Sediment pond helps clear water

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Regan McRandle, from the Roading Company (left), ICC Parks Operations manager Chris McAulay and ICC Capital Projects project technician Leonardo Ramirez at a planting day. PHOTO: SUPPLIED

ONCE a dumping site but now an area supporting biodiversity, an Invercargill sediment pond is helping to keep the region’s waterways healthy.

Completed late last year, the $500,000 pond project, near the Beatrice St entrance to the Waihopai estuary, seems to be doing its job.

Invercargill City Council capital projects project technician Leonardo Ramirez said the main purpose of the pond was to retain sediments and other particles contained in the first flush (first 25mm of rainfall) catch from a total of 82 hectares in the north area of town.

While there was an existing one metre ditch, it was not big enough to retain water long enough for particles to sink; most sediment collected from sumps was pumped “almost immediately” into the estuary.

A planting day was held and volunteers added more than 9000 plants to support biodiversity.

“Since the pond was built, a very positive outcome has been realised, with a noticeable reduction of sediment before the water is pumped into the estuary. Once the planting is more fully established we expect even better performance,” Mr Ramirez said.

He said a second stage of the project could be constructed, depending on the performance on the first.

“This will increase the retention time of the current water discharging in the existing ditch linked to the new pond.

“Before the pond was built, the surrounding area was becoming a dumping site, where people were illegally dropping off unwanted stuff like tyres and couches, which caused a negative impact in the security and landscaping of the area.”

Environment Southland land and water services manager Fiona Young said sediment ponds and constructed wetlands were not something new in Southland.

“Across New Zealand, when there is new infrastructure going in for things like a new subdivision, these sorts of treatment devices are required to manage stormwater runoff.”

She described it as a settling area for water to remove sediment, nutrients and heavy metals.

“High levels of sediment are a major contributor to poor water quality, can harm biodiversity and impact on various uses for waterways like swimming and fishing. Sediment is often contaminated with chemicals and nutrients causing further environmental issues.

“A well designed sediment trap can therefore lead to significant improvements in water quality.”

They could also provide benefits during floods by holding back or storing sediment-laden water and releasing it during lower flows.

She said NIWA had estimated a sediment trap which measures about 1% to 5% of its catchment can collect anywhere from 30% to 90% of the sediment coming from the catchment.

“There are possibly hundreds of sediment traps around Southland in various forms, commonly used with roading, forestry, urban land development and more often on farms.”

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