Module 5: Highway Drainage and Highway Economics

HIGHWAY DRAINAGE
Prof. DHANANJAY M
1

HIGHWAY DRAINAGE
Highway drainage consists of removing or
controlling surface water and subsurface water away
from the road surface and the subgrade supporting
it.
2

IMPORTANCE OF HIGHWAY DRAINAGE
 Excess water on shoulders and payment edge causes considerable
damage to the pavement structure.
 Stagnation of water on the pavement surface or edge and increase in
moisture content in the pavement layers can cause reduction in
strength characteristics of most of the flexible pavement component
materials.
Stagnation Of Water Edge Cracking
3

 In some clayey soils variation in water content causes considerable variation
in volume of subgrade. Alternate swelling and shrinkage of the supporting
layers may also contribute to pavement failures in the form of cracking.
 One of the important types of flexible pavement failure due to poor drainage
is by progressive development of undulations on road surface in the form of
rutting along the wheel paths of heavy vehicles and formation of ' waves and
corrugations‘.
4

 Sustained contact of water with bituminous pavements causes failures
due to 'stripping' of bitumen from aggregates and consequent
loosening or detachment of some of the bituminous pavement
materials, 'ravelling' and formation of pot-holes.
STRIPPING
POT HOLES
RAVELLING
5

 The prime cause of failures in rigid pavements by 'mud pumping' is due the
presence of excess water in fine subgrade soil.
 Excess moisture causes increase in stress and simultaneous reduction in strength of
the soil mass of the earth slopes, resulting in landslides on some stretches of the hill
roads.
6

 In places where freezing temperatures are prevalent in winter, the
presence of water in the subgrade and a continuous supply of water
from the ground water due to capillary action can cause considerable
damage to the pavement due in 'Frost action'.
 Ineffective and improper surface drainage causes erosion of soil from
top of unsurfaced roads and slopes of embankment, cut and hill side.
7

REQUIREMENTS OF HIGHWAY DRAINAGE
 The surface water from the carriageway and Shoulder should
effectively be drained off without allowing it to percolate to subgrade
and weaken the soil.
 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 the surface water collected from the roadway.
 Flow of surface water across the road and shoulders and along slopes
should not cause erosion or form cross ruts.
8

 Seepage and other sources of underground water should be effectively
intercepted and drained off by the suitable subgrade drainage system.
 Highest level of groundwater table should be kept well below the level
of subgrade, preferably by at least 1.2m. If the highest level of
groundwater level is closer than 1.2m, it is desirable to lower the same
with a well planned and laid subsurface drainage system.
 In waterlogged areas special precautions should be taken, especially if
detrimental salts are present or if flooding is likely to occur.
9

TYPES OF HIGHWAY DRAINAGE SYSTEM
The highway drainage system consists of,
• Surface Drainage System
• Subsurface Drainage System
10

SURFACE DRAINAGE SYSTEM
Removal and diversion of surface water from the roadway and
adjoining land is termed as 'surface drainage’
Objects of surface drainage system of roads:
The surface drainage system enables to drain off the water from
the pavement surface and the shoulders during the rains and to divert
it to the roadside drains such that the entry of water into the pavement
layers and the subgrade soil is minimised. During rains, one portion of
the rain water flows along the surface as 'runoff water' and the
remaining portion of the rain water percolates through the soil mass
under the action of gravity until it reaches the ground water-table.
11

SUBSURFACE DRAINAGE SYSTEM
Diversion or removal of excess soil-water from the subgrade
is termed as subsurface drainage.
Objects of surface drainage system of roads:
The subsurface drainage system enables intercepting the
‘seepage flow’ of water and diverting the same away from the
roadway to the nearest water course. The subsurface drainage system
also helps in lowering the ground water level well below the subgrade
and in controlling the capillary rise of water.
12

COMPONENTS OF SURFACE DRAINAGE SYSTEM
Surface drainage of the roadway is to be effected with the help of a
well designed and constructed surface drainage system consisting
of components such as:
a) The cross slope or camber of the pavement surface and the
Shoulders
b) Cross drains and
c) The roadside drains
13

