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Flood Maps of the Lower Mekong Basin
  An example of flood map derived from RADARSAT-1 ScanSAR image acquired October 17, 2007

 

Flood Change Map LMB
 

An example of flood change map

 

Flood expansion
Flood expansion and flood recession around Chau Doc gauge station between October 10 and 17, 2007
 
../assets/images/Hydrograph_CDO_20071024.jpg
Observed water level at Chao Doc gauge station (Bassac River) on October 24, 2007, published at MRC Flood Forecasting Site

Flood Maps of the Lower Mekong Basin Delivered in Near-real-Time

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)