Implementation of Mitigation Works to Date

To date, there have been a number of flood mitigation measures from the Hewitts Creek Floodplain Risk Management Plan (December 2002) which have been implemented. These are summarised below.

Hewitts Creek planning matrix

A planning matrix for the combined Hewitts Creek catchment has been adopted by Council and forms part of DCP 2009 (Chapter E13 – Floodplain Management). 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 in this area, you will need to refer to DCP 2009 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 concept design phase.

Voluntary Purchase

Under the Floodplain Risk Management Plan, three properties are earmarked for voluntary purchase.  To date, two properties along the Princes Highway, Bulli, have been purchased and demolished by Council.