Minnegang Creek

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

Minnegang Creek Floodplain Risk Management Study and Plan (FRMP) has been prepared for Council by Kellogg Brown & Root Pty Ltd, Level 9, 201 Kent Street Sydney NSW 2000. The plan was adopted by Council in June 2004.

Catchment description
Minnegang Creek is located approximately 8.5km south of Wollongong, in Lake Heights. 80% of the catchment is developed, mostly with low density residential housing. The remaining 20% is made up of open space and recreational areas.

The creeks that make up the catchment of Minnegang Creek include a combination of natural open watercourses and piped drains. Minnegang Creek flows from the north-west of the catchments to the south east where it discharges into Lake Illawarra.

Minnegang Creek has two main tributaries. The first flows from the north of the catchment, follows Melinda Grove and then passes under Gilgandra Street, joining together with Minnegang Creek upstream of Mirrabooka Road. The second tributary commences in the south-west, from Gordon Crescent and flows under RanchbyAvenue, and joining with Minnegang Creek upstream of Lake Heights Road.

Flood problem
The steepness of the catchment results in flooding that is relatively well contained, with floods both large and small having similar results.

There have been five significant flood events in the catchment recorded over the last 20 years. Floods in December 1985 and October 1987 were well recorded but details of the other floods are limited.

One of the contributing factors of flooding in the catchment is the lack of dedicated overland flow paths, ie. areas where water is able to flow before it enters a drainage system. The worst affected area is immediately downstream of the Barina Park detention basin where there is no overland flow path to convey flood flows spilling over the embankment, which is predicted to occur on average at least once every five years.

It is estimated that approximately 60 properties would experience flooding in a PMF event (Probable Maximum Flood), 20 of which would be flooded above floor levels. For a more common storm event - 5 year ARI, 35 houses would suffer yard flooding, with five properties flooded above floor level.

Impact and cost of flooding
The number of flood affected properties in the catchment is relatively few, the highest concentration of affected properties lies directly downstream of Barina Park.

Average Annual Damage: $63,600**

* PMF = Probable Maximum Flood  
** Figure calculated in 2004                                    

Mitigation options
A range of mitigation options have been considered to improve the flooding situation in Minnegang Creek. It is not feasible to implement every mitigation option, rather those which offer the greatest benefit and value for money. Mitigation measures are comprised of a range of options from each of the following categories:

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

A summary of the adopted management options for Minnegang Creek are listed below:

• voluntary purchase of six flood affected properties (Mirrabooka Road and Weringa Avenue);
• house raising of one flood affected property (Barina Avenue);
• spillway for detention basin and flood warning signs at Barina Park;
• rock lined channel from Barina Park to Minnegang Creek (existing pipes used as culverts under roadway);
• community education and flood awareness initiatives for the entire catchment, but focused on properties at direct risk from flooding;
• maintenance of public areas and enforcement of existing drainage easements;
• incorporation of the Planning and Development Control Matrix into DCP 54;
• maintenance of catchment flow paths;
• emergency management;
• update and reissue Section 149 Certificates to the owners of flood-affected properties in the catchment.

The cost of implementing the mitigation works is estimated at approximately $2.1million**

** Figure calculated in 2004  

Benefits of mitigation options
The below figures outline the the number of flood affected properties, pre and post mitigation works.

* PMF = Probable Maximum Flood  

The existing flood problems in Minnegang Creek contributes to an average annual damages cost of $63,000**, however the proposed mitigation works will reduce this cost to $23,000** per year, representing a 64% saving in costs.

** Figures calculated in 2004  

Implementation of mitigation works

Voluntary purchase:
Under the Minnegang Creek FRMP six properties were earmarked for voluntary purchase. To date, two properties at Weringa Avenue have been purchased and demolished.

Spillway For Barina Park Detention Basin and Channel From Barina Park To Minnegang Creek:
This project involves the construction of basin spillway and provision of flow path downstream of the Barina Park Basin. An overland flow path to convey flows from downstream of the detention basin to the start of the open section of Minnegang Creek, downstream of Weringa Avenue is the key requirement in alleviating flooding problems in this zone. Flows for the 1 in 5 year event and larger spill over the embankment of the detention basin and therefore must be directed to Minnegang Creek along a defined flow path.

An engineering consultancy has been engaged to prepare detailed construction drawings for the Barina Park Basin Spillway and flow path.

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