COLLECTION OF SURFACE WATER
a) Camber or cross slope:
The water from the pavement surface and shoulders is first drained off to the
roadside drains with the help of the cross slope or camber. The rate of this cross
slope of the pavement or the carriageway is decided based on: 1) The type of
pavement surface and 2) amount of rainfall in the region.
b) Cross drains:
On rural stretches of highways, the water flowing along the roadside drains are
collected by suitable cross drains through cross drainage structures at locations of
natural valleys and streams and disposed off to the natural water course. 14

C) Road side drains:
The Road side drains of highways passing through rural areas are generally open,
'Katcha' drains of trapezoidal shape. On plane terrain with embankments the
longitudinal drains are provided on both sides. However, if the road passes through
sloping terrain then the longitudinal drain be provided on one side only. In cuttings,
the longitudinal drains are installed on either side of the formation.
On urban roads because of the limitation of land width and also due to the
presence of footpath, dividing islands and other road facilities, it is necessary to
provide underground longitudinal drains.
Drainage of surface water is all the is more important on hill roads. 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 that from numerous cross
streams is an important part of hill Road drainage system. 15

The design of surface drainage system may be divided into
two phases:
Hydrologic analysis
Hydraulic analysis
16
DESIGN OF SURFACE DRAINAGE SYSTEM FOR
HIGHWAY

HYDROLOGIC ANALYSIS
Objective :
The main objective of hydrologic analysis is to estimate the maximum quantity
of water, Q expected to reach the component of the drainage system under
consideration. A portion of the precipitation during the rain-fall infiltrates into the
ground as groundwater and a small portion gets evaporated. The remaining portion of
the water which flows over the surface is termed as 'Run-off'.
Various factors affecting runoff are:
a) Intensity or rate of rainfall
b) Type of soil
c) Moisture content in the soil
d) Topography of the area
e) Type of ground cover like the type of pavement surface, vegetation on the adjoining
land etc. 17

Principle of Hydrologic analysis:
The surface drainage system is to be designed to drain away the surface
run-off water reaching each component, such as the roadside drain and cross
drains.
The following four steps to be followed:
a) To collect the details of rainfall in the area including intensity, duration and
frequency of occurrence of storm.
b) To find drainage area from where water is likely to flow in.
c) To determine the run-off and maximum rate of run-off for the area under
consideration using any of the accepted approaches.
d) To estimate the peak quantity of run-off water reaching the component of the
drainage system to be designed. 18

CALCULATION OF RUNOFF
The rational formula, in its simplest form is given by,
Q = C i Ad
Where, Q = run off (m3/sec)
C = run off coefficient
i = intensity of rain fall (mm/sec)
Ad = area of drainage (1000 m2)
Note: Above expression is dimensionally not balanced.
19

RUN-OFF COEFFICIENT,C
Run off coefficient ‘C’ is the ratio of run-off to the rate of rainfall. So, it
is not same for all types of surfaces. C depends mainly on the type of
surface and its slope.
C values for different surfaces are as follows:
20
Type of Surface Coefficient of run-off, C
Pervious soil surface 0.05 – 0.30
Soil covered with turf 0.30 – 0.55
Impervious soil 0.40 – 0.65
Gravel & WBM roads 0.35 – 0.70
Bituminous & C.C roads 0.80 – 0.90

DRAINAGE AREA, Ad
If the drainage area consists of several types of
surfaces with different values of run-off coefficients
C1, C2, C3,…. and if their respective areas are A1 , A2,
A3,….. the weighed average value of run-off
coefficient, C is determined from the equation:
C = (A1 C1+A2 C2+A3 C3) / (A1+A2+A3)
21

DESIGN VALUES OF RAINFALL INTENSITY, i
To find the intensity of rainfall ‘i’, first we need to know the time taken by
water to reach drainage inlet from the drainage area. This can be found out from the
below graph. This is called as Inlet time.
22

Now we need to calculate the time required for water to
travel from inlet of drainage to the outlet which is called as
travel time This is calculated from the velocity allowed in the
drainage line and generally it is kept in between 0.3 – 1.5
m/sec.
After that both times (inlet time and travel time) are
added which finally gives us the time of concentration. From
this total duration, read the rain fall intensity from the below
graph by assuming frequency of rainfall occurrence (say for 5
years, 10 years etc.)
23

