Highway Drainage – prime objective to maintain all parts of the highway in
an excellent drainage condition. To prevent slip and congestion accidents
Hydrologic and Hydraulic requirements – among the facets must be
considered during the early phase pf the design process.
Highway Alignment- and its relationship with the drainage system is the
foremost concern of the hydraulic engineer
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Horizontal Alignment – where stream crossings will occur and where
the traverse and longitudinal encroachments
Vertical Alignment – commonly the profile on highway drainage
facilities; must be asses in comparing alternate locations;
determines the level of upstream flooding
1. Depressed roadways – acts as drainage interceptors;
most likely to cut through existing storm drainage
2. Sag Vertical Curves – critical profile areas, serve to trap
highway drainage
3. Steepness of the Highway – determines the spacing of
inlets in areas of roadway with curbs
Three types of Drainage Systems:
1. Surface Drainage
2. Groundwater Drainage
3. Structural Drainage
Basic Design Data – planning and design of drainage facilities should always
be preceded by accurate field surveys
Catchment area – determine the basin boundary from a 1:50,000 mao from
NAMRIA
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Rainfall Analysis – includes the formation of design hyetographs for
hydrological analysis
Rational Formula – most widely used methods for estimating peak flows for
smaller catchments; Area limits: 5km^2 for urban and 20km^2 for rural.
Qp = cIA/3.6
Coefficient of run off “c” – percentage to which the peak run off is reduced
due to transitionary storage’ ratio of run off to rainfall
Design Storm Frequency – considers estimates of the magnitude of floods
based on frequency of occurrence
Culvert – convey surface water across or from the highway right-of-way;
carry construction and highway traffic, and earth loads.
Culvert Location – horizontal and vertical alignment of the culvert with
respect to both the stream and highway
Deposition – the sediment transport capacity of flow within the culvert is
often less than the stream.
Closed Conduits – designed for the condition of flowing full
Hydrologic Analysis – most important step prior to hydraulic design of a
highway drainage structure
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Peak Discharges – occur at the peak of the stream’s flood hydrograph
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Estimation of Discharge
1. Catchment delineation
2. Design rainfall analysis
3. Choice and use of hydrological analysis techniques
Planimeter
Triangulation
Cross-section mm paper
CAD/GIS software
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Reinforced Concrete Pipes – most widely used conduits for carrying
water under the roadway
Box Culverts – adaptable to uses other than drainage such as
underpass for vehicles, pedestrians, or animals
Wing walls – where flow currents approach the inlet at an angle
Headwalls – stabilize the fill slopes at culvert entrance; prevents
undermining and sloughing of the bank into the stream
Velocity Limits
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0.8 m/s – minimum velocity for culverts
5 m/s – max velocity for piped drainage system
Minimum Sizing
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910mm – culverts under local roads
1m – under expressway
Cover
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0.6m – of fill over the top of the pipe
Shape and Cross Section
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Circular – most commonly used culvert shape is circular; preferred
due to available structural options for various fill heights
Pipe arch and elliptical shapes – used in lieu of circular pipe where
there is limited cover or overfill
Box or Rectangular – designed to pass large flood and to fit nearly
any site construction
Arches – less obstruction to a waterway is a desirable feature and
where foundations are adequate for structural support.
Multiple Barrels – useful in wide channels where concentration of
flow is to be kept to a minimum
Outlet Scour Control – to reduce flow velocities prior to discharging to
watercourses in order to avoid erosion
Rock pad outlets – commonly adopted for culvert outlets.
Material for Culverts:
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Concrete
Iron
Wood
Stainless Steel
Plastic
Steel
Culvert End Structures – are attached to reduce erosion etc
Projecting – when the culvert barrel extends beyond the face of the
roadway embankment
Mitered- is cut to conform with the plane of the embankment slope
Pipe End Sections – prefabricated metal or precast concrete sections
Headwater – the total flow depth in the stream measured from the culvert
inlet invert
Tailwater – is the flow depth in the downstream channel measured from the
invert at the culvert outlet
Two major conditions of Culvert Flow:
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Flow with inlet control
Flow with outlet control
Highway Storm Drain system – collect, convey, and release storm water
runoff.
