Mekong River Commission

1. Introduction

The 2006 Annual Flood Report aims to fulfil two primary roles: it provides a summary of the flood year 2006 and it collates important data on the flood regime. These data will, in time, accumulate to provide a primary regional resource for flood research and the collation of historical reference material.

2. About floods in the Mekong Basin

2.1 The size of Mekong floods in their global context

Based on the data in the World Catalogue of Large Floods (IAHS. 2003), extreme floods on the Mekong are compared to those upon other global river systems with catchments in excess of 500,000 km2, revealing that the river is amongst the world maxima classified upon the basis of peak discharge per unit area and very close to the global limit for rainfall generated flood runoff.

2.2 The historical geography of floods on the Mekong mainstream

An analysis is undertaken of the temporal and spatial nature of floods along the Mekong mainstream, revealing that the river basin is far from geographically homogenous with regard to the nature and severity of the flood season in any given year.

2.3 The nature and analysis of floods on large rivers

The quantitative definition of the magnitude of a flood exclusively in terms of its peak discharge is a useful and sufficient statistic in the case of small river basins where the duration of flood events is usually only a matter of several days. On large rivers the flood hydrograph has a much longer duration, which in the case of the Mekong is a matter of several months. The multivariate aspects of the hydrograph therefore need to be simultaneously taken into account in the assessment of flood risk and magnitude. In this report modern multivariate statistical technology is used; this brings together the peak flood discharge, the volume of the flood hydrograph and the duration of flows above critical thresholds.

2.4 Temporal aspects of the Mekong flood regime

The onset and duration of the flood season in the Lower Mekong Basin is clearly an important variable from year to year. In keeping with broader definitions of hydrological seasonality on the mainstream adopted by the Environment and Fisheries Programmes within the Mekong River Commission (MRC), four flow seasons are identified, namely the flood season, the dry season and two transition seasons. The hydrological indices adopted to define the onset and closure of each one are presented along with a historical assessment of how these dates have varied from year to year. It is demonstrated that these temporal variables have been remarkably consistent over the last 80 to 90 years and have a surprisingly small inter-annual variability. Studies of the palaeoclimate during the Holocene are quoted that suggest these temporal aspects of the Mekong flood regime have been unchanged over the last 5,000 to 6,000 years.

Throughout Part 1 the events of 2006 are set within this wider geographical, historical and temporal context and a number of graphical techniques are presented that could be adopted as standards for the comparative assessment of the Mekong flood for any year that is under consideration.

3 The 2006 flood season

3.1 Hydrological and meteorological aspects

The total volume of flows during the flood season of 2006, throughout the mainstream, was below average and in parts, significantly so. This deficit becomes more evident towards the downstream regions, particularly at Pakse and Kratie. The defining feature of the year’s flood hydrology, however, is the second peak to the hydrograph during October, which brought about the only time of the year that discharges consistently approximated or exceeded their long term daily averages. This second peak was the response to Severe Tropical Storm Xangsane, which tracked over the Mekong Basin during the first week of October. Without this event, the flood volumes in the lower regions of the basin would have been amongst the lowest on record. Peak flows in response to Xangsane were not excessive.

3.2 Water levels in the Tonle Sap floodplain of Cambodia and the Delta in Viet Nam

Clearly these reflect the magnitude of discharges entering these lower regions of the Mekong system and were therefore generally below average. Once again, the conditions of 2006 are firmly set within their historical context. In this regard, the water levels for the Tonle Sap system as far back as 1924 are reviewed and some exploratory statistical analyses undertaken. The incidence and role of typhoon incursions into these regions of the Lower Basin are considered and some and their impacts upon the distribution of storm rainfall revealed.

3.3 Regional summary and tributary flood hydrology during 2006

Apart from the mainstream, arguably the major regional flooding took place in Northern Thailand where flash flooding occurred along the northern tributaries, such as the Nam Mae Kham, Nam Mae Kok and Nam Mae Ing, causing very considerable damage. The period between July and October saw a number of such events, which also caused excessive water levels in Northern Lao PDR, along the Luang Namtha, for example. These episodes are synthesized and summarised in terms of cause and effect.

4 Overview of annual flood reports

4.1 Lao PDR

During 2006 localised flash flooding in Luang Namtha and Attapeu Provinces were the only noteworthy flood incidents that occurred in Lao PDR. Events in Luang Namtha during the second week of August were in response to orographically induced monsoonal storms which produced 230 mm to 270 mm of rainfall over the first 10 days of the month. The flooding in Attapeu Province during the first week of October was attributable to the incursion of Severe Tropical Storm Xangsane. Elsewhere the only meteorological episodes of any significance were local storms over Vientiane which caused brief urban flooding in March, May and October and a highly localised ‘whirlwind’ which moved at high speed through the Chanthabouly district of the Capital at 7:25pm on the 5 May 2006, with wind speeds of over 100 km/h (30 m/sec) causing severe but very local structural damage.

