Narrabundah was established in the 1950s and development in the suburb has continued, like much of Canberra, into the current day. There are two prominent reasons behind these upgrades.

1. At the time the suburb was built, original houses were typically elevated off the ground and the water simply flowed around structures in the path of natural water movement. With the change in size of houses in Narrabundah and coupled with typical construction methodology of today (slab on ground), the area of pervious ground cover (grassed areas) capable of absorbing water has reduced, which further exacerbates flooding risks within the Narrabundah catchment.
2. Stormwater infrastructure constructed decades ago was built applicable to conditions of the day however is now out of sync with today’s standards (depth/size of pipes). Whilst the capacity of the pipes was suitable to transport water in the past, recent urban infill (newer houses) in Narrabundah has shown the network to be strained and incapable of providing flood protection to some residential properties during extreme storm events.

Climate forecasts indicate Canberra is expected to experience more extreme and frequent weather events which will continue to keep properties susceptible to flooding at risk if the necessary upgrades in the stormwater network aren’t made in catchments where deficiencies are evident.

Flood modelling for the entire Narrabundah catchment was undertaken in 2019 to identify priority sequencing of proposed upgrades in the suburb. This modelling is the leading method of assessing stormwater management. Using analysis and detailed survey, through this modelling we are able to input ‘worst case’ rain events and proposed upgrades (i.e. a retarding basin) and understand how the upgrades will manage the conditions and improve impacts to ‘at-risk’ properties.

Click on the links below for more details on the flood modelling specific to each project:

• Sprent Park retarding basin (PPT 11MB).
• Narrabundah playing fields retarding basin (PDF 4.4 MB)
• Throsby Park retarding basin (PDF 2.1 MB)

While a number of options have been raised, the majority of the options raised DO NOT achieve 1% AEP (1 in 100 year) flood protection.

The basin is considered the preferred option as it provides 1% AEP flood protection to impacted areas of the community and can be completed for approximately 10-20% of the total project cost of any of the pipe augmentation options. It is noted the pipe augmentations are also highly disruptive to the surrounding suburb for a period of at least six months, if all necessary funding was available.

Existing pipes can be enlarged, but the cost and disruption can be significant. Also, localised upgrades would simply move the problem to the end of the finished work so the entirety of the affected network would need to be considered applicable to stormwater pipes requiring enlargement.

Any improvements to the stormwater infrastructure discreetly in each area would not provide comparable benefits to the water detention availed through the proposed retarding basins. To meet the same threshold of benefit to the broader suburb, the pipe network would require a significant investment for hundreds of metres. Upgrading the existing pipe network within the road reserve is definitely achievable but the cost would be considerably higher than the proposed retarding basin and the timeframe to complete would be in the ‘years’ timeframe instead of ‘months’.

Enlarging the pipes is also an undertaking that would be subject to funding availability with TCCS obliged to demonstrate why a viable solution like a retarding basin is not preferred and instead, a capital injection of severalfold is preferred. This scenario would create a situation whereby value for money would be queried and providing a clear rationale would be challenging. The current situation in Narrabundah is not one that was created, but rather, one that was inherited. As such, correcting the apparent deficiencies of the past are required but not at the expense of using taxpayer funds equitably across the Territory. Had the topography of Narrabundah not presented an option for a retarding basin, piping the works through the road reserve would have been the next logical decision.

The Territory’s stormwater infrastructure consists of sumps, stormwater pipes, stormwater channels, water quality pond embankments, cut‐off drains, retarding basins, gross pollutant traps, dams, and weirs.

Transport Canberra and City Services manages the stormwater network in the ACT within government land but also inclusive of inter-allotment drainage (stormwater connections within dedicated easements of some residential blocks).

Maintenance alone is not able to address the flooding issues – the piped network is not sized for 100-year events and even running at optimal capacity does not protect leases from inundation in the target areas. Maintenance is not meant to replace the benefits of infrastructure upgrades and it can be seen as a band-aid solution at best. In a major storm event in Canberra, TCCS stormwater maintenance personnel and contractors need to spread their time and focus over the entirety of the ACT. It is thus not practical to affix their attention solely to Narrabundah through a proactive approach. However, they certainly focus their efforts noting issues in Narrabundah are well established.

Upgrade work to infrastructure must be accepted as a rolling program throughout the ACT and Narrabundah is presently in line for the necessary works to ease the identified burden on the stormwater network. Through this, flooding will be alleviated for impacted residents in Narrabundah and free up TCCS maintenance personnel to respond to other problem areas in Canberra until funding is available to improve flooding issues elsewhere.

Regular and routine inspections are carried out throughout the Territory on the stormwater network. In the course of these works, areas beyond ‘typical’ maintenance are identified and investigated further. The locations are then captured in a priority list whereby funding to complete designs (and future construction) are progressed through Treasury. It is to be noted funding sought does not always eventuate (in the short term) as the Territory must strategically grant funding for a range of projects and services to the broader ACT community.

If the ACT Government does not upgrade stormwater in known problem sites, it could be exposed to compensation claims if flooding occurred because it had failed to augment stormwater infrastructure to help manage increased flows.

When issues are identified, all efforts to remedy are undertaken but it is noted addressing a problem may take time due to funding availability. In the meantime, maintenance would be undertaken (as practical) to minimise flooding but it is also noted that residents/businesses impacted by flooding would remain susceptible until such time as upgrade works are complete.

Development applications are required for all three proposed retarding basins and have been granted conditional approval.

Any/all other relevant approvals are progressed in parallel with the DA and include agreement from service providers (e.g. ICON), agreement from various areas within Transport Canberra and City Services including Urban Trees (tree removal and replacement), playground compliance agreement from City Services and stormwater/retarding basin acceptance from Roads ACT.

In general, design elements for flood management around Canberra will only consider outright tree removal if no other means to achieve the overall intention of flood management is possible.

Sprent Park

In total, 27 trees will be removed. These will be offset with the planting of 38 trees, intended to be more mature than
saplings. The trees will be a combination of natives and deciduous species. In addition, approximately 200 square metres of groundcover (including native grasses and shrubs) will be planted which, when established, will see more vegetation in the park than there was prior.

Throsby Park

To facilitate construction of the retarding basin, approximately 21 trees will be removed. These trees have been inspected and identified as being small and in poor condition, providing limited urban amenity. Replacement of these trees will occur at a minimum of 1:1, with an intention to plant more trees than what is removed where possible. The tree removal and species replacement document (PDF 1.8 MB) provides information on the indicative locations of tree removal and replacement species.

Yes. Only during extreme storm events when there may be water present in the basin will usage be compromised. Noting, though, the proposed new playground will be relocated away from any potential lingering water in the basin so it should be accessible following virtually any rain event, excluding a massive storm.

Retarding basins are similar to other bodies of water in Canberra including water quality ponds located at Lake Tuggeranong, North Weston Pond, Lake Ginninderra and Yerrabi Pond as well as playing fields which are often used for retardation.

Whilst education is a key component of teaching children and adults to take care around bodies of water, signage will be erected once construction is complete warning of potential hazard when flooded.