Heed the danger signs from the Boulder Bank
As part of a series of articles prepared by Nelson strategy group ACE (Actions for a Climate Emergency), Kindra Douglas looks at the possible impacts of climate change on Nelson’s Boulder Bank.
Ishudder at the thought of what’s happening to Nelson’s wonderful Te Pokohiwi/Boulder Bank, and at the profound changes undermining its form, and maybe even its capacity to protect our lovely city. Could this mighty natural feature be our version of the canary in the coal mine? Could it be giving us its own warning signals about the climate emergency biting harder, about the escalating effects of extreme weather events, and even of sea level rise?
Land Information New Zealand shows Nelson having a highest recorded astronomical tide of 4.68m, and highest mean high water springs of 4.31m. This must make the low-lying parts of our city especially vulnerable to the combination of king tides, storm surge and sea level rise.
The shelter offered by Te Pokohiwi – the longest natural boulder bank in the world – was a major factor in Ma¯ ori first settling at Whakatu¯ (Nelson), and now it physically protects many city assets.
The protection the Boulder Bank offers from northerly winds and seas seems largely unmeasured. However, there are profound changes going on, and there seem to be significant reductions in the bank’s capacity to protect our city centre, State Highway 6, and the residential areas of northern suburbs. A map will show how exposed the Boulder Bank is to the north.
I’ve been walking the Boulder Bank for over 18 years, and it was my happy place until three years ago. Now I’ve become sensitised to its dynamic nature, and understand the DOC sign stating ‘‘Despite its robust appearance, it has a delicately balanced ecology’’.
I’m most familiar with the first few kilometres south from Boulder Bank Drive, and this has changed so much as to be almost unrecognisable.
Many will remember the torrential rain and storm surges in February 2016, and the huge quantities of driftwood washed on to Tahunanui Beach. It wasn’t until taking an extended walk along the Boulder Bank that I realised with alarm how much of it was decades-old driftwood from the top of the bank.
My NZ Tides app also shows that summer’s tides went above the usual highest of 4.68m to 4.8-4.9m.
Did this explain the movement and removal of so much material?
In winter 2017, I realised that larger changes were again afoot, so I recorded what I was noticing. A polar blast and storm surges had been through in June, momentous weather events were happening around New Zealand, and the shape of the Boulder Bank was unrecognisable in places. Was I witnessing the effects of climate change, with storm surges clearly washing directly over large extents of the bank?
In August 2017, I drew a cross-section of the general profile of the Boulder Bank near the drive. It had a distinctive and memorable shape, and had seemed entirely stable for the last 15 years. Vegetation in large parts attested to this, and longestablished sections of walking track were further evidence. By February 2018, all those had gone.
During the December 2018 storms, I was so alarmed that I again documented what I’d seen.
The profile of the Boulder Bank bore no similarity to my previous drawing – the sea had not only reached higher than before, but had gone over the whole bank many times, driving previously stable rocks and flotsam to the estuary side, with this forming large drifts in many places. Some areas were reduced by as much as approximately 1m in depth. The large concrete structure (roughly 4m square and 1.5m high) near the drive went from 95 per cent buried to 95 per cent exposed.
Three months later, during the storm surges of February this year, the damage was even greater. The height of the whole length I frequented was further reduced, and the earlier profile was entirely changed again. Storm surges had clearly overtopped the entire bank again, and more swathes of boulders had spewed on to the estuary side.
With further major height reductions, I fear we may be heading for breaches that will expose our city to greater threat of inundation.
It’s now time to better understand the Boulder Bank’s capacity to maintain our protection from the sea – and to also get more information on how it might be an indicator system for sea level change. We are being warned.
It’s now time to better understand the Boulder Bank’s capacity to maintain our protection from the sea – and to also get more information on how it might be an indicator system for sea level change.
Suggested Nelson City Council actions:
Easy: Regular monitoring of vehicle intrusions past the security fence.
Medium: Commission an in-depth study, supported by ongoing monitoring (drones?), to anticipate the medium- and long-term likelihood and implications of a major breach of the Boulder Bank.
Large: Repositioning large amounts of material from the estuary side back on the top of the Boulder Bank. Consider responses in the event of a complete breach of any portion. Relocate major infrastructure (like sewerage works) to make it less vulnerable.