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Drainage and heavy rainfall

Council officers have commissioned a flood modelling exercise to identify built-up urban areas subject to flooding which allowed an assessment of the potential flooding against desired service levels to be completed. The assessment has proposed an achievable '3-star' service level target in existing urban areas which is consistent with Melbourne Water’s service level and is representative of pre-existing local conditions.

A ‘5-star’ level of service is required in all new developments.

On-Site Storm Water Detention System (OSD)

An OSD system is designed to restrict the flow of stormwater from a development site to a rate equal or less than the pre-developed site conditions.

All new developments within Bass Coast, particularly unit developments, create an increase in hard surface areas resulting in a higher volume of run-off being discharged into Council's existing drainage system. The existing drainage system was not designed to cope with the increased volume of run-off.

An OSD system is required as part of all new developments across Bass Coast to limit stormwater flow rates to pre-development levels. Stored flows are released slowly thus not increasing the overall rate of discharge to maintain the designed capacity of the pipes.

1 in 100-year storm event

Storms are classified using historical rainfall data and statistical probability. A one in 100 year storm event is a storm with characteristics that are only expected to occur on average once every 100 years. Naturally with any statistical probability, there is a possibility that multiple events could occur in much shorter succession.

We could receive multiple 100 year storm events in consecutive years.

In fact a 100 year storm has a one per cent probability of occurring in any given year. For further information, visit the Bureau of Meteorology website.

Why does Council's drainage system struggle during heavy rainfall?

Council's underground drainage system design was based on industry best practice at the time of installation. For much of the municipality, this is identified as a one in five year storm frequency. As such, the underground drainage system should be able to contain up to 80 per cent of the rainfall events that occur during any year. However, during storm events that exceed this probability it is expected that the capacity of the Council stormwater drainage network will be exceeded and overland flows will result.

During extreme rainfall, these overland flows may result in flooding of roads and properties. Properties in low lying areas and natural gullies are most at risk from overland flows. Property owners are encouraged to keep overland flow paths clear of objects that will obstruct the overland flow of water increasing the risk of inundation to habitable areas.

The Victoria State Emergency Service is the control agency for flooding in Victoria and are responsible for planning for floods, and for managing flood response if they do occur.

If you believe that your property is under threat from flood waters, you should contact the Victoria State Emergency Service on 132 500 from anywhere in Victoria for flood, storm, tsunami and earthquake emergency.

For life-threatening emergencies call Triple Zero (000).

Experience shows those who plan and prepare for emergencies can reduce the impact of the emergency, and can recover more quickly afterwards. Taking the time to think about emergencies and making a plan helps you to think clearly, have a greater sense of control, and make better decisions when an emergency occurs.