by Geoffrey Churchman

The meeting of the Kapiti Historic Society last month featured a talk by Mark Hickson of Auckland University on how Kapiti’s coastline has changed over the decades. His job is establishing as accurately as possible what has happened around the whole New Zealand coast and why. The second part of that is tougher than the first.

In the last couple decades satellite photos have been available and further back in the past aerial photos have been taken for mapping purposes by firms like White’s Aviation. The problem is that, as people know, there is often a big difference between the high tide and low tide levels of the sea on the beaches. But by using a computer program they are able to determine at what point in the low tide/high tide continuum the photos were taken.

There are two basic, more or less constant changes to the coastline that happen: accretion and erosion. Accretion happens when the shoreline (beach) is expanding outwards, mostly through the build-up of gravel and sediment from the rivers, and it no surprise that there is a big bulge just south of the Waikanae River on which a large part of Paraparaumu is built because of that. But both accretion and erosion (where the sea ‘eats’ into the shoreline) can occur within relatively short distances and examples were given. Ocean currents must be a factor in explaining that.

Mark spoke of the general cycles in climate that have happened over the millennia and how sea levels have fluctuated because of the changing amounts of ice on land (ice on sea doesn’t affect liquid sea level). There have been regular ice ages and warming periods; one figure he gave was that 4000 years ago, sea level was about 2 metres higher than it is now. He also mentioned in answer to a question the difference between absolute and relative sea level rise — parts of the coastline have risen significantly because of earthquakes: the Miramar Peninsula, areas around Napier and Kaikoura are all well known examples. But is there evidence of inter-seismic event subsidence? There is some that it happens a little.

He wisely avoided the topic of Climatism, saying that serious scientists dislike prediction and those that do it are frowned upon. However, he seemed to think that the observed increases in Sydney of between 1.8 mm and 2.4 mm a year over the last several decades are reasonable expectations likely in the future. Thus by the end of the century the sea level could be about 15-20 cm higher than now.

A point that became clear is that earthquakes and volcanic activity affect coastline a lot more than fluctuations in temperatures — and who can predict those? Long term management of rivers near their mouths also has a big impact.

People can widen beaches by ‘renourishment’; dredging up sand and dumping it close to shore — Oriental Bay in Wellington is an example where that has been done, and it’s possible to reclaim land by more substantial measures as has been done over the last 180 years in central Wellington,

The Coastal Ratepayers United Annual General Meeting

This was held a few days later and covered this general subject too, as you would expect. This meeting was attended by about 100 CRU members, as well as Crs Wilson, Cooper, Kofoed and Prvanov. Pleasingly, in comparison to attitude of the former Mayor Guru, the new mayor also came, but not the regional councilor, Penny Gaylor.

The Keynote Speaker was Katharine Moody, who lecturers at Massey University and has post graduate qualifications in planning. Slides of her presentation are here (26 page-pdf).

As can be seen on the pdf, the major aspect of the presentation was the replacement of the massive Resource Management Act:

“Natural and Built Environments Plans combine/replace Regional and District Plans and Regional Policy Statements and will be developed regionally (i.e., it is intended that planning policies and
rules, particularly for the built environment, will become common across a region).”

Additionally the Ministry for the Environment indicates that a draft Climate Adaptation Bill will be introduced to Parliament by the end of 2023. This will support “managed retreat”.

“Managed retreat identifies areas considered of intolerable risk and reduces or eliminates exposure to extreme weather events. It enables people to relocate their houses, activities, and sites of cultural significance away from at-risk areas within a planned period.”

Few councils have tackled the development of a managed retreat policy; the main stumbling block is ‘who pays’, However, one council which has is Porirua City whose recent Flood Retreat Response Policy identifies 44 dwellings likely to meet the criteria for managed retreat. Homeowner decisions to retreat are voluntary. Specific criteria to qualify must be met. The PCC is planning to accumulate funding in its long-term plan to purchase, renovate (lift) and re-sell.

Observations on the official Council Takutai Kapiti

AEP = annual exceedance probability, so 1% is 1 in 100 years.

As can be seen, the dotted black line covers a huge chunk of the beach zones along the whole Kapiti Coast.

Whereas the notorious Shand reports some years back tagged some 1800 homes as subject to Sea Level Rise/climate change effects, these newly defined adaptation areas from Jacobs + Becca are even worse and likely capture in excess of 5000 residences!

There are always models used and policymakers choose which one they like the most. Katharine says we should tackle climate adaptation planning with a clear understanding of the likelihood of events unfolding, rather than from the perspective of the full range of possibilities. The official preference for the upper range IPCC 8.5 scenario is for one that the IPCC itself says is extreme and implausible. IPCC 4.5 is considered the most appropriate. For an explanation of this, see this article she composed for the interest.co.nz website

Thoughts raised by Katharine Moody:

  • Concern that these adaptation areas might be used to define areas of potential managed retreat in some future Long-term Regional Spatial Plan (under the new RMA).
  • CRU technical experts are yet to consider the methods used by Jacobs to determine the “Limit of coastal influence on flooding and groundwater levels” (i.e., the inland extent of the adaptation areas)
  • Question how these adaptation areas might be applied in future District Plan coastal hazard provisions.
  • Question the validity of this statement from the associated powerpoint presentation:

    Sea level is the main factor for coastal effects on flooding and groundwater.” (slide 3)

Katharine said that problem areas along the Kapiti Coast are essentially those where artificial structures have been put in place to mitigate the effects of storm surges. One point made was that isolated sections of seawall don’t work as wave surges just flow around and behind them: they need to be continuous through collaborative works by all those with properties on the shore.

Elsewhere, such as Waikanae Beach, which is accreting – not eroding – all that is needed for beach maintenance/management here is native planting of the foredune.  In other words, not renourishment, but native planting.