Research has shown that in the worst-case scenario, 440,000 New Zealanders would be living in regions affected by one-year coastal flood return levels by 2100. Christchurch, standing merely 20 meters above the sea level, faces an imminent threat as the sea level keeps rising.
Earth.Org has mapped what severe flooding would look like in Christchurch by 2100 if no effective measures are taken.
New Zealand is located in the collision zone between the Indo-Australian and Pacific plates, which makes it prone to earthquakes. One of these in 2011 aggravated Christchurch’s vulnerability to floods. Pre- and post-earthquake topological data revealed river channels narrowing and shallowing, reducing their water storage and funneling capacity. Along the large regional Waimakariri River north of Christchurch, the flood hazard is particularly evident. There is a 4% chance of water breakout and a severe flooding could cost US $800 million worth of damage. If flooding were to happen, up to 300,000 people would be severely affected.
The Waimakariri Flood Protection Project began in 2010 and was completed in late 2019. The two resulting 25 km embankments along the Waimakariri River aim to provide a back-up flood control for areas around Christchurch. The system is considered a high level of flood protection, aiming to protect people from a 5500 cumecs (cubic meter per second) flood. While this may effectively mitigate floods for the time being, the added effect of sea level rise will increase the risk of the system being overrun.
Earth.Org has mapped what extreme flooding could be like in Christchurch by 2100.
Global mean sea level is projected to rise by 2m at the end of this century. However, in order to determine local sea level rise (SLR), one has to take into account local coastal flood levels which could be 2.8m above Mean Higher-High Water (MHHW) at extreme forecasts. These local levels bring variability to the projected SLR from 1m to 6.5m (eg. Rio vs Kolkata).
The SLR scenarios used in this study are based on the forecasts from Climate Central – Coastal Risk Screening Tool with the following parameters:
- Sea level Projection Source
- Coastal Flood Level
- Pollution Scenario
Sea level Projection Source
From two highly cited journals by Kopp et al., estimating SLR mainly due to ocean thermal expansion and ice melt. The mid-range scenario projected 0.5-1.2m of SLR based on different representative concentration pathways (RCP) defined by the IPCC. While the pessimistic scenario added more mechanisms of ice-sheet melting, estimating SLR at 1m-2.5m in 2100, with a projection of 10m SLR at 2300.
More frequent coastal flooding is a direct impact of sea-level rise. Based on the Global tides and surge reanalysis by Muis et al., (2016), it is estimated that the extreme coastal water level could be from 0.2 – 2.8m over the mean level. While in extreme cases like China and the Netherlands it could experience 5-10m of extreme sea levels. Here, the coastal local flood level is added on top of the projected SLR.
Allows choosing the RCP, the greenhouse gas concentration trajectory defined by the IPCC. The mild level is based on RCP4.5, of 2°C temperature rise; while the Extreme level is based on RCP 8.5, of 4°C temperature rise.
Applies to the baseline SLR, defined in the “Sea level projection” section, upon which we add flooding. “Mild” refers to the mid-range scenario of 0.5-1.2m, and “extreme” to the pessimistic scenario of 1-2.5m. We used the high-end value of each scenario (mild = 1m; extreme = 2.5m).
Mapping and methodology by Braundt Lau. Article written by Jennie Wong and Owen Mulhern.
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