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CULVERTS
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IntroductionCulverts are commonly used both as cross-
drains for ditch relief and to pass water under a road at a natural drainage and stream crossings. In either case, they need to be properly sized and installed, and protected from erosion and scour.
Designed culverts need to have sizes large enough to pass the expected flow plus extra capacity to pass debris without clogging.
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IntroductionDischarge (design flow) of a culvert will depend on the:Watershed drainage areaRunoff characteristicsdesign rainfall intensity return period(frequency) of the design
storm.
The general purpose of a culvert is to transmit water flowing in natural streams or collected on the high side of the right-of-way from one side of the road to the other. 11
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A CulvertB HeadwallC WingwallD Apron
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Common Culvert SizesThe capacity is the most important deciding
factor – whichever is locally available and can be obtained at lower prices.
In Ghana pipe culvert sizes usually available for roads are 900mm, 1200mm & 1800mm.
A size 600mm can however be used where maintenance is not likely to be a problem.
Minimum allowable size is 900mm for reasons of maintenance.
For box culverts the minimum allowable is 1000mm x 1000mm.
Also available in Ghana is 2000mm x 1200mmMaximum of about 4000mm x 2000mm
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Field InvestigationsFor the purpose of analysis, culverts along a
road may be classified into two groups – major and minor culverts.
Major Culverts: All culverts whose total waterway cross-sectional areas are greater than 5m2 or whose total widths/span are 4m and above are classified as major culverts.
Minor Culverts: All culverts with spans less than 4m or total flow area less than 5m2 are classified as minor culverts.
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Field Investigations
Generally, inventory of existing structures will be taken. This will usually consist of the types of cross drains – bridges/culverts and longitudinal.
Type of cross drain1. Span of cross drain2. Size of drain. Adequate? Or increase?3. Material of longitudinal drain – lined or
unlined4. Structural competence5. Hydraulic competence6. Kind of maintenance required
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Field InvestigationsWhere an existing culvert is structurally
competent and can perform efficiently for the design life of the project, it is recommended that it be desilted, repaired, extended, etc. if found necessary. However, where an existing culvert is found to be structurally inadequate, they are constructed to not less than the dimensions and capacities of those they replace.
Pipe culverts less than 900mm diameter are replaced or supplemented with 900mm diameter culverts to meet the MRT/GHA current requirements for the minimum size required for cross-drainage pipe culverts.
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Design criteria for culvertsSpecifications approved by the
Ghana Highway Authority are used for the design of culverts. The return periods for the estimation of the design discharges through culverts are taken as follows.Highway System Return Period
International 50 years
Regional 25 years
Local 5 – 10 years
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Design criteria for culverts
Spanof Culvert Return Period
0.9 – 4.0 (minor culverts) 10 years
4 – 10m 25 years
10m and above 50 years
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Culvert Design – Sizing of Culverts
The purpose of hydraulic design of culverts is to provide a drainage facility or system that will adequately and economically drain away the estimated flow throughout the design life without unreasonable risks to the roadway structure or nearby property.
A Secondary objective is the prevention of scouring at the culvert outlet.
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Whenever a constriction such as a culvert is placed in a natural (open channel) stream, there is an increase in the depth of water just upstream of the constriction.
The allowable level of the headwater upstream of the culvert entrance is generally the principal control on the culvert size and inlet geometry. The allowable headwater depth depends on the topography and the nature of land use in the culvert vicinity.
In establishing the allowable headwater depth, consideration should be given to the possible harmful effects that flooding may cause, such as damages to the pavement, interruptions to traffic, and inundation of nearby property.
From experiments it is known that culvert hydraulics shows that there are two major types of culverts flow: flow with inlet control and flow with outlet control.
Culvert Design – Sizing of Culverts
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What is the difference between Bridge and Culvert?
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Inlet ControlOccurs when the culvert barrel can
convey more flow than the inlet will accept. The flow is only affected by headwater level, inlet area, inlet edge configuration, and inlet shape. Factors such as roughness of the culvert barrel, length of the culvert, slope and tailwater level have no effect on the flow when a culvert is under inlet control.
