Developing in a flood area

Mainstream flooding is inundation of normally dry land occurring when water overflows the natural or artificial banks of a stream, river, estuary, lake or dam.

Local flooding is flooding resulting from the local catchment system and can be described as local drainage and major drainage where:

(i)local drainage is smaller scale problems in urban areas; and

(ii) major drainage is:

• The floodplains of original watercourses (which may now be piped, channelised or diverted), or sloping areas where overland flows develop along alternative paths once system capacity is exceeded; and/or
• Water depths generally in excess of 0.3m (in the major system design storm as defined in current version of Australian Rainfall and Runoff). These conditions may result in danger to personal safety and property damage to both premises and vehicles; and /or
• Major overland flow paths through developed areas outside of defined drainage reserves; and or
• The potential to affect a number of buildings along the major flow path.

1 in 20 year flood (commonly known as a 20 year flood) is a statistical event to describe a flood of particular magnitude (or larger) occurring on average once in every 20 years, i.e. there is a 5% chance of a flood of this size or greater occurring in any given year.

1 in 100 year flood (commonly known as a 100 year flood) is a statistical event to describe a flood of particular magnitude (or larger) occurring on average once in every 100 years, i.e. there is a 1% chance of a flood of this size or greater occurring in any given year.

AEP is an abbreviation for the Annual Exceedance Probability, the chance of a given flood or larger size occurring in any one year, usually expressed as a percentage.

For example a 1 in 100 year ARI flood is also sometimes quoted as a 1% AEP flood or a 1 in 20 year ARI flood is also sometimes quoted as a 5% AEP flood.

The Probable Maximum Flood (PMF) is the largest flood that could conceivably occur at a particular location, usually estimated from the probable maximum precipitation (rainfall).

The PMF defines the extent of flood prone land, that is, the floodplain.

What is AHD?

AHD is the abbreviation for Australian Height Datum. It is a common national datum (or plain) of level corresponding approximately to mean sea level.

ARI is the abbreviation for Average Recurrence Interval.

It is defined as the long-term average number of years between the occurrences of a flood as big as or larger than, the selected event.

ARI is another way of expressing the likelihood of occurrence of a flood event.

A High Flood Risk Precinct is generally defined as the area of land below the 100 year flood that is either subject to a high hydraulic hazard or where there are significant evacuation difficulties.

Most land uses (with the exception of Open Space & Non Urban and Concessional Development) would not be suitable within this precinct.

A Medium Flood Risk Precinct is generally defined as the land below the 100 year flood that is not subject to a high hydraulic hazard and where there may be some evacuation difficulties.

Most land uses (with appropriate planning and building controls with the exception of Sensitive Uses and Facilities and Critical Uses and Facilities) would be permitted within this precinct only after proper compliance with all the relevant Planning and Development Controls outlined in Council’s Local Floodplain Risk Management Policy.

Detailed hydraulic modelling together with a flood impact report is typically required when proposing any development in these areas.

A Low Flood Risk Precinct is the area above the 100 year flood and includes all area up to and including the probable maximum flood.

Most land uses (with appropriate planning and building controls with the exception of Sensitive Uses and Facilities) would be permitted within this precinct.

Properties which are within a Grey Area are those identified as being within a catchment area which drains to a location of a known drainage problem area.

There may be additional development controls imposed on developments which are in a Grey Area.

Typically these controls include the need to provide on-site detention to reduce the impact of increased peek runoff in the catchment resulting from further development.

Properties which are within a Grey Hatched Area are subject to local runoff from the local catchment which results in flooding.

There are development control requirements that apply for development within a Grey Hatched area.

These controls may include increased boundary set backs and requirements to provide formal overland flow path(s) and easements to safely convey surface flows through the property.

There may be additional requirements imposed on developments within a Grey Area and a Grey Hatched Area.

These controls could include provisions imposed for:

• on-site detention
• increased boundary set backs
• requirements to provide:
  • an overland flow path
  • easements to convey stormwater drainage