EXAMPLE
The distance between the farthest point in the turf covered drainage area (with
an average slope of 1.5% towards the drain) and the point of entry to side drain is
200m. The weighed 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
540m. The velocity of flow in the side drain may be assumed as 0.6m/sec so that silting
and erosion are prevented. Estimate the design quantity of flow on the side drain for a
10- years period of frequency of occurrence of the storm.
Solution:
25

HYDRAULIC ANALYSIS OF HIGHWAY DRAINS
In hydraulic analysis of highway drains the dimensions of drainage
channels or culverts are designed based on ‘Q’ obtained in the
Hydrologic analysis. Now we have discharge, which is nothing but
designed run off ‘Q’.
Now, if we know the allowable velocity ‘V’ in the channel, then
the area of channel can be calculated from below formula:
Q = A.V
But the allowable velocity is not same for all types of channels. If the
channel is lined, then the allowable velocity can be kept at normal. But if
the channel is unlined it may cause severe damage to the channel in the
form of silting or scouring.
27

The allowable velocity of flow depends on the soil type of the open side
drain.
So, the allowable velocity for different cases of unlined materials is as
follows:
Now we can find out the area of channel in m2 .
28
Soil type Allowable velocity (m/sec)
Sand or silt 0.30 – 0.50
Loam 0.60 – 0.90
Clay 0.90 – 1.50
Gravel 1.20 – 1.50
Soil with grass 1.50 – 1.80

Next, the longitudinal slope of channel ‘S’ is to be calculated by
Manning’s formula:
Where, V = Allowable velocity (m/sec)
n = Manning’s roughness coefficient
R = Hydraulic radius (m)
S= Longitudinal slope of channel
In the above formula, we already know the ‘V’ value. Hydraulic
radius ‘R’ is the ratio of area of the channel to its wetted perimeter. Now
comes, the roughness coefficient values depend on the type of soil in
unlined channels. 29

Lining material Manning’s roughness coefficient, n
Ordinary soil 0.02
Soil with grass layer 0.05 – 0.10
Concrete lining 0.013
Rubble lining 0.04
30
Finally, longitudinal slope “S” is known and all the dimensions of
drainage channel are known. Thus, the design of surface drainage system
is complete. This method is mostly used for designing side drains of
roads.

EXAMPLE
The maximum quantity of water expected in one of the open
longitudinal drains on clayey soil is 0.9m³/sec. Design the cross section
and longitudinal slope of trapezoidal drain assuming the bottom width of
the trapezoidal section to be 1.0m and cross slope to be 1.0 vertical to 1.5
horizontal. The allowable velocity of flow in the drain is 1.2m/sec and
Manning’s roughness coefficient is 0.02.
Solution:
31

SUB-SURFACE DRAINAGE SYSTEM
Moisture changes in the subgrade occur due to
percolation of rain water and seepage flow, as also due to the
phenomenon of capillary rise. The aim of subsurface drainage
is to keep the ground water table (GWT) sufficiently below the
level of the subgrade – at least 1.2 m.
When the water table is almost at the natural ground
surface, the best option is to raise the formation of the
roadway on an embankment, such that it is 1.2 m above the
ground. If this is not possible for the reason of unfavourable
topography, the only option is to lower the ground water table
by means of subsurface drainage arrangements. 35

FEW DRAINAGE ARRANGEMENTS FOR
DIFFERENT SITUATIONS
Lowering of water table:
1) Longitudinal Drain Trenches and Pipes:
If the soil is relatively pervious, longitudinal drainage trenches with drain
pipe, backfilled with filter sand can be used. The depth of the trench
depends on the extent of lowering required, soil type, and distance
between the trenches.
36

2) Longitudinal and Transverse Drains for Lowering GWT:
If the soil is relatively less permeable, longitudinal as well as transverse
drains may be needed to lower the ground water table.
37

