Flood Protection

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Levels of Protection and Service

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Levels of service apply to stormwater provision, while levels of protection apply to flood protection systems.

Stormwater involves drainage of a locality or developed area, and may be provided by means of reticulation (pipes), open drains or natural watercourses, or on-site controls such as soak-holes and wetlands. Flood protection consists of those measures and infrastructure designed to protect property and life in the event of a major rainfall or storm event.

Historically, storms were expressed as having a specific return period, or Average Recurrence Interval (ARI) for a given combination of rainfall intensity and duration. The so-called 100 year storm being a storm which had a probability of occurring once in a 100 year period. This can be confusing as it is possible (but statistically unlikely) to get two "100 year storms" in the same year. The 200 or 500 year storm may also occur within one of the 100 year periods as well as the 100 year event.

To better explain the statistical nature of storm frequency, the probability is now expressed as a percentage of a particular storm event being exceeded in any one year. This is a similar format to the weather forecast projections which some television weathercasters give for the likelihood of rain for an area on a given day (in percentages).

For example, the level of service for residential property is to design stormwater infrastructure to cope with a 20 per cent Annual Exceedance Probability (AEP), whereas the level of protection for residential floors is for a one per cent AEP.

A 20 per cent AEP event therefore has a 20 per cent chance of being exceeded in any one year, and is approximately the equivalent of the five year storm. Similarly, a one per cent AEP event can be considered equivalent to the 100 year storm.

In the case of a major flood event, the expectation is that the roads and grounds of properties may become flooded, provided that floor levels are not inundated.

The improvements required for individual catchments or areas are identified through Catchment Management Plans. For each catchment these are prioritised on the basis of the current level of service offered versus the required level of service, and weighed against the cost required to achieve the improvement (a quasi benefit/cost appraisal).

Stormwater service level protection	Primary protection	Secondary Protection
Land use category	AEP	AEP
Parks, reserves, sports grounds	20%	20%
Residential land and accessory buildings	20%	20%
Residential floors	20%	1%
Commercial floors	10%	1%
Industrial floors	10%	1%
Public buildings	10%	1%
Road culverts	10%	2%

Related links

• Flood protection
• Stormwater overview