Vientiane, Lao PDR, 25th Aug 2008
Although water levels along the Mekong River above Phnom Penh have continued to decline since their peak so far in 2008, around August 17, when loss of life and damage to property were recorded in the Lao PDR and Thailand, many parts of the Lower Mekong Basin remain at risk of future flooding. People living in low-lying areas near rivers are advised to continue to be prepared for possible emergency situations.
While water levels have decreased, continuing storms mean they have not dropped to levels that preclude further flooding. The region is currently vulnerable to the impacts of additional tropical storms tracking across the basin during the remainder of the season.
The Mekong River Commission (MRC) continues to work with meteorological and water resource agencies in its member states of Cambodia, the Lao PDR, Thailand and Viet Nam to monitor river and weather conditions and forecast the likelihood of flooding. Daily updates on water levels are provided on the MRC website, and more detailed information is routinely sent to various government agencies, academic institutions, and international, regional and non-governmental organisations every day throughout the flood season and every week during the rest of the year. Organisations wishing to be directly supplied with such information are encouraged to contact the MRC Secretariat by email.
The information provided by the MRC is used to assist relevant national agencies in implementing their flood warning, preparedness and response plans, and in delivery of assistance to affected districts. These are national responsibilities, and national disaster coordination mechanisms are established in each member country. In light of the current situation the MRC Flood Mitigation and Management Programme has been communicating the likelihood of dangerous future conditions to provincial and district authorities across the Mekong floodplains of Cambodia and Viet Nam through the Asian Disaster Preparedness Centre and partners in both countries. These local authorities have activated Community-Based Early Warning Systems and Flood Preparedness Plans set up through an MRC pilot project.
The MRC has been analysing data from flood events and continual river measurements. The recent flood in northern parts of the Lower Mekong Basin resulted from a combination of high rainfall during the first months of the monsoon season together with the effects of tropical storm Kammuri. Kammuri tracked westwards across northern Laos and southern Yunnan on the 8th, 9th and 10th of August and produced 100-150 mm of rainfall, though in some areas as much as 250 mm of rain was recorded between those dates. The major impact of this storm occurred in the northern provinces of Viet Nam in the Red River Basin. As catchments in the Mekong region had already been saturated by the strong monsoonal conditions of May, June and July, flood runoff there was maximised, affecting communities on the banks of the Mekong and tributaries.
The flood runoff was almost entirely generated in the area between Jinghong in Yunnan and Luang Prabang, and thus the most critical flood conditions occurred in upstream reaches of the Mekong River. The origin of the floodwater was relatively evenly divided between China and the large left bank tributaries in northern Laos, and the volume of water in the river can be explained as a direct effect of extreme rainfall patterns.
Preliminary analysis of the available combined water storage volume behind the three hydropower dams that are operational or approaching completion on the Lancang-Mekong River and tributaries in China suggests this to be small compared to the volumes of natural runoff generated directly by rainfall. The combined active storage capacity of the Chinese dams, Manwan, Dachaoshan and Jinghong, is less than 1 km3, only a small part of which could be released within the period that the floodwater accumulated. Given that at Chiang Saen the flood peak on August 12 showed an accumulated flood runoff volume for the month of 8.5 km3, while at Vientiane on August 15 the figure was 23 km3, any release from these dams could not have been a significant factor in this natural flood event.
The recent floods were therefore the result of natural meteorological and hydrological circumstances. Similarities to the 2008 events can be seen with the flood of September 1966, when tropical storm Phyllis poured massive amounts of rain on Yunnan. A detailed scientific summary of the MRC’s initial analysis will be released soon.
The water level reached on August 15 at Vientiane was the highest recorded since records began in 1913. At 13.7 m above the gauge datum, the Mekong River was 1 m higher than the maximum levels recorded in 1966, 1971 and 2002. However, the only valid comparative figure among these previous extremes is that for 2002, since hydraulic conditions in the mainstream river at Vientiane were in earlier times significantly different. Although water levels at Vientiane were this year 1 m higher than 1966, when the city centre was extensively flooded, the peak discharge in the river was actually slightly lower in 2008.
The higher water levels for similar discharge conditions are explained by works undertaken in past years to raise flood protection levees on both banks of the river, and the resulting containment of the flood flow within the channel. In other words, although the flood embankments proved effective in limiting damage, containment of the flood within the river’s banks led to higher river levels. The raising of flood embankments can influence water levels and flow distribution elsewhere along the river, and therefore an integrated approach to flood management is required.
While peak discharges downstream of Vientiane have so far in 2008 been average, flood volumes were and remain considerably above normal. The situation within the Cambodian floodplain and the Mekong Delta should therefore be watched with care as water levels at Phnom Penh have approached, and at Chau Doc and Tan Chau have passed, the alarm stage. Any further significant flood water contributions later in the season either from northern, central or southern tributaries could result in a repeat of conditions that were experienced in 2000 and 2001, when socio-economic losses were considerable.
As the flood season progresses, the MRC will make available more detailed reports on flooding in the basin. It shall also review its data collection, analysis and distribution mechanisms in light of events across the whole 2008 flood season and discuss its performance with partners. In the meantime events and data exchange mechanisms will be discussed in detail at the upcoming meeting of the MRC Joint Committee in Vientiane, August 27-28, and at the subsequent Dialogue Meeting between the MRC Member States and their Dialogue Partners, China and Myanmar.
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