Control of Capillary Rise :
Two methods:
1) Granular Capillary Cut-off.
2) Impermeable Capillary Cut-off.
1) A layer of granular material of suitable thickness is provided during the construction of
embankment, between the subgrade and the highest level of subsurface water table. The
thickness of the granular capillary cut-off layer should be sufficiently higher than the
anticipated capillary rise within the granular layer so that the capillary water cannot rise
above the cut-off layer.
2) The capillary cut-off may even be an impermeable bituminous layer.
• The location of the cut-off should be above the level of capillary rise expected for the
subgrade. 38

Control of seepage flow:
When the general ground level as well as the impervious strata
below are sloping, seepage flow is likely to exist. If the seepage zone is
at depth less than 0.6m to 0.9m from the subgrade level, longitudinal
pipe drain in trench filled with filter material and clay seal may be
constructed to intercept the seepage flow.
39

DESIGN OF SUBSURFACE DRAINAGE SYSTEM
The size and spacing of the subsurface drainage system
would depend on the quantity of water drained off, the type of
soil and type of the drains. Mostly this is decided based on
experience and other practical considerations.
However proper filter material should be used for back
filling the drainage trenches and also for use in all subsurface
drainage system.
40

DESIGN OF FILTER MATERIAL
The filter material used in the subsurface drains should
be designed to have sufficient ‘Permeability’ offering
negligible resistance to the flow. The filter material should be
designed to resist the flowing of the fine foundation soil
resulting in problems like ‘ Piping’.
Hence the grain size distribution of the filter material is
decided based on these two criteria's:
1) Permeability and
2) Piping.
41

PROCEDURE FOR DESIGN OF FILTER
42

EXAMPLE
A pipe drain with circular perforated holes 10mm diameter is
to be designed. The soil gradation is given in the table below.
Design the filter material.
44
Sieve size Percentage passing
1.18 mm 100
425 micron 93
300 micron 85
150 micron 60
75 micron 15
53 micron 7

45
D15 (Protected soil) = 75 micron = 0.075 mm
D15 (Filter) ≥ 5 X 0.075 ≥ 0.375 mm
D85 (Protected soil) = 300 micron = 0.3 mm
D15 (Filter) ≤ 5 X0.3 ≤ 1.5mm
These two points have been plotted in Fig.12.29, as A and B
Size of perforated hole = 10 mm dia.
D85 (Filter) > 2 X 10mm > 20mm
This point is marked in the figure as C. A suggested grading
is freely sketched to lie to the right of point C and passing in
between points A and B.

CROSS DRAINAGE STRUCTURES
Roads have to be aligned often as to cross natural drainage
channels, streams and major rivers. Sometimes, the alignment will be
across man-made channels like those for irrigation.
In such cases, the need for constructing cross drainage structures
arises to ensure that the water flows beneath the road without causing
any inconvenience or instability to the highway structure.
Types of Cross-drainage Structures:
1. Culverts (waterway less than 6 m)
2. Minor bridges (waterway from 6-30 m)
3. Medium-sized bridges (waterway from 30-100 m)
4. Major bridges (waterway more than 100 m)
5. Causeways 47

The common type of culverts in use are:
i) Slab Culvert: In slab culvert RCC slab is placed over
abutments made of masonry and the span is generally
limited to 3m.
ii) Box Culvert: Box culvert of square or rectangular shapes
is made of RCC.
iii) Arch Culvert: Arch culvert is generally built using brick or
stone masonry or plain cement concrete.
iv) Pipe Culvert: Pipe culverts of minimum diameter 75cm
and made of steel or prefabricated RCC is used when the
discharge is low. 48

Various types of bridges are in use; the choice is based
on several considerations including span. Now a days RCC
and pre-stressed concrete bridges are commonly constructed .
On less important roads, in order to reduce the
construction cost of cross drainage structures, sometimes
submersible bridges or Cause-ways’ are constructed; during
the flood the water will flow over the road at the locations of
cause ways. The total period interruption to traffic has
however to be kept as low as possible, not exceeding about
15days in a year. Such roads where interruption to traffic
occur during floods are called ‘Fair weather roads’.
49

HIGHWAY USER BENEFITS
A highway affords several benefits to the general public or the
road-users.
Classification of Highway user benefits:
1) Direct or Primary or Tangible or Quantifiable Benefits
2) Indirect or Secondary or Intangible or Non-quantifiable
Benefits.
Primary benefits can be quantified in terms of their monetary
value, but secondary benefits cannot be treated thus.
51

