Mekong River Commission

 

1. Introduction

As in 2006, the 2007 Annual Flood Report aims to fill two primary roles, (i) to provide a summary of flood conditions in the Mekong Region over the year, and (ii) to collate the relevant data and information. The report also has an annual 'theme'. Having contributed significantly to the understanding of Mekong mainstream floods and flooding in the 2006 Report, this document has made the logical progression and taken the flood hydrology of the tributaries as its theme, with flash floods meriting particular attention.

2. The flood hydrology of the major tributaries in the Lower Mekong Basin

2.1 An introductory note on the impacts of land-use change and deforestation on the flood regime of the Mekong tributary rivers

The belief that deforestation causes an increase in the frequency and severity of major floods and compounds the damage that they do appears to be widespread. This view is as widely held within the Mekong Basin as it is throughout the tropical world. However, an authoritative Report from the UN’s Food and Agriculture Organization (FAO) and the Centre for International Forestry Research (Cifor) says that the evidence shows no link. That flood damage is increasing is unquestioned; however this is not linked to the increasing magnitude of events but is due to the growing global population and the consequent expansion of human settlements into areas which had once been marginal. As a result, each flood claims more lives than it would have done a century ago. In addition the human diversion of watercourses and the construction of flood protection works often moves the problem from upstream to downstream areas.

There are arguments and some evidence to suggest that the actual frequency of floods is beginning to increase, possibly in response to climate change, a major consequence of which, it is generally agreed, will be an increase in the incidence of extreme events. 

2.2 The geography of the major tributary contributions to the flood hydrology of the Mekong

The physical geographical region defined by the Lower Mekong Basin (LMB) is a variable one in terms of its topography, land cover, and rainfall. These three factors obviously have a major impact on regional hydrological response and its spatial variability, which is reflected in the contributions that the major tributaries make to the mainstream flows. These major tributaries are identified and, recognising the geographical distinctions in tributary hydrology, three reaches of the mainstream and their tributaries are considered: Chiang Saen to Vientiane, Vientiane to Pakse and Pakse to Kratie.

2.3 Chiang Saen to Vientiane — the regional history of tributary floods and flooding

In northern Thailand and northern Lao PDR flash floods have been much in the news in recent years with locally catastrophic events particularly in 2000 and 2001. Flash floods in these regions feature in the hydrological landscape to some extent virtually every year and there is a long history of extreme events. For example, the years between 1918 and 1920 each featured devastating events, as did 1953. Even further back in time the original capital of the Lanna Kingdom had to be moved to Chiang Mai at the end of the 13th Century because of frequent flood inundation. These events obviously occurred long before the regional forests were reduced by logging, which has been blamed for a supposed increase in the frequency of the flood hazard since the 1960s. The logging of natural forests was banned in Thailand following the national flood emergency of 1988.

In the northern Lao provinces of Huaphan, Pongsaly, Luangnamtha and Luang Prabang significant flooding has been similarly frequent, having occurred in 1991, 1995, 1996, 2000, 2002, 2005 and 2006. Prior to that the largest regional flood was in September 1966, which was associated with the incursion of Cyclone Phyllis over the large northern tributaries.

2.4 Chiang Saen to Vientiane — the regional tributary systems

A schematic of the regional tributary rivers is presented along with summaries of their areas and the hydrological data that are available.

• The reach comprises a complex of right bank tributaries in north and northeast Thailand, principally the Nam Mae Khan, Nam Mae Ing, Nam Mae Kok and Nam Loei and a number of generally larger left bank tributaries in northern Lao PDR. Of these the Nam Ou is by far the most significant in terms of drainage area and discharge.
• The total tributary area is 110,000 km2, of which stream-flow data are available for almost 60,000 km2.
• Annual tributary runoff is less than in other parts of the Lower Mekong Basin to the south, particularly from the large left bank tributary systems downstream of Vientiane.
• The sub-reach between Luang Prabang and Vientiane is unique in the LMB in that there are no large left bank tributaries, a fact which largely explains the small contribution it makes to the overall flow of the Mekong.
• Flash floods in this area of the Lower Mekong Basin have been a key feature of its hydrological history. It has also been an area where deforestation and 'slash and burn' agriculture have been widespread, leading to arguments that the consequent landscape changes have been a major contributory factor to flooding.

2.5 Chiang Saen to Vientiane — are flash floods becoming more common?

The view that deforestation has increased the frequency and severity of flash floods is widely held, particularly in northern Thailand. There appears to be sufficient evidence on the basis of the analyses undertaken that extreme floods have become more frequent over the last 15 years or so. Whether this is a response to external influences such as climate change is arguable. It may simply be part of the natural periodicity of such processes - the fact that there were no significant events at all during the 1980s might be evidence in this regard. However, the timescale for which data are available is far too short to establish any such possible periodic patterns one way or the other. Nevertheless, it seems reasonable to conclude that after the logging ban in Thailand during the late 1980s, deforestation appears not to be a factor in the increased frequency of events post 1990.

