Developing in a Flood Area

A flood enquiry application provides you with information relating to the flood extent and flood levels for an area or specific property. It is required in addition to a 149 Certificate when lodging a development application (DA) in a floodrisk area. We recommend you discuss your development proposal with City of Parramatta’s Town Planner, Development Services Engineer and Catchment Management Engineer before submitting your application. 

flood information for local catchments

City of Parramatta does not have flood extent or flood level information for local catchment systems. It is the responsibility of anyone proposing to do building work or any development to check the local catchment system and determine if the property on which works are proposed is affected by stormwater runoff from the local catchment.

If the property is affected by stormwater runoff then a hydrological and hydraulic catchment study to determine the impacts to the proposed development and surrounding properties needs to be undertaken by a qualified and experienced Drainage Engineer.

The safe conveyance of stormwater runoff and demonstration that the development will not result in any adverse increased stormwater runoff to adjoining properties will need to be adequately addressed in any development application submitted.

Flood Study and flood modelling

Generally you will need to undertake a flood study and flood modelling if you are proposing to do building extension works or proposing to redevelop your property and your property is:

  • located within a Medium or High Flood Risk Precinct Area. Under City of Parramatta’s Local Floodplain Risk Management Policy, building within a High Flood Risk Precinct area within a high hydraulic hazard is not suitable

  • located within a Grey Hatched Area

  • potentially impeding, changing or restricting the movement of flood waters or reduces the flood plain storage

  • affected by stormwater runoff from the local catchment.

For more information please consult with a suitably qualified and experienced professional Drainage Engineer and with City of Parramatta’s Catchment Management Engineer. It is important that any DA be accompanied with adequate hydrological and hydraulic modelling to demonstrate that the proposed development does not adversely impact flooding at this property or any other property.

What is mainstream flooding?

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.

What is local overland flooding?

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, channelized 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.

What is a 1 in 20 year flood?

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.

What is a 1 in 100 year flood?

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.

What is AEP?

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.

What is the Probable Maximum 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.

What is ARI?

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.

My property is located in a High Flood Risk Precinct. What does this mean?

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.

My property is located in a Medium Flood Risk Precinct. What does this mean?

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.

My property is located in a Low Flood Risk Precinct. What does this mean?

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.

My property is located in a Grey Area. What does this mean?

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.

My property is located in a Grey Hatched Area. What does this mean?

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.

My property is located in a Grey Area. What development controls are required?

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 or requirements to provide an overland flow path and easements to convey stormwater drainage.

Contact

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