It has already been established that, from a hydrological perspective, flood conditions along the mainstream during 2006 were geographically quite variable.
The daily discharge hydrographs for 2006 are illustrated in Figure 15, the key feature of which is the second peak and the fact that, without it, flood conditions would have been much very below average, particularly at Kratie.
Figure 15. The 2006 daily discharges
observed at Chiang Saen,
Vientiane and Kratie, compared with the long term mean daily
discharge hydrograph.
This picture of generally below average conditions, specifically from a volumetric viewpoint, is confirmed from an examination of the 2006 flood season flow duration curves (Figure 16), which show the percentage of time that a given discharge was exceeded.
Figure 16. 2006 flood season daily discharges
observed
at Chiang Saen, Vientiane and Kratie,
compared to their historical range and average.
Overall, these results indicate that during 2006 the peak discharge thresholds that were exceeded for any length of time were unexceptional. Only at Chiang Saen were severe flood discharges and water levels exceeded, though only briefly.
Figure 17. Water levels at the Mekong
mainstream at Jinghong on
Yunnan Province, China; 1stSeptember to 15th October 2006.
The peak
water level occurred on the 11th October.
The 2006 monsoon was generally rather weak overall, which is reflected in the hydrology of the flood season. Three major tropical storm systems tracked across the Mekong Basin during the year; their tracks and associated rainfall are presented in Figures 18 to 20:
Figure 18. Tropical Storm Prapiroon, moved in over the Upper Mekong Basin during the last week of August, 2006, but rapidly weakened to become a tropical depression. Intense storm rainfall (150 to 200 mm) was confined to northern Thailand and eastern Cambodia.
Figure 19. Severe Tropical Storm Xangsane, moved in over the Central Mekong Basin during the first week of October, 2006. Very intense storm rainfall (250 to over 500 mm) was widespread over southern Lao PDR, Cambodia and Viet Nam.p
Figure 20. Tropical Storm Durian, moved in over the Mekong Delta in Viet Nam during the first week of December, 2006, but rapidly weakened to become a tropical depression. The associated rainfalls for the week were generally less than 100 mm.
Figure 21. Cambodian Great Lake.
Annual maximum
water levels observed at Kampong Luong.
The generally below average magnitude of the mainstream seasonal flood hydrograph during 2006 carried over into the Cambodian floodplain and the Mekong Delta in Viet Nam, where seasonal maximum water levels were similarly below normal. This apparently did not apply to maximum water levels achieved in the Tonle Sap/Great Lake system, however, where as Figure 21 illustrates the 2006 maximum water level was only a few centimetres less than that achieved in 2000, an extreme flood year which led to the largest regional loss of life and socioeconomic damage witnessed in recent decades.
The 2006 maximum was observed on the 14th October and was a clear response to the storm rainfall and second peak to the regional flood hydrograph associated with Severe Tropical Storm Xangsane. As Figure 19 clearly shows most of the excessive rainfall generated by this system was confined to eastern Cambodia and the Great Lake’s own system of tributaries, which would have caused significant volumes of local runoff, given that during the first week of October these areas received 400 to 500 mm of rainfall. Three other factors contribute to the fact that the 2006 maximum lake level almost matched that of 2000:
Figure 22. Comparison of the spatial
extent of floodplain
inundation over the Cambodian. Floodplain and the Mekong
Delta for the years 2000 (delimited in yellow) and 2006
(area shaded blue).
In the face of these factors therefore, the extent of flood inundation over the Cambodian floodplain in 2000 and 2006 were quite similar, as Figure 22 confirms.
Downstream from the Great Lake itself, at Prek Kdam on the Tonle Sap river and at Phnom Penh Port on the Mekong mainstream, the maximum water levels for 2006 were much closer to average, though only three to four centimetres lower than the maximum observed on the Great Lake. They were, however, much less than those observed during the 2000 flood at each of these sites (Figure 23). Quite why this should be so, when the level of the Great Lake in 2000 and 2006 were so similar, is not entirely clear, but the fact that the rainfall over the Great Lake basin and its tributaries was so extreme due to Severe Tropical Storm Xangsane, may go some way towards an explanation.
Figure 23.Annual maximum water
levels for 2006 at Phnom Penh Port and
Prek Kdam in their historical context. They were average and
in 50% of years
they would have been exceeded.
Over the Delta in Viet Nam the total area inundated in 2000 was much greater than during 2006, as can be seen from Figure 22. Maximum water levels for the year were marginally below average, as the data for Tan Chao and Chao Doc confirm in Figure 24. As regards the timing of these annual maximum water levels, they were two weeks later than normal at all sites in these floodplain and delta regions, as illustrated in Figure 25.
Figure 26 summarises these downstream water levels for 2006. The maxima for the year were above the average for the day on which they occurred, but only marginally so. The fact that they occurred two weeks later than average is confirmed.
This moderate to below average 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 over a duration of one to three hours, though actual inundation was often longer due to poor drainage capacity (VNMC 2006 Annual Flood Report).
Figure 24. Annual maximum water levels for 2006 at Tan Chao and Chao Doc in their historical context. They were average and in 65% of years they would have been exceeded.
Figure 25. Historical (1980 –2006) probability distribution of the date of occurrence of annual maximum water level at Prek Kdam, Tan Chao and Chao Doc, which 50% of the time would be expected to occur not later than the beginning of October. During 2006, it’s occurrence was atypically late, since maximum water levels were not observed until the latter half of October.
Figure 26. Long term distribution of daily water levels at Phnom Penh Port, Prek Kdam, Tan Chao and Chao Doc, the historical average onset and end dates of the ‘flood season’ and the comparative magnitude of the maximum water levels reached in 2006.