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Rocky ridge built in estuary to protect and restore vital ecosystem

Nelson City Council says a rocky barrier built in the Waimea Estuary is designed to “dampen wave action” near an area of saltmarsh under restoration.

Posted on November 29, 2022

A barrier made of rocks has been built in the Waimea Estuary to protect and enhance an area of saltmarsh – an ecosystem instrumental in helping mitigate and adapt to climate change.

Nelson City Council said it erected the barrier, known as a chenier sill​, along a section of the estuary’s 65 kilometre margin, just south of Monaco.

The sill would “dampen wave action” to protect plantings and reduce shoreline erosion at the site, close to the mouth of the Orchard Stream in Stoke, the council said.

It would also trap sediment, increasing the height of the estuary bed over time and changing the shape of the shoreline to make it more suitable for saltmarsh vegetation to grow.

Coastal ecosystems including saltmarsh overseas have been found to sequester much more carbon than equivalent areas of forest.

Coastal vegetation also provided greater protection to low-lying coastal areas from sea level rise and storm surges, regulated coastal water quality and provided a vital habitat for estuarine species.

Saltmarshes were some of the most productive ecosystems in the world, and a significant part of the detrital food chain (a type of food chain that started with dead organic matter), the council said.

The habitat in the Waimea Inlet had “been in significant decline” in recent years, it said.

Its ecosystems were under threat from excess silt from land clearance, pollution from sewage, industrial wastes and agricultural runoff, introduced pest species and land reclamation and extraction, according to information from the Department of Conservation.

A barrier of rocks has been erected in Waimea Estuary to “dampen wave action” on an area of the inlet being restored.

The outcomes of the “Waimeha/Waimea Inlet Saltmarsh Enhancement Project Pilot”, would be used to inform future work to restore and protect Nelson’s saltmarsh ecosystems, the council said.

The pilot would help determine appropriate locations and species for future plantings, it said.

Around 1800 plants had already been planted within a trial area of the site, focusing on three species; salt marsh rush, jointed wirerush, and knotted/knobby club rush.

Other research underway in the inlet included a study of the amount of carbon stored at sites dominated by jointed wire rush and glasswort, led by the Tasman Enviornmental Trust.

The inlet was also one of several coastal weland areas scoped by non-governmental organisation, The Nature Conservancy (TNC), which was exploring the possibility of a voluntary “blue carbon” credit market to finance large scale restoration activities in New Zealand.

People buying credits to help fund the restoration in such a market could use the credits to offset their emissions.

TNC earier this year identified tidal salt marsh as “a real opportunity” for blue carbon restoration projects in Aotearoa.

Nelson City Council (NCC) said the pilot also “connected with” several other activities across the Inlet – the largest semi-enclosed estuary in the South Island spanning Nelson and Tasman District.

Activities included the Waimeha Inlet Enhancement Project funded by Ministry for the Environment (MfE) through Tasman District Council (TDC) (with MfE providing some funding towards NCC’s saltmarsh pilot via TDC, and from the Waimeha Inlet Billion Trees project towards plants and planting), The Waimea Inlet Restoration ProjectBattle for the banded rail,] NCC’s Heathy Streams programme: and the Kotahitanga mō te Taiao strategy.


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