This document discusses the design of highway drainage for the Thankot-Chitlang road section in Nepal. It covers the importance of drainage, requirements for highway drainage systems, and the specific design of surface and subsurface drainage features for this road project. The surface drainage design includes determining stormwater discharge amounts, dimensions for side drains and culverts using Manning's equation. The subsurface drainage design considers lowering the water table and controlling seepage and capillary moisture.

1. Highway Drainage
Manas Thapa

2. Contents
 Introduction
 Significance of Drainage
 Requirements of Highway Drainage
 Design of drainage in Thankot-Chitlang road
section
Sub-surface drainage
Surface drainage

3. What is Highway Drainage?
 Highway drainage is a process of
removing and controlling excess
surface and sub-soil water within
the right of way.
 It includes interception and
diversion of water from the road
surface and sub grade.

4. Significance of Drainage
 Excess moisture in soil subgrade causes
considerable lowering of its stability.
 Increase in moisture cause reduction in strength
of many pavement materials like stabilized soil
and water bound macadam.
 Sustained contact of water with bituminous
pavements causes failure due to stripping of
bitumen from aggregates like loosening or
detachment of some of the bituminous pavement
layers and formations of pot holes
 Excess water on shoulders and pavement edges
causes considerable damage.

5. Significance of Drainage
 In clayey soil variation in moisture content
causes considerable variation in volume of sub
grade.
 High moisture content causes increases in
weight and thus increase in stress and
simultaneous reduction in strength of soil
mass.
 Erosion of soil from top of unsurfaced roads
and slopes of embankment, cut and hill side is
also due to surface water.
 In cold regions presence of water in the
subgrade and a continuous supply of water
from the ground water can cause considerable
damage to the pavement due to frost action.

6. Requirements of Highway
Drainage
 The surface water from the carriageway and
shoulder should effectively be drained off
without allowing it to percolate to sub grade.
 Surface water from adjoining land should be
prevented form entering the roadway.
 Side drain should have sufficient capacity and
longitudinal slope to carry away all surface
water collected.
 Flow of surface water across the road and
shoulders and along slopes should not cause
formation of cross ruts or erosion.

7. Requirements of Highway
Drainage
 Seepage and other sources of under
ground water should be drained off by
the subsurface drainage system.
 Highest level of ground water table
should be kept well below the level of
subgrade, preferably by at least 1.2m.
 In waterlogged areas special
precautions should be taken,
especially if detrimental salts are
present or if flooring is likely to occur.

8. Subsurface Drainage
The removal of
excess water from
the subgrade of
the pavement is
called sub-surface
drainage. The
change in moisture
content in
subgrade are
shown alongside
Fluctuation
in ground
water table
Seepage
flow
Percolation
of rain
water
Movement
of capillary
water

9. Methods of sub-surface
drainage
Lowering of water table
• In permeable soil, longitudinal drain trenches are
provided.
• Transverse drains are provided in less permeable soil.
Seepage control
• Longitudinal drain pipes in trench filled with filter &
clay seal is constructed.
Capillary control
• Granular capillary cut-off
• Impermeable capillary cut-off

10. Design of surface drainage
Design of side
drain and pipe
culverts in
Thankot–Chitlang
road section

11. Codes and data followed in
design
 NRS(2070) .
 Road Safety Notes 2-Design of side drains-2053.
 Some guidelines of NRRS (2071).
 Monthly rainfall data of Thankot.
 Some data are taken from Google earth.

12. Location and Catchment

13. Some Parameters of
design
 Discharge
 Calculated by rational method.
 Q=
𝐶∗𝐼𝑐∗𝐴
360
(m3/s)
 C=Run- off coefficient. Given by:
 C=
𝐶1
𝐴1
+𝐶2
𝐴2
+⋯𝐶𝑛𝐴𝑛
𝐴1
+𝐴2
+⋯.𝐴𝑛
 Ic=Critical intensity of rainfall(mm/hr.),given by:
 Ic=
𝑃(𝑇+1)
𝑇(1+𝑡𝑐)
 where, p=Daily maximum rainfall(mm)
 T=Maximum duration of storm(hrs.)

14. Some parameters of
design
 tc=time of concentration in hrs.it is the
time taken by the rain to reach the
point of outlet from the remote point
of the catchment area. And it is given
by:
 tc=(0.87*
𝐿3
𝐻
)0.385
 L=length of the watercourse(m)
 H=elevation difference between
remote and outlet point.(m)
 A=area of the catchment in
hectare.

15. Determination of channel
dimension
 The obtained discharge is equated with the manning's
equation to obtain the channel dimension as follows.
Q=
1
𝑛
*A*R2/3*S1/2
where,
 A=area of channel(m2) given by:
 A=BD+D2/1.5
 R=Hydraulic radius given by:
 R=A/P
 Where, P=wetted perimeter given by:
 P= B+2D√13 /3
 With these values we can solve the above equation and
obtain suitable values of B and D.(using the above
equation for B=0.25 m the value of D is 0.26

16. Determination of channel
dimension
Adding some
freeboard of 30 cm
to the above
obtained depth of
channel. Final cross
section of the
channel dimensions
are obtained as in
the figure
alongside.Fianlly,the
obtained total depth
of the channel is 56
cm.

17. Principle of pipe culvert
design
 The design of the pipe culvert is based on the principle
of partial flow in pipes.

18. Design procedure of
Dimension of Pipe culvert
 Consider pipe conveys design discharge at
the depth of 80% of the diameter.so that
d=0.8D.
 Determine angle subtended θ as shown above
fig: using relation Cos (
ө
2
) = (1-2d/D).
 Now find the flow area and wetted
perimeter using the relation as
follows in terms of D.
Flow area (a) =
π∗𝐷2
4
(
ө
360
-
sinө
2𝜋
)
Wetted perimeter(p) = πD*
ө
360

19. Design procedure of
Dimension of Pipe culvert
 Now find the Hydraulic radius using the relation
Hydraulic radius(r) = a/p in terms of the pipe
diameter.
 Apply following equation to calculate the
discharge. From Manning's
equation,
v= (
1
𝑛
*r2/3 *s1/2)
Here, n=Manning's rugosity (Depends on material of
culvert) .
r= Hydraulic radius calculated in terms of D.
S= Slope in which the culvert is to be laid(7% as per
the guidelines of NRS-2070)
 Now use Q=Area *velocity

20. Design procedure of
Dimension of pipe culvert.
 Substitute the value of Q from rational formula.and
other parameters in terms of D .Then solve the equation
to find the required value of the culvert diameter.
 Calculation gives the dia. Of 74 cm. The commercially
available diameter is 90cm.So we use 90cm dia. Pipe
culvert.

21. Thank you for
your kind
Attention!!!!