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Since 1999, the RADARSAT-1 satellite has been used
by the Mekong River Commission to accurately map the extent of the
annual floods, and supplement and complement data from river gauges
and flood models. RADARSAT-1 is particularly useful for flood mapping
because of its ability to see through clouds and sensitivity to
flooding.
In 2007, the regular annual flood mapping took
a step forward through the implementation of a near-real-time flood
mapping service implemented in partnership with the Canadian Space
Agency and Hatfield Consultants (Canada). Every 5 to 7 days, this
service provides a set of flood map products to MRC, which are made
available to users of the MRC Flood Forecast web portal within 24
hours of acquisition (http://ffw.mrcmekong.org/).
Flood extent and flood area change products are produced for the
Lower Mekong Basin and individual Hydrological Stations, to compliment
gauge data. Between June and December 2007, flood maps will have
been delivered on 24 individual dates, with a maximum of 42 individual
flood maps per date.
The weekly acquisition of radar imagery over the
Lower Mekong Basin since July, has provided an unprecedented amount
of information on the pattern of flooding. October is usually the
month when the monsoon floods in Southeast Asia start to recede.
In 2007, however, October appears to be the month of peak flooding
in the lower Mekong floodplain and the delta area.
The October 10th and October 17th RADARSAT-1 images
show the “flood extent” and “flood change”
products provided in the flood mapping service. The "flood
change" product reveals the flood dynamics, with flood expansion
and flood recession between October 10th and 17th as identified
in the colour scheme. The course of the Mekong, Tonle Sap and Bassac
rivers, as well as other permanent water bodies, are coloured black;
the huge extent of the area flooded on both October 10th and 17th
is shown in a light blue colour; the greater extent or expansion
of the October 17th flood situation is shown by the red colour;
and those areas that were flooded on the October 10th image but
not on the October 17th image, representing flood recession, are
shown in green. There are several areas in the lower Mekong basin
where the October 17th flood extent (red colour) has exceeded the
October 10th flood extent. Examples include the western fringe of
the Tonle Sap or Great Lake, in the floodplain south of Phnom Penh,
and in the delta area.
A flood change map centred on Chau Doc gauge station
can be used in conjunction with actual river gauge measurements
from the MRC flood forecasting site. The latest gauge information
can be found at: http://ffw.mrcmekong.org/stations/cdo.htm#map.
The flood gauge information (see the hydrograph of Chau Doc gauge
station on October 24) confirms the interpretation of the RADARSAT–1
imagery, and the flood change product. A belt of monsoon rain was
active in the southern part of the lower Mekong basin, and the river
gauges at Chau Doc, Tan Chau and Koh Khel along the border between
Cambodia and Viet Nam recorded water levels that crossed the alarm
level around September 28th. During the month of October, much of
the accumulating flood water is typically draining toward the rice
growing areas to the east and to the west of the main stem of the
Mekong and Bassac rivers; however, the monsoon rains caused unexpected
flooding. At the Chau Doc and Tan Chau gauges, water levels have
reached very close to the flood level mark of 3.5 m and 4.2 m, respectively.
The overall trend of this year's flood season suggests that 2007
may be considered as a “normal” flood year in the lower
Mekong basin.
Flood mapping using RADARSAT-1 is valuable for
Component 1 of FMMP to support the Annual MRC Flood Reports. Near-real-time
flood map products are also potentially a valuable part of future
emergency flood response and mitigation.
High resolution jpeg images:
» RADARSAT-1
ScanSAR image derived flood map, 17 October 2007 (1 MB)
» Changes
in flooded areas between 10 and 17 October 2007 (1 MB)
» Changes
in flooded areas around Chao Doc gauge station between 10 and 17
October 2007 (1 MB)
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