5.Result and discussion
Figure 4 shows the map overlaid between the
water body and flood process ones, which presents
the movement of the flood. In the map the most
parts of the Huolin River and the Taoer River Basin
were flooded on Aug.16. Actually, those regions
had been flooded on Aug.9. Such inconsistency
resulted from the partial unavailability of the radar
data.
The usual shape of the Nenjiang River is
narrow and tortuous, and many oxbow lakes are
located in the riverbed. The riverbed was already
submerged on Aug.9. With the flood rising, the
floodplain and low terraces were drowned. The
flood kept spreading until it reached the national
dikes.
Zhenlai County of Jilin Province is located at
the juncture of the western part of the Nenjiang
River and the northern part of the Taoer River.
Because the dike of the Nenjiang River in Tailai
County broke on Aug.15, the flood moved south
and entered into Zhenlai County on Aug.16. The
central region of the county was submerged on
Aug.20. Although the flood peak had passed on
Aug.23, the flood continued spreading around,
especially towards the Nenjiang River. Except for
the submerged low lands, several plots of upland
were isolated in the flood.
In the downstream area of the Huolin River and
the surrounding areas of the Chagan Lake, where
Danan, Qianan and Qianguo County are situated,
the external flood converged with the internal one.
The flood from the Huoklin River approached the
Chagan Lake on Aug.9 and entered into the Lake
on Aug.16. It kept expanding in both directions
until Aug.20. After that the flood gradually
subsided. The flood from the Taoer River joined
with the one from the Nenjiang River at
Yueliangpao Lake and continued expending
around.
Fig.5 Dynamic increase of flooded areas in the counties of Northeast Jilin
Taonan and Tongyu County lie in the Taoer and
the Huolin River Basin. The flood stopped
spreading there on Aug.16 for their higher terrains.
The dynamic extension of inundated area in
northwestern counties of Jilin Province
corresponding to time is illustrated in Figure 5. For
the partial shortage of the radar data on Aug. 9, the
inundated areas is 0 in Tongyu and Taonan County.
Because the dike in Tailai County broke, the
inundated area of Zhenlai County adjacent to Tailai
County increased at the greatest rate and was above
the ones of other counties. In addition, Daan,
Qianan and Qianguo County situate in the
downstream areas of the Taoer and the Huolin
River near the Nenjiang River, thus, the increasing
rates of the inundated areas were greater.
Fig.6 Dynamic changes of various landcover areas flooded in Zhenlai
Figure 6 shows the dynamic changes of
landcover areas flooded during the monitoring
periods in Zhenlai County. Due to its low elevation,
the swamps were drowned first and their inundated
area reached the top on Aug.9 and began to
decrease afterwards. The saline lands with low
terrain were drawn after the swamps were done, the
maximum value occurred on Aug.20 for their
higher terrain than swamp’s, the condition of the
medium-density grasslands resembled the saline
lands. Since dry land, paddy land and high-density
grassland distribute on much higher uplands, their
inundated areas reached the maximum until Aug.23.
With the flood recession on Aug.29, the inundated
areas began to decrease. The values listed in Table
2 indicate the damages in Zhenlai County.
Table.2 The inundated area of different landcover types in Zhenlai (Ha)
| |
Aug.9 |
Aug.16 |
Aug.20 |
Aug.23 |
Aug.29 |
| Forest Land |
5924 |
1324 |
3821 |
3700 |
1755 |
High-density
Grassland |
7119 |
21567 |
102926 |
36210 |
19142 |
Middle-density
Grassland |
27315 |
62834 |
148352 |
246162 |
85904 |
Low-density
Grassland |
24390 |
13181 |
41691 |
18418 |
10350 |
| Floodplain Land |
4424 |
0 |
0 |
0 |
0 |
| Settlement |
2391 |
2027 |
10996 |
10910 |
4602 |
| Saline |
122174 |
294203 |
661452 |
138713 |
28226 |
| Swamp |
786610 |
176121 |
161656 |
85619 |
34496 |
| Paddy |
368 |
2086 |
10370 |
42827 |
34676 |
| Dry land |
31599 |
56030 |
60762 |
144116 |
72627 |
6.Conclusion
Radarsat data are very useful for flood
monitoring. The paper demonstrates that multi-temporal
Radarsat data of wide-scanning mode can
be utilized to dynamically monitor the flood
movement and evaluate the damages quickly and
accurately. The conclusions drawn from the study
are as follow:
-
According to the characteristics of wide-scanning
radar data and the region features in
Northwest Jilin, the data preprocesses of the
multi-temporal Radarsat images were carried
out. The flood information was extracted
from them, not only were the flood areas
vectorized through automatic tracing, but also
the attributes of the features in the flood
process maps were quickly added. Owing to
its higher precision and efficiency, the method
can be adopted to acquire quick and accurate
flood information.
- The limits of process techniques led to some
logical inconsistencies between different
temporal flood images. An approach similar
to MVC, which is often used to remove cloud
contamination on NOAA images, is proposed
to evaluate the flood pixels of different
temporal flood images. When the multi-temporal
flood images were integrated, the
maximum value at the same location was
selected, thereafter the errors of flood
boundaries were avoided.
- The landcover map is overlaid with the multi-temporal
flood maps so that the dynamic
monitoring of the flood movement and the
damage change can be performed better. In
contrast to the fusion of multi-temporal radar
images, the map of flood monitoring from
multi-temporal Radarsat image is more
suitable dynamically to analyze the flood
process and damages quantitatively.
Acknowledgement:
-
The author wish to thank the Institute of Remote Sensing Application, Chinese Academy of Sciences and
Ground Receiving Station of Satellite for their providing multi-temporal radar data. Contributions of Prof.
Hua Runkui and Prof. Sun Guangyou were notably appreciated.
