Towradgi Creek

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

Towradgi Creek Floodplain Risk Management Study and Plan was adopted by Council in September 2003.

Catchment description
The Towradgi Creek catchment drains an area of about 7.2km2 through the northern Wollongong suburbs of Tarrawanna, Corrimal, Towradgi and Fernhill. The catchment lies approximately 5.5km north of the Central Business District of Wollongong.

About 50% of the catchment is steep and forested and forms part of the Illawarra Escarpment. The lower half of the catchment is much flatter and mostly urbanised. Towradgi Creek flows into the Pacific Ocean at Corrimal Beach.

Six main tributaries flow into Towradgi Creek. For this study, in the absence of other information, and as agreed to by the Floodplain Risk Management Committee, the six tributaries were given the following names:

1. Towradgi Creek;
2. South Angels Creek;
3. North Angels Creek;
4. South Corrimal Creek;
5. North Corrimal Creek;
6. Carr Creek (also known locally as Jones Creek);
7. Parker Creek.

Flood problem
The Towradgi Creek catchment has suffered regular flooding with 24 instance of flooding occurring in the district from 1958 to 1989. However, because there had been no systematic collection of flood data until the early 1990s, the details on any one particular flood are quite limited.

On 17 August 1998 the largest flood on record was experienced within the catchment. At least 40 houses and units were flooded above floor level and the retail area in Corrimal was severely damaged. Vehicles were swept off flooded roadways and some residents battled very hazardous conditions whilst evacuating their flooded properties.

Analysis of this flood showed that the peak flood flow of the August 1998 flood was smaller than the 100 year flood in the upper and middle parts of the catchment and just larger than the 100 year flood in the lower part of the catchment. The flood levels of the August 1998 flood were found to be typically much higher than the levels predicted by previous studies of the catchment. The primary reason for these higher than predicted flood levels was that almost every pipe and culvert crossing was blocked by vegetation and other debris.

As a consequence of this flood event, Wollongong City Council decided that the flood study being conducted at the time should be reviewed in light of the information received from the August 1998 flood.

Impact and cost of flooding
The following table summarises the number of properties that would be flooded in different flood sizes, together with the flood damage that would be suffered by the Towradgi Creek catchment community.  This data is determined from the Flood Study component of the floodplain management process.

* PMF = Probable Maximum Flood                              
** Figures calculated in 2003.  

The following points summarise some of the impacts of flooding in the catchment:

• in a 100 year flood, the majority of residential properties flooded above floor level would be located in Towradgi Creek (33 properties), Parker Creek (27 properties), North Corrimal Creek (20 properties) and Carr Creek (20 properties). Most (10) of the commercial properties would be located in North Corrimal Creek;
• in North Corrimal Creek there would be 12 residential properties and 9 business properties flooded above floor level in a 5 year flood. This represents more than one third of the residential properties and most of the business properties in the Towradgi Creek catchment flooded in the 5 year flood; 
• of the more than 600 residential properties that would have their yards inundated in a probable maximum flood, 250 of these properties would be located in the Towradgi Creek (Branch 1) floodplain;
• the maximum depth of flooding above floor level of residential properties would be 1.4m in a 5 year flood, 1.7m in a 20 year flood, 1.8m in a 50 year flood, 2.0m in a 100 year flood, and 3.2m in a PMF;
• the average depth of flooding above floor level of residential properties would be 0.33m in a 5 year flood, 0.35m in a 20 year flood, 0.42m in a 50 year flood, 0.42m in a 100 year flood, and 1.0m in a PMF.

Mitigation options
The following is a summary of the adopted mitigation options as detailed in the Towradgi Creek Floodplain Risk Management Study and Plan (FRMP), June 2003. Please refer to the FRMP for more detailed information.

• updating planning controls and policies;
• preparation of Riverine Corridor Strategy;
• preparation of flood proofing guidelines;
• improved Emergency Management Planning;
• improved Community Flood Awareness;
• development of Policy - Management of Escarpment Runoff;
• voluntary Purchase of nine properties;
• voluntary house raising  of 16 properties;
• voluntary reconstruction/redevelopment of 12 properties;
• embankment and channel improvements;
• construction of debris control structures (9);
• management of ocean entrances;
• culvert enlargements;
• improving and maintaining overland flow paths.

Benefits of mitigation options
The implementation of the mitigation options will see a significant reduction in the number of properties adversely affected by flooding. The total cost of the Towradgi Creek Floodplain Risk Management Plan is in the order of $14.5 million**. Implementation of the entire plan would result in an approximate saving of 40% of the average annual flood damages in the catchment (a saving of $500,000** per annum).

** Figures calculated in 2003. 

Implementation of mitigation measures
To date, there have been a number of mitigation works undertaken in the Towradgi Catchment.

