Coastal flooding occurred earlier this week in the Mekong Delta (Vietnam) due to a combination of high tides, heavy rainfall and high-water levels which eroded embankments on the outskirts of Can Tho City.
Streets in the city were inundated to a depth of 50cm, with 91 hectares of crops damaged. The high tides carried seawater into freshwater zones and posed a threat to farmlands for vegetables and rice. According to a local resident, the tides were the highest seen over the past decade (Straits Times). The floods provided a stark reminder that the area remains particularly vulnerable to coastal flooding.
Flood risk in the Mekong Delta
Areas of Southeast Asia are highly susceptible to sea level rise: 3.3% of the world’s landmass is in this region yet it accounts for 11% of the world’s coastline (New York Times). One such area is the Mekong River Delta in Southern Vietnam.
If the sea level increased by 0.9m, over 30% of the delta would be submerged and up to 17 million people would potentially be at risk of flooding. Although these estimates fail to account for preventative measures (such as dikes), such barriers are often the source of additional concerns due to their retention of industrial waste water, fertilisers and pesticides - the accumulation of which can be harmful to agricultural activities in the river delta (New York Times).
The Mekong Delta is perpetually exposed to flood risk as it lies only slightly above sea level. Floods occur annually, typically in the wet season. The four main components of this flooding are flood water carried downstream by the Mekong River; high intensity localised rainfall over a short period; tidal floods due to storms and high tides and human intervention.
In recent years, a growing concern for many has been flooding related to the management of hydropower dams upstream (Duong et al.). However, it’s important to be mindful that some flooding is an important component of maintaining the fertility of farmlands and aquatic farms in the Mekong. Overall, it’s estimated that 85% of the population in the Mekong Delta rely on it for their livelihood (New York Times).
When flooding does occur, the mean annual economic losses are USD $71 million in the Mekong Delta in Vietnam and USD $88 million in the Lower Mekong Basin in Thailand (Mekong River Commission, 2018).
Figure 1: An extract of the JBA 1 in 20-year Vietnam River Flood Map. The area illustrated is Can Tho city and the Mekong Delta in southern Vietnam. The 2011 flood event in Can Tho is estimated to have a return period of between 10 and 20 years. (JBA Risk Management Limited™).
Severe Floods in Vietnam – 2011
In 2011, the Mekong Delta experienced one of the most severe flood events in the past decade. The Cambodian floodplain and Mekong Delta were both inundated. A total of 265 fatalities were reported, with at least 449,000 houses damaged and an estimated economic loss of USD $600 million (Mekong River Commission, 2015).
Several factors combined to cause the flooding. Two tropical storms brought heavy rainfall, subsequently resulting in high-water levels in the lower Mekong Delta. Following this, the Intertropical Convergence Zone (ITCZ) stalled over the region in September. The presence of the ITCZ and a strong southwest monsoon resulted in high precipitation (Mekong River Commission 2011, Mekong River Commission, 2015).
The maximum water level recorded in the Lower Mekong Basin during the 2011 wet season was 12-13% higher than the historical average recorded from 1960–2010. The 2011 floods were estimated to have between a 10 and 20-year return period (Mekong River Commission, 2011).
Table 1: Maximum water levels for various sites in Cambodia and the Mekong Delta in Vietnam (Source: Mekong River Commission 2014, Mekong River Commission river gauge data)
Table 2: Maximum water levels in metres recorded at Chao Doc and Tan Chau stations in 2018 in comparison with the major flood events in 2000 and 2011 (Source: Mekong River Commission 2014, Mekong River Commission river gauge data)
The threat of sea level rise
The rising sea level is threatening the livelihoods of many who rely on the fertile delta of the Lower Mekong Basin. On various occasions, the dikes built by the Vietnamese government have successfully prevented salt water intrusion into the farms owned by locals. Unfortunately, not everyone has the same protection for their land. Numerous homes are not protected by the dikes, resulting in occasions where sea inundation has occurred and drowned mangroves and eucalyptus trees (Mekong River Commission, 2016).
One of the principle reasons for this risk is that the lower basin has an average elevation of only 2–4m above Mean Sea Level. Climate change estimates foresee sea levels rising by 0.8–1m by 2100 and as a result, 38% of the delta may be inundated. For many, the worst-case scenario for the delta is a high astronomical tide coinciding with major flooding of the Mekong (Mekong River Commission, 2016).
JBA Risk Management has nationwide return period flood maps for Vietnam at 30m resolution. Please get in touch for more information.
References
Duong, HTV., Van, T., Nestmann, F. and Oberle, P., 2014, 'Land Use Based Flood Hazards Analysis for the Mekong Delta', Proceedings of the 19th IAHR-APD Congress 2014, viewed 16 October 2018
Mekong River Commission, Flood and Drought web page, viewed 16 October 2018
Mekong River Commission, 2011, 'Flood Situation Report 2011', Mekong River Commission Technical Paper No. 36, viewed 15 October 2018
Mekong River Commission, 2015, 'Annual Mekong Flood Report 2011', viewed 15 October 2018
Mekong River Commission, 2016, ‘Living On The Edge of the Rising Sea’, viewed 15 October 2018
Mekong River Commission, 2018, 'Flood Sector Key Findings Report: Flood Protection Structures and Floodplain Infrastructure', viewed 18 October 2018
Mydans, S., 2009, ‘Vietnam finds itself vulnerable if sea rises', The New York Times, viewed 15 October 2018
The Straits Times, 2018, ‘Mekong Delta fights high-tide flooding’, viewed 15 October 2018