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North Brother Local Catchments

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North Brother Local Catchments Flood Study

The North Brother Local Catchments Flood Study has been prepared to define the behaviour of local catchment flash flooding from North Brother Mountain and to identify the flood risk that it poses to the community. This study will ultimately be used to assist Council to develop a range of measures to manage the impact of local catchment overland flooding and guide strategic planning for future development of the area via a Floodplain Risk Management Study and Plan.

The North Brother Local Catchments Study forms part of the NSW Floodplain Management Process and has been undertaken within funding and support for the NSW office of Environment and Heritage (OEH). The flood study report provides information on flows, depths, extents, levels and velocities of flows originating from the North Brother Mountain for a range of different storm events.

The North Brother Local Catchments Flood Study has resulted in the modelling of a range of design storm events including the 20% and 5%, 2%, 1% and 0.5% Average Exceedance Probability (AEP) flood events and the Probable Maximum Flood (PMF).

NOTE - The Average Exceedance Probability (or AEP) defines the likelihood of a flood/storm of a given size or larger occurring in any year; expressed as a percentage.

In addition to the above, flood behaviour was estimated for a climate change scenario comprising the 1% AEP plus 10% increase in rainfall plus 0.9m sea level rise.

Flood mapping of depth and flow velocity was undertaken for all event AEPs.

Provisional Flood Mapping of Flood Risk and Hazard have also been completed.

North Brother Study Area

The study area comprises the northern and eastern faces of the North Brother Mountain and the associated urban areas between the foot of the mountain and the adjoining receiving waters. It has an approximate area of 1,852ha, with the North Brother Mountain extending to a height of 490m AHD, dominating the landscape. The upper reaches of the study area are predominantly the Dooragan National Park, containing the North Brother Mountain itself, below which is situated the Laurieton CBD, various vegetated natural gullies and flow paths as well as significant established low and medium density residential, caravan parks and holiday accommodation precincts.

Development of the study area has been occurring from the early 1900's through to the present day with the majority of development having occurred between 1970 - 2000. The construction of associated drainage infrastructure has also primarily dated from this time, with the result being that the majority of watercourses stemming from the North Brother Mountain have either been built over, filled, redirected, piped or crossed by road embankments, often resulting in urban development occurring on flood prone lands.

Urban development at the foot of the Mountain is typically bounded by diversion drains and largely natural gullies which generally direct large volumes of stormwater runoff safely around developed lands and into the downstream waterways. However, developments have occurred in some locations in close proximity to natural watercourses and manmade surface drainage and are at risk to flooding when the drainage capacities are exceeded. In addition, localised flooding in some areas are exacerbated by mainstream flooding in Queens Lake, Stingray Creek and Camden Haven River.

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Why do we need a Flood Study?

  • Council is responsible for the management of flood liable land and has an obligation to implement the NSW Floodplain Management Program and Flood Policy.
  • Council needs to ensure that any new development is compatible with identified flood hazards and flood risks.
  • Council needs to ensure that existing flood risk is minimised, future flood risk is managed and that new development does not create additional flooding problems.
  • To aid in our development of management and mitigation measures to manage flood problems.
  • To enable Council to qualify for State Government subsidises for flood mitigation works to alleviate existing problems. 

The Next Steps

Utilising the Adopted Flood Study, Council is currently in the process of preparing a Floodplain Risk Management Study and Plan, as a means of identifying, developing and implementing a range of measures to manage the impact of local catchment overland flooding and guide strategic planning for future development of the area. At this stage, it is anticipated that a Floodplain Risk Management Study and Plan for the North Brother Local Catchments will be completed by April 2021. Further information will be added to the webpage as this ongoing project continues.

This page was last updated on: 29 May 2020