Allans Creek

Content: Catchment description | Flood problem | Impact and cost of flooding | Mitigation options | Property modification actions | Flood modification actions | Emergency response modification actions | Implementation of migration works | Status

Allans Creek Floodplain Risk Management Study and Plan has been prepared for Council by Cardno Lawson Treloar Pty Ltd, Level 2, 910 Pacific Highway, Gordon NSW 2072. The plan was adopted by Council in November 2006.

Catchment description
Allans Creek catchment is located southwest of Wollongong’s CBD, rising up to the Illawarra Escarpment in the west. The catchment is 42km2 and is made up of a mix of residential housing, light and heavy industry and environmentally protected areas. The table below gives an overview of the catchments characteristics. The main natural tributaries that drain the catchment are:

• Allans Creek;
• American Creek;
• Brandy and Water Creek (and tributary);
• Branch Creek (and tributary);
• Byarong Creek;
• Charcoal Creek (and tributaries);
• Ghost Creek;
• Jenkins Creek; 
• Nudjia Creek;
• Running Brook. 

The catchment is characterised by steep upper slopes with limited development and a floodplain with a mix of residential, commercial and industrial development.  The Creek systems are highly modified in some locations, consisting of concrete pipes and open channels draining ultimately through the BHP industrial area to Port Kembla Inner Harbour. 

Development of Port Kembla Harbour in the mid 1900's has resulted in the lowland and former estuarine areas of the Allans Creek catchment being more modified than in any other catchment of the Illawarra region (Forbes Rigby, 2002).

In the majority of other areas the creeks are in a natural condition but their integrity is compromised by the impacts of catchment development. 

Major transport routes through the catchment include the F6 Freeway, the Princes Highway and the Illawarra Railway Line.  These major links cross the various tributary creeks with bridges and/or culverts, which are major controls to flooding. 

Flood problem
In the past, flooding within the Allans Creek catchment has caused property damage and posed a high hazard to the residents living in close proximity to major watercourses in the catchment.  Additionally, major transport links and local roads have been inundated by flood waters making evacuation and emergency access difficult.
 
Flooding of developed areas within the catchment has been reasonably frequent in recent times.  Over the past decades, Allans Creek has experienced significant flood events including those in March 1975, March 1978, March 1983, October 1983, February 1984, June 1991, August 1998 and October 1999. 

Due to the large nature of the catchment and the significant variation in the amount of rainfall across the catchment, the August 1998 and October 1999 events were of varying magnitudes depending on the locality within the catchment.  For example, the August 1998 event was most severe in the northern parts of the catchment, whilst the October 1999 event was most severe in the southern parts of the catchment.  During both of these events a number of properties were affected and a substantial cleanup operation was required following the flood. 

The floods in 1998 and 1999 were intensified due to a number of culverts being blocked by debris. This resulted in higher flood levels being experienced in areas upstream of the blockages, and in some cases, the creation of flow diversion paths, causing water to flow in usually dry areas. Floods in the catchment are primarily caused by intense rainfall over a short duration (typically less than six hours).

Impact and cost of flooding

Average Annual Damage: $13,500,000**

*PMF = Probable Maximum Flood          
** Figure calculated in 2006

Mitigation options
Using the merit-based approach advocated in the NSW State Government’s Floodplain Development Manual (2005) and in consultation with the community, Council and state agency stakeholders, a number of potential mitigation options were identified.

Mitigation measures are comprised of a range of options from each of the following categories:

• property modification measures;
• response modification measures;
• flood modification measures.

Property modification actions (ie. changes to property or policies that affect property).

• adoption and implementation of development control matrix to be included in Council’s DCP54 Managing Our Flood Risks (PM1);
• caravan park / manufactured home estate policy (PM9);
• cumulative Development Impact Study and review of On-Site Detention Policy (PM10);
• detailed investigation of possible zoning modifications (PM6);
• house raising program (PM3) (eligible property owners will be contacted by Council);
• preparation of flood data collection forms and undertaking of data collection (PM11);
• provision of a flood refuge within the Figtree Gardens Caravan Park (PM5);
• public awareness and education for property matters (creek maintenance in private property) (PM13);
• rewording of section 149(5) certificates (PM7) and/or adapting Towradgi Creek Flood Certificate (PM8);
• updating relevant council documents to include guidelines for public domain infrastructure (PM2);
• voluntary purchase program (PM4);
• additional property ground survey (PM14).

Flood modification actions (ie. some form of constructed works, see map for locations).

