The Narrandera Shire Council has endorsed its urban stormwater upgrade design for Narrandera and will place it on public exhibition until July 1 for the community to review the design and funding options and make submissions to Council.

The combined construction cost to achieve the ultimate upgrade outcome is estimated at $5,900,000.

The Council was faced with three options:

1. Resolve to accept the drainage design and place on public exhibition, as detailed within the recommendation.

2. Reject the proposal and seek alternate designs for the drainage.

3. Resolve not to pursue the drainage improvement options and accept the current arrangements.

Council had already allocated $4 million in the 2022/23 budget. This was made up of 50% grant funding and the remaining funds from Council allocation that would be derived from external loan funding.

Deputy General Manager (Infrastructure) Shane Wilson said the ultimate solution was $1,900,000 over the original pre-design estimate, which was never thought to be able to achieve such a substantial mitigation of the urbanised flooding.

“Consideration will need to be given to the desire to source additional funds to complete the ultimate solution, or vary the scope, or defer the project until funding becomes available,” he said.

“The required loan funds also need to be further considered, as the loan repayment liability would need to be identified and included within the operational budgets moving forward. The loan payments would be around $125,000 per year based on a $2million loan, with one of the funding options being the introduction of a special rate on all affected properties.”

Mr Wilson said given the complexity and timing of the loan repayment options, a further report specifically considering the funding would be presented to Council at its June meeting.

The report to the May meeting was intended for Council to consider the recommended design solution to remediate the urbanised flash flooding experienced in January 2022 and to resolve support for the project and the required funding allocation.

During the January 2022 storm events the Narrandera urban stormwater system was unable to cope with the volumes of water emanating from intense storms, resulting in the flood inundation of several properties.

The week after the last storm event, Council staff engaged the services of consultant hydrologists and engineers with the intent of designing solutions to mitigate the impacts of these types of local flash flood storm events on the urban areas of Narrandera.

Mr Wilson explained that the proposed works had to be designed to consider the future usability of the road networks and public places and at the same time be financially achievable.

The proposed design solution proposes three drainage line improvements that will provide flood mitigation to cater for a one per cent storm event, such as those experienced in early 2022.

While the ultimate design will include all three sections of the upgrade, it was recommended that the minimum works should include the two-drainage line 1 and 2A works as detailed on the plan and it had an estimated combined construction cost of $5,350,000.

Council engaged the services of Spiire (consulting engineers) and Lyall & Associates (stormwater and flood hydraulic engineers) to undertake hydraulic modelling and design of a stormwater drainage system for the urban area of Narrandera to cater for the flash flooding events of recent times.

“Council has worked for several years to develop design solutions for overland and riverine flooding. However, the changing weather in more recent times has seen an increased importance on catering for localised urban storm events that result in flash flooding,” Mr Wilson said.

“The hydraulic modelling and design are now complete, with new drainage lines proposed to run from Bolton Street to the canal system south of Larmer Street in a combination of open and closed drainage cells.”

Mr Wilson said the proposed works had been broken up into components and while some needed to be undertaken to ensure a continued and completed drainage system is available, others could be undertaken as a separate project.

Modelling for the upgrades identifies that the design will be able to cater for a near 1-in-100 rain event such as experienced in early 2022, with all the buildings within the central urban areas being free from flooding whilst the outer areas will have a substantial reduction.

Further stormwater detention on the outer areas as identified within the Narrandera Flood Plain Study will provide further protection for these properties, but such works were not included in these cost estimates.

The draft design presented to Council has been endorsed and will be placed on public exhibition during which time the community can review the design and funding options and make submissions to Council.

A further report will be prepared for Council to consider the matters raised if substantial objections are received.

Mr Wilson said that as could be seen from the storm events in January 2022, the impacts on infrastructure and the community were substantial and there was a high likelihood that these types of events would occur more frequently, putting Council infrastructure and the community at risk of further damage and loss.

“Given that Council is aware of the issues and does not have a design to mitigate any of the events, failure to act could result in insurance claims against Council,” he said.

This article appeared in the Narrandera Argus, 26 May 2022.

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