The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Minhdu Nguyen by e-mail or on (08) 9323 4541.
To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.
Main Roads Standard culvert drawings show skewed culverts with the wingwalls on a skew but the headwall parallel to the road centerline to maintain a constant offset from road centerline to the culvert headwall.
As shown on Figure 2.1 and 2.2, two flow conditions are possible in culvert flow and they are referred to as inlet control and outlet control flow conditions. Inlet control refers to the situation where the capacity of the culvert is entirely influenced by inlet factors such as the width of the inlet and approach conditions. The water surface profile within the culvert approaches the normal depth profile and the culvert behaves as an open channel.Outlet control refers to the situation where a combination of the culvert size, roughness and tailwater conditions influence the discharge capacity of culvert. The flow conditions are similar to pressurised pipe under full flow condition. In the design process, the headwaters corresponding to both inlet control (HWi) and outlet control (HWo) situations need to be analysed. The higher of the two values defines the flow condition, i.e. inlet control is deemed to be the flow condition when HWi is higher than HWo and vice versa.
Figure 2.1 Flow Profiles for Culvert Under Inlet Control (AUSTROADS,1994)
Figure 2.2 Flow Profiles for Culvert Under Outlet Control (AUSTROADS,1994)
For details on culvert operating conditions refer to Section 7.3 of Austroads Waterway Design Guide (1994).
For the design event, the maximum headwater level that can be generated at a culvert location without encroaching onto the roadway is the level at the pavement batter (i.e. top of subgrade surface). The Designer needs to determine the maximum headwater level that can be achieved at the location. In order to do this the Designer will need either existing plan / profile drawings or survey information (if an existing road) or proposed plan / profiles if new works. The information must be sufficient to enable the Designer to locate the low point on the profile and therefore identify the maximum headwater level.In instances where the headwater level remains at the top of road subgrade level or higher for periods longer than 24 hours the pavement should be designed using soaked CBR values for the subgrade.
2.6.1 Inlet, Outlet and Friction Losses
2.6.2 Inlet Losses in Multi Barrel Culverts
These losses can reduce the capacity of the culverts by 30% and as high as 70%. When designing multi-barrel culverts the Designer should undertake some sensitivity analysis to determine what effect this reduced capacity will have. If the reduced capacity will significantly increase the headwater in a susceptible area for the design event or cause excessive damage to the road pavement then the Designer should increase the capacity of the culverts to cater for these additional losses.
The existing ground conditions at drainage locations must be assessed as "Aggressive" or "Non Aggressive" for the purpose of selection of the appropriate cover to the reinforcement in precast concrete culvert and pipe units. The aggressive environments include the following conditions:
Refer Section 4 Design for Durability, of AS 3600 Concrete Structures for more details.Main Roads does not allow use of standard (galvanised) corrugated steel culverts in the areas with aggressive ground conditions, but does permit the use of approved polymer-coated corrugated steel pipe culverts in these conditions.
The design Nomographs provided in Chapter 7 of the Austroads Waterway Design Guide can be used.
Type of Culvert
Roughness or Corrugation
Concrete Pipe (RCP)
Concrete Boxes (RCB)
Corrugated Steel Pipe-Arch and Box
150 by 50mmStructural Plate230 by 64mmStructural Plate
Table 2.1 Manning's n value for RCP, RCB, and Corrugated Steel Arches (Source NHI; 2001)
Manning's Coefficient (n)
Corrugations - Pitch X Depth (mmxmm)
1950 & Larger
Table 2.2 Manning's n value for Helically Wound Corrugated Steel Pipes (Source: AISI; 1999)
Note: # Indicates that no pipes are available in this pitch x depth.
Culverts usually result in outlet velocities which are higher than the natural stream velocities. These outlet velocities may require energy dissipation to prevent down stream erosion. Main Roads generally uses rock protection to prevent down stream erosion. If required, rock protection shall be designed for the same ARI as the culvert design.
Where the culvert is subject to velocities in excess of 3 m/s, the apron must be designed to extend beyond the hydraulic jump lengths. Where the velocities are 3 m/s at the edge of the exit apron rock protection works are necessary.
