Rising sea levels

Sea levels have been rising since the early 1900s. The rate is accelerating, doubling approximately every 10 years. The initial rise was due primarily to thermal expansion as the seas warmed. Today, glaciers and ice-caps are melting, decanting into the ocean faster than at any time in the last 10,000 years. NIWA currently estimates a rise in sea levels up to 2m by 2100. It will rise at least 30cm before 2050, a rate that is ‘locked in’, ie it will happen regardless of what actions we take (see video below) because of lag between the amount of CO2 already in the atmosphere and the effect on rising temperatures.

When ice caps have melted in the past, sea levels have abruptly jumped as much as 2m in 50 years, so the minimum rate may need to be revised upwards as the rate that ice shelves holding back glaciers and ice caps in Greenland and the Antarctic collapse (see for example the Thwaites glacier).

As seas rise, river mouths migrate inland as the land is inundated. Because braided rivers mouths form complex estuaries and hapua, the morphology of these areas will undergo physical re-arrangement as they move inland. In some places estuaries will become harbours and hapua will become estuaries, the locations of which will vary or vanish altogether depending on landform, land use, and flood control measures used by local and regional governments to prevent inundation and the loss of critical infrastructure.

Where river-mouths can freely retreat inland and there is sufficient gravel, new hapau will readily form (or retreat) in low-lying areas behind historic and extant dune systems (see the NIWA slide below).

Why is it a problem for braided rivers?

  • Short term – saltwater influx into existing brackish or freshwater hapua, leading to degradation of existing ecology
  • Mid-term – increasing frequency of inundation leading to permanent inundation of existing hapua (see Table 3.2 below)
  • Progressive – coastal squeeze in some areas will prevent the natural inland migration of estuaries and hapua, with flood control measures built or reinforced to protect existing land, built structures and critical infrastructure. Hapua and lagoons will disappear in these circumstances, reducing the habits of species dependent upon them.
  • See discussion in relation to the Ashley estuary
The inundation map above is based on a 1.5°C rise in global temperature leading to a 2.9m rise in sea level over time. Note: this is an inundation map based on topography. It does not factor in how coastal processes will change the form of estuaries and hapua as the river mouth migrates inland. Nor is engineering to prevent inundation or drainage of low-lying areas factored in. Click on the image for more detailed information.
In this example of the Ashburton River, gravel eroded from the beaches will likely allow the hapua to reform as the river mouth migrates inland. The availability of this gravel is subject to no engineering works being undertaken to prevent coastal erosion, ie built structures and land is lost. This slide is taken from a NIWA presentation at the 2019 Braided River Seminar. Click on the image to download the entire presentation.

Research and references

The New Zealand Coastal Policy statement is 9 years old and based research and knowledge that has since been superseded.  Because the impact of rising sea levels on cities and infrastructure is so critical, research, plans, and policies are moving at a very fast pace. Below are references to material specifically used in this page or are direct relevance. If you are interested in this topic, for the latest information | we recommend Climate Central or the journal Nature Climate Change.