Hewitts Creek

Content: Catchment description | Flood problem | Impact and cost of flooding | Mitigation measures | Benefits of mitigation measures | Implementation of mitigation measures | Status

The Hewitts Creek Floodplain Risk Management Study and Plan was adopted by Council in February 2003.

Catchment description
The catchments of Slacky, Tramway, Woodlands, Hewitts and Thomas Gibson Creeks are collectively known as the Hewitts Creek catchment. Located approximately 10kms to the north of the city of Wollongong, they drain a catchment area of approximately 7.5km2.

The catchment comprises of a mix of developed land (residential, commercial, industrial) and natural environment with the escarpment forest on the steep western half of the catchment.

Urban expansion in the catchment has generally reached its limits, although there is ongoing redevelopment of industrial land along with intensification of densities in areas close to town centres.

Flood problem
Flooding in this catchment is well known with a number of significant floods occurring in the past. August 1998 was the most recent and significant flooding event in the catchment. The limited catchment size and the steep terrain of Hewitts Creek, often resulting in flash flooding, provides resident’s very little warning time of an approaching flood.

Past development in the lower reaches of the catchment has resulted in the construction of many culverts and bridges on streams which have had the effect of impeding flows in these streams in times of flood. In addition, infringement of development onto floodplains has in several locations reduced the effective width (and waterway area) of flow in larger storm events, further worsening the situation.

Ocean conditions and sand build up at the ocean outlets also contribute to flooding in the lower reaches of Slacky, Tramway, Hewitts and Thomas Gibson Creeks.

Hewitts Creek catchment has a number of known “hot spots” with regards to flooding. They include:

• sites upstream of the rail line and other north-south orientated roadways and embankments;
• site adjacent to culvert structures which are generally undersized and prone to blockage in larger events;
• sites that have poorly defined overflow paths such as the smaller tributaries of Hewitts Creek and in older area or Thomas Gibson Creek where creek systems have been piped;
• sites downstream of the uncontrolled flow diversions such as in the Tramway Creek at the eastern end of Hobart Street.

Impact and cost of flooding

Number of properties with yard and above floor flooding 

Average annual direct damage: $750,000**.

* PMF = Probable Maximum Flood         
** Figures calculated in 2002.

Mitigation options
A consolidated summary of the recommended mitigation options for Hewitts Creek Catchment are listed below:

• implementing planning and development controls (minimum floor heights, maximum building heights, appropriate building materials, etc);
• flood education program;
• flood signage;
• update Local Flood Plan (SES);
• formalise over flow and diversion paths;
• install sediment basins;
• channel enlargements and stabilisation;
• ocean outfall policy;
• culvert modifications;
• voluntary house purchase;
• debris control structures;
• property modification such as house raising;
• creek rehabilitation;
• raising of kerb and guttering.

The total cost of the recommended mitigation options is close to $9 million**.

** Figures calculated in 2002.

Benefits of mitigation measures
The following tables outline the benefits of the proposed mitigation options by comparing the number of flood affected properties, pre and post mitigation works.

Properties affected by above floor flooding:

Properties affected by below-floor flooding (yard flooding):

Implementation of mitigation measures
To date, there have been a number of mitigation measures undertaken in the Hewitts Creek Catchment. These are summarised below.

Hewitts Creek planning matrix
A planning matrix for Hewitts Creek has been adopted by Council and forms part of DCP 54 “Managing Our Flood Risk”. The matrix considers the range of land uses, and the potential risk to flooding, within the floodplain up to the level of the probable maximum flood. Using this approach, a matrix of development controls, based on the flood exposure and the land use, has been developed that balances the risk of exposure across the floodplain.
If you are developing within the Hewitts Creek floodplain you will need to refer to DCP 54 (including the planning matrix) to ensure that Council’s requirements with respect to floodplain management are understood and met.

