Jim Salinger: We need big-picture fix for floods

United approach needed to protect major centres threatened by high rainfall caused by global warming.

Subsiding land has exposed Christchurch to greater flood risk, but the same fate awaits many other areas. Photo / Getty Images
Subsiding land has exposed Christchurch to greater flood risk, but the same fate awaits many other areas. Photo / Getty Images

From my perch in Rome I have observed that some poor residents of Christchurch are being hammered by the fourth bout of flooding in less than two months. This is very much a warning of what will happen in a warming world as seas continue to rise. The question is what to do about it.

Is this something Christchurch City Council must respond to, or should there be a wider all-of-government response?

First, let's look at some stark facts:

• The Christchurch earthquakes of 2011 lowered some parts of the city by 40cm.

• Sea levels have risen by more than 20cm over the past century because of melting glacier ice and warming seas.

• The atmosphere is warmer so holds more moisture and when it rains the amounts are higher.

• The city endured its wettest March on record (back to 1863) and on March 4 Riccarton recorded the greatest one-day March fall on record with 123mm.

• There was a further 60mm from April 16-18.

• On April 28-29 there was another 60mm, bringing the April total to more than 160mm, the fifth-wettest on record.

Fourteen of New Zealand's 17 largest towns and cities are either along the coast or at - or near - sea level.

Even without earthquakes, the increased incidence of flood-producing rainfalls will produce more occurrences of what is happening in Christchurch over coming decades.

Christchurch is the "canary in the coalmine" because the battleground from a warming world will occur along the coasts and in low-lying towns and cities.

It is clear we will have to adapt to rising sea levels and flooding throughout this century and beyond. Options include:

Retreat - abandoning parts of the coast and low-lying areas that are simply too expensive to protect or relocating people from coastal regions or islands where it may become too dangerous to live.

Accommodate - live with rising sea levels by changing infrastructure and appropriate planning for use of coastal and low-lying areas.

Protect - building, extending and using more frequently barriers such as dykes, or soft protection measures such as flood banks, beach nourishment, and increased drainage.

Some regions are so valuable to society that protection is the only real choice. But is, for example, the cost of protecting the entire coast from Napier to Hastings, which is open to storm surges and flooding, worth the housing and infrastructure to be protected? These engineering measures do not protect the natural environment and lead to catastrophic situations when they fail, such as in New Orleans.

Currently, any response to flooding is with local bodies through the Civil Defence and Emergency Management Act. But as such occurrences will become more widespread, a whole-of-government approach to decision-making is required.

In the United Kingdom, the Government in the 1990s addressed this by making some hard decisions on the areas it would protect and others it would abandon from flooding.

The wisdom of the 1990s was put into practice in the last UK winter when record-breaking rainfalls and flooding doused southern counties.

The UK Government has developed policies and legislation in discussion with local authorities that set out the risks of flooding. Its Budget from March 2011 to March 2014 has included 2.4 billion ($4.6 billion) to improve forecasting and early warning systems, maintain existing flood defences and improve flood protection for 165,000 households.

It is now time for our Government to engage with local authorities through Local Government New Zealand. What is happening in Christchurch is simply a warning of grimmer natural disasters to come in future decades. The Christchurch situation has been accelerated owing to land subsidence from the earthquakes.

The current approach from central Government to pressure Christchurch into hasty stopgap solutions is very irresponsible. The bigger picture must be addressed where whole-of-government engages with local authorities on what will be enduring strategies and solutions for the future.

After all, increased flooding from sea-level rise and higher-intensity rainstorms is with us to stay.

Jim Salinger is an Auckland climate scientist, currently on an exchange as a visiting scientist at the Council for National Research in Rome.

- NZ Herald

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