Calls for rain radar in Nelson as regions vulnerability to flooding intensifies

Changes have been made to the monitoring of Nelson's rivers since the damaging floods of August 2022, but council staff say the region remains vulnerable to storm events without a rain radar.

Nelson City Council hydrologist Chris Strang said in the last 18 months, work had been underway to build resilience in the flood monitoring network and bring it back in-house.

River management services across Nelson and Tasman have been managed by the Tasman District Council since 1992, after being transferred from the Nelson/Marlborough Regional Council in 1989 and before that, the Nelson Catchment Board.

Since last July, the council had been in charge of its river data, maintaining a network of flow monitoring sites in a number of rivers, with water level data collected and sent back through telemetry into Civic House.

The monitoring stations used gas bubbler sensors or float and weight systems to measure river levels, transmitting data in real time back to the operations centre via cell networks or digital radio.

In advance of bad weather, Strang said his team were out checking the river flow sensors and getting the flood gauging gear and equipment ready to capture data.

He said Nelson was surrounded by hills on three sides and its river catchments were quite short compared to larger rivers in other regions, and the rivers flowed through urban areas - so being able to provide early flood warnings was crucial.

"During a flood event, we have a series of stage alarms so when the river rises to a certain stage, it alarms and then there's actions that are taken from those stage levels."

If things were looking serious, Nelson Tasman Emergency Management and wider council staff would get involved to make decisions on evacuations or other public safety matters.

Three days of high intensity rainfall during the storm in August 2022, led to flooding and widespread slips across the region.

The Maitai River burst its banks and reached a flood flow of 322 cumecs (cubic metres per second).

Strang said that was the equivalent of 322,000 one litre milk bottles flowing past someone, per second.

He said it was the sub-tropical weather systems that came towards Nelson from the north that caused the biggest trouble, like the one bearing down on the country this Easter, especially if they included high intensity rainfall in a short space of time.

"There's a lot of ocean between us and the tropics for it to collect rain and moisture and Nelson is open to the north, that's why we historically have had very large floods in the Nelson-Tasman region."

The need for radar

A report to the Tasman District Council in June 2023 said the unpredictable nature of rain that year had highlighted the poor coverage in Tasman of the Wellington weather radar, with "isolated convective rain cells sneaking in between our rain gauges".

Environmental information manager Rob Smith said in the report that a good example was the large Wai-iti flood in May 2023, which was the largest since records began in that area in 1987, with no advance warning from the rain gauge network.

He noted there had been an increase in storms and unpredictability, with heavy rain appearing when it wasn't expected.

Because of the poor coverage, a marine radar at a cost of $30,000 had been installed, which gave a visual output of heavy rain but no numerical data, so it could not be ingested into flood warning models.

It was not a permanent solution, as the unit had limited power for Nelson, Riwaka and the Waimea Basin.

Tasman District Council principal hydrologist Martin Doyle said the council had been pushing for a rain radar in the region since 2010, after a major flood in the Aorere Valley in Golden Bay.

The region was considered a blank spot in the national radar coverage.

He said there were 50 rain gauges, around the size of dinner plates, situated around the region that collected rainfall samples, but in some places they were up to 50 kilometres apart.

"We're actually quite blind between the location of those rain gauges and so the radar just fills in the gaps, it gives us a visual picture and it also gives us data we can feed into our flood forecasting models.

"We've got areas of Nelson City, Richmond and Riwaka that are highly vulnerable to short duration high intensity rainfall, and so that just gives us an extra bit of information and we can see incoming storm cells which we weren't able to see very well in 2013, when Richmond was very badly flooded."

He said there had been an upsurge of flood events in the last decade, including ex-tropical cyclones Fehi and Gita in 2018, the latter which caused numerous slips that closed the Tākaka Hill, and significant flooding in the Riwaka Plains and Takaka Valley.

The 2021 storm caused the Motueka River to burst its banks, and the 2022 storm also badly affected Nelson City.

He said in his 45 years in the industry, he'd learnt extreme care was needed when it came to dealing with ex-tropical cyclones, like the one bearing down on the country this Easter.

"With a small change in direction or speed, that weather system can cause vastly different effects, so whenever we've got a system like this coming through we actually take it very seriously and take quite a conservative approach - and that was particularly borne out with recent experiences with ex-cyclone Gita."

Without four experienced flood forecasting staff who could fill in the gaps, he said the council would be in a very poor position.

"We kind of feel like it's our time to have something located here just because of that vulnerability and the huge advantages that weather radar brings to not only us and the flood forecasting team, but the whole community, the farmers, the aviation industry," he said.

It had been one of his goals to see a weather radar installed in the region before his retirement.

With Doyle planning to hang up his raincoat in the next year, he said a rain radar for Nelson Tasman would make a better retirement gift than a gold watch.

MetService has been approached for comment.

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