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.
1.2.1 Culvert Design Events
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 townsites 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:
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.
The following minimum culvert sizes are used by Main Roads:
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.
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.
ReferencesAUSTROADS, 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).