Flood Zones
All urban areas within Stonnington are serviced by an underground drainage system with pipe sizes generally designed to run full on average once every five years. That is, the underground drainage system is designed to cope with a one in five year storm event.
This is considered an efficient and cost-effective design method, as constructing a stormwater system to cope with greater storm events would cost a disproportionate amount of money for pipe sizes which would very rarely reach their full capacity.
This design method works well when excess stormwater resulting from the occasional storm event greater than the one in five year storm is able to drain effectively and safely by complementarily designed surface floodways allowing excess stormwater to successfully transfer to the nearest creek or river. Properly designed roads or open grassed drainage reserves make acceptable floodways.
Unfortunately, as with many long established urban areas within Melbourne, some of the older underground drains do not deliver a 5 year design discharge and the accompanying floodway is either ineffective or non-existent. This may be due to past developments and changing design standards. In some instances moderately low lying areas were developed without recognition or understanding of likely flooding problems. Consequently, some allotments are prone to flooding during heavy rain.
The widely accepted industry standard for a drainage system is one which can cope with a "one in one hundred year" storm event, generally referred to as a 1% probability flood event. A well designed drainage system has underground drains and associated floodways which adequately transfer stormwater from a 1% probability storm without serious damage or flooding of private property and with only moderate inconvenience to the road system.
Stonnington Council in co-operation with the Melbourne Water Corporation has now identified and mapped the areas within the municipal district which are likely to be flooded by 1% probability storm events. Some are well known areas which have long been recognised as liable to flooding, identified by a Land Subject to Inundation Overlay under the Stonnington Planning Scheme. Others have been identified by analysis of drainage capacities, with areas captured in a Special Building Overlay (SBO) listed under the Stonnington Planning Scheme. Landowners affected by planning overlays for flooding are likely to require a planning permit for proposed building works to allow the design to take account of the relevant flood level.
The Building Regulations also require Council to map areas liable to flooding. The areas designated are the same as those found in the Planning overlays.
Identify areas liable to flooding.