General Standards

Document No:  67-08-79
Revision:  2A
Date amended:  29-Mar-2019

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 Eric Cheung by e-mail or on (08) 9323 4402.

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.


Revision Register


Ed/Version Number Clause Number Description of Revision Date
 1 All Guideline Developed and Approved. 29-May-2003 
 2 Header Contact person changed. 23-Jan-2014
​2AHeader​​Contact person changed. ​29-Mar-2019


Table of Content

1. Chapter 1 of 3. GENERAL STANDARDS

1.1 General
Culverts are used to convey water under roads as temporary and permanent structures. Any structure that constricts the waterway will cause the rise in upstream water level. The incremental increase is referred to as the backwater while any rise in specific energy or total energy due to the width of waterway being narrower than that required to pass the discharge at critical velocity with respect to natural stream level is termed afflux. The depth of water above the invert of the culvert at the inlet is referred to as the headwater as shown in Figures 2.1 and 2.2.

The hydraulic design of culverts is based on Bernoulli's equation of open channel flow. The Designer determines the most economical size of structure within the restrictions given by the allowable velocity, head water and other site specific conditions. To further optimise the culvert performance the design includes culvert end treatments, scour protection, energy dissipation structures, levees, channel bed and bank stabilisation in the both approach and departure channels of the culvert.
Guidance on the design of open drains associated with culverts can be found in the Guide to Design of Open Drains.
1.2 Design ARI
Culverts shall be designed for the nominated recurrence interval.  Refer Section 2.5 Headwater Levels for the other factors which should be considered when determining design ARI.

1.2.1 Culvert Design Events

In determining the recurrence interval (serviceability level) that should be used for the design for culverts the consequences of failure need to be carefully considered.  The mode of failure of the culvert and the existing site conditions greatly influences the design recurrence interval e.g. the design of a culvert on a section of road with limited sight distance which fails by overtopping of the road will require a higher design recurrence interval than a road which fails by overtopping but has adequate sight distance.

For roads outside the metropolitan area where flows in excess of design will not effect the serviceability of the road (ie. failure mechanism does not overtop the road or there is no likelihood of property damage in the event of failure) the culvert should be designed to pass at least 1 in 10 year ARI. 

Culverts which are at low points of the road alignment (or the failure mechanism is to overtop the road or there is a likelihood of property damage in the event of failure) should be designed for the 20 year ARI event, but preferably for the 50 year ARI.

Within the metropolitan area or within town sites all culverts shall be designed for the 20 year ARI event but preferably for the 50 year ARI because of the consequences of any backwater affects. 

Example of typical serviceability levels are presented below.  However, these are indicative only, and the required serviceability level for any road must be assessed on a case-by-case basis.

Typical serviceability levels:

National Highways, State Highways and Freeways     50-100 ARI
Main Roads 20-50  ARI
Minor roads 10-20  ARI
In some situations consideration may need to be given to using a 100 year ARI. Typical examples are contained in Table 2.1 of ARRB report No. 34 and would include emergency services such as police, ambulance, fire stations or large retail and department stores, venues for entertainment etc.

Culverts that have a waterway area greater than 3.0m2 require a bridge number that can be obtained from the Bridge Branch and are subject to a formal Waterways Analysis as outlined in the Procedure for Waterways Investigation and Design section of the "Structures Engineering Management System Manual".

1.2.2 Scour Protection

If the waterway area is more than 3.0m2, consideration should be given to provide rock protection at the down stream end of the culvert. In general no rock protection is provided for smaller culverts. However due to some specific site conditions scour protection may be required.
For minor culverts it is not expected that flow duration for low frequency flows will be extended for periods that could cause serious scour problems.  More details on culvert protection are provided in Section 2.11.

1.3 Design Flow
The required design flow shall be determined in accordance with the Guide to Obtaining Design Flows unless specified otherwise.
1.4 Minimum Culvert Sizes

The following minimum culvert sizes are used by Main Roads:

  • pipe culvert size 375 mm nominal diameter
  • minimum box culvert size 1200 x 375 mm.

In determining the minimum size of the culvert the Designer should consider the type and size of likely debris which can ultimately cause blockages.  Refer Clause 2.2 Site Investigation.

1.5 Special Applications

Culverts are occasionally designed to fulfil special functions.  For example:

  • environmental requirements may require culverts to cater for Mulga habitats in addition to their primary function as drainage structures
  • a Culway site 

The criteria for these culverts are therefore not necessarily based on capacity requirements for the design event and the Designer should design the culverts accordingly.

AUSTROADS, Waterway Design, A Guide to the Hydraulic Design of Bridges, Culverts and Floodways (1994).

John Argue, Storm Drainage Design in Small Urban Catchments, A Hand Book for Australian Practice, Special Report No 34, Australian Road Research Board, (1986).