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Extent of flooding and water level rise from dam break in southern Laos
Initial and preliminary analysis of the extent of the water level rise in Stung Treng, Cambodia, due to the dam break shows about 20 cm compared to normal conditions
Vientiane, Laos, (Mekong River Commission) -- On July 23, 2018, Saddle dam D, one of the five auxiliary dams of the Xe Pian-Xe Namnoy hydropower project in Attapeu province, Laos, failed and flooded several villages along the Xe Pian river (5 km from the dam), a tributary of the Xekong River, which flows into the Mekong mainstream.
The distance from the dam to the confluence of the Xekong and Mekong is about 283 km. Located right below the confluence is Stung Treng in Cambodia with Kratie about 124 km further down.
The flood damage to lives and livelihoods in Laos itself have been widely reported. In addition, it was also reported that communities in Stung Treng were evacuated due to the flooding situation.
What was the extent of the flooded areas? What was the extent of the water rise in Cambodia because of the dam break on top of other factors? As previously reported, MRC monitoring shows significant rainfall and water rises throughout the Mekong basin.
Extent of flooding
The MRC, working with its partners, has produced possible flood maps created by analysing data taken from Synthetic Aperture Radar (SAR) satellite imagery. They show inundated areas and the situation caused by the collapsed saddle dam and flooded villages downstream as of July 25, 2018. SAR, attached to PALSAR-2 aboard the ALOS-2, emits microwaves and receives reflections from the ground to acquire information and can be used day or night.
The maps showed a vast brown inundation swamping houses and jungle-like areas.
Partners supporting the analysis include the Center for Research and Application for Satellite Remote Sensing of Yamaguchi University in Japan, Asian Disaster Preparedness Center (ADPC), the Japan Aerospace Exploration Agency (JAXA), the European Space Agency (ESA), the US National Aeronautics and Space Administration (NASA) Marshall Space Flight Center, and Sentinel Asia network. Yamaguchi University and ADPC provided maps of the possible flooded areas through remote sensing technology by digital image processing of the Synthetic Aperture Radar from ALOS-2. Remote Sensing Technology can be used to strengthen the decision support system to member countries in preparing for possible disasters and allocating relief supplies to assist in the emergency evacuation with recovery efforts.
Extent of water level rise
According to the MRC monitoring station in Siem Pang district (Stung Treng province in Cambodia) on the Xekong River, the observed water level recorded automatically in 15-min intervals increased by more than 4 metres from 8.39 metres at 3:15pm on July 22 to 12.47 metres at 2 pm on July 27. It was caused by heavy rains and then later by a large amount of water released from Saddle dam D.
Initial and preliminary analysis of the extent of the water level rise in Stung Treng due to the dam break shows about 20 cm compared to normal condition. Why?
The flows at Stung Treng are dominated by:
Inflows from upstream of the Mekong River from Pakxe; inflows from the 3S area of Xekong, Sesan and Srepok Rivers; and the catchment rainfall.
Since the Stung Treng Station is dominated by the Mekong inflow upstream, the direct inflow from Pakxe has a strong connection with its downstream part. If normal condition flow at Stung Treng was based on the actual flows at Pakxe, the correlation between Pakxe and Stung Treng was developed, and the increased volume during the dam break (critical condition) was calculated based on this correlation.
The water level of normal and critical conditions at Stung Treng from July 24 to 27 is showed in Table 1.  Table 2 showed the increased water volume at the same period.

(Latest Update August 06, 2018)

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