• corridor management and maintenance for all creek systems (FM21/FM41/FM54);
• bridge construction to replace culverts at American Creek crossing of the F6 Freeway (FM55);
• bridge construction to replace The Avenue culverts (FM20);
• Byarong Creek modification works between Princes Highway and The Avenue (FM18);
• creek modification works - Charcoal Creek downstream of Lindsay Maynes Park (FM46);
• creek modification works between Blackman Parade and Tallegalla Street (FM43);
• creek modification works upstream of Koloona Avenue (FM3);
• creek rehabilitation along length in upper reaches of American Creek (FM24);
• creek rehabilitation works between Gibsons Road and Princes Hwy American Creek (FM30);
• creek rehabilitation works between The Avenue and Freeway (FM38);
• creek works and flood detention area between Tallegalla Street and Princes Highway, Unanderra (FM48);
• debris control structure upstream of Cordeaux Road (FM23);
• debris control structure upstream The Avenue (FM58);
• debris control structure upstream of Alukea Street culvert (FM60);
• debris control structure upstream of Princes Highway Bridge on Allans Creek (FM56);
• debris control structure(s) - Unanderra Industrial area – Upstream of Berkeley Road (two crossings) (FM57);
• detention basin (FM2) and debris control structure for the park upstream of Koloona Avenue (FM4);
• Figtree Gardens access road culvert amplification (FM34);
• formalise channel for flow between Lysaght Oval and freeway bypass (FM37);
• improvements to Princes Highway bridge, debris control structure, modification of creek system at rear of Arrow Avenue Properties (FM15);
• levee embankment - Govett Crescent (FM25);
• removal of a portion of the F6 median strip (FM59);
• further investigations for upper Byarong Creek scheme (FM61/FM63/FM64/FM65);
• ring levee around O'Donnell Drive (FM35);
• stormwater drainage and overland flow path improvements Arrow Avenue-Bellevue Avenue (FM16);
• realignment of pipe outlet upstream of Koloona Avenue (FM1);
• minor levee upstream of Baker Crescent (FM62);
• Brandy and Water Creek- investigation and implementation of floodplain lowering (FM66).

Emergency response modification actions (ie. changes to emergency management).

• electronic information transfer agreements between relevant government agencies (EM5);
• enhancing existing flood alert systems (using additional rainfall gauges within the Bureau of Meteorology’s ALERT system) (EM3);
• issue of reports and laminated flood extent plans to SES (EM6);
• locality based ‘Flood Safe’ brochure with fridge magnet (EM7/EM8);
• periodic revision of DISPLAN / Flood Sub Plan (EM1);
• preliminary assessments for the establishment of a trial SMS service (EM2);
• public awareness and education - Annual Remembrance Day (eg. 17th August) (EM9);
• public awareness and education - School Information Package (EM10).

The implementation of the proposed flood mitigation measures would see a reduction of approximately 200 houses being flooded above floor level in a 100 year flood event.

The next step in the Floodplain Management process is the implementation of the adopted Plan. This will depend on Council receiving financial assistance from the Federal and State Governments. The implementation of the proposed mitigation works is a lengthy and expensive process and therefore will take many years to complete.

Implementation of mitigation works
Removal of a portion of the F6 median strip:
A large vegetated earth mound forms the median strip of the F6 Freeway in the Figtree area. The removal of this median mound for the entire length of overtopping will allow for flood flows pass the Freeway with less restriction and hence reduce flood risk upstream of the Freeway.

The removal of this median mound was completed by the Roads and Traffic Authority in 2005.

Upper Byarong Creek  Scheme - Investigation and Concept Design

The main objective of this project is to assess and design a number of concept flood mitigation options that were identified in the Allans Creek Floodplain Risk Management Study & Plan (Cardno Lawson Treloar, 2006) and recommend the suitable concept options to reduce the effects of flooding on the community and development within the upper Byarong Creek catchment.

The recommended flood mitigation options (known as the upper Byarong Creek scheme) comprise:

• the installation of debris control structure upstream of Koloona Avenue;
• creek works upstream of Koloona Avenue;
• contouring Park on the left bank upstream of Kolona Avenue to form an offline dry detention area;
• ring levee in the vicinity of Casuarina and Whelan/Coronata Intersection ;
• flood storage area immediately upstream of Koloona Avenue following voluntary purchase of property;
• flood storage area immediately downstream of Koloona Avenue following voluntary purchase of property and 
• flood storage area in the vicinity of Casuarina Place and Eureka street.

An engineering consultancy has been engaged to undertake this project.

Voluntary purchase
Under the FRMPeighteen properties were earmarked for voluntary purchase.  To date one property on Princes Highway, Figtree, adjacent to Byarong Creek, has been purchased and demolished by Council.

The implementation of further work is dependent on the Federal and State Governments' funding. Council has submitted application for funding for the 2008-09 financial year. No advice has been received to date. 

Status:
Floodplain Risk Management Study and Plan has been completed and adopted by Council.