What lies beneath: The problem with the Modellers Pond
Fill it, save it, or convert it?
The Nelson City Council's been grappling with questions over the future of Tahunanui's Modellers Pond for nearly two decades, as weed and algae continue to plague the pond.
The council is now back at the drawing board after the latest agreed fix came back with a higher than expected price tage of $1.73 million.
Group manager of infrastructure Alec Louverdis said the pond was a two-pronged issue; on one hand, it holds a recreational function for model boats.
"There are a number of councillors who used to do their little boats there and there is a nostalgic view to that," he said.
However, dating back to 1998 it also holds a stormwater function. That year the council decided to deal with persistent flooding in Tahunanui by piping stormwater via a pump station under nearby Centennial Park, through 600mm pipes into the Modellers Pond.
The pond has a small island with two pipes sticking out, which produce a fountain of stormwater during heavy rain. The stormwater mixes with tidal water and ends up in the estuary aat Tahunanui's Back Beach.
Prior to 2000, the pond was kept clear of weed and algae by using the chemical copper sulphate. However, new Resource Management Act standards banned chemicals as they end up in the estuary.
The council, with encouragement by the Modellers Society has spent the last 17 years looking for alternatives.
What the council has tried
Louverdis said the addition of stormwater hadn't caused the algae and weed problems; rather the issue was the pond environment itself.
The base is sludge and mud that allows weeds to grow, and summer sun and warmth causes algae to thrive.
Years of chemical treatment has contaminated the soil underneath.
Workers can't uproot the weed as they disturbing the Hazardous and Industrial Land (HAIL).
They can do temporary clean-ups by sweeping weeds and algae off the top – one is planned ahead of the Modeller's Convention this summer – but weed and algae can return within a week.
Following the ban of copper sulphate, Louverdis said staff did "many, many reports" and tried to get a resource consent to use other chemicals and colourants.
The discovery of a beetle, "which is quite unique to the Back Beach", ensured the use of any chemicals is permanently off the table.
Louverdis said Nelmac suggested they introduce fish but it "was very, very complicated" because of the presence of fresh and tidal water.
The fish wouldn't eat the weed and couldn't survive in a mix of salt and fresh water. Councillor Kate Fulton said at a recent meeting, "if only the carp had worked", to which Louverdis replied, "if only".
Another idea that has been bandied around for years is the "modify" option voted through earlier this year. It would put a concrete base on the pond.
However it was given an unexpectedly high $1.73m price tag – over and above the $1.2m budget for the project.
The "specialist work" modifying the pond would involve putting a massive concrete slab on the bottom, sealing the HAIL underneath. It would need to be thick and have "pressure release valves" because the ground water level is high, meaning pressure from water could push the concrete up.
It would also need a "lagoon master" to create circulation and stop weeds growing, and the council would need to replenish the pond with both fresh and tidal water.
The council put that option on hold while they take another look options from 1998.
What's back on table
Before any decisions are made about the pond's future, the council has to address where the stormwater could go.
In 1998, the council considered a swale drain for stormwater – an open channel directly to the estuary.
"One of the reasons that the swale wasn't adopted at that stage was because it was a perceived safety risk, because when the pumps work it's about 1.8 cubic metres a second, so that's two VW [beetles] coming towards you, it's a lot of water," Louverdis said.
Another option is to pipe the stormwater to an area further away from pond and Tahunanui's recreational spots.
Louverdis said in considering these 1998 options, they had to bear in mind "the environment has changed".
There are new environment standards and a good chance the council would now be required to treat the stormwater before it reaches the estuary, and mitigate erosion caused by water gushing from the drain.
Any changes to stormwater would also need a new "fully notified" resource consent, seeking public feedback.
The council is also again considering a "return to estuarine" option.
Louverdis said there is an argument it should have been explored at the same time as the modify option – a concern raised by councillors Brian McGurk and Matt Lawrey at a recent council meeting.
However Louverdis said it was not common for engineers to do a "detailed design" on more than one option. A rough estimate has the estuarine at $690,000 but it's thought it could reach $1.1m to $1.3m.
An estuarine environment could have positive environmental aspects, given wetlands are often used to treat water before it ends up in rivers or harbours, but it would also require a new resource consent.
The other option is to "fill it in". This would require re-routing stormwater to the estuary, emptying the pond, topping and sealing the HAIL, and filling the area with a large volume of soil.
Where to from here
There has been criticism from councillors and from the Modellers Society about the length of time it's taken to get to this point, and successive councils' failure to enact a solution.
But Louverdis said it has always been a complex issue.
"There's a history behind this and there's a logical progression. It's not as though we've been sitting on our hands, we've tried a whole lot of things," he said.
Council staff will spend the next four to five months taking another look at the 1998 options, and doing a detailed investigation into the "return to estuarine" option.