One-in-five year rain event overloads stormwater network
Saturday’s significant weather event saw 20mm of rain in the Gore township in less than three hours, overwhelming the town’s ageing stormwater network.
3 Waters Asset Manager Matt Bayliss said that amount of rain equated to a one-in-five-year storm event.
Currently, about 40% of the Council’s stormwater network is combined with wastewater. This means there is limited capacity within the network to cope with more significant rainfall events.
“At this stage, it appears the majority of the flooding was in areas where we have known capacity issues in our drainage network, that is, the pipes are too small.”
However, within a few hours of the event, most of the water had receded – “this shows the network was functioning but was overwhelmed by the intensity of rainfall during the peak of the event”.
Mr Bayliss said the Ardwick Street pump station was working during the event. The Council’s other pump stations are designed to prevent floodwaters from entering the network and only operate when the Mataura River is in flood.
The Council has identified significant investment in its long-term plan to address capacity and capability issues.
In 2018 a study found it would cost $175 million to resolve all the capacity issues in the Gore stormwater network.
“The speed with which we can make the necessary upgrades to resolve these flooding issues is currently limited by the Council’s ability to borrow money to fund the work and resources to complete the work.”
The Council has budgeted $11.2 million in its 2021-2031 Long Term Plan to improve Gore’s stormwater network over the next decade.
“The first project included in this funding has been upgrading Elizabeth Street’s stormwater, wastewater and water pipelines.
“Despite the Elizabeth St Project not being finished, it appears the new infrastructure installed to date coped well with the rainfall on Saturday evening. Properties that would normally have flooded didn’t.”
The Council will start looking at its next stormwater improvement projects in the first half of next year, Mr Bayliss said.
One of the 3 Waters Reform drivers is improving services such as stormwater networks.
“Incidents such as the one at the weekend highlight the need for Government assistance to get the work done.
“We have asked for $186 million to be included in the budget over the next 30 years to improve the capacity of the District’s stormwater networks,” Mr Bayliss said.