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important effect of the volume scattering. This can
give a very strong return if the ground is covered
with water. (Ahern). Double-bounce scattering is a
geometrically similar situation to a two dimensional
corner reflection. Buildings and trees can redirect a
radar beam which was backscattered from a smooth
water surface back to the radar sensor. This is why
flooded towns and forests can look even brighter
than unflooded areas.
S. RESULTS :
The northern Moravia was imaged twice with
RADARSAT. The first image revealed the flood
peak in the lowlands while the second image
featured the post-flood situation. The mountainous
area of northern Moravia, which was also the main
source of water volume suffered the most
destruction. Destruction due to the flood water
included roads, railways, bridges, houses, and many
other types of infrastructure features. The affected
communities within the narrow valleys of the high
mountain ranges could not be analyzed by
RADARSAT imagery for two reasons. Radar
shadow was a factor and the duration of the flood in
this region was short and had ended before the first
RADARSAT image was obtained.
The central Moravia and southern Poland,
lowland regions were then studied on the image of
July 10. An example of flooded areas, and
permanent water bodies, are shown as black areas
(Figure 1). Water surfaces without waves act as a
smooth surface. When the radar sensor transmits a
beam of radar energy towards this smooth surface
the result was no backscatter return to the radar
sensor but rather the scattering of the radar energy
away from the sensor. Pixels for these areas have
zero values and water areas are black solid
phenomena on images (Leconte et al).
Pixel classification techniques, an often
used method in image processing was performed for
surface water. They detected not only areas with
surface water excluding forest and urban regions
but also shadows in high relief areas. These
shadows have the same values of reflection as water
bodies: their measured values are the same - zero or
very low values in both cases.
It was necessary to use two images from
two different time intervals in order to distinguish
flooded areas from permanent surface water. A
multitemporal color composite (RGB) of the two
images (one of them must be used twice) can
distinguish permanent versus flood water
immediately. Permanent surface water was black
(Figure 3) whereas flooded areas were lighter (blue
in color version).
Figure 2 represents the same area on the
image for July 27, 1997. Brighter features within
the imagery coincided with previously flooded
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
areas. Brighter features were related to either terrain
with greater surface roughness due to ploughing or
the sedimentation of course materials or higher soil
moisture content (RADARSAT Illuminated,Brown,
Engman et al.). Sedimentation of coarse materials
did not occur as the result of rather low water
velocities in this area. Nor was ploughing the reason
for higher backscatter values. The region was
divided into small long private fields which were
not significantly damaged by the flood because
crops continued in their growth after flood levels
declined. It was therefore concluded that excessive
soil moisture was the reason for the brighter
backscatter values
Pixel values on the image of July 24 are
lower for 6 per cent compared to July 14. Pixels
values on the image of July 27 are higher for about
21 per cent in comparison to July 14 in forest
targets. Higher pixel values of forest in inundation
is a proof of higher reflectance of flooded forest
region. Comparison of unflooded and flooded forest
is on Figure 5.
There was an area around Olomouc (town
in central Moravia) which did not show the same
effect on the same RADARSAT images. This area
is quite flat, similar to southern Poland but probably
with different soil perviousness. Hydrogeological,
geological and pedological conditions for the area
around Olomouc are different. The previously
flooded fields could not be detected on the post-
flood image.
Southern Moravia was imaged by
RADARSAT at three time intervals. On July 14,
1997 the flood peak was captured on the first image
(Figure 4a, b, and c). The same area was brighter on
the images of July 24, and July 27. Comparison of
these two images can be a
confirmation of the fact that steep incidence angles
(July 27) provide the greatest amount of
information regarding soil moisture and minimize
roughness effects (Ulaby). The image of July 24
had a shallower incidence angle (36? - 42?) and that
was why the area was not as bright as the same area
on the image of July 27 ( with incidence angle 20? -
31?) if compared to its surroundings. This was
another example of possibility to delineate flooded
area on a post-flood radar image. To determine
reliably these areas required images from a given
area at the moment of existing higher soil moisture.
This moment differs for various soil types,
hydrogeological conditions, terrain slopes, and
canopy. To determine the time when soil moisture
levels were due to the previous flood must be a
subject of more detailed studies in areas of interest.
More frequent post-flood images could offer this
information.
A multitemporal color composite (RGB) of
the three images can distinguish permanent and
flood water immediately (Figure 5). Permanent
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