Rapid Assessment for Resilient Recovery and ... - GFDRR

systematically analyzed using accepted international methods of dam portfolio risk assessment.
There is also an urgent need for a good river-reservoir optimization model and a
flood early warning model with improved weather forecasting.
Behind the Floods
Over the course of the monsoon
season from June to October 2011,
Thailand experienced floods due to
heavy rainy in the mountains to the
north. The total rain for July, August
and September was about 1,156
mm – the highest amount of rain
recorded since recordkeeping began
in 1901. The probability of such a
rainy event has been estimated at 1
in 250 years.
Figure 4: The observed quantity of rain in July, August and
September in northern Thailand
Source: GPCC v5 analysis 1901–2009, monitoring/first
guess analysis 2010–2011. From: knmi.nl accessed
November 2011.
This high-intensity rainy generated
unprecedented flood peaks in
Figure 5: Extremes statistic of rainfall in July, August and
the Chao Phraya River Basin near
September in northern Thailand.
Nakhon Sawan of about 4,686 m 3 /s Under the assumption that the climate does not change,
against the maximum river capacity the return period of the quantity rain as measured in
of 3,500 m 3 2011 is higher than 250 years (upper green line of the
/s (figures 4 and 5).
95% reliability interval). The middle green line indicates
the extrapolation of the observations over 1901–2010,
Based on the current river discharge the blue line indicates the value of 2011.
it has been estimated that the 2011
flood event is a 1 in 50–100 year
event. However, there is a need for
further analysis. The preferred
approach would be to analyze rain
level statistics for the entire Chao
Phraya catchment and translate
these into discharge and overland
flow volumes by means of a hydrological
model of the river catchment.
There is also a need for a flood risk
analysis including the quantification of
probabilities of rainy, river discharge
and sea water level, as well as their
influence on water levels in the river/drainage system and on inundation depths in the city
of Bangkok.
THAI FLOOD 2011 RAPID ASSESSMENT FOR RESILIENT RECOVERY AND RECONSTRUCTION PLANNING
77