What to do with the Modellers Pond?

Nelson Mail - - INSIGHT - HAN­NAH BARTLETT

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 tag 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 at 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 would dis­turb the Haz­ardous and In­dus­trial Land (HAIL).

They can do tem­po­rary cleanups 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 the 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 the 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 re­cently.

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.

BRADEN FASTIER/NEL­SON MAIL

So­ci­ety sup­port­ers get a first-hand look at how dirty the Modellers Pond is.

BRADEN FASTIER/NEL­SON MAIL

Model maker and Nel­son So­ci­ety of Modellers mem­ber Noel Dyer with one of his boats that he can no longer sail.

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