2.6 Chiang Saen to Vientiane — the hydrological assessment of extreme regional tributary flood events

It is proposed that there are two weather processes linked to the occurrence of floods, one that generates average and significant floods and one that generates floods that are far more extreme. This distinction may be linked to:

• typical monsoonal storms and average antecedent catchment conditions that result in the more typical or basic events, and;
• more extreme weather systems, such as intense tropical depressions, severe tropical storms and typhoons, which combine with wet or saturated antecedent catchment conditions to generate extreme or outlier events.

The picture that emerges from the assessment of the most extreme historical regional flood events is that, except for the larger catchments in Lao PDR such as the Nam Ou, they are of short duration, typically three to four days. In addition:

• times to peak are extremely rapid, typically less than one day;
• antecedent flow conditions are often not linked in any significant way to the peak magnitude, though such is the rapid rate of recession of the flood hydrograph this may not of itself be a meaningful indication of the actual state of the catchment in terms of wetness and its ability to generate maximum rates of runoff during a storm.

2.7 Chiang Saen to Vientiane — the hydrological assessment of extreme

In this reach the large left bank tributaries in Lao PDR begin to exert their dominant influence on the flood hydrology of the Mekong, starting with the entry of the Nam Ngum 50 km downstream of Vientiane. The combined mean annual flow of all tributaries between Vientiane and Pakse is 171 km3, almost 40% of the total for the Mekong. The major contributions to this figure are made from the left bank by the Nam Ngum, Nam Kading/Nam Theun and the Se Bang Hieng, while the Mun - Chi Basin accounts for by far the greater part of the floodwater arising from the right bank in Thailand. Within this reach:

• The tributary catchment areas are generally very large, such that flash floods do not dominate the flood hydrology as they do in the tributaries upstream of Vientiane. They are locally important on some of the smaller, steeper, tributaries and in the upper reaches of the larger rivers, and on the larger tributaries conditions may arise where extremely rapid increases in discharges are followed by equally fast flood recessions.
• In their lower reaches backwater effects from the confluence with the Mekong mainstream can extend very considerable distances upstream and exacerbate the depth and duration of inundation during extreme events.
• The accelerating development of hydropower schemes in this sub-region of the LMB will in time significantly modify the flood regime of many of the major tributaries. The impact upon flood peaks and volumes will depend upon the scale of reservoir development and whether any such storages are operated in a way that mitigates the flood hazard, for example by the provision of operational flood storage during the flood season.
• It is anticipated that one of the major changes to the flood regime will be a delay in the start of the flood season and the onset of the higher discharges associated with it. Early season flood water will be withheld in the reservoirs which will typically be drawn down at the end of the dry season.
• The cumulative impacts of the numerous proposed schemes will in time modify the flood regime of the Mekong mainstream itself.

2.8 Vientiane to Pakse — the hydrological assessment of extreme regional tributary flood events

Because the major tributaries in this Mekong hydrological sub-region are large, flood risk should be assessed both in terms of flood peak and volume, thereby acknowledging the importance of both the depth and duration of flood inundation. A highly variable flood hydrology both within and between years is characteristic of the large left bank tributaries and there is, in effect, nothing that can be defined the 'typical' flood season hydrograph. Extremely rapid increases and decreases in flood discharge, generally associated with severe tropical storms can, even in very large catchments, cause flash flood conditions.

Due to the flat landscape, the lower rainfall and to some extent the presence of large reservoirs, flash floods are not a feature of the hydrology of the Mun — Chi Basin, except in the far upstream reaches. In recent years the events of 2000, 2001 and 2002 were the largest both in terms of flood peak and volume. On the Nam Mun the highest peak discharge occurred in 1978, corresponding with the year that the largest flood peak was observed on the Mekong mainstream at Kratie since records began in 1924.

The evidence from the Nam Chi Basin suggests that below 'normal' flood years, when peak and volume are significantly less than average, tend to cluster. During the 13 years between 1967 and 1979, eight of the most extremely deficient annual floods were observed.

With a total basin area of 120,000 km2 , equivalent to 22% of the Mekong drainage area at Pakse, the Mun — Chi system contributes only 10% to the average flood volume at that point on the mainstream and 6% to the total at the Delta.

2.9 Vientiane to Pakse — the estimation of flood risk at ungauged locations

The accelerating pace of hydropower development in the LMB means that there is a growing need for the development of a reliable procedures for the estimation of design flood risk at ungauged dam sites. Over recent years 'best practice' has involved the use of regional methods based upon the pooling of data in regions and sub-regions that might be considered to be homogenous with respect to their flood hydrology. The Mekong between Vientiane and Pakse can be considered to be one such, though the left and right bank tributaries require separate treatment. A statistical procedure is developed.

2.10 Pakse to Kratie — the regional tributary systems

This reach of the Mekong, which accounts for 18% of the total basin drainage area, the tributary contribution to the mean annual flood volume at the Delta is about 20%. At Kratie 90% of floodwater has already entered the system; downstream of there most of the balance is made up by the contribution of the Tonle Sap Basin in Cambodia. Of this 20% around 18% is accounted for by the combined Se Kong, Se San and Sre Pok Basins, which combined make the largest single contribution to the total Mekong flood in most years.

