The latest report by the Parliamentary Commissioner for the Environment (PCE) provides useful guidance on how to break the news to home owners that they may be affected by sea level rise. The PCE website also provides regional land elevation maps which could be valuable to councils.
The full report is available at
http://www.pce.parliament.nz/publications/preparing-new-zealand-for-rising-seas-certainty-and-uncertainty, and some of the key messages for councils are summarised below.
Process for informing home owners of riskPlanning in the face of uncertainty is never easy, but is particularly difficult when choices will affect people’s homes. Yet Councils could be found negligent if they hold relevant information and fail to provide it clearly, fairly, and accurately.
Accurate measurement of land elevation above sea level is an essential first step in considering the potential impacts of sea level rise. These maps provide a starting point for councils beginning to engage with their communities on this challenging issue. The PCE recommends taking time to discuss this before introducing hazard zones, or including information on LIMs.
Assessment of the vulnerability of a particular area generally requires information about a range of local characteristics, which is a follow up step. A clearer separation between scientific assessment and decision-making would increase clarity around risk levels and assumptions. Judgments, such as those involved in adding safety margins or setting restrictions on development, should be made transparently by decision-makers, rather than rolled into technical assessments.
Clear communication is a vital component of a good process. One particular need is to avoid referring to ‘one-in-50 year’ or ‘one-in-100 year’ flood events. Not only is this terminology difficult to understand, it is not a stable measure over time. For example, after a rise in sea level of 30 centimetres, an extreme high water level (which has previously been a 1 in 100 year event) would be expected to occur about:
The report includes a number of other useful descriptions of complex concepts, including the following.
Areas close to river mouths can experience the ‘double whammy’ of river flooding coinciding with the sea pushing its way upriver at high tide. As high tides become higher because of sea level rise, such floods will become more likely.
In some coastal areas, the water table is not far below the ground and is connected to the sea. As the level of the sea rises, the water table will rise in these areas.
High groundwater causes a number of problems:
Areas of land reclaimed from the sea are especially likely to experience problems caused by high groundwater.
LidAR and RiskScape
LiDAR technology - pulses of light from a laser on an aeroplane are bounced off the ground, and the time taken for the reflected pulse to return is used to measure the elevation of the ground. Topographic surveys using LiDAR are typically accurate to 10 to 15 centimetres, so can be used as a basis for analysing the impacts of sea level rise.
RiskScape – this software programme has been used to find how many buildings, and which roads, railways, and airports are located within the different elevation bands.
Elevation maps availableThe PCE commissioned NIWA to convert the available LiDAR data into a standardised form. Once this was done, NIWA used RiskScape software to estimate how much of the built environment is at risk from sea level rise.
The report contains maps showing low lying coastal land in Auckland, Wellington, Christchurch, and Dunedin. The impact of sea level rise will be felt in many other areas outside of these four major cities. Maps of these and other coastal areas have also been prepared in the course of this investigation, and are available at www.pce.parliament.nz
Five other cities and towns that have significant areas of low-lying coastal land – Napier, Whakatane, Tauranga, Motueka, and Nelson.
The situation in Auckland, Wellington, Christchurch and Dunedin
Vulnerable transport links include the Northern Motorway just north of the Harbour Bridge and the causeway on the Northwestern Motorway where it crosses the mud flats at Waterview.
Most low-lying areas in Wellington are on the floodplain of the Hutt River – in Petone, Seaview, and Waiwhetū. The more pressing issue for this area is river and stream flooding. However, rising sea level will exacerbate such river floods by reducing the fall to the sea.
Much of Wellington city’s shoreline is protected by concrete seawalls and/or rock armouring. Such hard defences will require increasingly expensive maintenance as the sea rises.
A considerable amount of low-lying land shown on the map is in the Residential Red Zone and so has been largely cleared of buildings.
Dunedin is notable for the large built-up area in the city’s south that is very low-lying. Of the nearly 2,700 homes that lie less than 50 centimetres above the spring high tide mark, over 70% are lower than half that elevation.
The low elevation of South Dunedin along with its high water table makes it prone to flooding after heavy rain. The water table also rises and falls with the tides, so these problems will increase as high tides become higher.
National direction and guidanceThe Ministry for the Environment began work on a national environmental standard in 2009. However, this work has now stopped. The Minister for the Environment’s view is that there is “too much uncertainty for a rigid standard to be applied”. The Ministry is now working on an update of the 2008 MfE Guidance Manual. This provides an opportunity to address matters that emerged during the PCE's investigation. The revised guidance should be a ‘living document’, so it can be readily updated.
Recommendations to the Minister for the Environment include: