Chris de Freitas: Coastal property rules must be set beach-by-beach

For all coastal management purposes, the important statistic is not average global sea level; rather it is local relative sea level (LRSL) change.

At this time of year, thousands of Kiwis enjoy lazy days at the beach. Coastal recreation and all it involves is a way of life in New Zealand. With around 15,000km of coastline there is much to enjoy. There is also much at stake.

This is because coasts are high energy environments that are vulnerable to the impacts of storm surge, high onshore winds, erosion and flooding. Low-lying coastal land is most susceptible. The prospect of sea level rise from climate change adds another dimension, a major concern, especially for owners of valuable coastal property. All this has set coastal planners thinking.

Global warming leads to ice melt and thermal expansion of oceans that lead to sea level rise. Unlike warming, which can have beneficial effects, sea level rise cannot; but public discussion of the problem is beset by poor data and at times extremely misleading analyses.

Currently the global average rate of sea level rise is about 1.8mm per year.

But sea level rises around New Zealand have never reflected changes in the worldwide average, other than in those few places where land movement and net coastal sediment movement is zero.

For all coastal management purposes, the important statistic is not average global sea level; rather it is local relative sea level (LRSL) change. LRSL rise will be greater than 1.8 mm per year for locations where the coast is sinking. LRSL rise will be less than 1.8mm per year for locations where the coastal land is uplifting. LRSL will fall at an increasing rate for increasing rates of uplift or where the effects of shoreline progradation are greater than 1.8mm per year.

That these relativities are poorly understood is reflected in local body plans for sea level rise and legislation that require plans for managed coastal retreat, limits development on certain coastlines and refuses consents for extensions to existing buildings in the coastal zone. Such legislation based on any global figure of sea level rise has little or no practical policy value. LRSL change alone is relevant for coastal planning.

Sea level rise alarmism is promoted by government agencies, including local authorities.

The Ministry for the Environment recommends considering the consequences of a mean sea level rise of at least 800mm over the coming 50-100 years relative to the 1980-1999 average.

Many local authorities have already started to plan for this, some producing coastal hazard assessment maps that undermine coastal property values. Much of their advice is based on selective interpretation of the available data. There are however signs that public reaction may be pushing back the tide of government intervention.

The Kapiti Coast District Council has agreed to remove controversial 50- to 100-year hazard shorelines from the Lim reports of about 1800 properties. Coastal land owners and developers, fearing devaluation of their property and increased insurance premiums, were shocked when the council announced new 50- and 100-year predicted shorelines last year and had the information recorded on Lim reports. The council has also indicated that building consent processes related to properties in designated coastal hazard zones will be relaxed.

The same is happening in Australia, where the New South Wales government will order local councils to consider evidence for sea level rise on a beach-by-beach basis. The move is reaction to punitive planning conditions set by local authorities based on sea level rise predictions contained in reports by the UN Intergovernmental Panel on Climate Change. It is also a reaction to observations that showed local shoreline sedimentation rates in many places were making beaches bigger.

One hopes all this may be the beginnings of a common-sense approach to coastal planning based in part on the science of what is actually happening in each location.

Chris de Freitas lectures at Auckland University and is author of the book New Environmentalism: Managing New Zealand's Environmental Diversity.