4.9Drainage#
Temporary roads are typically required for durations of less than 12 months. Recommendations for drainage design in Western Australian and the US both allow use of a storm event frequency of 1 in 2 years for temporary roads in use for periods of less than 12 months.
The guidance in Western Australia further states that in situations for temporary alignments in place for up to twelve months the following guidelines shall apply to temporary drainage systems:
- Where culverts are provided for drainage they shall be designed for an Annual Exceedance Probability (AEP) of 39%.
- Spread limits; Retain the widths accepted by the jurisdiction with the AEP event being 39% rather than 10%.
- Underground piped network system being designed for an AEP 39% event.
- Check on failure mechanisms and impacts on property damage. A risk assessment should be undertaken to determine the consequences for the temporary road and surrounding environment should the design event be exceeded (extreme event), and measures implemented to mitigate any unacceptable risks. Furthermore a risk assessment of any additional hydraulic impacts due to the temporary road/works should also be undertaken and evaluated against the likelihood of occurrence for the period the temporary road/works are in place.
Guidance in the US also suggests the use of an AEP 39% design frequency for temporary drainage facilities for a temporary roadway in use for less than one year and an AEP 18% design frequency for a road in use for more than a year.
