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Flood study

Green Square Catchment Floodplain Risk Management Plan

GREEN SQUARE CATCHMENT

The Green Square and West Kensington study catchment has an area of approximately 2.5 km2 and drains predominantly from east to west (refer Figure 1). South Dowling Street runs northsouth through the middle of the catchment dividing the City of Sydney and Randwick City Local Government Areas (LGAs). 57% of the study catchment lies within the City of Sydney LGA, with 43% being within the Randwick City Council LGA. The City of Sydney portion of the catchment includes parts of the suburbs of Zetland, Waterloo, Alexandria and Rosebery.

Urbanisation has dramatically altered the nature of available drainage within the catchment.

Flood problems typically result from insufficient capacity in the formal drainage system and ponding in trapped low-points such as those found in Joynton Avenue, Lachlan Street, South Dowling Street and Botany Road. A number of these locations are known to have experienced severe flooding in the past.

The NSW Government’s Flood Policy provides for:

  • a framework to ensure the sustainable use of floodplain environments, solutions to flooding problems,
  • a means of ensuring new development is compatible with the flood hazard.

Implementation of the Policy requires a four stage approach, the first of which is preparation of a Flood Study to determine the nature and extent of the flood problem. This is followed by a Floodplain Risk Management Study which examines management measures. The subsequent Floodplain Risk Management Plan details the adopted measures and ultimately the works are undertaken in the final stage. This report documents the Floodplain Risk Management Plan for the Green Square catchment.

GREEN SQUARE WEST KENSINGTON FLOOD STUDY

The Green Square-West Kensington (GSWK) catchment is known to have experienced flooding of roads, residential properties and commercial areas. The most recent significant floods occurred in November 1984. The GSWK Flood Study was jointly undertaken by both City Of Sydney (COS) and Randwick City Council (RCC) to quantity existing flood behaviour and identify flood risk management issues within the area. The specific aims of the Green Square-West Kensington Flood Study were to:

  • define flood behaviour in the Green Square-West Kensington catchment,
  • prepare flood hazard and flood extent mapping,
  • prepare suitable models of the catchment and floodplain for use in a subsequent Floodplain Risk Management Study and Plan.

Hydrologic and hydraulic investigations have been undertaken to determine the response of the catchment and drainage system to 50% AEP (1 in 2 year), 20% AEP (1 in 5 year), 5% AEP (1 in 20 year), 2% AEP (1 in 50 year), 1% AEP (1 in 100 year) and 0.2% AEP (1 in 500 year) events and the Probable Maximum Flood (PMF). The results of these investigations were quantified as peak pipe capacities in addition to peak flood levels, flows and velocities within the floodplain.

EXISTING FLOOD PROBLEM

A flood damages assessment for existing development was undertaken for a range of design events. This assessment was based on a detailed survey of building floor levels in the Green Square area. The estimated number of residential building floors which are likely to be inundated in the 20% AEP is 8 and 17 in the 1% AEP. The corresponding number of commercial properties inundated is 27 and 56 for the 20% AEP and 1% AEP events respectively. In the PMF a total of 130 building floors would be inundated. The annual average damages were estimated to be close to $1.03million. No consideration has been given for damages to public structures or utilities (bridges, roads, pumping stations) or for the complete collapse of structures due to flooding.

FUTURE DEVELOPMENT

The area of the catchment immediately west of South Dowling Street was once dominated by industrial premises. Re-development of this area in the form of medium and high density housing/mixed use premises has been undertaken in recent years, including ongoing development within the Victoria Park precinct. Significant re-development has commenced (or is planned to commence within the next five to ten years) for other areas within the catchment including the Green Square Town Centre (GSTC) precinct, the Mid-block Precinct (bounded by O’Dea Avenue and Lachlan Street) and Precinct E and the adjacent area between Link Road and Joynton Avenue.

GREEN SQUARE-WEST KENSINGTON CATCHMENT FLOODPLAIN RISK MANAGEMENT

STUDY The specific aims of this study were to: review the Flood Study results to determine flood damages and flood hazard for existing conditions, identify development and planning controls to regulate redevelopment in the flood affected properties and to ensure that future redevelopment does not significantly add to the overall potential damage, make recommendations to adopt Flood Planning Levels (FPL) appropriate for the catchment, investigate available floodplain risk management measures along with prioritisation, and staging of works.

FLOODPLAIN RISK MANAGEMENT MEASURES

A list of all possible floodplain risk management measures which could be applied in the study area were initially developed for consideration. The assessment extended to examination of potential future development and its possible adverse impacts on flows and water quality. The measures were then assessed in terms of their suitability and effectiveness for minimising social, ecological, environmental, cultural and economic impacts. As part of this process a number of measures were identified as not being worthy of further consideration.

A summary of the various floodplain management measures considered during the course of the Floodplain Risk Management Study is presented in Table (i) and Figure A together with a brief assessment of their viability for implementation as part of the Floodplain Risk Management Plan for the Green Square catchment.

Sydney Water and other floodplain stakeholders raised objections to the “Option 1a” works originally documented as part of the public exhibition phase of the Floodplain Risk Management Study in June 2011. Between 2011 and 2013, City of Sydney and Sydney Water undertook ongoing investigations to develop potential options to address the issues relating to the “Option 1a” works, resulting in a significantly revised trunk drainage option. The details of the revised option are documented in Sections 4 and 5.3.8 of this Plan.

Additional Information

Field Value
Title Green Square Catchment Floodplain Risk Management Plan
Publication Date 1 July 2013
Themes Land and Resource Management
Spatial Extent
Map data © OpenStreetMap contributors
Council/LGA Council Of The City Of Sydney
Author/ Prepared by WMA Water
Publish date 9 June 2017
Update date 9 June 2017
River Basin 213 - Georges
Place Name Kensington
Approval State Approved
Submitted for approval 9 June 2017
Submitted by John Silk
Approved 27 June 2017
Approved by dstazic
Data Comment

Report only; no other data. Legacy data; may have been superseded.

Identifier deaffd77-c187-4aff-899c-4abb3b3ed1f1