4.2 Thailand

During 2006 Thailand was badly affected nationwide by floods from several storms, most particularly from Severe Tropical Storm Xangsane, which turned into a tropical depression in the country. Out of 75 provinces, 46 were locally inundated. By mid-October, Thailand’s Department of Disaster Prevention and Mitigation (DDPM) reported that 47 people had been killed, two were missing and more than 2.4 million people had been affected to various degrees over the country as a whole. Approximate losses are estimated to be of the order of US$8 million.

4.3 Cambodia

In Cambodia conditions during the 2006 flood season were below average both in terms of peak and volume. The flood peak was in fact amongst the lowest recorded over the past 80 or more years. The maximum discharge for the year occurred in mid-August, after which water levels decreased considerably until early October and the passage across the region of Severe Tropical Storm Xangsane. This weather system generated a slightly lower second peak in mid- October, an uncommon feature of the annual hydrograph. As a result of these below normal seasonal flows no significant crop losses were reported, with the exception of the fact that the unseasonally late second peak led to the inundation of some low lying areas. Some early flood recession rice plantings were lost and a second replanting was required. No flood damage to infrastructure took place during the year.

4.4 Viet Nam

The moderate to below average 2006 flood regime in the Delta during the year meant that direct flood damage was not severe. However, the late appearance of flood peaks did bring about some unfavourable conditions for agriculture. What widespread inundation there was occurred when high tides combined with incoming flood flows from upstream, which occurred three times during August and September. The associated water levels at most stations were higher than Alert Level 3 (see Appendix 8) over a duration of one to three hours, though actual inundation was often longer due to poor drainage capacity (VNMC 2006 Annual Flood Report). By far the major damage during the year was associated with Tropical Storm Durian during the first week of December which generated extreme wind speeds and tidal surges. Immediate needs following this national disaster were estimated by the UN to be of the order of US$60 million.

5. Summary conclusions and recommendations

5.1 Summary conclusions

Regionally, the flood season of 2006 saw below average conditions both in terms of flow volumes and peak discharges, most particularly on the lower mainstream downstream of Vientiane. To the north, however, flash flooding in Thailand and Lao PDR resulted in significant damage and loss. Deep monsoonal depressions that were largely confined to these northern Provinces were generally responsible, though the wider regional impacts of Severe Tropical Storm Xangsane played a major role in early October, particularly in the south of Lao PDR. The major regional disaster in 2006 was the result of Tropical Storm Durian during December when extreme windspeeds and tidal surges caused immense damage in the Delta and southern coastal regions of Viet Nam.

5.2 Recommendations and lessons

The hydrological and water level data and information available for the mainstream are more than sufficient for a comprehensive assessment of the annual flood from year to year. Analyses of these historical data that have been reported here provide a framework for the objective and more perceptive evaluation of annual floods on the mainstream within their wider temporal and geographical context.

Tributary data analysis and information are far less complete at present, which amounts to a significant shortcoming, given the hazard of flash flooding in these river systems. The FMMP T2 Flood Risk Mapping Project on the Nam Mae Kok in northern Thailand is a recognition of this. Its modest extension to provide a regional flood risk analysis for the tributary systems upstream of Vientiane in both Thailand and Lao PDR would provide substantial ‘add on’ value. The HYCOS Project will also contribute much to the understanding of the flood hydrology of these tributary rivers.

In the longer term HYCOS will also add to the meteorological knowledge base and the nature of the linkages between regional storm rainfall and flood runoff. For the present purposes of the Annual Flood Report, however, daily satellite based rainfall estimates at the regional scale are sufficient and appropriate.

Finally, it is recommended that the Annual Flood Report be ‘theme’ based. Here the emphasis is on data analysis and the temporal and spatial nature of floods and flooding in the Mekong region. Such material provides an important supplement to the framework of knowledge within which the FMMP is being undertaken as well as contributing basic insight into the regime of the Mekong that are a necessary in many other contexts, for example the assessment of the environmental impacts of basin development. Other annual themes that should be considered are the socio-economic benefits, or otherwise, of the flood regime, meteorological aspects and the potential consequences of climate change, including links with El Nino Southern Oscillation (ENSO). The flood report should also provide a medium for reporting FMMP progress in the interests of dissemination to the wider audience and stakeholders.