. Control of seepage flow.
When both ground surface and
impervious layers are sloping
toward road, seepage flow is
likely to reach road surface.
If vertical distance between sub
grade and seepage level is less
than 60 cm to 90 cm, we need
to construct sub surface drain to
prevent seepage flow from
entering road sub grade.
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61. Control of capillary rise.
To control capillary rise,
we can provide a layer of
granular material of
suitable thickness.
By providing an
impermeable or a
bituminous layer in place
of granular material rise of
capillary water can be
controlled.
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65. Cross drainage structure.
Cross drainage structure.
Whenever streams have to cross the road way, cross
drainages are provided.
Sometimes, water from side drains is to dispose in stream or
valley. In such case we provide a drainage system called
cross drainage system.
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66. Cross drainage system.
Pipe culverts.
If stream carries low discharge and
there is requirement of high
embankment in such case pipe
culverts are considered more suitable.
While providing pipe culverts, pipe is
laid slightly inclined and there should
be atleast 50 cm cover of soil so that
traffic load transmitted on pipe is of
small intensity.
Pipe may be made of stone, RCC,
concrete. Standard size of pipes are
0.5m, 0.75m, 1.25m, 2.0m in diameter.
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70. Functions of pipe culvert
Collecting and leading the water across the road so as not to
cause damage to road bank.
Allowing sufficient water ways to prevent heading up of water
above road surface.
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71. Box culverts
Box culvert can be made in large size to accommodate
increase flow rate.
Size of rectangular should not be less than 60cm x 60cm for
easy cleaning of debris.
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76. Slab culverts
These culverts have masonry
abutment with stone slab over
them.
In those localities where
stones is easily available, slab
culverts are mostly used.
If stone is not available in
such case RCC slabs are
used. RCC slabs are
designed as simply supported
slabs and span of RCC slab
may be about 3 m. 30 August 2021
76
79. Arch culvert.
This culvert is suitable when more filling is to be done and there
are heavier load on culvert.
Here arch may be built from brick or stone masonry or PCC.
Span of each arch should be less than 3m.
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84. Cause way.
A types of cross drainage structure provided instead of culverts.
This type of drainage system save the construction cost but
during flood water flows over the road and traffic on the both
sides is stopped.
Thus during heavy floods cause way may be under water.
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85. Erosion control and energy dissipating
structure
The high velocity of water causes erosion of stream bed and
removal of vegetation layer. The control of erosion is directly
concern with the dissipation of energy which is ultimately
means the reduction of velocity.
The following measures are taken for erosion control and
energy dissipation
A. Energy dissipation measures
Drain lining
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86. Bed slope Types of soil Types of lining
Upto 1% Sandy No lining
Upto 2 % Clay No lining required
1-2% Sandy Turf/turfing
2-4% Clay Turf
Upto 5% All types of soil Dry stone paving
More than 5% All types of soil Stone masonry with ditch
check
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87. ii. Ditch check
Ditch checks are used in channels to reduce water
velocity, dissipate energy, and contain sediment in ditches.
The slope covered with turf and bottom covered by gravels of
desired size will reduce the velocity of flowing water. These
structure is called ditch check.
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88. iii Fall or drop structure
These structures are
provided in hill roads where
the bed slope of existing
drainage is very high and
are provided both upstream
and downstream of the
cross drainage structures.
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89. B. Erosion control measures
Vegetation
Stone pitching, lining and protection walls
Bank protection
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