PRIMARY OR DIRECT BENEFITS
(i) Reduction in Vehicle Operation Cost (VOC)
(ii) Increase in revenue from the motor vehicles
(iii) Saving in travel time
(iv) Reduction in accident rate
(v) Benefits to general public
52

SECONDARY OR INDIRECT BENEFITS
(i) Increase in comfort, convenience and safety of the road-
user.
(ii) Increase in general amenities, medical services, social,
educational and recreational facilities.
(iii) Increased value of natural resources owing to convenient
access by road.
(iv) Improved mobility of essential goods and services,
making them accessible even to remote places.
(v) Improved mobility for defence forces contributing to the
security of the country.
(vi)Improved mobility enabling reduced suffering and pain of
those involved in road accidents, fire hazards and other
calamities. 53

VEHICLE OPERATION COST (VOC)
Vehicle operation cost is the cost of owning and
operating vehicle due to its use on roads.
Factors affecting vehicle operating cost (VOC):
1) Vehicle Type
2) Vehicle Speed
3) Speed Changes
4) Gradient
5) Curvature
6) Road Surface
54

EFFECT OF ROAD CONDITION ON VOC
55
PSI – Present Serviceability Index
VOC – Vehicle Operating Cost

ECONOMIC ANALYSIS
Economic analysis of a highway improvement aims at determining the
monetary benefits to the road users and others due to the additional
expenditure.
The following are some of the specific objectives in carrying out an
economic analysis:
1) Whether the plan under consideration is worth investment or not.
2) To rank schemes competing for scares resources in order of priority.
3) To compare mutually exclusive schemes and select the most economic
one.
4) To assist in phasing the programme over a time period depending
upon the availability of resources. 56

METHODS OF ECONOMIC ANALYSIS
There are several methods of economic analysis of
highway projects. Some of the simple methods are:
1) Annual-cost method
2) Benefit-cost ratio method
3) Net- present value (NPV) method
4) Internal rate of return(IRR) method
57

ANNUAL COST METHOD
The annual cost of each element of capital improvement is found by
multiplying by the appropriate Capital Recovery Factor (CRF) value calculated
for the assumed life span.
This is based on the capital recovery formula of uniform series.
A = P (CRF)
Here, A stands for annual cost
P = The present capital.
n = Number of years of design life of the component
i = Appropriate interest rate for the component.
The average annual costs of different alternatives/proposals of a highway
scheme/project are worked out and compared.
The alternative for which the average annual cost is the lowest is the most
economical option. 58

BENEFIT- COST RATIO METHOD
The principle of this method is to assess the merit of a particular scheme by comparing
the annual benefits with the increase in annual cost.
Benefit cost ratio = Annual benefits from improvement / Annual cost of the
improvement
= (R-R1)/(H1-H)
Where, R = Total annual road user cost for existing highway
R1 = Total annual road user cost for proposed highway improvement
H = Total annual cost of existing road
H1 = Total annual cost of proposed highway improvement.
The benefit-cost ratios are determined between alternate proposals and those plans
which are not attractive are discarded. Then the benefit cost ratios for various
increments of added investment are computed to arrive at the best proposal. In order to
justify the investment, the ratio should be greater than 1.0
62

EXAMPLE
63
It is proposed to widen a stretch of a single lane road of length 40km
to two lanes with earthen shoulders at a total cost of Rs.125 lakhs per
km and the rate of interest is 10% per year. The annual cost of
maintenance of the existing single lane road is Rs.21,000 per km and
that of the improved two lane road is Rs.75,000 per km. The average
vehicle operation cost on the existing road is Rs.4.0 per vehicle-km
and that on the widened road is estimated to be Rs.3.0 per vehicle-
km. If present traffic is 6000 motor vehicles per day and by the end
of 15 years design period the traffic is estimated to be doubled,
determine whether the investment on the improvement of the road is
economically viable, during the 15 years period.

