Rising sea level

If greenhouse gas emissions are not slowed down or limited, sea level rise could have profound consequences for Waitaha/Canterbury’s coastline. Coastal settlements – including Christchurch, our region’s largest city – have been built on low-lying or flat land which is flood-prone. In those places, flood risk will increase over time, both from sudden storms and gradual changes to the shoreline and tides. Some pockets of Waitaha/Canterbury are already exposed to coastal erosion and coastal flooding. These areas will become increasingly vulnerable as the sea level continues to rise, and communities will need to decide how to respond.

Rising sea level may lead to more saturated soils beneath the water table, increasing the chance of liquefaction in coastal areas during an earthquake.

Someday, aspects of our heritage may be under water. Waitaha/Canterbury has a rich history and many of us have parts of the coast that are dear to us or to our families. This includes places where memories have been made, as well as places of archaeological, cultural and historical importance.

Our relationship with the coast is part of our identity, and it creates economic opportunities for investment and jobs in tourism, conservation, research, education and the arts. As we adapt to our changing coastline, we will be writing new chapters of history. This will be a story of loss and reinvention.

There are Ngāi Tahu marae and settlements all along the coast. Rising sea level could affect buildings, land and investments that provide for the tribe. Land, water, taonga species, mahinga kai and landscapes are all important to history and identity. Although consequences may be minimal now, these could become more marked in 30 years, affecting cultural, spiritual and economic wellbeing.

Sea level rise will change our foreshores, estuaries, dunes, and coastal lakes and wetlands. Although climate change is having consequences on these environments now, scientists think transformations could become more extreme by the end of the century. This would affect flora and fauna, including migratory and coastal birds. Erosion of dunes, changing tides in rock pools, and water quality may drive changes in species distribution and threaten vulnerable populations of coastal species.

Sea level rise – and rising, saltier groundwater tables – could affect septic tanks and water bores near the coast. If wastewater systems rely on gravity to work, sewers may back up or spill. We could be dealing with public health and environmental issues well before waves are lapping at waterfront properties.

From time to time, flooding, storms, slips and wash outs affect coastal roads and railway lines. For example, in 2020 a series of high tides gave the railway south of Timaru a battering, disrupting freight and requiring KiwiRail to undertake emergency repairs. We can expect more of this as the sea rises. We may need to build or enhance protection such as seawalls or rock revetment. New, safer routes may be possible (if expensive) in some places, but not all. Ports, wharves and boat ramps could be affected but those consequences are not considered imminent and might be addressed as marine facilities are renewed.

Sea level rise could increase the risk of flooding during coastal storms, particularly in low-lying parts of Christchurch. Some coastal homes may become more expensive to insure, or even uninhabitable. A recent study says about 4500 coastal homes in Waitaha/Canterbury that are currently within 1km of the coast could face insurance difficulties in coming decades, with flow-on effects for the ability to raise a mortgage. This will depend on how insurers and banks respond to climate change risk.

Coastal inundation/flooding may occur when high tides combine with a storm surge and larger than normal waves or swell. Just how big and deep a flood gets depends on the size and timing of the storm and the characteristics of the coastline. Coastal flooding will be exacerbated by the effects of climate change, such as sea level rise.

Coastal erosion can occur either suddenly or slowly and can be temporary or permanent. Erosion only becomes a hazard when it threatens people’s activities or settlements, or other things they value. One of the impacts of climate change is that councils and landowners need to undertake more complicated hazard assessments that look at how land might behave under the different possible futures under climate change. For example, a recent coastal erosion assessment for Timaru showed that, generally, eroding areas will continue to erode, possibly faster. In some parts of the Timaru coast, sea level rise has little influence on erosion rates.

One way to glimpse the shoreline that our children and grandchildren may experience is to observe what happens during very high tides. What we call a very high tide now – something equivalent to king tide level - may become a normal high tide by the end of the century because of sea level rise. These very high tides happen about twice as often today as they did in 1900, increasing the potential for coastal inundation.