Mekong River Commission

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.

Figure 19 shows the times series of annual flood peak and volume for three large left bank Mekong tributaries in Lao PDR:

• The data for the Nam Ngum at Tha Ngon clearly indicate the impact of the upstream reservoir on the flood regime. Since the dam was commissioned in 1971 only two significant flood events have occurred, while the most extreme regional flood events in recent regional hydrological history, those of 2000 and 2001 are not evident. Such impacts upon and scale of change to the downstream flood hydrology may be anticipated more widely as more reservoirs are developed regionally.
• Elsewhere, the evidence suggests that the tributary flood hydrology is broadly concordant with no significant events at during the 1980s and early 1990s followed by the events of 2000, 2001 and 2002, which dominate the regional picture.

Figure 20. Se Bang Hieng at Ban Keng Done (catchment area: 19,400 km2). Annual flood hydrographs of 1961, 1974, 1997
and 1999, indicating the highly variable nature of the annual flood regime and that between flood peak and volume.

• Other notable regional tributary events were those of 1961, 1974, 1997 and 1999. In comparing these four annual flood hydrographs, as in Figure 20 for the Se Bang Hieng, their highly variable nature from year to year becomes apparent, as does the often inconsistent relationship between flood peak and volume. The regularity of flash flood conditions, even within a very large catchments also emerges.
• The event of 1961 combined an exceptional 50 year flood volume with a series of peak flows, none of which exceeded a magnitude to be expected once in five years.
• On the Se Bang Hieng the flood season usually begins in mid June and extends to the end of October. During 1974 it got off to a false start during June but sustained flood flows did not begin until mid August. The flood season also ended a month early in the last week of September. It was therefore short and the volume of floodwater low. In the mean time the peak discharge reached almost 8500 cumecs, expected once on the average in 25 years.
• This type of flash flood hydrograph, from a catchment of almost 20,000 km2 is even more evident during the flood of 1997, when discharges increased four fold over less than a week also to 8500 cumecs.
• The most remarkable seasonal hydrograph is that of 1999, when to all effect the flood season ended two months early at the end of August. Then during November, outside of the usual flood season, a severe tropical storm caused a massive increase in flows over two days from less than 500 cumecs to 8700 cumecs and the classic flash flood hydrograph.

This 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 on 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. Historically, the events of 2000, 2001 and 2002 dominate, both in terms of flood peak and volume (Figure 21), as they do elsewhere in the LMB. 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 contributes only 10% to the average flood volume at that point on the mainstream (Figure 22) and 6% to the total at the delta. It is never the less a key tributary system within the context of the flood hydrology of the Mekong, though overall the contributions of the left bank tributaries dominate:

Table 7. Mean annual tributary contributions to the Mekong flood.

China (Upper Mekong)		Lower Mekong
	Left bank tributaries		Right bank tributaries
16%	60%		24%