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Rising Sea levels

Braided River Status

All braided river estuaries and coastal lagoons/hapua throughout New Zealand will be affected by rising sea levels.

Description

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 collapse accelerates (see video below).

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. Estuaries will become harbours, hapua will become estuaries, the locations of which will vary or vanish altogether depending on landform, land use, and flood control measures used to prevent this inundation.

Where river-mouths can freely retreat inland, new hapau will readily form in low-lying areas behind historic and extant dune systems. Where engineering prevents this, hapua will disappear.

The following was released Dec 11, 2014 by the Parliamentary Commissioner for the Environment, Dr Jan Wright, warning that flooding and erosion from rising sea levels will have a large impact on many New Zealanders in their lifetimes.

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

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Conservation Activities

  • Reduction of greenhouse gasses (GHG). Regardless of this, a minimum 30cm sea-level rise over the next 35 years is now ‘locked in’, that is, nothing can be done to prevent or reverse this.

More information

The inundation map below is set to the UK at 7m by default. Scroll to New Zealand and adjust to 2m or less.

Note: this is an inundation map based on topography; it does not factor in coastal erosion or engineering to prevent inundation or drain low-lying areas. In areas where the coast is 10-20m above sea level but composed of easily eroded material, rising sea levels will undercut cliffs or wash away dunes, leading to sudden collapses of up to several metres inland, often during a single storm. Conversely, areas below sea level that are protected by sea-walls or artificial bunds are unlikely to be inundated if storm surges do not overtop them and flooding from the land is not occurring simultaneously (for example during a storm). 

 Research papers

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