During the rainy season, from May to October, Ho Chi Minh experiences regular flooding. A combination of high tides, heavy rains, overflow in the Saigon and Dong Nai rivers and land subsidence puts the city in the top 10 most vulnerable to sea level rise.
Earth.Org has mapped what a severe flood could look like in Ho Chi Minh by 2100.
Flooding is a common part of life in Ho Chi Minh city. 40 to 45% of the city stands less than a meter above sea level and land subsidence due to rapid groundwater extraction is making this worse. Record breaking river tides, reaching 1.77 metres, are increasingly frequent because tide-draining swamps have been filled up to build housing.
A 1-in-100 year flood (whose intensity has a 1% chance of occurring in any given year) could cause hundreds of millions of US dollars in infrastructure damage and up to USD $1.5 billion in real estate damage. A recent paper published in Nature estimates that, under high emissions scenarios, global sea levels could rise by an average 1 to 2 meters by 2100. This phenomenon could put a large part of the city under water without any kind of water surge, meaning that a 1-in-100 year flood would be disastrous.
A 2017 study states that such an event would cost USD $7.8 billion in infrastructure damage and $18 billion in real estate costs. The knock-on effects of the city’s shutdown were estimated at $45 billion, or a fifth of Vietnam’s GDP. Earth.Org has modelled what such a flood would result in by 2100.
Sea Level Rise Methodology
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: is 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 projection of 10m SLR at 2300.
Coastal Flooding: 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 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.
Pollution Scenario: allows to choose 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 Extreme level is based on RCP 8.5, of 4°C temperature rise.
Luck: applies to the 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).
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Kopp, R. E., Horton, R. M., Little, C. M., Mitrovica, J. X., Oppenheimer, M., Rasmussen, D. J., Strauss, B. H. & Tebaldi, C. (2014). Probabilistic 21st and 22nd Century Sea-Level Projections at a Global Network of Tide-Gauge Sites. Earth’s Future, 2(8), 383–406
Kulp, S. A. & Strauss, B. H. (2019). New Elevation Data Triple Estimates of Global Vulnerability to Sea-Level Rise and Coastal Flooding. Nature Communications, 10(1), 4844. Retrieved June 21, 2020, from http://www.nature.com/articles/s41467-019-12808-z
Muis, S., Verlaan, M., Winsemius, H. C., Aerts, J. C. J. H. & Ward, P. J. (2016). A Global Reanalysis of Storm Surges and Extreme Sea Levels. Nature Communications, 7.
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