Mekong River Commission

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

Figure 7. Regional annual number of significant and extreme flood events
- 1970 to 2003. (See text for definitions).

The view that deforestation has increased the frequency and severity of flash floods is widely held, particularly in northern Thailand. Defining a significant event as a flood that was more than one standard deviation above the mean annual maximum flood peak, Figure 7 shows the regional incidence of such events between 1970 and 2003. Even if the figures are corrected for the number of station years available in each decade, the increased proportion and incidence of such events remains apparent from 1990 onwards. Of the 25 significant events observed since 1970, 16 or 65% occurred after 1990. Interestingly, none at all occurred during the 1980s. The regional median annual maximum event remained unchanged over the period as a whole.

There appears to be sufficient evidence on the basis of this simple analysis to suggest that significant 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 would appear not to be a factor to the increased frequency of events post 1990.

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

 
Figure 8. Nam Mae Chan at Ban Huai Yano Mai. Definition of significant and extreme floods
in the context of the events of 2001 and 2002.

Figure 8 shows a plot of the annual maximum flood events of 2001 and 2002 on the Nam Mae Chan at Ban Huai Yano Mai in northern Thailand. These are by far the largest on record for the river over the 24 years for which data are available (1980 – 2003), with peak discharges three standard deviations above the mean annual flood over the same period. Such events are quite distinct from the rest of the sample, which might be classified into significant events (greater than one standard deviation above the mean) and extreme events (greater than two standard deviations above the mean). Clearly the suspicion is that there are two weather processes at work, one that generates average and significant floods and one that generates those 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 following 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.

Figure 9. Nam Mae Kok at Chiang Rai and at Ban Tha Ton. Annual maximum peak
discharges for the periods of available data and their annual recurrence intervals.

Figure 9 shows the annual maximum discharges on the Nam Mae Kok at Chiang Rai and upstream at Ban Tha Ton over the years for which data are available. The indication is that flood peaks in excess of ten years occurred in 1972, 1988, 1995, and 2004, the latter being greater then the 1:30 year event at Ban Tha Thon (no data are available for Chiang Rai). Of the observed data at Chaing Rai, the largest event occurred in 1995 (though the 2004 event would have exceeded it significantly if the flows upstream at Ban Tha Ton are scaled accordingly). This 1995 event at Chiang Rai is shown in detail in Figure 10:

• It was not a flash flood in any real sense, the overall flood season hydrograph being quite coherent, with several independent peaks superimposed upon it rather than rising rapidly from a relatively low flow state. The associated daily rainfalls are also shown, with the maximum daily depth of 130 mm occurring on the 5th August. This depth has estimated annual recurrence interval of 1:5 years.

Figure 10. Nam Mae Kok at Chiang Rai - 1995 flood hydrograph with associated daily rainfall.

• It is the accumulated daily rainfalls that were responsible for the relative severity of the event, with the 5- and 10-day totals observed at Chaing Rai being exceptional, with recurrence intervals of 20 years.

The mean annual and monthly tributary flow contributions to the mainstream are summarised in Figure 4, these being set out geographically between key locations along the mainstream itself. For the sake of consistency we consider the Upper Mekong in China as the ‘tributary’ at Chiang Saen:

Table 4. Chiang Rai - cumulative rainfall, 5th to 14th August, 1995 and their recurrence interval.
Storm duration	Dates August 1995	Cumulative depth (mm)	Recurrence interval
1 day	5th	130	5 years
2 day	5-6th	150	10 years
3 day	5-7th	200	10 years
5 day	5-9th	280	20 years
10 day	5-14th	400	20 years

Figure 11.Nam Mae Kok at Ban Tha Ton - the 2004 flood
hydrograph with associated daily rainfall at Mae Chan.

The later period of storm rainfall at the end of August and during the first few days of September produced a slightly higher flood peak. However, the cumulative depth over five days of 200 mm was unexceptional in terms of its recurrence interval, but it was far more effective in runoff terms since flows in the river were already high and the upstream catchment saturated.

The 2004 event in the Nam Mae Kok was much more severe with a flood peak of 650 cumecs at Ban Tha Thon. The corresponding peak discharge at Chiang Rai could have been anywhere between 800 and 1000 cumecs, possibly therefore as high as a 1:50 year event. It takes on much more of the appearance of a flash flood with discharges rising by over 400 cumecs in two days from a relatively modest initial flow condition (Figure 11). Finding daily rainfall conditions to link to the event proved a problem, indicative of the highly localised nature of intense storm rainfall bursts. The only ‘local’ site with a storm rainfall pattern consistent with the flood runoff during September was that at Mae Chan, though even here the daily rainfalls were anything but extreme, even in terms of the accumulated total over five days. The later period of storm rainfall at the end of August and during the first few days of September produced a slightly higher flood peak. However, the cumulative depth over five days of 200 mm was unexceptional in terms of its recurrence interval, but it was far more effective in runoff terms since flows in the river were high and the catchment saturated.

The classic regional example of the extreme fl ash fl ood is provided by events on the Nam Loei, Nam San and Nam Man during August 1978 (Figure 12):

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