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Overland Flow and Flooding
*** ISC provides the information below in good faith to assist in the general knowledge regarding overland flows and flooding. It does not in any way or shape imply or infer any design intent, or council approval. The accuracy of the advice is based on third party information which cannot be warranted. Its accuracy is based on information at hand, and does not constitute what a local council may invoke.
Floods can happen at anytime. There are no easy answers - the key is to plan ahead.
Assessment of what potential flooding can occur is conducted by a civil engineer with expertise in the field of hydrology and hydraulics. ISC can provide you with professional advice on how to safe guard your property from potential flooding, and ensure your development doesn't impact on your neighbours, as required by law.
Sometimes our profession confuses, the difference between what the community perceives as overland flow and flooding. Generally councils refer to both terms as flooding. The mainstream community refers to Flooding as rising waters where depths of water exceed 1m. Overland flow is considered as natural flowing waters that flow through properties via their natural valleys. Neither is really wrong. But it always helps to clarify the difference.
To explain it in more detail, you'll need to understand a few basic reasons why they occur.
The most common are:
a) rivers and creeks overtop their banks due to excessive flows from upstream catchments.
b) drainage pipe and culvert systems back up because they can not cope with the volume of water entering them or they get blocked by debris and rubbish.
c) developments of properties are constructed without consideration to natural flows such as retaining walls and fences.
The worst flooding occurs after prolonged rainfall when the soil is saturated and the water levels in the rivers and creeks are elevated.
Then, if an intense rainfall burst occurs, causing a large amount of rain within a brief period, flash flooding may occur with little or no warning.
The New South Wales government introduced a Flood Plain Management Manual in 1986, which has been amended several time since to provide a guide for local councils on how to address the issues of overland flow and flooding in New South Wales.
Most councils are now implementing the new guidelines and imposing high restrictions on what extent a property can be developed based on the severity of the flooding or even minor overland flow. It is crucial to have your property assessed before preparing plans to ensure that the proposal will not be knocked back by council, which could mean months of planning wasted and costs associated, partially wasted also.
Contact your local council and ask to speak to an engineer to advise you whether your property is subject to overland flow. If your not confident in contacting council, ask ISC to conduct a site assessment. Your choice, but either way, it'll save everyone a lot of time.