Tahunanui Stormwater Diversion

Preventing erosion of the beach

The coastal erosion that threatened to wash away Tahuna Beach has been thwarted – the new seawall and stormwater outlet has seen a gradual build-up of beach, instead of it being carried away by the waves.

Dune levels near the tree-line have risen considerably since August, 2005, when 9,000 cubic metres of sand was shifted by trucks and loaders from the spit to the beach, giving the dunes a healthy head start.

How do we know?

Council engineers are monitoring sand levels of the dune and spit, as required by the coastal permit. Every three months they measure several ‘profiles’ (areas of the beach) and compare them to previous measurements.

If necessary, more sand can be shifted mechanically from the spit to the dunes – up to 10,000 cubic metres a year over the next thirteen years. However, unless a combination of severe storms and exceptional high tides causes significant erosion, this shouldn’t be needed.

Profiles of the spit, from where the dune restoration sand was dredged, show it is gradually regaining the sand removed.

Not just sand

An important contribution to restoring and maintaining the dunes is made by the Nelson community – Coast Care volunteers planting thousands of the native pingao and spinifex contributed by local business Bowater Honda, which last year donated $11,000 for the project from their Honda Tree Fund. These ‘pioneer’ grasses help trap the sand and rebuild the dunes.

In September 2005, 180 Tahunanui School students, 30 Bowater Honda employees, Coast Care and Nelson Tree Planters volunteers planted more than 5,000 grasses; more plantings are planned for later this year. Environmental Reserves Supervisor Bruce Dippie explains, “We’re going to start a bit earlier in the year this time around, which will allow more time for the grasses to get established.” In some areas the grasses have become well-established, while in other parts of the shoreline they didn’t take hold.

What's been done so far?

The 174-metre-long seawall has two purposes.
First, to reduce the impact of waves bouncing off the wall, a sloping concrete shelf was built on top of the existing concrete wall. Second, the seawall serves as a culvert to carry stormwater - which used to drain out at the top of the beach between the carpark and the playground - further out to sea.

As a further gain, the stormwater channel that used to separate the Abel Tasman carpark from the Lions Playground has become a grassy plot connecting the two.

What gets planted - and why

The growing dune area is being planted with pioneer foredune plants spinifex (Spinifex sericeus) and pingao (Desmoschoenus spiralis), which trap wind blown sand in the frontal dune. This sand serves as a reservoir for the beach during periods of wave erosion. If sand-trapping dune vegetation is not present, wind-blown sand from the beach moves inland and is lost from the beach/dune system.

The above-ground parts of these dune plants act as wind filters, slowing the wind down and depositing more sand on and around these plants, naturally rebuilding dunes after wave attack. Spinifex and pingao can grow through accumulations of wind blown sand. Cycles of sand deposition and plant growth result in dune formation and buildup.

Beach cycles come and go

Beach changes are cyclical in nature:

• Storm waves move large quantities of sand from the beach and dunes to build offshore storm bars
• Subsequent calm weather produces onshore movement of the sand to re-establish the volume of sand on the beach
• Prevailing onshore winds blow sand back into a dune system where it can be trapped by dune vegetation, particularly the native sand binding grasses spinifex and pingao. The dune steadily builds up as these plants trap the wind blown sand. Gradually the plants become buried and grow up through the sand, continuing to trap sand as they do so. Over time, the eroded dune is repaired. This natural process of dune repair has maintained coastal dunes over centuries, despite episodes of severe coastal erosion.

What you can do

Dunes are our protection from coastal erosion.

If we protect the native dune plants, dunes and beaches will recover and rebuild after storms. Please use beach access ways where provided.