Presentation to Thames Community Board – 6 November 2017
In the presentation, I covered the following topics:-
I began with slides showing the current annual rate of sea level rise, – 3.3 mm globally, and between 4.4mm – 4.8mm in New Zealand since 1993. The last 7 years of records at the nearest tidal gauge – at Tararu near Thames show 10mm per annum, but this is not officially confirmed.
The next slides show the latest projections for sea level rise from the Ministry for the Environment and the November 2017 US Climate Science report. The MfE projections are based on the IPCC Report from 2014 and show lower projections than the latest US report which takes account of the most recent data. The slide on the right has data taken from the US Climate Science Report and shows that a sea level rise of 0.2m to 0.4 m by 2050 is very likely. The US Report confirms that range of sea level rise is locked in regardless of the steps we take in the next few decades to reduce emissions.
Comparison with Dunedin
The next section looks at what is happening in Dunedin regarding sea level rise and flooding due to rising groundwater. The first video is from the Otago Regional Council. The second video is a news item about how the Dunedin City Council has commissioned an overseas study studying how other places are dealing with rising groundwater issues due to sea level rise.
The following slides show that Dunedin does not have an immediate threat from direct flooding from the sea but only from rising groundwater. It also shows some of the possible engineering “solutions” proposed by engineering consultants Beca involving very expensive large-scale pumping operations. The second slide shows the likely extent of surface groundwater ponding after 0.3 m of sea level rise. (this is compared later with the potentially greater extent of groundwater flooding in Thames with just 0.2m of sea level rise)
Coastal Threats to Thames
I then point out that whilst the news media has extensively covered flooding in Dunedin with its groundwater problem, Thames has both a groundwater and a direct inundation/coastal flooding problem. But we hear little about Thames’ challenges. The photos show that Thames already has groundwater intrusion through stormwater drains and direct inundation during king tides.
Maps of Potential Sea Flooding For Thames
This slide covers the components of increased water levels on the coast which are included in the Waikato Regional Council’s coastal inundation tool. The tool shows normal and very high (King) tides and also the effects of storm surges. The tool also allows the viewer to add various levels of sea level rise up to 1 m. Note the tool does not show the additional effects of wave setup and wave run-up. For more information on how to use the inundation tool please see here.
The following slides show various scenarios for Thames for different levels of sea level rise and the effect on rising groundwater – the green areas on the maps – and on direct flooding from the sea – the blue areas on the maps. Note that with just 0.2 m of sea level rise there are extensive areas within Thames Township that are likely to be adversely affected by rising groundwater. These areas would seem to be much more extensive with 0.2 m of sea level rise than the areas on the previous slide showing the effects in Dunedin of 0.3 m of a rise.
Frequency of Extreme Events
This section shows that much more frequent extreme sea flooding events due to sea level rise will occur. The table is taken from the Parliamentary Commissioner for the Environment’s report which shows that a one in 100-year event such as that which caused $3million of damage to houses in Moanataiari in Thames in 1997, and Tamaki Drive in Auckland in 2014, and the foreshore at Whitianga to flood, would become a six monthly event with 0.5 m of sea level rise and a daily event with 1 m of rise.
Subsidence of Thames Foreshore Adds to Risk
These photos were taken in the 1870s and 1940s show how flat and low-lying parts of Thames are, and how the present site of the Toyota car factory, the Richmond Villas retirement complex, the Goldfields Shopping centre, and Moanataiari suburb have all been reclaimed over marine mud/sediment (just like South Dunedin).
Numerous reports have recorded substantial and rapid subsidence of the Thames foreshore. The most up-to-date data comes from the Richmond Street site which is sinking at the rate of around 13 mm a year. This is a large rate of subsidence – compared to say Dunedin which is sinking at only 1 mm a year. The slide explains how subsidence of the land next to the coast adds to the risk of sea level rise because both the rise in sea level and the rate of subsidence must be added together to establish the local sea level rise extent. Thus, Thames has a third concerning issue – not only is it affected by groundwater problems and direct inundation, but the subsidence of the foreshore adds to the risk also. Further research is required to confirm the rate of subsidence.
This table shows a comparison between the actions taken by the Dunedin City Council since at least 2010 to obtain expert reports and review their District Plan, and the inaction of the Thames Coromandel District Council.
The New Guidelines for Councils
Draft Ministry for the Environment Guidelines for Local Authorities on climate change and sea level rise had been prepared but have not yet been officially released. The guidelines were leaked prior to the election and are available here. It is expected that the Guidelines will be released shortly. The Guidelines update the latest science and contain substantial new content on risk assessment, community engagement and adaptive planning. They outline a ten-step decision-making process that councils and communities can follow when planning for the effects of climate change on coastal hazards. TCDC should adopt these comprehensive Guidelines as its own Strategy for dealing with coastal hazards.
One of the significant new provisions in the new Guidelines are “minimum transitional values” which required councils to “stress test” any new major developments or intensification of existing development by considering a sea level rise of 1.9 m and a timeframe out to 2150. The following map shows the 1.9 m line imposed on Thames and gives a stark indication of the type of totally new thinking that councils and the community will have to consider when new development proposals are brought forward.
How Does the Risk to Thames Compare With Other Cities and Towns?
In 2015 NIWA carried out a nationwide risk exposure of buildings and roads in low-lying coastal areas. A count was carried out of buildings, and building replacement costs were assessed (based on $2011 values) in these low-lying coastal areas. Figures for the Thames urban area, and for other towns and cities were provided. This has made it possible to compare the risk to Thames with other places in various elevation bands above sea level.
In the lowest 0 – 0.5 m elevation band Thames is rated the eighth most at risk urban area compared to all other cities and towns in New Zealand. In the 0 – 1.5 m elevation band Thames is rated the 13th most at risk town in New Zealand and has 1600 properties at risk from sea level rise which are valued at $0.4 billion
Flooded Roads and Other Impacts From Rising Seas
Thames has 18.7 km of roads in the 0m – 1.5 m elevation band which are at risk from sea level rise. There are other potential impacts from rising seas.
Dunedin City Council and its Mayor have long recognised that it has some serious challenges with sea level rise and has been actively asking for assistance from Central Government. However, Thames has perhaps an even greater range of challenges including not just rising groundwater, but direct flooding from the sea, and a subsiding foreshore. Therefore Thames has a very strong case for funding and expert advice. The new Government has a policy of providing funding and expertise to urban communities subject to sea level rise. Help is available but the community, the Council, and the local Member of Parliament need to come together and ensure that Thames gets the help it needs.
Leadership and Action From TCDC
There are some simple, very effective actions TCDC can take. The first would be to show leadership and join with over 50 other Councils and sign the Local Government Leaders Declaration on Climate Change. Here are some other actions Council can take –
The Thames Community Board will make a submission to TCDC on the Coastal Management Strategy. This is a most welcome initiative from the Board.
We have to face the challenge that rising seas make low lying parts of Thames a slowly unfolding red zone. We all must start to plan and adapt. Denial and inaction will lead to more damage, hardship and stress.
A .pdf version of the presentation to the Thames Community Board is viewable here – Presentation6November2017