NET PRESENT VALUE (NPV) METHOD
In this method, the stream of costs/benefits associated with the project
over an extended period of time is calculated and is discounted at a
selected discount rate to give the present value. Benefits are considered
positive and costs negative, and their summation gives their net present
value (NPV). Any project with positive NPV is treated as acceptable. In
comparing more than one project, a project with the higher NPV should
be accepted.
Where, NPV0 = Net present value in the year
Bn = Value of benefits which accrue in the year n
Cn = Value of costs which occur in the year n
i =discount rate per annum
n = number of years considered for analysis 65

EXAMPLE
The cost of improving an existing road , 25Km long is
Rs.4.00 lakhs per km. The i) road user costs, with and
without the improvements, ii) Accident costs, with and
without improvements and iii)Maintenance costs, with and
without improvements are given for a 10 year period after
the completion of the improvements. Assuming a discount
rate of 10 percent, find out whether the project is
economically justifiable. Use the NPV method.
66

INTERNAL RATE OF RETURN (IRR) METHOD
The internal rate of return is the discount rate which makes the
discounted future benefits equal to the initial outlay. In other words, it is
the discount rate which makes the stream of cash flows zero.
Assuming B0 = 0,
69

• This equation may be solved by trial and error, but it is
rather tedious. With a computer programme, the solution
becomes very simple.
• If the IRR obtained is greater than the rate of interest
obtainable by investing the capital in the open market, the
project is considered acceptable.
70

EXAMPLE
An investment of 1,36,000 yields the following cash
inflows. Determine the internal rate of return.
71
Year Amount , Rs
1 30,000
2 40,000
3 60,000
4 30,000
5 20,000
Total 1,80,000

HIGHWAY FINANCING
Highway financing is the aspect of raising the funds necessary for the
construction and maintenance of a new highway project or
improvements to an existing road.
The construction of highway and their maintenance is the
responsibility of the Government in India, as it is in many other
countries. The funds necessary for this are realised from the main
beneficiaries – the road-users – in the form of direct and indirect taxes.
Two general methods of highway financing are:
(i) ‘Pay-as-you-go’ method.
(ii) ‘Credit Financing’ method.
76

(i) ‘Pay-as-you-go’ method :
In pay-as-you-go method, the payment for highway
improvements, maintenance and operation is made from the central
revenue. The automobile users pay indirect levies in the form of taxes
and other levies on petrol, diesel, oil, tyres, spare parts, etc.
(ii) ‘Credit Financing’ method :
In credit financing method, the payment for highway
improvement is made from borrowed money and this amount and the
interests are re-paid from the future income. The income after
completing the road up-gradation project may be from toll
collections.
77

BOT AND BOOT CONCEPTS
BOT and BOOT are the Type of PPP (Public Private
Partnership) Models.
78

PUBLIC PRIVATE PARTNERSHIP (PPP)
With a view to attract private investment in road
development , maintenance and operation, National
Highways act (NH Act) 1956 was amended in June 1995.
In terms of the amendments made, the private sector can
invest in the NH projects, levy collect and retain fee from
the road users; they are also empowered to regulate traffic
on such highways in terms of provisions of Motor Vehicle
Act-1988.
Several incentives have been announced by the
government to attract private sector participation and
foreign direct investment. Theses schemes are called as
‘Public Private Partnership’ or PPP schemes.
79

BUILD OPERATE AND TRANSFER (BOT)
• In this model of public-private partnership, private
partner has to build, operate, maintain the facility for the
period specified in the agreement.
• In most cases, the private partner has to arrange part or
full finance for the project, which can be recovered by
revenue generated by people using those services or it is
given by the government in the form of an annuity.
• As a private partner doesn't own the property he has to
pay monthly or yearly fees to the public partner during
the operation period.
• As in this project, the risk is mainly taken by the private
partner and also arranging finance is difficult task.
80

BUILD, OWN, OPERATE, AND TRANSFER (BOOT)
• In this PPP model, the private partner has to build, operate
and then transfer the facility to the public partner after the
stipulated period of time mentioned in the contract
agreement.
• It is different from the BOT in the sense that in the BOOT
model private partner owns the facility and therefore
doesn't required to pay a fee to the public partner during
the operation period.
81

Module 5: Highway Drainage and Highway Economics

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Module 5: Highway Drainage and Highway Economics