What was the most important lesson from the January 5 sea flood?  We got a timely warning as to what to expect when sea levels rise further and faster, and these extreme events become ever more frequent.   But will we heed those warnings? 

Thames/Coast is at high risk of sea flooding

There have been warnings about this threat for decades. The lower Firth of Thames, particularly Kaiaua, Thames Township and the Thames coast are extremely vulnerable to storm surge and coastal inundation (sea flooding). In her 2015 report, the Parliamentary Commissioner for the Environment told us how at-risk these low-lying areas are from sea level rise.  Much of the land in these communities is less than half a metre above sea level.  She published maps and tables showing that Thames was in the top 10 of all towns and cities in New Zealand from the risk of coastal flooding to buildings and infrastructure.  All this was well known, yet the extent of the flooding and the damage from the storm surge took many local people, the District Council, and Civil Defence by surprise.

Water levels were highest for decades

The water levels reached 2.8 m above normal spring tide. This was the highest level for many decades and came very close to the highest ever recorded level in 1938 of 3.0m when the Hauraki Plains was flooded. The height of the storm surge itself was about 1 m which is near the upper range of storm surge levels. The Regional Council has described this as a 1-in-200-year event.

Sea Flooding with Minimal Impact from Rain

The storm surge was almost entirely due to coastal influences. We have become accustomed in Thames to dealing with river flooding from the land due to very heavy rain, but this storm surge flooding is an entirely different beast. Many people still wrongly confuse land flooding with coastal flooding. There was minimal rain yet there was extensive flood damage.  Maybe this will help people to understand what sea flooding due to accelerating sea level rise looks like?

3 Factors Combined on Same Day to Cause Surge

This storm surge of about 1 m and coastal flooding on 5 January was due a combination of several effects coming together on the same day –

• The very low-pressure system of itself raised sea levels.

Everyone 1 HPA of air pressure below the average of 1013 HPA raises the sea level by 1 cm. The very low-pressure system did not make a direct hit on the North Island but was centred off the West Coast. At the height of the storm air pressure in the centre of the low was 976 HPA. Over Thames the air pressure was around 990 HPA. This meant that the low-pressure system itself raised the level of the sea by about 23 cm (1013-990). If the low-pressure system had tracked directly over the Firth of Thames the storm surge could have been a lot worse – perhaps adding another 15 cm or so to water levels.

• Strong winds from a northerly direction pushed up waves in the Firth of Thames and caused wave effects and wave run-up.
• King tides on 5 January will be the highest tides for the whole of 2018.

for more – see this

What Contribution Did Climate Change Make to the Storm?

• Since 1900 sea levels have risen by about 20 cm. This doesn’t sound a lot, but in many places, if the water levels had been 20 cm lower, the existing seawalls would not have been overtopped, and the flood damage would have been avoided or much reduced. In other places without a seawall, if the water level was 20 cm lower, the surge would not have reached as far inland and caused as much damage to houses, businesses and the Thames Coast road.  For example look at this video filmed near Richmond Villas

The seawall may not have been breached if the water level was 20cm lower?

• NIWA have confirmed that the unusually very high sea temperatures in the North Tasman Sea (up to 6° higher than normal in some places) made the low-pressure system much more intense and strong. Those very high sea temperatures are due in part to global warming.

A Glimpse into a Future with Accelerating Sea Level Rise

The storm surge on 5 January gave us a glimpse into the future. We saw in graphic detail what the impacts will be from ever-increasing storm surges as the sea level rises. Climate scientists and coastal scientists are telling us with high confidence that January 5 events will no longer be just 1-in-100-year events, but far more frequent. Just half a metre of sea level rise will have these current 1-in-100-year events occurring every six months.  NIWA scientist Dr Brett Mullan (HH 19 Jan) warns us that accelerating sea level rise forms the new baseline, and that future storm tide levels will be on top of this new escalating baseline.   40-50cm of rise is already “locked in” to the climate system. No one is projecting that extreme river floods will happen every 6 months but extreme flooding from the sea that often is exactly what we are facing.

With just 15 cm of sea rise, these extreme events will happen every 20 years.  At that frequency – perhaps sooner – insurance companies raise premiums and excesses or even withdraw cover, and banks refuse to lend.  (More on this in a later blog.)

The scientists are also telling us that the inland extent and depth of the flooding will be greater and greater as the sea rises. This can be seen from the Regional Councils excellent coastal flooding simulator.

We Must Look Forward and Plan

It’s pointless to compare the 5 January event with past events such as ex-tropical cyclone Drena or land-based storms like Bola.  Those past events cannot be used as a gauge as to what we face in the future. We are entering uncharted territory.

The storm gave us a timely warning about the slowly unfolding red zone that’s coming. How do we respond and adapt?   Fortunately, we have a comprehensive blueprint in the just-published Coastal Hazard Guidance for Councils.  TCDC, WRC and MP Scott Simpson must now lead the way by securing funding for research into options, and by starting active community engagement. There is no need to panic but it’s reckless to delay careful planning and adaptation.