The major constraint to hydrological analysis in this sub-region is a lack of representative data, which are distributed amongst only seven sites. With the exception of the gauge on the Se Done at Kong Se Done, which records the flows from 65% of the basin, and the gauge on the Se Kong at Attapeu, the others on the Se Kong and Sre Prok are far upstream and only provide information about the hydrology of the headwaters. Meaningful estimates of the flood hydrology of these key regional river systems is therefore dependent upon hydrological modelling.

2.11 Pakse to Kratie — the hydrological assessment of extreme regional tributary flood events

This lack of representative data means that only a limited assessment of the tributary flood hydrology is possible, which is unsatisfactory given the pivotal role of the Se Kong, Se San and Sre Pok in the generation of extreme flood conditions across the Cambodian floodplain and in the Mekong Delta. Their contribution to the extreme regional flood of 2000 was a major one. This event was characterised by a peak flow at Kratie on the mainstream of the Mekong that was only marginally above average. It was the total flood volume over the flood season that was extreme, being the highest observed since 1924.

3. The 2007 flood season

3.1 Hydrological aspects — the mainstream of the Mekong

In terms of peak discharges and seasonal flood volumes, conditions along the Mekong mainstream during the 2007 flood season were comparable to those that occurred during 2006. As then, both variables fell below their long-term average values; the degree to which they did so became more pronounced downstream, so that at Kratie both flood peak and volume were significantly below normal. In fact, flood conditions at Kratie during 2006 and 2007 are amongst the eight lowest observed since 1924.

These below average flood conditions were not, however, the definitive hydrological feature of the 2007 season. This lies with the fact that throughout the mainstream the start of the flood season was generally the latest observed over the last 80 to 90 years. Typically, the season begins in the last week of June or the first week of July in those reaches above Kratie and a week or two later at Phnom Penh and in the Delta in Viet Nam. As a consequence of the three week to one month delay in 2007, water levels at the end of July were amongst the lowest ever observed. 

3.2 Hydrological Aspects — The Cambodian Floodplain and Mekong Delta

Self evidently, hydrological conditions over the Cambodian floodplain and within the Delta during the 2007 flood season — indicated in terms of water level rather than discharge — reflected those that prevailed upstream. Water levels were considerably below average throughout most of the years and amongst the lowest observed at the end of July. Only in October did they increase to anywhere near average.

3.3 The 2007 Flood Season — Meteorological Aspects

With the exception of most of Cambodia and the Delta region, rainfall during the SW Monsoon of 2007 over the Mekong region was moderately below average, with only one extensive storm system that was large enough to bring the Mekong up to average discharges and water levels. This was Sever Tropical Storm 'Lekima', which passed over the central regions of Lao PDR and Thailand during the first week of October.

4. Lao PDR 2007 Country Report

The central and southern parts of Lao PDR were the most affected by floods in 2007, which were mainly associated with impacts of Tropical Storm 'Lekima'. The resultant national damage and loss exceeded that for 2006, particularly with respect to the rice crop. Elsewhere in the country the only events of note in 2007 were heavy rainfall during mid-September in Luangnamtha Province and a local storm in Vientiane during April which caused some damage. Two people were killed and over 600 villages were affected in some way by the floodwater. Almost 160,000 ha of rice crop was damaged to some extent by prolonged submergence along with 30% of the planted vegetable crop.

5. Thailand 2007 Country Report

The southern parts of the Mun and Chi River Basins around Nakon Phanom, Kalasin Mukdahan, Roi Et Yasothon and Ubonrachatani were the most affected by the storm rainfall associated with 'Lekima'. Most rainfall occurred on the 4th and 5th of October, by which time the event had been downgraded to a tropical depression. Rainfall decreased from in excess of 250 mm in the east of the Mun Chi Basin to less that 150 mm in the west during the first week of October as the intensity of the system decreased as it tracked westwards. Flooding in NE and central Thailand during October 2007 was described by the Bangkok Post as the worst in 40 years, with troops and rescue workers dispatched to assist stranded residents. A number of people died as a result of being swept away and thousands of hectares of crop lands were inundated to a depth of 1 m and more. There was widespread disruption of road links and many communities were cut off. Other reports suggest that the total number of people killed was as high as 67 and that 17,000 people had to be evacuated.

6. Cambodia 2007 Country Report

Events in Cambodia during 2007 centred around the flooding caused by Tropical Storm 'Pabuk' in the northern and coastal areas of the country on the 4th and 5th of August. Otherwise, the fact that the annual flood on the Mekong mainstream was a month late in starting and that water levels, once it had, remained significantly below average until October following 'Lekima', meant that levels in the Great Lake were considerably lower than average. This delay in the increase in lake levels until early August and the fact that inundation of the riparian forest covered a much smaller area than usual resulted in a large fall in the annual fish catch. Reports from those provinces affected by 'Pabuk' during August indicate that more than 160,000 persons were affected, 5 people were killed, 37 km of rural roads were seriously eroded, along with some locally significant damage to public building, temples and residential areas. A number of dykes and irrigation canals eroded or filled with sediment. Over 8000 ha of rice and other crops were damaged or altogether lost due to prolonged submergence.