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Outlet Control Occurs when the culvert barrel can not convey
more flow than the inlet can accept. The flow is a function of the headwater elevation, inlet area, inlet edge configuration, inlet shape, barrel roughness, barrel shape and area, slope, and tailwater level.
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Inlet Control in design
Most often inlet conditions exist therefore most designs are based on inlet flow control.
Nomographs are available for the design of inlets control when the slope of the culvert is steep and the outlet is not submerged.
Under inlet control, the discharge capacity of a culvert depends primarily on the depth of headwater at the entrance and the size and entrance geometry (barrel shape, cross-sectional area, and type of inlet edge).
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Design Considerationsminimizing channel modifications;avoiding constriction of the bankfull flow
channel width;maintaining the natural grade and
alignmentusing quality, well compacted bedding and
backfill materialusing inlet, outlet, and stream bank
protection measuresTrash racks are often desirable in channels
with significant amounts of debris to prevent pipe plugging 25
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Design Considerations
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Concrete box culvert with concrete wing walls for inlet/outlet protection and fill retention.
Normal metal culvert installation using riprap around the inlet and outlet of culverts
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Design ConsiderationsFlared ends improve efficiencyUse culverts as wide as stream widthUse same gradient as stream channelUse same alignment as stream channelSingle large culvert is better for debris
passage than several small onesArea Reduction Factor for areas >2km2 the
area should be multiplied by Cr.\
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Design ConsiderationsUse single large pipes or a concrete box in
favor of multiple smaller diameter pipes to minimize plugging potential in most channels (unless roadway elevation is critical). In very broad channels, multiple pipes are desirable to maintain the natural flow spread.
Align culverts in the bottom and middle of the natural channel so that installation causes no change in the stream channel alignment or stream bottom elevation. Culverts should not cause damming or pooling or increase stream velocities significantly.
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Poor – Single pipe concentrates flow in the broad channel or floodplain.
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Better – Multiple pipes disperse the flow across the channel. Middle pipe may be slightly lower to pass the normal low flow and to promote fish passage.
Design Considerations
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Design Considerations
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Poor Make road crossings of natural drainages perpendicular to the drainage to minimize pipe length and area of disturbance
Adequate
Best
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Design ConsiderationsAt culvert outlets where pipe velocities are
accelerated, protect the channel with either a plunge pool (on gentle slopes), rock armoring (riprap) or with a splash apron with a rough or rock inset surface and cutoff key.
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Design ConsiderationsAvoid culvert outlets in the middle of a fill
slope. Use culverts long enough to extend to the toe of the slope, or use headwall structures to retain the fill material and minimize the pipe length.
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Design Discharge
This is calculated using the Rational method.Q = CIA/3.6 = 0.278CIAQ = design discharge (m3/s)C = Runoff coefficientI = Rainfall intensity for the given return period
(mm/h)A = Catchment area (km2)
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Catchment Areas
Catchment areas used in the Rational method are mapped out from topographic sheets obtained from the Survey Department of Ghana (1:10,000 – 50,000).
The areas are calculated using a planimeter; and the slope for each catchment area is also determined from the topographical sheets – profiles, GIS, etc.
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Typical I-D-F equations for Kumasi and Wenchi
For Kumasi
For Wenchi
I in (mm/hr) and Tc in hrs is given by
0.7T 140 = I
C10 0.7T
165 = IC
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0.44T 96 = I
C10 0.44T
116 = IC
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Time of Concentration
S A
58.5L = Tc 0.20.1
Tc = Time of concentration (min)L = Catchment length (km)A = Catchment Area (km2)S = Catchment slope (m/km)
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Nomograph Design
In built up areas Hw/D = 0.8 – 1.0, this is to avoid any ponding leading to overflow
For Feeder roads, the rule of thumb is usually Hw/D = 1.2
Select a suitable D, select suitable Hw/D for the area under design.
For example D = 900mm, Hw/D = 0.8Q=0.68m3/s from Chart
Draw a line between D and Hw/D. The intersection is the value of Q
If Q graph > Q rational then the dimensions are acceptable
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Nomograph Design
Different charts are available for box and pipe culverts.
The charts are also based on the type of material used and the design of the entry
If multiple barrels are to be used there are specific charts available for that too.
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