Inlets – enable storm water to be removed from the roadway area
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Grate inlets – commonly used inlet structures which are available in
a wide variety of shapes and sizes
Weir or Curb-opening Inlets – primary opening in the face of the
curb.
Slotted Drainage inlets – intercept sheet flow, gutter flow with or
without curbs, modify existing drainage systems
Combination Inlets -combination of all
Head loss – energy change in momentum or turbulence in inlets
Hydraulic Gradeline – last important feature; aids the designer in
determining the acceptability of the proposed system.
Manhole – provide access to a storm drain for inspection and maintenance
Junction Chambers – used to connect two or more separate storm drains
together. Ditches and Gutters
Pipe Underdrains – known as French Drains, a trench loosely backfilled with
graded stones . Stabilization Trenches – stabilize saturated foundation area.
Pavement – portion of the road, place above the subgrade and to provide a
running surface for vehicles
Rigid – layers of cement concrete. two layers only; Pavement slab & Subbase Course
Pavement Process – process of determining the thickness and strength of a
pavement laid on a soil foundation
Pavement slabs – maybe constructed without reinforcements. Designed as
continuous structural elements or jointed structural elements
Type of Pavement
Sub-base Course – provides a stable working platform of a rigid pavement
structure consists of one or more compacted layers of granular or stabilized
material placed between the subgrade and the rigid slab
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Flexible
Rigid
Unbound, gravel surfaces, unsealed or unpaved roads
Flexible Pavements
1. Bituminous surface course
2. Base course
3. Sub-base course – surface of subgrade is the formation level
Essential to know the basic limitations:
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Empirical Soil support scale
Soil support Value
Structural Number
Regional Factor
Traffic analysis
Bituminous Surface Course – surface course of a flexible pavement structure
consists of mineral aggregates and bituminous materials
Bituminous Prime Coat – a prime coat is an application of low viscosity
cutback bituminous asphalt to an absorbent surface
Bituminous Tack Coat – a tack coat is a very light application of bituminous
asphalt. Usually with water, to bond between surface being paved and
overlying course
Designing new road pavements:
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Traffic
Subgrade support strength
Water
Pavement material performance
Quality of construction
Common materials used:
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Portland Cement Concrete
Reinforcing Steel
Joint Sealing Materials
1. Liquid Sealants – asphalt, hot-poured rubber, polymers
2. Performed Elastomeric Seals – neoprene seals
Development of the Rigid Pavement Design Procedure
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Modulus of Subgrade Reaction (K) – equation valid for only one
value of modulus of subgrade reaction
Westergaard’s Modulus of Subgrade reaction (k) – loads in pounds
per square inch
Concrete properties
Modulus of Elasticity (Ec) of 29 GPa – average value for concrete
Base Course – portion of flexible pavement immediately after the surface
course; structural portion of the pavement
Expansion Joints – prevent the development of damaging compressive
stresses due to volume change
Sub-base Course – portion of flexible pavement between the subgrade and
base
Contraction Joints – provide for an orderly arrangement of the cracking that
occurs
Longitudinal Joints – used to prevent the formation of irregular longitudinal
cracks
Load transfer Devices
Tie Bars – designed to hold the faces of abutting slabs in the firm contact;
withstand max. tensile force
Pavement structure – layered system designed to distribute concentrated
traffic loads to the sub grade
Resilient Modulus – measures the stiffness and it is actually an estimate of
the Modulus of Elasticity (elastic stiffness)
Axle Load – portion of the total vehicle load transmitted to the road though
a single axle
Load equivalence Factor (LEF) – damaging effect per pass to a pavement by
a type of axle to the damage per pass of a standard axle
Design Reliability (R) – the probability that a pavement section using the
process will perform satisfactory over the traffic and environmental
conditions for the design period
Serviceability – defined as the ability to serve high-speed, high-volume
automobile and truck traffic.
Load Transfer Coefficient – factor used in rigid pavement design to account
for the ability of a concrete pavement structure to transfer load
Jointed Pavements (J) – recommended for a plain jointed pavement or
jointed reinforced concrete pavement with some type of load transfer
Stabilized Subgrade – improve the performance of plain and reinforced
jointed pavements