Railway Street, Corrimal - culvert amplification works
This project involves the design of culvert amplification works in the piped section of North Corrimal Creek between the eastern (downstream) side of the Northen Distributor and No.29 Railway Street. The proposed works aim to reduce the incidence of flooding in the local area via the amplification of the existing Railway St culvert system and associated outlet works.  
 
The proposed works principally comprise removal of the existing twin 1,800mm diameter pipes and replacement with culverts of the same size to those beneath the Northern Distributor (3 x 3,000mm wide x 1,800m high). The length of the proposed drainage system is approximately 95m.

The design of the works, including the resolution of conflicts with existing services and the preparation of a Traffic Management Plan to minimise the disruption to traffic during construction, has been completed.  

During the design process, a detailed estimate of the construction cost was prepared. As this cost significantly exceeded the benefits to be derived from the project, implementation of the works was terminated as it was not considered viable.

However, Council is pursuing the investigation of alternate flood mitigation options in order to provide flood benefits to the local community. This includes potential improvements to the downstream section of north Corrimal creek.
 
Railway Street, Corrimal - debris control structure
This involves the installation of a debris control structure and channel works along North Corrimal Creek, immediately upstream (on the western side) of the Northern Distributor culverts near Railway Street.   The aim is to reduce the frequency with which the existing Northern Distributor culverts block with debris.
 
The proposed works comprise:

• the installation of vertical posts, across the base of the creek, to act as a debris trap;
• reshaping and rock-lining of the culvert inlet area;
• an off-line elevated storage area to collect floating debris which would otherwise be carried downstream by surcharging floodwaters;
• improvement of an existing track to provide access down to the base of the creek to allow debris to be removed;
• landscaping works to revegetate the area.

Design and construction of these works have been completed.
 
Colgong Crescent, Towradgi - footbridge
The proposed works involve the demolition of the existing footbridge over Towradgi Creek and replacement with a new footbridge at a higher elevation to achieve flood mitigation benefits for the area.  The objective of the elevated footbridge, which provides a pedestrian link between Colgong Crescent and Linga Street, is to allow more floodwaters to flow through and reduce the amount of debris trapped beneath the footbridge.
 
The footbridge also includes vertical screens added to both sides of the structure for the purpose of providing privacy to the adjacent residential development.
 
Design and construction of these works have been completed.
 
Bellambi Street, Justine Ave and Cheryl Place, Tarrawanna - flood mitigation works
The aim of the project is to mitigate the impacts of flooding in the area by providing well defined overflow paths in combination with measures to reduce the potential for the existing underground culvert system to block with debris.

In order to achieve this, the proposed works include:

• improvement of the existing culvert in Justine Avenue to increase its capacity to accept water and to reduce the potential for debris blockages;
• cutting of an opening in the rear wall of 2 Bellambi Street to allow flow through the wall then onto the yard and Bellambi Street. This will reduce the diversion of flow from the rear of the property southwards towards existing
• development, thereby reducing potential damage during floods;
• construction of a low earth mound at Cheryl Place to deflect floodwaters away from nearby houses.

Design and construction of these works have been completed.
 
Lemrac Avenue, Corrimal - flood mitigation works
The objective of the project is to mitigate the effects of floodwaters conveyed by the existing channel (part of North Corrimal Creek) between No’s 8 and 10 Lemrac Avenue and along Lemrac Avenue south of the intersection with Willow Grove.

The proposed works include:

• provision of flood proof walls on the boundaries of a number of properties to reduce the frequency of ingress of surcharging floodwaters onto the properties, divert floodwaters downstream towards the open creek that runs between No 1 and 5 Lemrac Avenue, whilst also limiting the potential damage caused by waterborne debris
• installation of a debris trap upstream of the rear boundary of No 8 Lemrac Avenue and reshaping of the confluence of North Corrimal Creek and an unnamed watercourse immediately upstream.  This aims to improve the alignment of creek flows, minimise the amount of debris transported downstream to the existing culvert under Lemrac Avenue and thereby reduce its potential for blockage
• installation of a trash rack at the inlet of the Lemrac Avenue culvert to further reduce its potential to block with debris.

Design and construction of these works have been completed.

Underwood Street, Corrimal - flood mitigation works
These works, which aim to mitigate the flood problems associated with the Corrimal retail area, involve the implementation of a scheme of flood mitigation measures along North Corrimal Creek between Underwood St and downstream of the Princes Hwy.  

The outcome of investigations undertaken indicate that the existing underground drainage system needs to be upgraded in order to implement the mitigation measures. Separate investigations will be undertaken to determine the feasibility of upgrading the existing drainage system.

Voluntary Purchase
Under the FRMP nine properties were earmarked for voluntary purchase. To date, two properties along Pioneer Road, Towradgi, have been purchased and demolished by Council.

Status:
Towradgi Creek Floodplain Risk Management Study and Plan was adopted by Council in September 2003.