Highway drainage system and how it works

1. Highway Drainage
Transportation Engineering - I

2. Highway Drainage
• A means by which surface water is
removed from pavement and ROW
• Redirects water into appropriately
designed channels
• Eventually discharges into natural
water systems

3. Inadequate Drainage
• Damage to highway structures
• Loss of capacity
• Visibility problems with spray and
retro-reflectivity
• Safety problems, reduced friction and
hydroplaning

4. Highway Drainage
• Transverse slopes
– Removes water from pavement surface
– Facilitated by cross-section elements
(cross-slope, shoulder slope)
• Longitudinal slopes
– Minimum gradient to maintain adequate
slope in longitudinal channels
• Longitudinal channels
– Ditches along side of road to collect
surface water after run-off

5. Transverse slope

6. Longitudinal slope

7. Longitudinal channel

8. Drainage System
Three phases
1. To Estimate the quantity of water to
reach the system
2. Hydraulic design of system elements
3. Comparison of different materials to
serve the purpose
Steep slopes provide good hydraulic
capacity and lower ROW costs, but
reduces safety and increases
erosion and maintenance costs

9. Hydrologic Analysis
Q = CIA (english) or Q = 0.0028CIA (metric)
Q = runoff (ft3
/sec) or (m3
/sec)
C = coefficient representing ratio or runoff to
rainfall
I = intensity of rainfall (in/hour or mm/hour)
A = drainage area (acres or hectares)

10. Undivided traveled ways (two- and multilane)
on tangents and flat curves have a crown in the
middle and slope downward toward both edges
(camber). The downward cross slope may be a
plane or rounded section (parabolic), or a
combination of the two.
One-way traveled ways on divided highways
may be crowned separately or may have a
unidirectional cross slope/cross fall.
Transverse Slope

11. Transverse Slope

12. Transverse Slope

13. Transverse Slope

14. Crowns vs. Unidirectional Slopes
Type of
Roadway
Pros Cons
Crowned
separately
•rapid drainage during
rainstorms
•difference between low and
high points is minimal
•inlets and underground drainage
(drainage towards the median)
•difficult design of at-grade
intersection elevation
•use of such sections should be
limited to regions with high rainfall
Unidirectional
cross slopes
•more comfortable for drivers
changing lanes
•drainage away from the
median saves inlets and
drains
•simplifies treatment of
intersections
•drainage is slower
•difference between low and high
points of the cross section is
larger

15. Contradictory design controls
A steep lateral slope reduces water ponding and the width of the water
flow along the curb.
A flat lateral slope reduces vehicles' drift towards the low edge.
Recommended design controls
Lateral drift of vehicles at high speed is barely perceptible on cross slopes
up to 2%. The slope of 1.5-2.0% is acceptable on high-speed highways. In
the areas of intense rainfall a maximum cross slope is 2.5%.
Crown section
Change in the cross slope of 3-4% causes swaying of high body vehicles.
Rounded crowns reduce discomfort.
Cross Slopes on Tangents

16. Curbed Highways
The minimum slop values of 1.5-2 % in areas with intense
rainfalls will cause wide sheet of water on the curbed
traveled way.
Possible improvements:
• parabolic cross section with increasing cross slope
towards the outer edges,
• gutter along the curb with the cross slope larger than on
the traveled way,
• on multilane traveled way, cross slope broken along
traffic lane edges, increasing from the minimum value on
the innermost lane up to the maximum value on the
outermost lane. This solution is used on uncurbed sections
as well.

18. Drainage Channels and
Sideslopes
Design considerations of highway drainage
includes
• safety
• good appearance
• control of pollutants
• economy in maintenance
This can be achieved by applying
• flat side slopes
• wide drainage channels
• rounding

19. Drainage Channels
Types of Drainage Channels
814
812
810
808
806
804
802
800
794
798
800
802
804
806
808
Roadside
channel
Intercepting
channel
FlumeToe-of-slope
channel

20. Drainage Channels
Drainage channels should:
• have adequate capacity for the design
runoff,
• minimize damage to the highway caused
by unusual storm water,
• minimize risk for motorists,
• be resistant to the high speed water flows
where expected,
• prevent sedimentation of the particles
carried by water.

21. Side slopes
Side slopes should:
• insure the stability of the roadway
• provide opportunity for recovery of an out-of-control
vehicles

22. Roadside Channels
• Steep sides improve hydraulic efficiency and reduce right of way
costs
• Flatter sides improve slope stability and traffic safety, reduce
maintenance costs
• Sideslopes 1:4 or flatter provides a good chance of recovery for
errant vehicles and relax drivers' tension (roadside channel is
visible to drivers)
• Sideslopes of 1:5 or 1:6 are recommended in the flat areas
• Intercepting channels have a flat cross section form by a dike
made with borrow material
• Median drainage channels are shallow depressed areas with
inlets
• Flumes are open channels or pipes used to connect intercepting
channels or shoulder curbs with roadside channels
• Channel lining prevents channels erosion caused by fast stream
of water.
Examples: grass (where possible), concrete, stone etc.

23. Side slopes
Safety consideration
• Rounded hinge point reduces the chance of an errant
vehicle becoming airborne
• Foreslopes 1:6 or flatter can be negotiated by errant
vehicles
• Foreslopes 1:3 with liberal rounding provide a good chance
for recovery
• Slopes steeper than 1:3 can be used only where justified by
local conditions. The use of roadside barriers should be
considered
Maintenance consideration
• Flat and well-rounded side slopes simplify establishment of
turf and its maintenance
• Slopes 1:3 or flatter enable the use of motorized equipment

24. Side slopes
Other rules
Flat, well-rounded side slopes create a streamlined
cross section. Advantages for the streamlined
cross sections are:
• natural, pleasant appearance,
• improved traffic safety,
• snow drift prevented,
• easy maintenance.
Retaining walls should be considered where slopes
would be steeper than 1:2.
Standard slope for rock cuts is 2:1. In good-quality
rock, slopes ranges from 6:1.

25. Sideslopes