Black Diamond Place, Bulli - modifications to detention basin
The Slacky Flat detention basin is located at the eastern end of Black Diamond Place, and is positioned off-line to Slacky Creek. The purpose of this project is to modify the configuration of the basin outlet to reduce the frequency of shallow flooding downstream of the basin, and in particular along Beacon Avenue.   

The basin modifications include closure of the existing spillway located at the northern end of the basin, and the incorporation of a new spillway by lowering a section of the basin embankment located closer to the creek.  The effect of these works is to encourage the direction of basin overflows away from Beacon Avenue and back into Slacky Creek.

Design and construction of these works have been completed. 

The Esplanade, Thirroul - new overland flow path
The subject site is a public reserve of open space located between The Esplanade and Flanagans Creek to the north. During times of flood, this area becomes an overflow path for surface waters flowing from the south and towards the creek.
 
The purpose of the overland flow path is to reduce the incidence of shallow flooding within the public reserve and adjoining properties in the general vicinity of The Esplanade.
 
The works comprise reshaping and lowering of a section of the open space area to create an overflow swale approximately 5m wide at the base, 25m wide at the top and a maximum of 0.9m deep. This provides a path for water to be drained from The Esplanade to Flanagan’s Creek. The gentle slopes associated with the swale ensure that the area can continue to be used for recreation, with revegetation of the area also included.

Design and construction of these works have been completed.

Lachlan Street, Thirroul - improvements to existing culvert
The objective of the project is to reduce the existing flooding problems along Hewitts Creek in the general vicinity of Lachlan Street through improvements to both the inlet and outlet of the existing culvert under the road. The improvement works aim to reduce the frequency with which the culvert blocks with debris, thereby increasing the amount of flow through the culvert and reducing the flow that overtops the roadway and enters properties.

The works comprise:

• gabion walls along both creek banks upstream (northern side) of Lachlan Street;
• concrete apron and wingwalls added to the upstream face of the culvert;
• lowering of the existing culvert headwall on the downstream (southern) side of Lachlan Street;
• concrete wingwall added to the downstream face of the culvert;
• installation of safety handrails and provision of landscaping works.

Design and construction of these works have been completed.

Corbett Avenue, Thirroul - levee scheme
This project aims to reduce the flood risk to the local community and development in the Corbett Avenue/Hamilton Road area, through the construction of a levee on the northern bank of Hewitts Creek near its entrance.

Council has carried out a detailed investigation into the feasibility of such a levee scheme, including an assessment of various levee options and associated site constraints.  The report has recommended that the levee scheme not be implemented as the project cannot be justified on purely economic grounds given the limited benefit that will be gained from its implementation. 

Tramway Creek, Bulli - upgrade of railway culvert
The area immediately upstream of the Illawarra Railway line embankment, and in particular Allenby Parade, is subject to significant flooding due to the inadequate capacity of the existing culvert beneath the railway line and its potential to block with debris.  

The objective of this project is to reduce the incidence of flooding along the section of Tramway Creek immediately upstream of the railway embankment through an upgrade to the existing culvert to significantly increase the capacity of the current system.  The upgraded system will therefore have a lower potential to block, and result in lower flood levels upstream of the railway line. 

The project is currently in the preliminary design phase.

Woodlands Creek, Bulli - upgrade of railway culvert
Significant flooding is also known to occur in the area immediately upstream of the Illawarra Railway line embankment (including the Princes Highway) due to the inadequate capacity of the existing culvert beneath the railway line and its potential to block with debris.     

This project aims to reduce the incidence of flooding along the section of Woodlands Creek immediately upstream of the railway embankment by upgrading the existing culvert system.  As a result, the capacity of the upgraded system will be significantly increased, therefore reducing its potential to block, and resulting in lower flood levels upstream of the railway line. 

The project is currently in the preliminary design phase.

Voluntary purchase
Under the FRMP three properties were earmarked for voluntary purchase.  To date, two properties along the Princes Highway, Bulli, have been purchased and demolished by Council.

Status: The Hewitts Creek Floodplain Risk Management Study and Plan was adopted by Council in February 2003.