Rock protection should be in accordance with Table 6.1 of the Austroads Waterway Design Guide.
Multiple barrel culverts may be necessary due to certain site conditions, stream characteristics, or economic considerations. With multi-barrel culverts sedimentation and build up of debris can be issues. As a part of the design process the culvert inverts should be pegged on site as a check to ensure that the culverts are located correctly and at the right levels.
2.13.1 Box Culverts
2.13.2 Reinforced Concrete Pipes (RCP)
2.13.3 Corrugated Steel Pipes
The minimum cover to corrugated metal pipes should be determined in accordance with AS 1762 Helical Lock-Seam Corrugated Steel Pipes Design and Installation 1984, and AS/NZS 2041 Buried Corrugated Metal Structures: 1998. Maximum height of fill may vary from 10 metres to 50 metres, depending on the site loading conditions and the structural strength of the pipe. Minimum cover for the Helical Lock-Seam Corrugated Steel Pipes are :
Where Ss is the internal diameter of the pipe.
2.13.4 Corrugated Aluminium Pipes
Where culverts are liable to settle due to a compressible subgrade or a high fill, measures should be taken to ensure that the pipe assumes the design grading after settlement has occurred.In broad terms, the Designer should consider:
As shown on Main Roads Standard Drawing (Drawing No. 200131-061) for RCP's the headwall height measured from the obvert, can vary from a minimum 300mm to a maximum of 600mm. For the box culverts (Drawing No. 0130-2875) the maximum height of headwall is 500mm. Culverts requiring a headwall height over the above upper limits shall be designed individually.
Culvert length, apron lengths and headwall height must be designed to suit the embankment batter at the culvert site. It is ideal to locate the culvert headwall outside the clear zone (as specified in AASHTO Roadside Design Guide Table 3.1) whenever it is possible. When this criterion cannot be met, the headwall shall be located either outside the pavement batter or at an absolute minimum of 1.0m from the edge of finished shoulder whichever is greater. The top surface of headwall must not project above the finished batter level. Headwall faces must generally be parallel to the road shoulder.
Some culverts (depending on the size of the culvert) may require the installation of a safety barrier. The requirement for a safety barrier should be determined in accordance with Main Roads Guide to Safety Barriers.
Floatation is the term used to describe the failure of a culvert due to the uplift forces caused by buoyancy. The buoyant force is produced when the pressure outside the culvert is greater than the barrel. This occurs in a culvert acting in inlet control with a submerged upstream end. The phenomenon can also be caused by debris blocking the culvert end or damage to the inlet. The resulting uplift may cause outlet or inlet ends of the barrel to rise and bend. Occasionally, the uplift force is great enough to dislodge the embankment.
Large projecting or mitred corrugated steel pipe culverts are the most susceptible to floatation. In some instances, high entrance velocities will pull the unanchored inlet edges into the culvert barrel causing blockage and additional damage.
A number of precautions can be taken by the Designer to guard against floatation and damage due to high inlet velocities. These include:
Main Roads' generally use three different end treatments for CSP culverts, with endwalls, cut-ends and uncut-ends. CSP culverts with cut-ends are to be cut in accordance with Drawing No. 200131-066 also refer to 200131-065 for details of endwall. Therefore it is necessary to provide end treatment details for each culvert on the culvert schedule. A typical example for the detail required is as follows:
ReferencesAUSTROADS, Waterway Design, A Guide to the Hydraulic Design of Bridges, Culverts and Floodways (1994).National Highway Institute (NHI); Federal Highway Administration, Hydraulic Design of Highway Culverts; Hydraulic Design Series Nimber 5; (2001).American Iron and Steel Institute (AISI), Modern Sewer Design, (1999).AS/NZS 2041 Buried Corrugated Metal Structures, (1998).AS/NZS 1734 Aluminium and Aluminium Alloys - Flat Sheet, Coiled Sheet and Plate, (1997)AS/NZS 1762 Helical Lock-Seam Corrugated Steel Pipes Design and Installation, (1984).AS/NZS 3600, Concrete Structures; (2001).