Mekong River Commission

The Mekong flood of 2002 provided a classic illustration of the fact that in years when extreme water levels and discharges occur, they tend not to do so throughout the Lower Basin. The synoptic scale of the weather generating mechanisms such as typhoons and tropical storms that cause such conditions are such that their direct impacts are limited to just a part of the basin. Typically extreme annual events are confined either to the northern parts of the lower basin upstream of Vientiane, as in 1966 and 2008 or to the southern parts as in 2000, 2001 and 2002.

The events of 2008 saw the highest water levels observed at Luang Prabang, Vientiane and Nong Khai since 1966, with considerable inundation of peri urban and rural areas with flood damages, particularly in Lao PDR, the highest incurred in many years. The conditions, which peaked in mid August, were the result of the passage of cyclone Kamurri across northern Lao PDR and Thailand during the first week of the month. Analyses revealed that almost all of the floodwater originated within the Nam Tha, Nam Ou, Nam Soung and Nam Khan river basins, with only a modest trans-boundary contribution from China.

Downstream of Vientiane and Nong Khai flood conditions rapidly moderated since the flood contributions to the mainstream from the large left bank tributaries were not exceptional. As a consequence discharges and water levels across the Cambodian flood plain were average at best. Little or no damage was reported in these southern regions of the basin, though flash flooding did occur in the upper Se San and Sre Pok river basins in Viet Nam during May and August, though overall damage and losses were small compared to those of recent years.

The events of August 2008 provided the first real opportunity to apply and test the RFMMC’s flood forecasting system, which performed tolerably well when predicting the potential situation over a one and two day lead time, but less well over the longer lead times. This suggests that the flood routing component works well once the floodwater is in the major tributaries and the mainstream. However, estimating several days in advance the volumes of flood runoff that will be generated by the storm rainfall is recognized as needing improvement.

The major constraint to bringing this about appears to be the fact that the satellite imagery used to estimate the storm rainfall depths is apparently leading to systematic underestimation. Certainly the accumulated volumes of flood runoff during the first two weeks of August, which amounted to 30 km3 in the Mekong at Vientiane, could not have been produced by the total rainfall for the period indicated from satellite imagery. The figures are far too low and the areal coverage of maximum storm rainfall too small. There is a need therefore to extend the ‘ground truthing’ of the figures indicated from the satellite image.

In this context the major recommendation is the need to improve the rainfall recording network in the key river basins in Northern Lao PDR, although this need has already been recognized by the Department of Meteorology and Hydrology (DMH) in Vientiane. New gauges have been installed at Phongsali, Oudomxai and Luang Namtha.

It was also recognized during the post flood evaluation that the clarity and coverage of the flood forecasts as they are posted from day to day on the MRCS web site needs to be improved. The issue has been addressed.
Turning to the observations made in the Annual National Flood Reports:

• The August damage in Vientiane emphasized the need for improved urban planning such that residential development is controlled in flood prone areas, while the ongoing removal of natural flood storage such as wetlands will only lead to increased future losses.
• The inventory of equipment available to the emergency response agencies is inadequate.
• In Thailand, the call for greater emphasis on preventative measures such as early warning systems and the use of geo-informatics to identify and monitor areas at risk from flash floods and landslides applies regionally. The installation of the monitoring network to protect vulnerable villages provides a model that could be extended throughout the basin.
• The difficulty of mitigating the impacts of flash flooding has been long recognized in Viet Nam and a call is made for increased investment in research regarding the quantitative meteorological and hydrological thresh holds that trigger them in various kinds of landscape.
• Flood induced erosion, particularly in the Delta, and its linkage with ‘hard’ flood control measures is also an area that needs better understanding since it is emerging as a serious issue.
• As in previous years all four national reports recommend ongoing improvement towards effective disaster management through better inter-agency coordination and the provision of the appropriate levels of funding.